Unstable standard candles. Periodic light curve modulation in fundamental mode classical Cepheids [SSA]


We report the discovery of periodic modulation of pulsation in 51 fundamental mode classical Cepheids of the Magellanic Clouds observed by the Optical Gravitational Lensing Experiment. Although the overall incidence rate is very low, about 1 per cent in each of the Magellanic Clouds, in the case of the SMC and pulsation periods between 12 and 16d the incidence rate is nearly 40 per cent. On the other hand, in the LMC the highest incidence rate is 5 per cent for pulsation periods between 8 and 14d, and the overall amplitude of the effect is smaller. It indicates that the phenomenon is metallicity dependent. Typical modulation periods are between 70 and 300d. In nearly all stars the mean brightness is modulated, which, in principle, may influence the use of classical Cepheids for distance determination. Fortunately, the modulation of mean brightness does not exceed 0.01 mag in all but one star. Also, the effect averages out in typical observations spanning a long time base. Consequently, the effect of modulation on the determination of the distance moduli is negligible. The relative modulation amplitude of the fundamental mode is also low and, with one exception, it does not exceed 6 per cent. The origin of the modulation is unknown. We draw a hypothesis that the modulation is caused by the 2:1 resonance between the fundamental mode and the second overtone that shapes the famous Hertzsprung bump progression.

Read this paper on arXiv…

R. Smolec
Fri, 17 Mar 17

Comments: 13 pages, 14 figures, accepted for publication in MNRAS

SPHERE / ZIMPOL observations of the symbiotic system R Aqr. I. Imaging of the stellar binary and the innermost jet clouds [SSA]


R Aqr is a symbiotic binary system consisting of a mira variable, a hot companion with a spectacular jet outflow, and an extended emission line nebula. We have used R Aqr as test target for the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the Very Large Telescope (VLT).
We compare our observations with data from the Hubble Space Telescope (HST) and illustrate the complementarity of the two instruments. We determine from the Halpha emission the position, size, geometric structure, and line fluxes of the jet source and the clouds in the innermost region (<2″) of R Aqr and determine Halpha emissivities mean density, mass, recombination time scale, and other cloud parameters.
Our data resolve for the first time the R Aqr binary and we measure for the jet source a relative position 46+/-1 mas West of the mira. The central jet source is the strongest Halpha component. North east and south west from the central source there are many clouds with very diverse structures. We see in the SW a string of bright clouds arranged in a zig-zag pattern and, further out, more extended bubbles. In the N and NE we see a bright, very elongated filamentary structure and faint perpendicular “wisps” further out. Some jet clouds are also detected in the ZIMPOL [OI] and He I filters, as well as in the HST line filters for Halpha, [OIII], [NII], and [OI]. We determine jet cloud parameters and find a very well defined anti-correlation between cloud density and distance to the central binary. Future Halpha observations will provide the orientation of the orbital plane of the binary and allow detailed hydrodynamical investigations of this jet outflow and its interaction with the wind of the red giant companion.

Read this paper on arXiv…

H. Schmid, A. Bazzon, J. Milli, et. al.
Fri, 17 Mar 17

Comments: 24 pages, 14 figures (accepted for publication in Astronomy and Astrophysics)

Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results [SSA]


Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast-rotating objects with apparently normal surface nitrogen abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing. Methods: We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity. Results: This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way.

Read this paper on arXiv…

C. Cazorla, T. Morel, Y. Naze, et. al.
Fri, 17 Mar 17

Comments: accepted for publication by A&A

Spin alignment of stars in old open clusters [SSA]


Stellar clusters form by gravitational collapse of turbulent molecular clouds, with up to several thousand stars per cluster. They are thought to be the birthplace of most stars and therefore play an important role in our understanding of star formation, a fundamental problem in astrophysics. The initial conditions of the molecular cloud establish its dynamical history until the stellar cluster is born. However, the evolution of the cloud’s angular momentum during cluster formation is not well understood. Current observations have suggested that turbulence scrambles the angular momentum of the cluster-forming cloud, preventing spin alignment amongst stars within a cluster. Here we use asteroseismology to measure the inclination angles of spin axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The stars within each cluster show strong alignment. Three-dimensional hydrodynamical simulations of proto-cluster formation show that at least 50 % of the initial proto-cluster kinetic energy has to be rotational in order to obtain strong stellar-spin alignment within a cluster. Our result indicates that the global angular momentum of the cluster-forming clouds was efficiently transferred to each star and that its imprint has survived after several gigayears since the clusters formed.

Read this paper on arXiv…

E. Corsaro, Y. Lee, R. Garcia, et. al.
Fri, 17 Mar 17

Comments: 14 pages, 3 figures, 1 table. Published in Nature Astronomy

Evolutionary sequences for hydrogen-deficient white dwarfs [SSA]


We present a set of full evolutionary sequences for white dwarfs with hydrogen-deficient atmospheres. We take into account the evolutionary history of the progenitor stars, all the relevant energy sources involved in the cooling, element diffusion in the very outer layers, and outer boundary conditions provided by new and detailed non-gray white dwarf model atmospheres for pure helium composition. These model atmospheres are based on the most up-to-date physical inputs. Our calculations extend down to very low effective temperatures, of $\sim 2\,500$~K, provide a homogeneous set of evolutionary cooling tracks that are appropriate for mass and age determinations of old hydrogen-deficient white dwarfs, and represent a clear improvement over previous efforts, which were computed using gray atmospheres.

Read this paper on arXiv…

M. Camisassa, L. Althaus, R. Rohrmann, et. al.
Fri, 17 Mar 17

Comments: 40 pages, 13 figures. To be published in ApJ

Contribution of mode coupling and phase-mixing of Alfvén waves to coronal heating [SSA]


Phase-mixing of Alfv\’en waves in the solar corona has been identified as one possible candidate to explain coronal heating. While this scenario is supported by observations of ubiquitous oscillations in the corona carrying sufficient wave energy and by theoretical models that have described the concentration of energy in small scale structures, it is still unclear whether this wave energy can maintain the million degree solar corona. The aim of this work is to assess how much energy can be converted by a phase-mixing process triggered by the propagation of Alfv\’enic waves in a cylindric coronal structure, such as a coronal loop, and to estimate the impact on the coronal heating. To this end, we run 3D MHD simulations of a magnetised cylinder where the Alfv\’en speed varies through a boundary shell and a footpoint driver is set to trigger kink modes which mode couple to torsional Alfv\’en modes in the boundary shell. These Alfv\’en waves are expected to phase-mix and the system allows us to study the subsequent thermal energy deposition. We run a reference simulation to explain the main process and then we vary simulation parameters. When we take into consideration high values of magnetic resistivity and strong footpoint drivers, we find i) that phase-mixing leads to a temperature increase of the order of $10^5$ K or less, depending on the structure of the boundary shell, ii) that this energy is able to balance the radiative losses only in the localised region involved in the heating, iii) and how the boundary layer and the persistence of the driver influence the thermal structure of the system. Our conclusion is that due to the extreme physical parameters we adopted and the moderate impact on the heating of the system, it is unlikely that phase-mixing can contribute on a global scale to the heating of the solar corona.

Read this paper on arXiv…

P. Pagano and I. Moortel
Fri, 17 Mar 17

Comments: N/A

Time Dependent Models of Magnetospheric Accretion onto Young Stars [SSA]


Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner and the source of this variability remains an open problem, particularly on the shortest (~ day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ~ day long time-scales (e.g., due to turbulence) this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star the observed time-dependence will be a rapid increase in accretion luminosity followed by a slower decline and could be an explanation for some of the short period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated, multi-dimensional simulations, and observations.

Read this paper on arXiv…

C. Robinson, J. Owen, C. Espaillat, et. al.
Fri, 17 Mar 17

Comments: 16 Pages, 12 figures

Evidence of chaotic modes in the analysis of four delta Scuti stars [SSA]


Since CoRoT observations unveiled the very low amplitude modes that form a flat plateau in the power spectrum structure of delta Scuti stars, the nature of this phenomenon, including the possibility of spurious signals due to the light curve analysis, has been a matter of long-standing scientific debate. We contribute to this debate by finding the structural parameters of a sample of four delta Scuti stars, CID 546, CID 3619, CID 8669, and KIC 5892969, and looking for a possible relation between these stars’ structural parameters and their power spectrum structure. For the purposes of characterization, we developed a method of studying and analysing the power spectrum with high precision and have applied it to both CoRoT and Kepler light curves. We obtain the best estimates to date of these stars’ structural parameters. Moreover, we observe that the power spectrum structure depends on the inclination, oblateness, and convective efficiency of each star. Our results suggest that the power spectrum structure is real and is possibly formed by 2-period island modes and chaotic modes.

