Estimating the chromospheric magnetic field from a revised NLTE modeling: the case of HR7428 [SSA]

In this work we use the semi-empirical atmospheric modeling method to obtain the chro-mospheric temperature, pressure, density and magnetic field distribution versus height in the K2 primary component of the RS CVn binary system HR 7428. While temperature, pressure, density are the standard output of the semi-empirical modeling technique, the chromospheric magnetic field estimation versus height comes from considering the possibility of not im-posing hydrostatic equilibrium in the semi-empirical computation. The stability of the best non-hydrostatic equilibrium model, implies the presence of an additive (toward the center of the star) pressure, that decreases in strength from the base of the chromosphere toward the outer layers. Interpreting the additive pressure as magnetic pressure we estimated a magnetic field intensity of about 500 gauss at the base of the chromosphere.

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I. Busa, G. Catanzaro, A. Frasca, et. al.
Fri, 10 Mar 17

Comments: 11 pages, 13 figures, 1 table

Constraining proto-planetary disc evolution using accretion rate and disc mass measurements: the usefulness of the dimensionless accretion parameter [SSA]

We explore how measurements of protoplanetary disc masses and accretion rates provided by surveys of star forming regions can be analysed via the dimensionless accretion parameter, which we define as the product of the accretion rate and stellar age divided by the disc mass. By extending and generalising the study of Jones et al (2012), we demonstrate that this parameter should be less than or of order unity for a wide range of evolutionary scenarios, rising above unity only during the final stages of outside in clearing by external photoevaporation. We use this result to assess the reliability of disc mass estimates derived from CO isotopologues and submm continuum emission by examining the distribution of accretion efficiencies in regions which are not subject to external photoevaporation. We find that while dust based mass estimates produce results compatible with theoretical expectations assuming canonical dust/gas ratio, the systematically lower CO based estimates yield accretion efficiencies significantly above unity in contrast with the theory. This finding provides additional evidence that CO based disc masses are an under-estimate, in line with arguments that have been made on the basis of chemical modelling of relatively small samples. On the other hand, we demonstrate that dust based mass estimates are sufficiently accurate to reveal distinctly higher accretion efficiencies in the Trapezium cluster, where this result is expected given the evident importance of external photoevaporation. We therefore propose the dimensionless accretion parameter as a new diagnostic of external photoevaporation in other star forming regions.

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G. Rosotti, C. Clarke, C. Manara, et. al.
Fri, 10 Mar 17

Comments: 9 pages, 8 figures. Accepted to MNRAS

A time dependent relation between EUV solar flare light-curves from lines with differing formation temperatures [SSA]

Extreme Ultraviolet (EUV) solar flare emissions evolve in time as the emitting plasma heats and then cools. Although accurately modeling this evolution has been historically difficult, especially for empirical relationships, it is important for understanding processes at the Sun, as well as for their influence on planetary atmospheres. With a goal to improve empirical flare models, a new simple empirical expression has been derived to predict how cool emissions will evolve based on the evolution of a hotter emission. This technique was initially developed by studying 12 flares in detail observed by the EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO). Then, over 1100 flares observed by EVE were analyzed to validate these relationships. These results have practical implications in improving flare irradiance modeling and for identifying key emission lines for future monitoring of flares for space weather operations, and also provide insight into the cooling processes of flare plasma.

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E. Thiemann, F. Eparvier and T. Woods
Fri, 10 Mar 17

Comments: N/A

K2 Observations of 95 Vir: delta Scuti Pulsations in a Chromospherically Active Star [SSA]

We have searched for photometric variability in 95 Vir, a fast rotating, chromospherically active early F-type star, which was observed in the framework of Campaign 6 of the Kepler K2 mission. Available literature information on 95 Vir were procured, and well-established calibrations were employed to verify the derived astrophysical parameters. We have investigated the location of our target star in the M(Bol) versus log T(eff) diagram, which provides information on evolutionary status. We have discussed our results in detail, drawing on literature information and the theoretical predictions of state-of-the-art pulsation models, with the aim of unraveling the underlying variability mechanisms. From an analysis of 3400 long-cadence measurements, we have identified two main frequencies and several harmonics in our target star. We attribute the main frequency, f1 = 9.53728 d**-1, to delta Scuti pulsations. The origin of the secondary signal, f2 = 1.07129 d**-1, is less clear. We have investigated three possible interpretations of the low-frequency variation: binarity, pulsation and rotational modulation. Current evidence favours an interpretation of f2 as a signature of the rotational period caused by the presence of cool star spots, which goes along well with the observed chromospheric activity. However, phase-resolved spectroscopy is needed to verify this assumption. We briefly consider other chromospherically active delta Scuti stars that have been presented in the literature. A search for star spot-induced photometric variability in these objects might be of great interest, as well as an investigation of the interplay between chromospheric and pulsational activity.

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E. Paunzen, S. Huemmerich, K. Bernhard, et. al.
Thu, 9 Mar 17

Comments: 7 pages, 7 figures, accepted for publication in MNRAS

VLTI/PIONIER images the Achernar disk swell [SSA]

Context. The mechanism of disk formation around fast-rotating Be stars is not well understood. In particular, it is not clear which mechanisms operate, in addition to fast rotation, to produce the observed variable ejection of matter. The star Achernar is a privileged laboratory to probe these additional mechanisms because it is close, presents B-Be phase variations on timescales ranging from 6 yr to 15 yr, a companion star was discovered around it, and probably presents a polar wind or jet. Aims. Despite all these previous studies, the disk around Achernar was never directly imaged. Therefore we seek to produce an image of the photosphere and close environment of the star. Methods. We used infrared long-baseline interferometry with the PIONIER/VLTI instrument to produce reconstructed images of the photosphere and close environment of the star over four years of observations. To study the disk formation, we compared the observations and reconstructed images to previously computed models of both the stellar photosphere alone (normal B phase) and the star presenting a circumstellar disk (Be phase). Results. The observations taken in 2011 and 2012, during the quiescent phase of Achernar, do not exhibit a disk at the detection limit of the instrument. In 2014, on the other hand, a disk was already formed and our reconstructed image reveals an extended H-band continuum excess flux. Our results from interferometric imaging are also supported by several H-alpha line profiles showing that Achernar started an emission-line phase sometime in the beginning of 2013. The analysis of our reconstructed images shows that the 2014 near-IR flux extends to 1.7 – 2.3 equatorial radii. Our model-independent size estimation of the H-band continuum contribution is compatible with the presence of a circumstellar disk, which is in good agreement with predictions from Be-disk models.

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G. Vedova, F. Millour, A. Souza, et. al.
Thu, 9 Mar 17

Comments: N/A

The origin of dust polarization in molecular outflows [SSA]

Aims: In this paper we present a case study to investigate conditions necessary to detect a characteristic magnetic field substructure embedded in a large-scale field. A helical magnetic field with a surrounding hourglass shaped field is expected from theoretical predictions and self-consistent magnetohydrodynamical (MHD) simulations to be present in the specific case of protostellar outflows. Hence, such an outflow environment is the perfect for our study. Methodes: We present synthetic polarisation maps in the infrared and millimeter regime of protostellar outflows performed with the newly developed RT and polarisation code POLARIS. The code, as the first, includes a self-consistent description of various alignement mechanism like the imperfect Davis-Greenstein (IDG) and the radiative torque (RAT) alignment. We investigate for which effects the grain size distribution, and applied alignement mechanism have. Results: We find that the IDG mechanism cannot produce any measurable polarization degree (< 1 %) whereas RAT alignment produced polarization degrees of a few 1 %. Furthermore, we developed a method to identify the origin of the polarization. We show that the helical magnetic field in the outflow can only be observed close to the outflow axis and at its tip, whereas in the surrounding regions the hourglass field in the foreground dominates the polarization. Furthermore, the polarization degree in the outflow lobe is lower than in the surroundings in agreement with observations. We also find that the orientation of the polarization vector flips around a few 100 micron due to the transition from dichroic extinction to thermal re-emission. Hence, in order to avoid ambiguities when interpreting polarization data, we suggest to observed in the far-infrared and mm regime. Finally, we show that with ALMA it is possible to observe the polarization emerging from protostellar outflows.

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S. Reissl, D. Seifried, S. Wolf, et. al.
Thu, 9 Mar 17

Comments: 15 pages, 9 figures

Identification of coronal heating events in 3D simulations [SSA]

The solar coronal heating problem is an open question. One model for the transport and release of mechanical energy generated in the sub-phorospheric layers and photosphere is the nanoflare model that incorporates Ohmic heating which releases a part of the energy stored in the magnetic field. The problem with the verification of this model is that we cannot resolve small scale events. Histograms of observable characteristics of flares, show powerlaw behavior, for both energy release rate, size and total energy. Depending on the powerlaw index of the energy release, nanoflares might be an important candidate for coronal heating; we seek to find that index. In this paper we employ a numerical 3D-MHD simulation produce by the numerical code {\it{Bifrost}}, and a new technique to identify the 3D heating events at a specific instant. The quantity we explore is the Joule heating, which is explicitly correlated with the magnetic reconnection because depends on the curl of the magnetic field. We are able to identify 4136 events in a volume $24 \times 24 \times 9.5 \ \textrm{Mm}^3$ (i.e. $768 \times 786 \times 331$ grid cells) of a specific snapshot. We find a powerlaw slope of the released energy per second, and two powerlaw slopes of the identified volume. The identified energy events do not represent all the released energy, but of the identified events, the total energy of the largest events dominate the energy release. Most of the energy release happens in the lower corona, while heating drops with height. We find that with a specific identification method that large events can be resolved into smaller ones, but at the expense of the total identified energy releases. The energy release which cannot be identified as an event favours a low energy release mechanism.

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C. Kanella and B. Gudiksen
Thu, 9 Mar 17

Comments: 10 pages, 6 figures

Uncrowding R 136 from VLT-SPHERE extreme adaptive optics [SSA]

This paper presents the sharpest near-IR images of the massive cluster R136 to date, based on the extreme adaptive optics of the SPHERE focal instrument implemented on the ESO/VLT and operated in its IRDIS imaging mode. Stacking-up a few hundreds of short exposures in J and Ks spectral bands over a FoV of 10.9″x12.3″ centered on the R136a1 stellar component, enabled us to carry a refined photometric analysis of the core of R136. We detected 1110 and 1059 sources in J and Ks images respectively with 818 common sources. Thanks to better angular resolution and dynamic range, we found that more than 62.6% (16.5%) of the stars, detected both in J and Ks data, have neighbours closer than 0.2″ (0.1″). Among newly resolved and detected sources R136a1 and R136c are found to have optical companions and R136a3 is resolved as two stars separated by 59mas. The new set of detected sources were used to re-assess the age and extinction of R136 based on 54 spectroscopically stars that have been recently studied with HST slit-spectroscopy. Over 90% of these 54 sources identified visual companions (closer than 0.2″). We found the most probable age and extinction for these sources within the photometric and spectroscopic error-bars. Additionally, using PARSEC evolutionary isochrones and tracks, we estimated the stellar mass range for each detected source (common in J and K data) and plotted the generalized histogram of mass (MF with error-bars). Using SPHERE data, we have gone one step further and partially resolved and studied the IMF covering mass range of (3-300) Msun at the age of 1 and 1.5 Myr. The density in the core of R136 is estimated and extrapolated in 3D and larger radii (up to 6pc). We show that the stars in the core are still unresolved due to crowding, and the results we obtained are upper limits. Higher angular resolution is mandatory to overcome these difficulties.