Read this paper on arXiv…

S. Forteza, T. Cortes, A. Hernandez, et. al.
Thu, 16 Mar 17

Comments: N/A

The early B-type star Rho Oph A is an X-ray lighthouse [SSA]


We present the results of a 140 ks XMM-Newton observation of the B2 star $\rho$ Ophiuchi A. The star exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to the one we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is almost only due to an increase of the plasma emission measure, while the second increase of rate is mainly due is a major flare, with temperatures in excess of 60 MK ($kT\sim5$ keV). From the analysis of its rise we infer a magnetic field of $\ge300$ G and a size of the flaring region of $\sim1.4-1.9\times10^{11}$ cm, which corresponds to $\sim25\%-30\%$ of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface, or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of $\ge$2.5 years, and suggests an overall large scale dipolar magnetic field that produce an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by both its young age and the tight orbit like in RS Cvn systems and $\rho$ Ophiuchi would constitute an extreme system worth of further investigation.

Read this paper on arXiv…

I. Pillitteri, S. Wolk, F. Reale, et. al.
Thu, 16 Mar 17

Comments: 10 pages, 7 figures, 2 tables, A&A accepted

The long-period binary central stars of the planetary nebulae NGC 1514 and LoTr 5 [SSA]


The importance of long-period binaries on the formation and evolution of planetary is still rather poorly understood, in part due to the lack of central star systems known to comprise such long-period binaries.
Here, we report on the latest results from the on-going Mercator-HERMES survey for variability in the central stars of planetary nebulae.
We present a study of the central stars of NGC 1514, BD+30$^\circ$623, the spectrum of which shows features associated with a hot nebular progenitor as well as a possible A-type companion. Cross-correlation of high-resolution HERMES spectra against synthetic spectra shows the system to be a highly eccentric ($e\sim0.5$), double-lined binary with a period of $\sim$3300 days. Previous studies indicated that the cool component might be a Horizontal Branch star of mass $\sim$0.55 M$_\odot$ but the observed radial velocity amplitudes rule out such a low mass. Assuming the nebular symmetry axis and binary orbital plane are perpendicular, the data are more consistent with a post-main-sequence star ascending towards the Giant Branch.
We also present the continued monitoring of the central star of LoTr 5, HD 112313, which has now completed one full cycle, allowing the orbital period (P$\sim$2700 days) and eccentricity ($e\sim0.3$) to be derived.
To date, the orbital periods of BD+30$^\circ$623 and HD 112313 are the longest to have been measured spectroscopically in the central stars of planetary nebulae. Furthermore, these systems, along with BD+33$^\circ$2642, comprise the only spectroscopic wide-binary central stars currently known.

Read this paper on arXiv…

D. Jones, H. Winckel, A. Aller, et. al.
Thu, 16 Mar 17

Comments: 4 pages, 4 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics Letters

The quest for blue supergiants: binary merger models for the evolution of the progenitor of SN 1987A [SSA]


We present the results of a detailed, systematic stellar evolution study of binary mergers for blue supergiant (BSG) progenitors of Type II supernovae. In particular, these are the first evolutionary models that can simultaneously reproduce nearly all observational aspects of the progenitor of SN 1987A, $\text{Sk}-69\,^{\circ}202$, such as its position in the HR diagram, the enrichment of helium and nitrogen in the triple-ring nebula, and its lifetime before its explosion. The merger model, based on the one proposed by Podsiadlowski 1992 et al. and Podsiadlowski 2007 et al., consists of a main sequence secondary star that dissolves completely in the common envelope of the primary red supergiant at the end of their merger. We empirically explore a large initial parameter space, such as primary masses ($15\,\text{M}_{\odot}$, $16\,\text{M}_{\odot}$, and $17\,\text{M}_{\odot}$), secondary masses ($2\,\text{M}_{\odot}$, $3\,\text{M}_{\odot}$, …, $8\,\text{M}_{\odot}$) and different depths up to which the secondary penetrates the He core of the primary during the merger. The evolution of the merged star is continued until just before iron-core collapse and the surface properties of the 84 pre-supernova models ($16\,\text{M}_{\odot}-23\,\mathrm{M}_{\odot}$) computed have been made available in this work. Within the parameter space studied, the majority of the pre-supernova models are compact, hot BSGs with effective temperature $>12\,\text{kK}$ and radii of $30\,\text{R}_{\odot}-70\,\mathrm{R}_{\odot}$ of which six match nearly all the observational properties of $\text{Sk}-69\,^{\circ}202$.

Read this paper on arXiv…

A. Menon and A. Heger
Thu, 16 Mar 17

Comments: Submitted to MNRAS. 21 pages, 11 figures, 7 tables

Multi-fluid Approach to High-frequency Waves in Plasmas. II. Small-amplitude Regime in Partially Ionized Media [SSA]


The presence of neutral species in a plasma has been shown to greatly affect the properties of magnetohydrodynamic waves. For instance, the interaction between ions and neutrals through momentum transfer collisions causes the damping of Alfv\’en waves and alters their oscillation frequency and phase speed. When the collision frequencies are larger than the frequency of the waves, single-fluid magnetohydrodynamic approximations can accurately describe the effects of partial ionization, since there is a strong coupling between the various species. However, at higher frequencies, the single-fluid models are not applicable and more complex approaches are required. Here, we use a five-fluid model with three ionized and two neutral components, which takes into consideration Hall’s current and Ohm’s diffusion in addition to the friction due to collisions between different species. We apply our model to plasmas composed of hydrogen and helium, and allow the ionization degree to be arbitrary. By means of the analysis of the corresponding dispersion relation and numerical simulations, we study the properties of small-amplitude perturbations. We discuss the effect of momentum transfer collisions on the ion-cyclotron resonances and compare the importance of magnetic resistivity, ion-neutral and ion-ion collisions on the wave damping at various frequency ranges. Application to partially ionized plasmas of the solar atmosphere are performed.

Read this paper on arXiv…

D. Martinez-Gomez, R. Soler and J. Terradas
Thu, 16 Mar 17

Comments: 24 pages, 15 figures. Revised version published in The Astrophysical Journal

Interferometric confirmation of "water fountain" candidates [SSA]


Water fountain stars (WFs) are evolved objects with water masers tracing high-velocity jets (up to several hundreds of km s$^{-1}$). They could represent one of the first manifestations of collimated mass-loss in evolved objects and thus, be a key to understanding the shaping mechanisms of planetary nebulae. Only 13 objects had been confirmed so far as WFs with interferometer observations. We present new observations with the Australia Telescope Compact Array and archival observations with the Very Large Array of four objects that are considered to be WF candidates, mainly based on single-dish observations. We confirm IRAS 17291-2147 and IRAS 18596+0315 (OH 37.1-0.8) as bona fide members of the WF class, with high-velocity water maser emission consistent with tracing bipolar jets. We argue that IRAS 15544-5332 has been wrongly considered as a WF in previous works, since we see no evidence in our data nor in the literature that this object harbours high-velocity water maser emission. In the case of IRAS 19067+0811, we did not detect any water maser emission, so its confirmation as a WF is still pending. With the result of this work, there are 15 objects that can be considered confirmed WFs. We speculate that there is no significant physical difference between WFs and obscured post-AGB stars in general. The absence of high-velocity water maser emission in some obscured post-AGB stars could be attributed to a variability or orientation effect.

Read this paper on arXiv…

J. Gomez, O. Suarez, J. Rizzo, et. al.
Thu, 16 Mar 17

Comments: To be published in Monthly Notices of the Royal Astronomical Society. 13 pages, 10 figures, 7 tables

SIGS – Seismic Inferences for Glitches in Stars [SSA]


The increased amount of high precision seismic data for solar-like stars calls for the existence of tools that can extract information from such data. In the case of the study of acoustic glitches there are no publicly available tools and most existing ones require a deep knowledge of their implementation. In this work a tool is presented that aims to both simplify the interaction with the user and also be capable of working automatically to determine properties of acoustic glitches from seismic data of solar-like stars. This tool is shown to work with both the Sun and other solar analogs but also shows that are still severe limitations to the methods used, when considering smaller datasets.