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Z. Khorrami, F. Vakili, T. Lanz, et. al.
Thu, 9 Mar 17

Comments: 15 pages, 17 figures, 5 tables, Accepted to A&A

Imaging a Central Ionized Component, a Narrow Ring, and the CO Snowline in the Multi-Gapped Disk of HD 169142 [SSA]

We report Very Large Array observations at 7 mm, 9 mm, and 3 cm toward the pre-transitional disk of the Herbig Ae star HD 169142. These observations have allowed us to study the mm emission of this disk with the highest angular resolution so far ($0\rlap.”12\times0\rlap.”09$, or 14 au$\times$11 au, at 7 mm). Our 7 and 9 mm images show a narrow ring of emission at a radius of $\sim25$ au tracing the outer edge of the inner gap. This ring presents an asymmetric morphology that could be produced by dynamical interactions between the disk and forming planets. Additionally, the azimuthally averaged radial intensity profiles of the 7 and 9 mm images confirm the presence of the previously reported gap at $\sim45$ au, and reveal a new gap at $\sim85$ au. We analyzed archival DCO$^+$(3-2) and C$^{18}$O(2-1) ALMA observations, showing that the CO snowline is located very close to this third outer gap. This suggests that growth and accumulation of large dust grains close to the CO snowline could be the mechanism responsible for this proposed outer gap. Finally, a compact source of emission is detected at 7 mm, 9 mm, and 3 cm toward the center of the disk. Its flux density and spectral index indicate that it is dominated by free-free emission from ionized gas, which could be associated with either the photoionization of the inner disk, an independent object, or an ionized jet.

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E. Macias, G. Anglada, M. Osorio, et. al.
Thu, 9 Mar 17

Comments: 12 pages, 6 figures, accepted for publication in ApJ

Asteroseismology of the {\it Kepler} target KIC\,9204718 [SSA]

The high precision data obtained by the {\it Kepler} satellite allows us to detect hybrid type pulsator candidates more accurately than the data obtained by ground-based observations. In this study, we present preliminary results on the new analysis of the {\it Kepler} light curve and high resolution spectroscopic observations of pulsating Am star KIC\,9204718. Our tentative analysis therefore show that the star has hybrid pulsational characteristics.

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C. Ulusoy, I. Stateva, I. Iliev, et. al.
Thu, 9 Mar 17

Comments: ‘Proceedings of Wide Field variability surveys : a 21 st Century 22nd Los Alamos Stellar Pulsation Conference San Pedro De Atacama ,Chile Nov 28-Dec 2, 2016’ to be published by the EPJ Web of Conferences

Investigating the past history of EXors: the cases of V1118 Ori, V1143 Ori, and NY Ori [SSA]

EXor objects are young variables that show episodic variations of brightness commonly associated to enhanced accretion outbursts. With the aim of investigating the long-term photometric behaviour of a few EXor sources, we present here data from the archival plates of the Asiago Observatory, showing the Orion field where the three EXors V1118, V1143, and NY are located. A total of 484 plates were investigated, providing a total of more than 1000 magnitudes for the three stars, which cover a period of about 35 yrs between 1959 to 1993. We then compared our data with literature data. Apart from a newly discovered flare-up of V1118, we identify the same outbursts already known, but we provide two added values: (i) a long-term sampling of the quiescence phase; and (ii) repeated multi-colour observations (BVRI bands). The former allows us to give a reliable characterisation of the quiescence, which represents a unique reference for studies that will analyze future outbursts and the physical changes induced by these events. The latter is useful for confirming whether the intermittent increases of brightness are accretion-driven (as in the case of V1118), or extinction-driven (as in the case of V1143). Accordingly, doubts arise about the V1143 classification as a pure EXor object. Finally, although our plates do not separate NY Ori and the star very close to it, they indicate that this EXor did not undergo any major outbursts during our 40 yrs of monitoring.

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R. Jurdana-Sepic, U. Munari, S. Antoniucci, et. al.
Wed, 8 Mar 17

Comments: 13 pages, 4 figures, accepted by A&A

Differences between Doppler velocities of ions and neutral atoms in a solar prominence [SSA]

In astrophysical systems with partially ionized plasma the motion of ions is governed by the magnetic field while the neutral particles can only feel the magnetic field’s Lorentz force indirectly through collisions with ions. The drift in the velocity between ionized and neutral species plays a key role in modifying important physical processes like magnetic reconnection, damping of magnetohydrodynamic waves, transport of angular momentum in plasma through the magnetic field, and heating. This paper investigates the differences between Doppler velocities of calcium ions and neutral hydrogen in a solar prominence to look for velocity differences between the neutral and ionized species. We simultaneously observed spectra of a prominence over an active region in H I 397 nm, H I 434 nm, Ca II 397 nm, and Ca II 854 nm using a high dispersion spectrograph of the Domeless Solar Telescope at Hida observatory, and compared the Doppler velocities, derived from the shift of the peak of the spectral lines presumably emitted from optically-thin plasma. There are instances when the difference in velocities between neutral atoms and ions is significant, e.g. 1433 events (~ 3 % of sets of compared profiles) with a difference in velocity between neutral hydrogen atoms and calcium ions greater than 3sigma of the measurement error. However, we also found significant differences between the Doppler velocities of two spectral lines emitted from the same species, and the probability density functions of velocity difference between the same species is not significantly different from those between neutral atoms and ions. We interpreted the difference of Doppler velocities as a result of motions of different components in the prominence along the line of sight, rather than the decoupling of neutral atoms from plasma.

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T. Anan, K. Ichimoto and A. Hillier
Wed, 8 Mar 17

Comments: 13 pages, 10 figures, accepted for publication n A&A

The effects of resistivity and viscosity on the Kelvin-Helmholtz instability in oscillating coronal loops [SSA]

Aims – Investigate the effects of resistivity and viscosity on the onset and growth of the Kelvin-Helmholtz instability (KHI) in an oscillating coronal loop. Methods – We modelled a standing kink wave in a density-enhanced loop with the three dimensional (3-D), resistive magnetohydrodynamics code, Lare3d. We conducted a parameter study on the viscosity and resistivity coefficients to examine the effects of dissipation on the KHI. Results – Enhancing the viscosity and resistivity acts to suppress the KHI. Larger values delay the formation of the instability and, in some cases, prevent the onset completely. This leads to the earlier onset of heating for smaller values of the transport coefficients. We note that viscosity has a greater effect on the development of the KHI than resistivity. Furthermore, when using anomalous resistivity, the Ohmic heating rate associated with the KHI may be greater than that associated with the phase mixing that occurs in an instability-suppressed regime (using uniform resistivity). Conclusions – From our study, it is clear that the heating rate crucially depends on the formation of small length scales (influenced by the numerical resolution) as well as the values of resistivity and viscosity. As larger values of the transport coefficients suppress the KHI, the onset of heating is delayed but the heating rate is larger. As increased numerical resolution allows smaller length scales to develop, the heating rate will be higher even for the same transport coefficients.

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T. Howson, I. Moortel and P. Antolin
Wed, 8 Mar 17

Comments: 13 pages, 12 figures

Perpendicular diffusion coefficient of cosmic rays in the presence of adiabatic focusing along background magnetic field [SSA]

The influence of adiabatic focusing on particle diffusion is an important topic in space and plasma physics. In the past several authors have explored the influence of along-field adiabatic focusing on parallel diffusion of charged energetic particles. In this paper by using the Unified NonLinear Transport (UNLT) theory by Shalchi (SH2010) and the method of He and Schlickeiser (HS2014) we derive a new nonlinear perpendicular diffusion coefficient for non-uniform background magnetic field. This formula demonstrates that particle perpendicular diffusion coefficient is affected by along-field adiabatic focusing. And we also find that perpendicular diffusion coefficient is dependent on the sign of adiabatic focusing. For the diverging background magnetic field along-field adiabatic focusing makes spatial perpendicular diffusion coefficient $\kappa_\bot$ increase, that is, more particles diffuse across the diverging mean field than that in the uniform mean field. For converging case the spatial perpendicular diffusion coefficient $\kappa_\bot$ decreases for the reason of adiabatic focusing.

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J. Wang, G. Qin, Q. Ma, et. al.
Wed, 8 Mar 17

Comments: N/A

Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry [SSA]

Context. Red giant branch (RGB) stars are very bright objects in galaxies and are often used as standard candles. Interferometry is the ideal tool to characterize the dynamics and morphology of their atmospheres. Aims. We aim at precisely characterising the surface dynamics of a sample of RGB stars. Methods. We obtained interferometric observations for three RGB stars with the MIRC instrument mounted at the CHARA interfer- ometer. We looked for asymmetries on the stellar surfaces using limb-darkening models. Results. We measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas, HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas. We detected departures from the centrosymmetric case for all three stars with the tendency of a greater effect for lower logg of the sample. We explored the causes of this signal and conclude that a possible explanation to the interferometric signal is the convection-related and/or the magnetic-related surface activity. However, it is necessary to monitor these stars with new observations, possibly coupled with spectroscopy, in order to firmly establish the cause.

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A. Chiavassa, R. Norris, M. Montarges, et. al.
Wed, 8 Mar 17

Comments: Accepted for publication as a Letter in Astronomy and Astrophysics, section 1. Letters to the Editor. The official date of acceptance is 06/03/2017

The Most Massive Heartbeat: An In-depth Analysis of ι Orionis [SSA]

{\iota} Ori is a well studied massive binary consisting of an O9 III + B1 III/IV star. Due to its high eccentricity (e = 0.764) and short orbital period (P orb = 29.13376 d), it has been considered to be a good candidate to show evidence of tidal effects; however, none have previously been identified. Using photometry from the BRITE-Constellation space photometry mission we have confirmed the existence of tidal distortions through the presence of a heartbeat signal at periastron. We combine spectroscopic and light curve analyses to measure the masses and radii of the components, revealing {\iota} Ori to be the most massive heartbeat system known to date. In addition, using a thorough frequency analysis, we also report the unprecedented discovery of multiple tidally induced oscillations in an O star. The amplitudes of the pulsations allow us to empirically estimate the tidal circularization rate, yielding an effective tidal quality factor Q $\approx 4 \times 10^{4}$ .

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H. Pablo, N. Richardson, J. Fuller, et. al.
Wed, 8 Mar 17

Comments: 11 pages, 5 figures, 3 tables Monthly Notices of the Royal Astronomical Society, in press

Identification of Young Stellar Variables with KELT for K2 I: Campaign 13 Taurus [SSA]

One of the most well-studied young stellar associations, Taurus-Auriga, will be observed by the extended Kepler mission, K2, in the spring of 2017. K2 Campaign 13 (C13) will be a unique opportunity to study many stars in this young association at high photometric precision and cadence. Using observations from the Kilodegree Extremely Little Telescope (KELT) survey, we identify “dippers”, stochastic variables, and periodic variables among K2 C13 target stars. This release of KELT data (lightcurve data in e-tables) provides the community with long-baseline observations to assist in the understanding of the more exotic variables in the association. Transient-like phenomena on timescales of months to years are known characteristics of young stellar objects, making contextual pre- and post-K2 observations critical to understanding the underlying processes. We are providing a comprehensive set of KELT light curves for known Taurus-Auriga stars in K2 C13. The combined data sets from K2 and KELT permit a broad array of investigations related to star formation, stellar variability, and protoplanetary environments.

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J. Rodriguez, M. Ansdell, R. Oelkers, et. al.
Wed, 8 Mar 17

Comments: 14 pages, 7 figure, 2 tables, Submitted to the AAS Journals

Turbulent kinetic energy in the energy balance of a solar flare [SSA]

The energy released in solar flares derives from a reconfiguration of magnetic fields to a lower energy state, and is manifested in several forms, including bulk kinetic energy of the coronal mass ejection, acceleration of electrons and ions, and enhanced thermal energy that is ultimately radiated away across the electromagnetic spectrum from optical to X-rays. Using an unprecedented set of coordinated observations, from a suite of instruments, we here report on a hitherto largely overlooked energy component — the kinetic energy associated with small-scale turbulent mass motions. We show that the spatial location of, and timing of the peak in, turbulent kinetic energy together provide persuasive evidence that turbulent energy may play a key role in the transfer of energy in solar flares. Although the kinetic energy of turbulent motions accounts, at any given time, for only $\sim (0.5-1)$\% of the energy released, its relatively rapid ($\sim$$1-10$~s) energization and dissipation causes the associated throughput of energy (i.e., power) to rival that of major components of the released energy in solar flares, and thus presumably in other astrophysical acceleration sites.