Read this paper on arXiv…

L. Pereira, J. Faria and M. Monteiro
Thu, 16 Mar 17

Comments: 4 pages; 5 figures; Submitted to the Proceedings of the joint TASC2/KASC9 workshop – SPACEINN & HELAS8 conference 2016

The Suppression and Promotion of Magnetic Flux Emergence in Fully Convective Stars [SSA]


Evidence of surface magnetism is now observed on an increasing number of cool stars. The detailed manner by which dynamo-generated magnetic fields giving rise to starspots traverse the convection zone still remains unclear. Some insight into this flux emergence mechanism has been gained by assuming bundles of magnetic field can be represented by idealized thin flux tubes (TFTs). Weber & Browning (2016) have recently investigated how individual flux tubes might evolve in a 0.3 solar-mass M dwarf by effectively embedding TFTs in time-dependent flows representative of a fully convective star. We expand upon this work by initiating flux tubes at various depths in the upper 50-75% of the star in order to sample the differing convective flow pattern and differential rotation across this region. Specifically, we comment on the role of differential rotation and time-varying flows in both the suppression and promotion of the magnetic flux emergence process.

Read this paper on arXiv…

M. Weber, M. Browning, S. Boardman, et. al.
Thu, 16 Mar 17

Comments: 8 pages, 5 figures, accepted for publication in the International Astronomical Union Proceedings Series for IAUS 328 – ‘Living Around Active Stars’

New Evidence for the Dynamical Decay of a Multiple System in the Orion Kleinmann-Low Nebula [SSA]


We have measured astrometry for members of the Orion Nebula Cluster with images obtained in 2015 with the Wide Field Camera 3 on board the Hubble Space Telescope. By comparing those data to previous measurements with NICMOS on Hubble in 1998, we have discovered that a star in the Kleinmann-Low Nebula, source x from Lonsdale et al. (1982), is moving with an unusually high proper motion of 29 mas/yr, which corresponds to 55 km/s at the distance of Orion. Previous radio observations have found that three other stars in the Kleinmann-Low Nebula (BN and sources I and n) have high proper motions (5-14 mas/yr) and were near a single location ~540 years ago, and thus may have been members of a multiple system that dynamically decayed. The proper motion of source x is consistent with ejection from that same location 540 years ago, which provides strong evidence that the dynamical decay did occur and that the runaway star BN originated in the Kleinmann-Low Nebula rather than the nearby Trapezium cluster. However, our constraint on the motion of source n is significantly smaller than the most recent radio measurement, which indicates that it did not participate in the event that ejected the other three stars.

Read this paper on arXiv…

K. Luhman, M. Robberto, J. Tan, et. al.
Thu, 16 Mar 17

Comments: Astrophysical Journal Letters, in press

DeepVel: deep learning for the estimation of horizontal velocities at the solar surface [SSA]


Many phenomena taking place in the solar photosphere are controlled by plasma motions. Although the line-of-sight component of the velocity can be estimated using the Doppler effect, we do not have direct spectroscopic access to the components that are perpendicular to the line-of-sight. These components are typically estimated using methods based on local correlation tracking. We have designed DeepVel, an end-to-end deep neural network that produces an estimation of the velocity at every single pixel and at every time step and at three different heights in the atmosphere from just two consecutive continuum images. We confront DeepVel with local correlation tracking, pointing out that they give very similar results in the time- and spatially-averaged cases. We use the network to study the evolution in height of the horizontal velocity field in fragmenting granules, supporting the buoyancy-braking mechanism for the formation of integranular lanes in these granules. We also show that DeepVel can capture very small vortices, so that we can potentially expand the scaling cascade of vortices to very small sizes and durations.

Read this paper on arXiv…

A. Ramos, I. Requerey and N. Vitas
Thu, 16 Mar 17

Comments: 9 pages, 5 figures, submitted to A&A

An Investigation of the Rotational Properties of Magnetic Chemically Peculiar Stars [SSA]


The magnetic chemically peculiar (mCP) stars of the upper main sequence exhibit strong, globally-organised magnetic fields which are inclined to the rotational axis and facilitate the development of surface abundance inhomogeneities resulting in photometric and spectroscopic variability. Therefore, mCP stars are perfectly suited for a direct measurement of the rotational period without the need for any additional calibrations. We have investigated the rotational properties of mCP stars based on an unprecedentedly large sample consisting of more than 500 objects with known rotational periods. Using precise parallaxes from the Hipparcos and Gaia satellite missions, well-established photometric calibrations and state-of-the-art evolutionary models, we have determined the location of our sample stars in the Hertzsprung-Russell diagram and derived astrophysical parameters such as stellar masses, effective temperature, radii, inclinations and critical rotational velocities. We have confirmed the conservation of angular momentum during the main sequence evolution; no signs of additional magnetic braking were found. The inclination angles of the rotational axes are randomly distributed, although an apparent excess of fast rotators with comparable inclination angles has been observed. We have found a rotation rate of $\upsilon/\upsilon_{\rm crit} \geq 0.5$ for several stars, whose characteristics cannot be explained by current models. For the first time, we have derived the relationship between mass and rotation rate of mCP stars, and provide an analysis that links mass and rotation with magnetic field strength. Our sample is unique and offers crucial input for forthcoming evolutionary models that include the effects of magnetic fields for upper main sequence stars.

Read this paper on arXiv…

M. Netopil, E. Paunzen, S. Hummerich, et. al.
Thu, 16 Mar 17

Comments: 13 pages, 9 figures, accepted for publication in MNRAS

The metal rich abundance pattern – spectroscopic properties and abundances for 107 main-sequence stars [SSA]


We report results from the high resolution spectral analysis of the 107 metal rich (mostly [Fe/H]$\ge$7.67 dex) target stars from the Calan-Hertfordshire Extrasolar Planet Search program observed with HARPS. Using our procedure of finding the best fit to the absorption line profiles in the observed spectra, we measure the abundances of Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Zn, and we then compare them with known results from different authors. Most of our abundances agree with these works at the level of $\pm$0.05 dex or better for the stars we have in common. However, we do find systematic differences that make direct inferences difficult. Our analysis suggests that the selection of line lists and atomic line data along with the adopted continuum level influence these differences the most. At the same time, we confirm the positive trends of abundances versus metallicity for Na, Mn, Ni, and to a lesser degree, Al. A slight negative trend is observed for Ca, whereas Si and Cr tend to follow iron. Our analysis allows us to determine the positively skewed normal distribution of projected rotational velocities with a maximum peaking at 3 km s$^{-1}$. Finally, we obtained a Gaussian distribution of microturbulent velocities that has a maximum at 1.2 km s$^{-1}$ and a full width at half maximum $\Delta v_{1/2}=$0.35 km s$^{-1}$, indicating that metal rich dwarfs and subgiants in our sample have a very restricted range in microturbulent velocity.

Read this paper on arXiv…

O. Ivanyuk, J. Jenkins, Y. Pavlenko, et. al.
Thu, 16 Mar 17

Comments: To be published in the Monthly Notices of the Royal Astronomical Society

Photometry of the long period dwarf nova GY Hya [SSA]


Although comparatively bright, the cataclysmic variable GY Hya has not attracted much attention in the past. As part of a project to better characterize such systems photometrically, we observed light curves in white light, each spanning several hours, at Bronberg Observatory, South Africa, in 2004 and 2005, and at the Observat\’orio do Pico dos Dias, Brazil, in 2014 and 2016. These data permit to study orbital modulations and their variations from season to season. The orbital period, already known from spectroscopic observations of Peters & Thorstensen (2005), is confirmed through strong ellipsoidal variations of the mass donor star in the system and the presence of eclipses of both components. A refined period of 0.34723972~(6) days and revised ephemeris are derived. Seasonal changes in the average orbital light curve can qualitatively be explained by variations of the contribution of a hot spot to the system light together with changes of the disk radius. The amplitude of the ellipsoidal variations and the eclipse contact phases permit to put some constraints on the mass ratio, orbital inclination and the relative brightness of the primary and secondary components. There are some indications that the disk radius during quiescence, expressed in units of the component separation, is smaller than in other dwarf novae.