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E. Kontar, J. Perez, L. Harra, et. al.
Wed, 8 Mar 17

Comments: 5pages, 4 figures, to be published in Physical Review Letters

The additional-mode garden of RR Lyrae stars [SSA]

Space-based photometric missions revealed a surprising abundance of millimagnitude-level additional modes in RR Lyrae stars. The modes that appear in the modulated fundamental-mode (RRab) stars can be ordered into four major categories. Here we present the distribution of these groups in the Petersen diagram, and discuss their characteristics and connections to additional modes observed in other RR Lyrae stars.

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L. Molnar, E. Plachy, P. Klagyivik, et. al.
Wed, 8 Mar 17

Comments: 4 pages, 4 figures, proceedings of the Joint TASC2-KASC9-SPACEINN-HELAS8 Conference “Seismology of the Sun and the Distant Stars 2016”, to be published in EPJ WoC

When flux standards go wild: white dwarfs in the age of Kepler [SSA]

White dwarf stars have been used as flux standards for decades, thanks to their staid simplicity. We have empirically tested their photometric stability by analyzing the light curves of 398 high-probability candidates and spectroscopically confirmed white dwarfs observed during the original Kepler mission and later with K2 Campaigns 0-8. We find that the vast majority (>97 per cent) of non-pulsating and apparently isolated white dwarfs are stable to better than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales, confirming that these stellar remnants are useful flux standards. From the cases that do exhibit significant variability, we caution that binarity, magnetism, and pulsations are three important attributes to rule out when establishing white dwarfs as flux standards, especially those hotter than 30,000 K.

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J. Hermes, B. Gaensicke, N. Fusillo, et. al.
Wed, 8 Mar 17

Comments: Accepted for publication in MNRAS; 7 pages, 4 figures, 2 tables

The Hubble Catalog of Variables [SSA]

We aim to construct an exceptionally deep (V ~< 27) catalog of variable objects in selected Galactic and extragalactic fields visited multiple times by the Hubble Space Telescope (HST). While HST observations of some of these fields were searched for specific types of variables before (most notably, the extragalactic Cepheids), we attempt a systematic study of the population of variable objects of all types at the magnitude range not easily accessible with ground-based telescopes. The variability timescales that can be probed range from hours to years depending on how often a particular field has been visited. For source extraction and cross-matching of sources between visits we rely on the Hubble Source Catalog which includes 10^7 objects detected with WFPC2, ACS, and WFC3 HST instruments. The lightcurves extracted from the HSC are corrected for systematic effects by applying local zero-point corrections and are screened for bad measurements. For each lightcurve we compute variability indices sensitive to a broad range of variability types. The indices characterize the overall lightcurve scatter and smoothness. Candidate variables are selected as having variability index values significantly higher than expected for objects of similar brightness in the given set of observations. The Hubble Catalog of Variables will be released in 2018.

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K. Sokolovsky, A. Bonanos, P. Gavras, et. al.
Wed, 8 Mar 17

Comments: 5 pages, 3 figures, 1 table, proceedings of the 22nd Los Alamos Stellar Pulsation Conference “Wide-field variability surveys: a 21st-century perspective” held in San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 2016

Cepheids with the eyes of photometric space telescopes [SSA]

Space photometric missions have been steadily accumulating observations of Cepheids in recent years, leading to a flow of new discoveries. In this short review we summarize the findings provided by the early missions such as WIRE, MOST, and CoRoT, and the recent results of the Kepler and K2 missions. The surprising and fascinating results from the high-precision, quasi-continuous data include the detection of the amplitude increase of Polaris, and exquisite details about V1154 Cyg within the original Kepler field of view. We also briefly discuss the current opportunities with the K2 mission, and the prospects of the TESS space telescope regarding Cepheids.

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L. Molnar, A. Pal and E. Plachy
Wed, 8 Mar 17

Comments: 5 pages, 2 figures, Proceedings of the 22nd Los Alamos Stellar Pulsation Conference, to be published in EPJ WOC

Confinement of the solar tachocline by a cyclic dynamo magnetic field [SSA]

The surprising thinness of the solar tachocline is still not understood with certainty today. Among the numerous possible scenarios suggested to explain its radial confinement, one hypothesis is based on Maxwell stresses that are exerted by the cyclic dynamo magnetic field of the Sun penetrating over a skin depth below the turbulent convection zone. Our goal is to assess under which conditions (turbulence level in the tachocline, strength of the dynamo-generated field, spreading mechanism) this scenario can be realized in the solar tachocline. We develop a simplified 1D model of the upper tachocline under the influence of an oscillating magnetic field imposed from above. The turbulent transport is parametrized with enhanced turbulent diffusion (or anti-diffusion) coefficients. Two main processes that thicken the tachocline are considered; either turbulent viscous spreading or radiative spreading. An extensive parameter study is carried out to establish the physical parameter regimes under which magnetic confinement of the tachocline that is due to a surface dynamo field can be realized. We have explored a large range of magnetic field amplitudes, viscosities, ohmic diffusivities and thermal diffusivities. We find that, for large but still realistic magnetic field strengths, the differential rotation can be suppressed in the upper radiative zone (and hence the tachocline confined) if weak turbulence is present (with an enhanced ohmic diffusivity of $\eta > 10^{7-8} \, cm^2/s$), even in the presence of radiative spreading. Our results show that a dynamo magnetic field can, in the presence of weak turbulence, prevent the inward burrowing of a tachocline subject to viscous diffusion or radiative spreading.

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R. Barnabe, A. Strugarek, P. Charbonneau, et. al.
Wed, 8 Mar 17

Comments: 10 pages, 7 figures, accepted in A&A

Science cases for a visible interferometer [SSA]

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized in January 15 \& 16, 2015 in Nice with the support of the Action Specific in Haute R\’esolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la C\^ote d’Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges.
This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41.

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P. Stee, F. Allard, M. Benisty, et. al.
Wed, 8 Mar 17

Comments: White Paper prospective. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41

Magnetically aligned dust and SiO maser polarization in the envelope of the red supergiant VY CMa [SSA]

We use Atacama Large Millimeter/submillimeter Array Band 5 science verification observations of the red supergiant VY CMa to study the polarization of SiO thermal/masers lines and dust continuum at ~1.7 mm wavelength. We analyse both linear and circular polarization and derive the magnetic field strength and structure, assuming the polarization of the lines originates from the Zeeman effect, and that of the dust originates from aligned dust grains. We also discuss other effects that could give rise to the observed polarization.
We detect, for the first time, significant polarization (~3%) of the circumstellar dust emission at millimeter wavelengths. The polarization is uniform with an electric vector position angle of $\sim8^\circ$. Varying levels of linear polarization are detected for the J=4-3 28SiO v=0, 1, 2, and 29SiO v=0, 1 lines, with the strongest polarization fraction of ~30% found for the 29SiO v=1 maser. The linear polarization vectors rotate with velocity, consistent with earlier observations. We also find significant (up to ~1%) circular polarization in several lines, consistent with previous measurements. We conclude that the detection is robust against calibration and regular instrumental errors, although we cannot yet fully rule out non-standard instrumental effects.
Emission from magnetically aligned grains is the most likely origin of the observed continuum polarization. This implies that the dust is embedded in a magnetic field >13 mG. The maser line polarization traces the magnetic field structure. The magnetic field in the gas and dust is consistent with an approximately toroidal field configuration, but only higher angular resolution observations will be able to reveal more detailed field structure. If the circular polarization is due to Zeeman splitting, it indicates a magnetic field strength of ~1-3 Gauss, consistent with previous maser observations.

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W. Vlemmings, T. Khouri, I. Marti-Vidal, et. al.
Wed, 8 Mar 17

Comments: 12 pages, 9 figures, submitted to astronomy and astrophysics, comments welcome

The metallicity dependence of WR winds [SSA]

Wolf-Rayet (WR) stars are the most advanced stage in the evolution of the most massive stars. The strong feedback provided by these objects and their subsequent supernova (SN) explosions are decisive for a variety of astrophysical topics such as the cosmic matter cycle. Consequently, understanding the properties of WR stars and their evolution is indispensable. A crucial but still not well known quantity determining the evolution of WR stars is their mass-loss rate. Since the mass loss is predicted to increase with metallicity, the feedback provided by these objects and their spectral appearance are expected to be a function of the metal content of their host galaxy. This has severe implications for the role of massive stars in general and the exploration of low metallicity environments in particular. Hitherto, the metallicity dependence of WR star winds was not well studied. In this contribution, we review the results from our comprehensive spectral analyses of WR stars in environments of different metallicities, ranging from slightly super-solar to SMC-like metallicities. Based on these studies, we derived empirical relations for the dependence of the WN mass-loss rates on the metallicity and iron abundance, respectively.

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R. Hainich, T. Shenar, A. Sander, et. al.
Wed, 8 Mar 17

Comments: 5 pages, 4 figures, to be published in the Proceedings of the IAU Symposium No. 329 “The lives and death-throes of massive stars”

Does the radial-tangential macroturbulence model adequately describe the spectral line broadening of solar-type stars? [SSA]

In incorporating the effect of atmospheric turbulence in the broadening of spectral lines, the so-called radial-tangential macroturbulence (RTM) model has been widely used in the field of solar-type stars, which was devised from an intuitive appearance of granular velocity field of the Sun. Since this model assumes that turbulent motions are restricted to only radial and tangential directions, it has a special broadening function with notably narrow width due to the projection effect, the validity of which has not yet been confirmed in practice. With an aim to check whether this RTM model adequately represents the actual solar photospheric velocity field, we carried out an extensive study on the non-thermal velocity dispersion along the line-of-sight (V_los) by analyzing spectral lines at various points of the solar disk based on locally-averaged as well as high spatial-resolution spectra, and found the following results. First, the center-to-limb run of V_los derived from ground-based low-resolution spectra is simply monotonic with a slightly increasing tendency, which contradicts the specific trend (an appreciable peak at theta~45 deg) predicted from RTM. Second, the V_los values derived from a large number of spectra based on high-resolution space observation revealed to follow a nearly normal distribution, without any sign of peculiar distribution expected for the RTM case. These two observational facts indicate that the actual solar velocity field is not such simply dichotomous as assumed in RTM, but directionally more chaotic. We thus conclude that RTM is not an adequate model at least for solar-type stars, which would significantly overestimate the turbulent velocity dispersion by a factor of ~2. The classical Gaussian macroturbulence model should be more reasonable in this respect.

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Y. Takeda and S. UeNo
Wed, 8 Mar 17

Comments: 25 pages, 14 figures, 1 table, accepted for publication in Publ. Astron. Soc. Japan

How Rotation Affects Masses and Ages of Classical Cepheids [SSA]

Classical Cepheid variable stars are both sensitive astrophysical laboratories and accurate cosmic distance tracers. We have recently investigated how the evolutionary effects of rotation impact the properties of these important stars and here provide an accessible overview of some key elements as well as two important consequences. Firstly, rotation resolves the long-standing Cepheid mass discrepancy problem. Second, rotation increases main sequence lifetimes, i.e, Cepheids are approximately twice as old as previously thought. Finally, we highlight the importance of the short-period ends of Cepheid period distributions as indicators for model adequacy.