Read this paper on arXiv…

A. Bruch and B. Monard
Thu, 16 Mar 17

Comments: N/A

WISE J080822.18-644357.3 – a 45 Myr-old accreting M dwarf hosting a pre-transitional disc [SSA]


WISE J080822.18-644357.3 (WISE J0808-6443) was recently identified as a new M dwarf debris disc system and a candidate member of the 45 Myr-old Carina association. Given that the strength of its infrared excess ($L_{\mathrm{IR}}/L_{\mathrm{\star}}$ ~ 0.1) appears to be more consistent with a young protoplanetary disc, we present the first optical spectra of the star and reassess its evolutionary and membership status. We find WISE J0808-6443 to be a Li-rich M5 star with strong H$\alpha$ emission ($-125 \lt$ EW $\lt -65$ Ang over 4 epochs) whose strength and broad width are consistent with accretion at a low level ($\sim 10^{-10} M_{\odot} yr^{-1}$) from its disc. The spectral energy distribution of the star is consistent with a two-temperature disc structure, with a hot inner disc ($T_{\mathrm{in}}$ ~ 1100 K) at orbital radius ~0.006 au and a warm outer disc ($T_{\mathrm{out}}$ ~ 230 K) at radius ~0.12 au. The dust belts likely correspond to the sublimation temperatures of silicates and icy planetesimals, respectively. We calculate an improved proper motion based on archival astrometry, and combined with a new radial velocity, the kinematics of the star are consistent with membership in Carina at a kinematic distance of $90\pm9$ pc. The spectroscopic and photometric data are consistent with WISE J0808-6443 being a $\sim 0.1 M_{\odot}$ Classical T-Tauri star and one of the oldest known accreting M-type stars. These results suggest that the upper limit on the lifetimes of gas-rich discs around the lowest mass stars may be longer than previously recognised, or some mechanism may be responsible for regenerating short-lived discs at later stages of pre-main sequence evolution. Low-level accretion for tens of Myr may provide a mechanism for the migration, eccentricity dampening, and resonance locking of small planets, like those recently reported for the TRAPPIST-1 exoplanetary system.

Read this paper on arXiv…

S. Murphy, E. Mamajek and C. Bell
Thu, 16 Mar 17

Comments: Submitted to MNRAS; 9 pages, 9 figures and 5 tables

A dynamo mechanism as the potential origin of the long cycle in Double Periodic Variables [SSA]


The class of Double Period Variables (DPVs) consists of close interacting binaries, with a characteristic long period that is an order of magnitude longer than the corresponding orbital period, many of them with a characteristic ratio of about 3.5×10^1. We consider here the possibility that the accretion flow is modulated as a result of a magnetic dynamo cycle. Due to the short binary separations, we expect the rotation of the donor star to be synchronized with the rotation of the binary due to tidal locking. We here present a model to estimate the dynamo number and the resulting relation between the activity cycle length and the orbital period, as well as an estimate for the modulation of the mass transfer rate. The latter is based on Applegate’s scenario, implying cyclic changes in the radius of the donor star and thus in the mass transfer rate as a result of magnetic activity. Our model is applied to a sample of 17 systems with known physical parameters, 10 also with known orbital periods. In spite of the uncertainties of our simplified framework, the results show a reasonable agreement, indicating that a dynamo interpretation is potentially feasible. At the same time, we note that the orbital period variations resulting from Applegate’s model are sufficiently small to be consistent with the data. We conclude that both larger samples with known physical parameters as well as potential direct probes of the magnetism of the donor star, including cold spots as well as polarization, will be valuable to further constrain the nature of these systems.

Read this paper on arXiv…

D. Schleicher and R. Mennickent
Tue, 14 Mar 17

Comments: 10 pages, 2 tables, 1 figure, accepted for publication with A&A

Non-LTE line formation of Fe in late-type stars IV: Modelling of the solar centre-to-limb variation in 3D [SSA]


Our ability to model the shapes and strengths of iron lines in the solar spectrum is a critical test of the accuracy of the solar iron abundance, which sets the absolute zero-point of all stellar metallicities. We use an extensive 463-level Fe atom with new photoionisation cross-sections for FeI as well as quantum mechanical calculations of collisional excitation and charge transfer with neutral hydrogen; the latter effectively remove a free parameter that has hampered all previous line formation studies of Fe in non-local thermodynamic equilibrium (NLTE). For the first time, we use realistic 3D NLTE calculations of Fe for a quantitative comparison to solar observations. We confront our theoretical line profiles with observations taken at different viewing angles across the solar disk with the Swedish 1-m Solar Telescope. We find that 3D modelling well reproduces the observed centre-to-limb behaviour of spectral lines overall, but highlight aspects that may require further work, especially cross-sections for inelastic collisions with electrons. Our inferred solar iron abundance is log(eps(Fe))=7.48+-0.04.

Read this paper on arXiv…

K. Lind, A. Amarsi, M. Asplund, et. al.
Tue, 14 Mar 17

Comments: 13 pages, 8 figures, accepted for publication in MNRAS

Little Bear's Pulsating Stars: Variable Star Census of dSph UMi Galaxy [SSA]


Recent observations and a photometric search for variable stars in the Ursa Minor dwarf spheroidal galaxy (UMi dSph) are presented. Our observations were taken at Apache Point Observatory in 2014 and 2016 using the 0.5m ARCSAT telescope and the West Mountain Observatory 0.9m telescope of Brigham Young University in 2016. Previously known RR Lyrae stars in our field of view of the UMi dSph are identified, and we also catalog new variable star candidates. Tentative classifications are given for some of the new variable stars. We have conducted period searches with the data collected with the WMO telescope. Our ultimate goal is to create an updated catalog of variable stars in the UMi dSph and to compare the RR Lyrae stellar characteristics to other RR Lyrae stars found in the Local Group dSph galaxies.

Read this paper on arXiv…

K. Kinemuchi, E. Jeffery, C. Kuehn, et. al.
Tue, 14 Mar 17

Comments: 3 pages, proceedings of the 22nd Los Alamos Stellar Pulsation Conference: “Wide-Field Variability Surveys: a 21st-century Perspective”, to be published in EPJ WOC

LSPM J1314: An oversized magnetic star with constraints on the radio emission mechanism [SSA]


LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of two nearly identical pre-main sequence stars of spectral type M7. The system is remarkable among ultracool dwarfs for being the most luminous radio emitter over the widest frequency range. Masses and luminosities are at first sight consistent with the system being coeval at age ~80 Myr according to standard (non-magnetic) evolutionary models. However, these models predict an average effective temperature, 2950 +/- 5 K, which is 180 K hotter than the empirical value. Thus, the empirical radii are oversized relative to the standard models by ~13%. We demonstrate that magnetic stellar models can account quantitatively for the oversizing. As a check on our models, we note that the radio emission limits the surface magnetic field strengths: the limits depend on identifying the radio emission mechanism. We find that the field strengths required by our magnetic models are too strong to be consistent with gyrosynchrotron emission, but are consistent with electron cyclotron maser emission.

Read this paper on arXiv…

J. MacDonald and D. Mullan
Tue, 14 Mar 17

Comments: Submitted to the Astrophysical Journal

Excitation of turbulence in accretion disks of binary stars by non-linear perturbations [SSA]


Accretion disks in binary systems can experience hydrodynamic impact at inner as well as outer edges. The first case is typical for protoplanetary disks around young T Tau stars. The second one is typical for circumstellar disks in close binaries. As a result of such an impact, perturbations with different scales and amplitudes are excited in the disk. We investigated the nonlinear evolution of perturbations of a finite, but small amplitude, at the background of sub-Keplerian flow. Nonlinear effects at the front of perturbations lead to the formation of a shock wave, namely the discontinuity of the density and radial velocity. At this, the tangential flow in the neighborhood of the shock becomes equivalent to the flow in in the boundary layer. Instability of the tangential flow further leads to turbulization of the disk. Characteristics of the turbulence depend on perturbation parameters, but alpha-parameter of Shakura-Sunyaev does not exceed ~0.1.