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R. Anderson, S. Ekstrom, C. Georgy, et. al.
Tue, 7 Mar 17

Comments: 5 pages, 4 figures, proceedings of the 22nd Los Alamos Stellar Pulsation Conference “Wide-field variability surveys: a 21st-century perspective” held in San Pedro de Atacama, Chile, Nov. 28 – Dec. 2, 2016

Accurate parameters for HD 209458 and its planet from HST spectrophotometry [SSA]

We present updated parameters for the star HD 209458 and its transiting giant planet. The stellar angular diameter $\theta$=0.2254$\pm$0.0017 mas is obtained from the average ratio between the absolute flux observed with the Hubble Space Telescope and that of the best-fitting Kurucz model atmosphere. This angular diameter represents an improvement in precision of more than four times compared to available interferometric determinations. The stellar radius $R_\star$=1.20$\pm$0.05 R$_{\odot}$ is ascertained by combining the angular diameter with the Hipparcos trigonometric parallax, which is the main contributor to its uncertainty, and therefore the radius accuracy should be significantly improved with Gaia’s measurements. The radius of the exoplanet $R_\text{p}$=1.41$\pm$0.06 $R_\text{J}$ is derived from the corresponding transit depth in the light curve and our stellar radius. From the model fitting, we accurately determine the effective temperature, $T_\text{eff}$=6071$\pm$20 K, which is in perfect agreement with the value of 6070$\pm$24 K calculated from the angular diameter and the integrated spectral energy distribution. We also find precise values from recent Padova Isochrones, such as $R_\star$=1.20$\pm$0.06 R$_{\odot}$ and $T_\text{eff}$=6099$\pm$41 K. We arrive at a consistent picture from these methods and compare the results with those from the literature.

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C. Burgo and C. Prieto
Tue, 7 Mar 17

Comments: 10 pages, 2 figures, published in MNRAS

Standard Galactic Field RR Lyrae. I. Optical to Mid-infrared Phased Photometry [SSA]

We present a multi-wavelength compilation of new and previously-published photometry for 55 Galactic field RR Lyrae variables. Individual studies, spanning a time baseline of up to 30 years, are self-consistently phased to produce light curves in 10 photometric bands covering the wavelength range from 0.4 to 4.5 microns. Data smoothing via the GLOESS technique is described and applied to generate high-fidelity light curves, from which mean magnitudes, amplitudes, rise-times, and times of minimum and maximum light are derived. 60,000 observations were acquired using the new robotic Three-hundred MilliMeter Telescope (TMMT), which was first deployed at the Carnegie Observatories in Pasadena, CA, and is now permanently installed and operating at Las Campanas Observatory in Chile. We provide a full description of the TMMT hardware, software, and data reduction pipeline. Archival photometry contributed approximately 31,000 observations. Photometric data are given in the standard Johnson UBV, Kron-Cousins RI, 2MASS JHK, and Spitzer [3.6] & [4.5] bandpasses.

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A. Monson, R. Beaton, V. Scowcroft, et. al.
Tue, 7 Mar 17

Comments: 23 pages, 6 figures (figure set 4 and machine readable tables available in published online journal )

Interaction of Two Active Region Filaments Observed by NVST and SDO [SSA]

Using high spatial and temporal resolution H$\alpha$ data from the New Vacuum Solar Telescope (NVST) and simultaneous observations from the Solar Dynamics Observatory (SDO), we present a rare event on the interaction between two filaments (F1 and F2) in AR 11967 on 2014 January 31. The adjacent two filaments were almost perpendicular to each other. Their interaction was driven by the movement of F1 and started when the two filaments collided with each other. During the interaction, the threads of F1 continuously slipped from the northeast to the southwest, accompanied by the brightenings at the junction of two filaments and the northeast footpoint of F2. Part of F1 and the main body of F2 became invisible in H$\alpha$ wavelength due to the heating and the motion of F2. At the same time, bright material initiated from the junction of two filaments were observed to move along F1. The magnetic connectivities of F1 were found to be changed after their interaction. These observations suggest that magnetic reconnection was involved in the interaction of two filaments and resulted in the eruption of one filament.

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L. Yang, X. Yan, T. Li, et. al.
Tue, 7 Mar 17

Comments: 18 pages, 6 figures, accepted by ApJ

The Milne problem in gas-dusty atmosphere [SSA]

We consider the Milne problem in plane-parallel optically thick atmosphere consisting of two components – the free electrons and small dust particles. This problem depends on three dimensionless parameters: $\eta$ – the ratio of optical depth due to free electrons to optical depth due to small dust grains; the absorption factor $\varepsilon$ of dust grains and two coefficients – $\overline b_1$ and $\overline b_2$, describing the averaged anisotropic dust grains. These coefficients obey the relation $\overline b_1+3\overline b_2=1$. The goal of the paper is to study the dependence of the radiation angular distribution and degree of polarization on these three parameters. Recall, that the solution of the Milne problem gives the angular distribution and polarization degree of emerging radiation when the sources of non-polarized radiation are located far from the boundary of optically thick atmosphere. Here we consider only continuum radiation.

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N. Silantev, G. Alekseeva and V. Novikov
Tue, 7 Mar 17

Comments: 13 pages, 6 Tables, 5 figures. In submission to Astrophysics and Space Science

Perspectives on Low-Mass Star Formation [SSA]

I review some recent work on low-mass star formation, with an emphasis on theory, basic principles, and unresolved questions. Star formation is both a gravitational fragmentation problem as well as an accretion problem. Molecular cloud structure can be understood as a fragmentation process driven by the interplay of turbulence, magnetic fields, and gravity (acting on either a dynamical or ambipolar-diffusion time scale). This results in a natural way to understand filamentary structure as magnetic ribbons that have an apparent width that scales differently than the Jeans length. Recent work also shows that stellar mass accretion through a disk is episodic. We show through numerical simulations that bursts of FU Ori type may be clustered, since the clump that accretes to the center is tidally sheared apart in its last stage of infall. Finally, we utilize a simplified model of stellar mass accretion and accretion termination to derive an analytic form for the initial mass function that has a lognormal-like body and a power-law tail. This scenario is consistent with an expectation of a larger number of substellar objects than may have been previously detected.