Read this paper on arXiv…

E. Kurbatov and D. Bisikalo
Tue, 14 Mar 17

Comments: Accepted in Astronomy Reports

A search for water maser emission from brown dwarfs and low-luminosity young stellar objects [SSA]


We present a survey for water maser emission toward a sample of 44 low-luminosity young objects, comprising (proto-)brown dwarfs, first hydrostatic cores (FHCs), and other young stellar objects (YSOs) with bolometric luminosities lower than 0.4 L$_\odot$. Water maser emission is a good tracer of energetic processes, such as mass-loss and/or accretion, and is a useful tool to study this processes with very high angular resolution. This type of emission has been confirmed in objects with L$_{\rm bol}\ge 1$ L$_\odot$. Objects with lower luminosities also undergo mass-loss and accretion, and thus, are prospective sites of maser emission. Our sensitive single-dish observations provided a single detection when pointing toward the FHC L1448 IRS 2E. However, follow-up interferometric observations showed water maser emission associated with the nearby YSO L1448 IRS 2 { (a Class 0 protostar of L$_{\rm bol}\simeq 3.6-5.3$ L$_\odot$)}, and did not find any emission toward L1448 IRS 2E. The upper limits for water maser emission determined by our observations are one order of magnitude lower than expected from the correlation between water maser luminosities and bolometric luminosities found for YSOs. This suggests that this correlation does not hold at the lower end of the (sub)stellar mass spectrum. Possible reasons are that the slope of this correlation is steeper at L$_{\rm bol}\le 1$ L$_\odot$, or that there is an absolute luminosity threshold below which water maser emission cannot be produced. Alternatively, if the correlation still stands at low luminosity, the detection rates of masers would be significantly lower than the values obtained in higher-luminosity Class 0 protostars.

Read this paper on arXiv…

J. Gomez, A. Palau, L. Uscanga, et. al.
Tue, 14 Mar 17

Comments: To be published in The Astronomical Journal. Eleven pages, four figures

The Yale-Potsdam Stellar Isochrones (YaPSI) [SSA]


We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of stellar evolution tracks and isochrones of solar-scaled composition. In an effort to improve the Yonsei-Yale database, special emphasis is placed on the construction of accurate low-mass models (Mstar < 0.6 Msun), and in particular of their mass-luminosity and mass-radius relations, both crucial in characterizing exoplanet-host stars and, in turn, their planetary systems. The YaPSI models cover the mass range 0.15 to 5.0 Msun, densely enough to permit detailed interpolation in mass, and the metallicity and helium abundance ranges [Fe/H] = -1.5 to +0.3, and Y = 0.25 to 0.37, specified independently of each other (i.e., no fixed Delta Y/Delta Z relation is assumed). The evolutionary tracks are calculated from the pre-main sequence up to the tip of the red giant branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI colors in the Johnson-Cousins system, and JHK colors in the homogeneized Bessell & Brett system, derived from two different semi-empirical Teff-color calibrations from the literature. We also provide utility codes, such as an isochrone interpolator in age, metallicity, and helium content, and an interface of the tracks with an open-source Monte Carlo Markov-Chain tool for the analysis of individual stars. Finally, we present comparisons of the YaPSI models with the best empirical mass- luminosity and mass-radius relations available to date, as well as isochrone fitting of well-studied ste

Read this paper on arXiv…

F. Spada, P. Demarque, Y. Kim, et. al.
Tue, 14 Mar 17

Comments: 17 pages, 14 figures; accepted for publication in the Astrophysical Journal

Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey [SSA]


We combine 131 new medium-resolution (R~2000) J-band spectra of M, L, and T dwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97 previously published BDSS spectra to study surface-gravity-sensitive indices for 228 low-mass stars and brown dwarfs spanning spectral types M5-T9. Specifically, we use an established set of spectral indices to determine surface gravity classifications for all M6-L7 objects in our sample by measuring equivalent widths (EW) of the K I lines at 1.1692, 1.1778, 1.2529 um, and the 1.2 um FeHJ absorption index. Our results are consistent with previous surface gravity measurements, showing a distinct double peak – at ~L5 and T5 – in K I EW as a function of spectral type. We analyze K I EWs of 73 objects of known ages and find a linear trend between log(Age) and EW. From this relationship, we assign age ranges to the very low gravity, intermediate gravity, and field gravity designations for spectral types M6-L0. Interestingly, the ages probed by these designations remain broad, change with spectral type, and depend on the gravity sensitive index used. Gravity designations are useful indicators of the possibility of youth, but current datasets cannot be used to provide a precise age estimate.

Read this paper on arXiv…

E. Martin, G. Mace, I. McLean, et. al.
Tue, 14 Mar 17

Comments: 33 pages, 13 figures, ApJ in press

On the Effectiveness of Multi-Instrument Solar Flare Observations During Solar Cycle 24 [SSA]


Our current fleet of space-based solar observatories offer us a wealth of opportunities to study solar flares over a range of wavelengths, and the greatest advances in our understanding of flare physics often come from coordinated observations between different instruments. However, despite considerable effort to coordinate this armada of instruments over the years (e.g. through the Max Millennium Program of Solar Flare Research), there are few solar flares that have been observed by most or all available instruments simultaneously, due to the combination of each instrument’s operational constraints. Here we describe a technique that retrospectively searches archival databases for flares jointly observed by RHESSI, SDO/EVE (MEGS-A and MEGS-B), Hinode/(EIS, SOT and XRT), and IRIS. Out of the 6,953 flares of GOES magnitude C1 or greater that we consider over the 6.5 years after the launch of SDO, 40 have been observed by six or more instruments simultaneously. The difficulty in scheduling co-ordinated observations for solar flare research is discussed with respect to instruments projected to begin operations during Solar Cycle 25, such as the Daniel K. Inouye Solar Telescope, Solar Orbiter and Solar Probe Plus.

Read this paper on arXiv…

R. Milligan and J. Ireland
Tue, 14 Mar 17

Comments: 17 pages, 6 figures, 4 tables. Submitted to Solar Physics

Multi-wavelength Spectral Analysis of Ellerman Bombs Observed by FISS and IRIS [SSA]


Ellerman bombs (EBs) are a kind of solar activities that is suggested to occur in the lower atmosphere. Recent observations using the Interface Region Imaging Spectrograph (IRIS) show connections of EBs and IRIS bombs (IBs), implying that EBs might be heated to a much higher temperature ($8\times10^{4}$ K) than previous results. Here we perform a spectral analysis of the EBs simultaneously observed by the Fast Imaging Solar Spectrograph (FISS) and IRIS. The observational results show clear evidence of heating in the lower atmosphere, indicated by the wing enhancement in H$\alpha$, Ca II 8542 \r{A} and Mg II triplet lines, and also by brightenings in the images of 1700 \r{A} and 2832 \r{A} ultraviolet continuum channels. Additionally, the Mg II triplet line intensity is correlated with that of H$\alpha$ when the EB occurs, indicating the possibility to use the triplet as an alternative way to identify EBs. However, we do not find any signal in IRIS hotter lines (C II and Si IV). For further analysis, we employ a two-cloud model to fit the two chromospheric lines (H$\alpha$ and Ca II 8542 \r{A}) simultaneously, and obtain a temperature enhancement of 2300 K for a strong EB. This temperature is among the highest of previous modeling results while still insufficient to produce IB signatures at ultraviolet wavelengths.

Read this paper on arXiv…

J. Hong, M. Ding and W. Cao
Tue, 14 Mar 17

Comments: 19 pages, 8 figures, accepted by ApJ

JVLA Observations of Young Brown Dwarfs [SSA]


We present sensitive 3.0 cm JVLA radio continuum observations of six regions of low-mass star formation that include twelve young brown dwarfs and four young brown dwarf candidates. We detect a total of 49 compact radio sources in the fields observed, of which 24 have no reported counterparts and are considered new detections. Twelve of the radio sources show variability in timescales of weeks to months, suggesting gyrosynchrotron emission produced in active magnetospheres. Only one of the target brown dwarfs, FU Tau A, was detected. However, we detected radio emission associated with two of the brown dwarf candidates, WL 20S and CHLT 2. The radio flux densities of the sources associated with these brown dwarf candidates are more than an order of magnitude larger than expected for a brown dwarf and suggest a revision of their classification. In contrast, FU Tau A falls on the well-known correlation between radio luminosity and bolometric luminosity, suggesting that the emission comes from a thermal jet and that this brown dwarf seems to be forming as a scaled-down version of low-mass stars.

Read this paper on arXiv…

L. Rodriguez, L. Zapata and A. Palau
Tue, 14 Mar 17

Comments: 24 pages, 7 figures, to appear in The Astronomical Journal

First evidence of external disc photoevaporation in a low mass star forming region: the case of IM Lup [SSA]


We model the radiatively driven flow from IM Lup — a large protoplanetary disc expected to be irradiated by only a weak external radiation field (at least 10$^4$ times lower than the UV field irradiating the Orion Nebula Cluster proplyds). We find that material at large radii ($>400$AU) in this disc is sufficiently weakly gravitationally bound that significant mass loss can be induced. Given the estimated values of the disc mass and accretion rate, the viscous timescale is long ($\sim10$Myr) so the main evolutionary behaviour for the first Myr of the disc’s lifetime is truncation of the disc by photoevaporation, with only modest changes effected by viscosity. We also produce approximate synthetic observations of our models, finding substantial emission from the flow which can explain the CO halo observed about IM Lup out to $\geq1000$AU. Solutions that are consistent with the extent of the observed CO emission generally imply that IM Lup is still in the process of having its disc outer radius truncated. We conclude that IM Lup is subject to substantial external photoevaporation, which raises the more general possibility that external irradiation of the largest discs can be of significant importance even in low mass star forming regions.

Read this paper on arXiv…

T. Haworth, S. Facchini, C. Clarke, et. al.
Mon, 13 Mar 17

Comments: 5 pages, 5 figures. Accepted for publication in MNRAS letters

A 2MASS/AllWISE Search for Extremely Red L Dwarfs — The Discovery of Several Likely L Type Members of $β$ Pic, AB Dor, Tuc-Hor, Argus, and the Hyades [SSA]


Young brown dwarfs share many properties with directly imaged giant extrasolar planets. They therefore provide unique laboratories for investigating the full range of temperature and mass encompasses by the growing collection of planets discovered outside our Solar System. Furthermore, if they can be tied to a particular group of coeval stars, they also provide vital anchor points for low-mass empirical isochrones. We have developed a novel procedure for identifying such objects based on their unique 2MASS and AllWISE colors. Using our search criteria, we have identified 50 new, late-type L dwarf candidates, 47 of which are spectroscopically confirmed as L dwarfs with follow-up near-infrared spectroscopy. We evaluate the potential membership of these objects in nearby, young moving groups using their proper motions, photometric distance estimates, and spectroscopic indicators of youth, and find seven likely L-type members belonging to the $\beta$ Pictoris moving group, the AB Doradus moving group, the Tucana-Horologium association, or the Argus association, in addition to several lower probability members. Also found are two late-type (L5 and L6) potential members of the nearby Hyades cluster (WISEA J043642.75$+$190134.8 and WISEA J044105.56$+$213001.5).

Read this paper on arXiv…

A. Schneider, J. Windsor, M. Cushing, et. al.
Mon, 13 Mar 17

Comments: Accepted for publication in the Astronomical Journal

L483: Warm Carbon-Chain Chemistry Source Harboring Hot Corino Activity [SSA]


The Class 0 protostar, L483, has been observed in various molecular lines in the 1.2 mm band at a sub-arcsecond resolution with ALMA. An infalling-rotating envelope is traced by the CS line, while a very compact component with a broad velocity width is observed for the CS, SO, HNCO, NH$_2$CHO, and HCOOCH$_3$ lines. Although this source is regarded as the warm carbon-chain chemistry (WCCC) candidate source at a 1000 au scale, complex organic molecules characteristic of hot corinos such as NH$_2$CHO and HCOOCH$_3$ are detected in the vicinity of the protostar. Thus, both hot corino chemistry and WCCC are seen in L483. Although such a mixed chemical character source has been recognized as an intermediate source in previous single-dish observations, we here report the first spatially-resolved detection. A kinematic structure of the infalling-rotating envelope is roughly explained by a simple ballistic model with the protostellar mass of 0.1–0.2 $M_\odot$ and the radius of the centrifugal barrier (a half of the centrifugal radius) of 30–200 au, assuming the inclination angle of 80\degr\ (0\degr\ for a face-on). The broad line emission observed in the above molecules most likely comes from the disk component inside the centrifugal barrier. Thus, a drastic chemical change is seen around the centrifugal barrier.

Read this paper on arXiv…

Y. Oya, N. Sakai, Y. Watanabe, et. al.
Mon, 13 Mar 17

Comments: accepted to ApJ

Investigation of Relationship Between High-Energy X-ray Sources and Photospheric Impact of X1.8 Solar Flare of October 23, 2012 [SSA]


The X-class solar flare of October 23, 2012, generated continuum photospheric emission and a strong helioseismic wave (“sunquake”) that points to an intensive energy release in the dense part of the solar atmosphere. We study properties of the energy release with high temporal and spatial resolutions, using photospheric data from Helioseismic Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO), and hard X-ray observations made by Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). For this analysis we use level-1 HMI data (filtergrams), obtained by scanning the Fe I line (6731 \AA) with time cadence of$\sim 3.6$ s and spatial resolution of $\sim 0.5^{\prime\prime}$ per pixel. It is found that the photospheric disturbances spatially coincide with the region of the hard X-ray emission but are delayed by $\lesssim 4$ seconds. This delay is consistent with predictions of the RADYN models. However, the models fail to explain the magnitude of the observed variations of the HMI lines. The data indicate that the photospheric impact might be caused by the electron energy flux substantially higher than that in the current flare radiative hydrodynamic models.

Read this paper on arXiv…

I. Sharykin, A. Kosovichev, V. Sadykov, et. al.
Mon, 13 Mar 17

Comments: 19 pages, 5 figures

Validation of Spherical Born Approximation Sensitivity Functions for Measuring Deep Solar Meridional Flow [SSA]


Accurate measurements of deep solar meridional flow are of vital interest for understanding the solar dynamo. In this paper, we validate a recently developed method for obtaining sensitivity functions (kernels) for travel-time measurements to solar interior flows using the Born approximation in spherical geometry, which is expected to be more accurate than the classical ray approximation. Furthermore, we develop a numerical approach to efficiently compute a large number of kernels based on the separability of the eigenfunctions into their horizontal and radial dependence. The validation is performed using a hydrodynamic simulation of linear wave propagation in the Sun, which includes a standard single-cell meridional flow profile. We show that, using the Born approximation, it is possible to accurately model observational quantities relevant for time-distance helioseismology such as the mean power spectrum, disc-averaged cross-covariance functions, and travel times in the presence of a flow field. In order to closely match the model to observations, we show that it is beneficial to use mode frequencies and damping rates which were extracted from the measured power spectrum. Furthermore, the contribution of the radial flow to the total travel time is found to reach 20% of the contribution of the horizontal flow at travel distances over $40^\circ$. Using the Born kernels and a 2D SOLA inversion of travel times, we can recover most features of the input meridional flow profile. The Born approximation is thus a promising method for inferring large-scale solar interior flows.

Read this paper on arXiv…

V. Boning, M. Roth, J. Jackiewicz, et. al.
Mon, 13 Mar 17

Comments: 11 pages, 9 figures, accepted for publication in ApJ

MARVEL analysis of the measured high-resolution rovibronic spectra of $^{48}$Ti$^{16}$O [SSA]


Accurate, experimental rovibronic energy levels, with associated labels and uncertainties, are reported for 11 low-lying electronic states of the diatomic \TiO\ molecule, determined using the \Marvel\ (Measured Active Rotational-Vibrational Energy Levels) algorithm. All levels are based on lines corresponding to critically reviewed and validated high-resolution experimental spectra taken from 24 literature sources. The transition data are in the 2 $-$ 22,160 \cm{} region. Out of the 49,679 measured transitions, 43,885 are triplet-triplet, 5710 are singlet-singlet and 84 are triplet-singlet transitions. A careful analysis of the resulting experimental spectroscopic network (SN) allows 48,590 transitions to be validated. The transitions determine 93 vibrational band origins of \TiO\, including 71 triplet and 22 singlet ones. There are 276 (73) triplet-triplet (singlet-singlet) band-heads derived from \Marvel\ experimental energies, 123 (38) of which have never been assigned in low or high resolution experiments. The highest $J$ value, where $J$ stands for the total angular momentum, for which an energy level is validated is 163. The number of experimentally-derived triplet and singlet \TiO\ rovibrational energy levels is 8682 and 1882, respectively. The lists of validated lines and levels for \TiO\ are deposited in the Supporting Information to this paper.

Read this paper on arXiv…

L. McKemmish, T. Masseron, S. Sheppard, et. al.
Mon, 13 Mar 17

Comments: N/A

Dwarf nova outbursts in intermediate polars [SSA]


The disc instability model (DIM) has been very successful in explaining the dwarf nova outbursts observed in cataclysmic variables. When, as in intermediate polars (IP), the accreting white dwarf is magnetized, the disc is truncated at the magnetospheric radius, but for mass-transfer rates corresponding to the thermal-viscous instability such systems should still exhibit dwarf-nova outbursts. Yet, the majority of intermediate polars in which the magnetic field is not large enough to completely disrupt the accretion disc, seem to be stable, and the rare observed outbursts, in particular in systems with long orbital periods, are much shorter than normal dwarf-nova outbursts. We investigate the predictions of the disc instability model for intermediate polars in order to determine which of the observed properties of these systems can be explained by the DIM. We use our numerical code for the time evolution of accretion discs, modified to include the effects of the magnetic field, with constant or variable mass transfer from the secondary star. We show that intermediate polars have mass transfer low enough and magnetic fields large enough to keep the accretion disc stable on the cold equilibrium branch. We show that the infrequent and short outbursts observed in long period systems, such as e.g., TV Col, cannot be attributed to the thermal-viscous instability of the accretion disc, but instead have to be triggered by an enhanced mass-transfer from the secondary, or, more likely, by some instability coupling the white dwarf magnetic field with that generated by the magnetorotational instability operating in the accretion disc. Longer outbursts (a few days) could result from the disc instability.

Read this paper on arXiv…

J. Hameury and J. Lasota
Mon, 13 Mar 17

Comments: 7 pages, 5 figures; submitted to Astronomy & Astrophysics

The star formation environment of V582 Aur [SSA]


We have studied the environment of the FU Ori type star V582 Aur. Our aim is to explore the star-forming region associated with this young eruptive star. Using slitless spectroscopy we searched for H alpha emission stars within a field of 11.5arcmin \times 11.5arcmin, centred on V582 Aur. Based on UKIDSS and Spitzer Space Telescope data we further selected infrared-excess young stellar object candidates. In all, we identified 68 candidate low-mass young stars, 16 of which exhibited H alpha emission in the slitless spectroscopic images. The colour-magnitude diagram of the selected objects, based on IPHAS data, suggests that they are low-mass pre-main-sequence stars associated with the Aur OB 1 association, located at a distance of 1.3 kpc from the Sun. The bright-rimmed globules in the local environment of V582~Aur probably belong to the dark cloud LDN~1516. Our results suggest that star formation in these globules might have been triggered by the radiation field of a few hot members of Aur OB 1. The bolometric luminosity of V582 Aur, based on archival photometric data and on the adopted distance, is 150-320 Lsun.

Read this paper on arXiv…

M. Kun, E. Szegedi-Elek and B. Reipurth
Mon, 13 Mar 17

Comments: 10 pages, 8 figures, 3 tables. Accepted for publication by MNRAS

Tidally driven mean flows in slowly and uniformly rotating massive main sequence stars [SSA]


We calculate tidally driven mean flows in a slowly and uniformly rotating massive main sequence star in a binary system. We treat the tidal potential due to the companion as a small perturbation to the primary star. We compute tidal responses of the primary as forced linear oscillations, as a function of the tidal forcing frequency $\omega_{\rm tide}=2(\Omega_{\rm orb}-\Omega)$, where $\Omega_{\rm orb}$ is the mean orbital angular velocity and $\Omega$ is the angular velocity of rotation of the primary star. The amplitude of the tidal responses is proportional to the parameter $f_0\propto (M_2/M)(a_{\rm orb}/R)^{-3}$, where $M$ and $M_2$ are the masses of the primary and companion stars, $R$ is the radius of the primary and $a_{\rm orb}$ is the mean orbital separation between the stars. For a given $f_0$, the amplitudes depend on $\omega_{\rm tide}$ and become large when $\omega_{\rm tide}$ is in resonance with natural frequencies of the star. Using the tidal responses, we calculate axisymmetric mean flows, assuming that the mean flows are non-oscillatory flows driven via non-linear effects of linear tidal responses. We find that the $\phi$-component of the mean flow velocity dominates. We also find that the amplitudes of the mean flows are large only in the surface layers where non-adiabatic effects are significant and that the amplitudes are confined to the equatorial regions of the star. Depending on $M_2/M$ and $a_{\rm orb}/R$, the amplitudes of mean flows at the surface become significant.

Read this paper on arXiv…

U. Lee
Mon, 13 Mar 17

Comments: 18 pages, 12 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society

The Effect of Unresolved Contaminant Stars on the Cross-Matching of Photometric Catalogues [SSA]


A fundamental process in astrophysics is the matching of two photometric catalogues. It is crucial that the correct objects be paired, and that their photometry does not suffer from any spurious additional flux. We compare the positions of sources in WISE, IPHAS, 2MASS, and APASS with Gaia DR1 astrometric positions. We find that the separations are described by a combination of a Gaussian distribution, wider than naively assumed based on their quoted uncertainties, and a large wing, which some authors ascribe to proper motions. We show that this is caused by flux contamination from blended stars not treated separately. We provide linear fits between the quoted Gaussian uncertainty and the core fit to the separation distributions.
We show that at least one in three of the stars in the faint half of a given catalogue will suffer from flux contamination above the 1% level when the density of catalogue objects per PSF area is above approximately 0.005. This has important implications for the creation of composite catalogues. It is important for any closest neighbour matches as there will be a given fraction of matches that are flux contaminated, while some matches will be missed due to significant astrometric perturbation by faint contaminants. In the case of probability-based matching, this contamination affects the probability density function of matches as a function of distance. This effect results in up to 50% fewer counterparts being returned as matches, assuming Gaussian astrometric uncertainties for WISE-Gaia matching in crowded Galactic plane regions, compared with a closest neighbour match.

Read this paper on arXiv…

T. Wilson and T. Naylor
Mon, 13 Mar 17

Comments: 10 pages, 6 figures; accepted for publication in the Monthly Notices of the Royal Astronomical Society

Predicting radio emission from the newborn hot Jupiter V830 Tau and its host star [SSA]


Magnetised exoplanets are expected to emit at radio frequencies analogously to the radio auroral emission of Earth and Jupiter. We predict the radio emission from V830 Tau b, the youngest (2 Myr) detected exoplanet to date. We model the host star wind using 3DMHD simulations that take into account its surface magnetism. With this, we constrain the local conditions around V830 Tau b that we use to then compute its radio emission. We estimate average radio flux densities of 6 to 24mJy, depending on the assumed radius of the planet (one or two Rjupiter). These radio fluxes are present peaks that are up to twice the average values. We show here that these fluxes are weakly dependent (a factor of 1.8) on the assumed polar planetary magnetic field (10 to 100G), opposed to the maximum frequency of the emission, which ranges from 18 to 240MHz. We also estimate the thermal radio emission from the stellar wind. By comparing our results with VLA and VLBA observations of the system, we constrain the stellar mass-loss rate to be <3e-9 Msun/yr, with likely values between ~1e-12 and 1e-10 Msun/yr. The frequency-dependent extension of the radio-emitting wind is around ~ 3 to 30 Rstar for frequencies in the range of 275 to 50MHz, implying that V830 Tau b, at an orbital distance of 6.1 Rstar, could be embedded in the regions of the host star’s wind that are optically thick to radio wavelengths, but not deeply so. Planetary emission can only propagate in the stellar wind plasma if the frequency of the cyclotron emission exceeds the stellar wind plasma frequency. For that, we find that for planetary radio emission to propagate through the host star wind, planetary magnetic field strengths larger than ~1.3 to 13 G are required. The V830 Tau system is a very interesting system for conducting radio observations from both the perspective of radio emission from the planet as well as from the host star’s wind.

Read this paper on arXiv…

A. Vidotto and J. Donati
Mon, 13 Mar 17

Comments: A&A, in press

On the methanol emission detection in the TW Hya disc: the role of grain surface chemistry and non-LTE excitation [SSA]


The recent detection of gas-phase methanol (CH$_3$OH) lines in the disc of TW Hya by Walsh et al. provided the first observational constraints on the complex O-bearing organic content in protoplanetary discs. The emission has a ring-like morphology, with a peak at $\sim 30-50$ au and an inferred column density of $\sim 3-6\times10^{12}$ cm$^{-2}$. A low CH$_3$OH fractional abundance of $\sim 0.3-4\times 10^{-11}$ (with respect to H$_2$) is derived, depending on the assumed vertical location of the CH$_3$OH molecular layer. In this study, we use a thermo-chemical model of the TW Hya disc, coupled with the ALCHEMIC gas-grain chemical model, assuming laboratory-motivated, fast diffusivities of the surface molecules to interpret the CH$_3$OH detection. Based on this disc model, we performed radiative transfer calculations with the LIME code and simulations of the observations with the CASA simulator. We found that our model allows to reproduce the observations well. The CH$_3$OH emission in our model appears as a ring with radius of $\sim60$ au. Synthetic and observed line flux densities are equal within the rms noise level of observations. The synthetic CH$_3$OH spectra calculated assuming local thermodynamic equilibrium (LTE) can differ by up to a factor of 3.5 from the non-LTE spectra. For the strongest lines, the differences between LTE and non-LTE flux densities are very small and practically negligible. Variations in the diffusivity of the surface molecules can lead to variations of the CH$_3$OH abundance and, therefore, line flux densities by an order of magnitude.

Read this paper on arXiv…

S. Parfenov, D. Semenov, T. Henning, et. al.
Mon, 13 Mar 17

Comments: Accepted for publication in MNRAS, 8 pages, 8 figures

OGLE Cepheids and RR Lyrae Stars in the Milky Way [SSA]


We present new large samples of Galactic Cepheids and RR Lyrae stars from the OGLE Galaxy Variability Survey.

Read this paper on arXiv…

A. Udalski
Fri, 10 Mar 17

Comments: 5 pages, 6 figures. Proceedings of the 22nd Los Alamos Stellar Pulsation Conference: “Wide-Field Variability Surveys: a 21st-century Perspective”

Petersen Diagram Revolution [SSA]


Over the recent years, the Petersen diagram for classical pulsators, Cepheids and RR Lyr stars, populated with a few hundreds of new multiperiodic variables. We review our analyses of the OGLE data, which resulted in the significant extension of the known, and in the discovery of a few new and distinct forms of multiperiodic pulsation. The showcase includes not only radial mode pulsators, but also radial-non-radial pulsators and stars with significant modulation observed on top of the beat pulsation. First theoretical models explaining the new forms of stellar variability are briefly discussed.

Read this paper on arXiv…

R. Smolec, W. Dziembowski, P. Moskalik, et. al.
Fri, 10 Mar 17

Comments: 5 pages; to be published in the proceedings of the 22nd Los Alamos Stellar Pulsation Conference “Wide-field variability surveys: a 21st-century perspective”, San Pedro de Atacama, Chile, Nov. 28 – Dec. 2, 2016

The ALMA-PILS survey: Detection of CH$_{3}$NCO toward the low-mass protostar IRAS 16293-2422 and laboratory constraints on its formation [SSA]


Methyl isocyanate (CH$_{3}$NCO) belongs to a select group of interstellar molecules considered to be relevant precursors in the formation of larger organic compounds, including those with peptide bonds. The molecule has only been detected in a couple of high-mass protostars and potentially in comets. A formation route on icy grains has been postulated for this molecule but experimental confirmation is lacking. We unambiguously identify CH$_{3}$NCO for the first time in a solar-type protostar through 43 unblended transitions found in the ALMA Protostellar Interferometric Line Survey (PILS) of the low-mass protostellar binary IRAS 16293-2422. The molecule is detected toward both components of the binary with a ratio HNCO/CH$_3$NCO $\sim$4–12. The isomers CH$_{3}$CNO and CH$_3$OCN are not identified, resulting in upper abundance ratios of CH$_{3}$NCO/CH$_{3}$CNO > 100 and CH$_{3}$NCO/CH$_3$OCN > 10. The resulting abundance ratios compare well with those found for related N-containing species toward high-mass protostars. To constrain its formation, a set of cryogenic UHV experiments is performed. VUV irradiation of CH$_{4}$:HNCO mixtures at 20 K strongly indicate that methyl isocyanate can be formed in the solid-state through CH$_{3}$ and (H)NCO recombinations. Combined with gas-grain models that include this reaction, the solid-state route is found to be a plausible scenario to explain the abundances found in IRAS 16293-2422.

Read this paper on arXiv…

N. Ligterink, A. Coutens, V. Kofman, et. al.
Fri, 10 Mar 17

Comments: Submitted for MNRAS; shared here on request by the editor. 13 pages, 8 figures

Proper motions of five OB stars with candidate dusty bow shocks in the Carina Nebula [SSA]


We constrain the proper motions of five OB stars associated with candidate stellar wind bow shocks in the Carina Nebula using HST ACS imaging over 9–10 year baselines. These proper motions allow us to directly compare each star’s motion to the orientation of its candidate bow shock. Although these stars are saturated in our imaging, we assess their motion by the shifts required to minimize residuals in their Airy rings. The results limit the direction of each star’s motion to sectors less than 90 degrees wide. None of the five stars are moving away from the Carina Nebula’s central clusters as runaway stars would be, confirming that a candidate bow shock is not necessarily indicative of a runaway star. Two of the five stars are moving tangentially relative to the orientation of their candidate bow shocks, both of which point at the OB cluster Trumpler 14. In these cases, the large-scale flow of the interstellar medium, powered by feedback from the cluster, appears to dominate over the motion of the star in producing the observed candidate bow shock. The remaining three stars all have some component of motion toward the central clusters, meaning that we cannot distinguish whether their candidate bow shocks are indicators of stellar motion, of the flow of ambient gas, or of density gradients in their surroundings. In addition, these stars’ lack of outward motion hints that the distributed massive-star population in Carina’s South Pillars region formed in place, rather than migrating out from the association’s central clusters.

Read this paper on arXiv…

M. Kiminki, N. Smith, M. Reiter, et. al.
Fri, 10 Mar 17

Comments: 13 pages, 5 figures, accepted for publication in MNRAS

Observational signatures of past mass-exchange episodes in massive binaries: The case of LSS 3074 [SSA]


The role of mass and momentum exchanges in close massive binaries is very important in the subsequent evolution of the components. Such exchanges produce several observational signatures such as asynchronous rotation and altered chemical compositions, that remain after the stars detach again. We investigated these effects for the close O-star binary LSS 3074 (O4 f + O6-7 :(f):), which is a good candidate for a past Roche lobe overflow (RLOF) episode because of its very short orbital period, P = 2.185 days, and the luminosity classes of both components. We determined a new orbital solution for the system. We studied the photometric light curves to determine the inclination of the orbit and Roche lobe filling factors of both stars. Using phase-resolved spectroscopy, we performed the disentangling of the optical spectra of the two stars. We then analysed the reconstructed primary and secondary spectra with the CMFGEN model atmosphere code to determine stellar parameters, such as the effective temperatures and surface gravities, and to constrain the chemical composition of the components. We confirm the apparent low stellar masses and radii reported in previous studies. We also find a strong overabundance in nitrogen and a strong carbon and oxygen depletion in both primary and secondary atmospheres, together with a strong enrichment in helium of the primary star. We propose several possible evolutionary pathways through a RLOF process to explain the current parameters of the system. We confirm that the system is apparently in overcontact configuration and has lost a significant portion of its mass to its surroundings. We suggest that some of the discrepancies between the spectroscopic and photometric properties of LSS 3074 could stem from the impact of a strong radiation pressure of the primary.

Read this paper on arXiv…

F. Raucq, E. Gosset, G. Rauw, et. al.
Fri, 10 Mar 17

Comments: N/A

A Test of the Neutron Star Hypothesis for Fomalhaut b [SSA]


Fomalhaut b is a directly imaged object in the debris disk of the star Fomalhaut. It has been hypothesized to be a planet, however there are issues with the observed colours of the object that do not fit planetary models. An alternative hypothesis is that the object is a neutron star in the near fore- or background of Fomalhaut’s disk. We test if Fomalhaut b could be a neutron star using X-ray observations with Chandra’s HRC-I instrument in the energy range of 0.08-10 keV. We do not detect X-ray emission from either Fomalhaut b or the star Fomalhaut itself. Our nondetection corresponds to an upper limit on the X-ray flux of Fomalhaut b of F_X < 1.3e-14 erg/cm/s^2 in the energy range 0.08-10 keV. For the A-type central star Fomalhaut, we derive an X-ray upper limit of L_X < 2e25 erg/s in the energy range 0.08-10 keV. Fomalhaut b’s X-ray non-detection constrains the parameter space for a possible neutron star significantly, implying surface temperatures lower than 91000 K and distances closer than 13.3 pc to the solar system. In addition we find that reflected starlight from the central star fits the available optical detections of Fomalhaut b; a smaller planet with a large ring system might explain such a scenario.

Read this paper on arXiv…

K. Poppenhaeger, K. Auchettl and S. Wolk
Fri, 10 Mar 17

Comments: 7 pages