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S. Basu
Tue, 7 Mar 17

Comments: 8 pages, 6 figures, to appear in proceedings of SFDE conference, eds. D. Johnstone, T. Hoang, F. Nakamura, Q. N. Luong, and J. T. Tranh Van

The multiplicity of massive stars: a 2016 view [SSA]

Massive stars like company. Here, we provide a brief overview of progresses made over the last 5 years by a number of medium and large surveys. These results provide new insights on the observed and intrinsic multiplicity properties of main sequence massive stars and on the initial conditions for their future evolution. They also bring new interesting constraints on the outcome of the massive star formation process.

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H. Sana
Tue, 7 Mar 17

Comments: To appear in Proceedings of IAY Symposium 329; 1 table, 7 figures

Kepler sheds new and unprecedented light on the variability of a blue supergiant: gravity waves in the O9.5Iab star HD 188209 [SSA]

Stellar evolution models are most uncertain for evolved massive stars. Asteroseismology based on high-precision uninterrupted space photometry has become a new way to test the outcome of stellar evolution theory and was recently applied to a multitude of stars, but not yet to massive evolved supergiants.Our aim is to detect, analyse and interpret the photospheric and wind variability of the O9.5Iab star HD 188209 from Kepler space photometry and long-term high-resolution spectroscopy. We used Kepler scattered-light photometry obtained by the nominal mission during 1460d to deduce the photometric variability of this O-type supergiant. In addition, we assembled and analysed high-resolution high signal-to-noise spectroscopy taken with four spectrographs during some 1800d to interpret the temporal spectroscopic variability of the star. The variability of this blue supergiant derived from the scattered-light space photometry is in full in agreement with the one found in the ground-based spectroscopy. We find significant low-frequency variability that is consistently detected in all spectral lines of HD 188209. The photospheric variability propagates into the wind, where it has similar frequencies but slightly higher amplitudes. The morphology of the frequency spectra derived from the long-term photometry and spectroscopy points towards a spectrum of travelling waves with frequency values in the range expected for an evolved O-type star. Convectively-driven internal gravity waves excited in the stellar interior offer the most plausible explanation of the detected variability.

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C. Aerts, S. Simon-Diaz, S. Bloemen, et. al.
Tue, 7 Mar 17

Comments: Accepted for publication in Astronomy & Astrophysics, 14 pages, 15 figures, 3 tables. High-quality figures had to be omitted in this preprint and will only be included in the final version of the paper

Physical properties of the very young PN Hen3-1357 (Stingray Nebula) based on multiwavelength observations [SSA]

We have carried out a detailed analysis of the interesting and important very young planetary nebula (PN) Hen3-1357 (Stingray Nebula) based on a unique dataset of optical to far-IR spectra and photometric images. We calculated the abundances of nine elements using collisionally excited lines (CELs) and recombination lines (RLs). The RL C/O ratio indicates that this PN is O-rich, which is also supported by the detection of the broad 9/18 um bands from amorphous silicate grain. The observed elemental abundances can be explained by asymptotic giant branch (AGB) nucleosynthesis models for initially 1-1.5 Msun stars with Z = 0.008. The Ne overabundance might be due to the enhancement of 22Ne isotope in the He-rich intershell. By using the spectrum of the central star synthesized by Tlusty as the ionization/heating source of the PN, we constructed the self-consistent photoionization model with Cloudy to the observed quantities, and we derived the gas and dust masses, dust-to-gas mass ratio, and core-mass of the central star. About 80 % of the total dust mass is from warm-cold dust component beyond ionization front. Comparison with other Galactic PNe indicates that Hen3-1357 is an ordinary amorphous silicate rich and O-rich gas PN. Among other studied PNe, IC4846 shows many similarities in properties of the PN to Hen3-1357, although their post-AGB evolution is quite different from each other. Further monitoring observations and comparisons with other PNe such as IC4846 are necessary to understand the evolution of Hen3-1357.

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M. Otsuka, M. Parthasarathy, A. Tajitsu, et. al.
Tue, 7 Mar 17

Comments: 24 pages, 7 figures, 12 tables, Accepted for publication by the Astrophysical Journal

Absolute Ages and Distances of 22 GCs using Monte Carlo Main-Sequence Fitting [SSA]

The recent Gaia Data Release 1 of stellar parallaxes provides ample opportunity to find metal-poor main-sequence stars with precise parallaxes. We select 21 such stars with parallax uncertainties better than $\sigma_\pi/\pi\leq0.10$ and accurate abundance determinations suitable for testing metal-poor stellar evolution models and determining the distance to Galactic globular clusters. A Monte Carlo analysis was used, taking into account uncertainties in the model construction parameters, to generate stellar models and isochrones to fit to the calibration stars. The isochrones which fit the calibration stars best were then used to determine the distances and ages of 22 globular clusters with metallicities ranging from -2.4 dex to -0.7 dex. We find distances with an average uncertainty of 0.15 mag and absolute ages ranging from 10.8 – 13.6 Gyr with an average uncertainty of 1.6 Gyr. Using literature proper motion data we calculate orbits for the clusters finding six that reside within the Galactic disk/bulge while the rest are considered halo clusters. We find no strong evidence for a relationship between age and Galactocentric distance, but we do find a decreasing age-[Fe/H] relation.

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E. OMalley, C. Gilligan and B. Chaboyer
Tue, 7 Mar 17

Comments: 17 pages, 8 tables, 6 figures, accepted in ApJ

Formation of Extremely Low-mass White Dwarfs Binaries [SSA]

Motivated by the discovery of a handful of pulsating, extremely low mass white dwarfs (ELM WDs, mass $M \lesssim 0.17\, M_\odot$) which likely have WD companions, this paper discusses binary formation models for these systems. Formation of an ELM WD by unstable mass transfer (MT) or a common envelope (CE) event is unlikely. Stable Roche-lobe overflow with conservative MT produces only $M \gtrsim 0.2\, M_\odot$. This paper discusses the formation of ELM WD using angular momentum losses by magnetic braking. The initially more massive star becomes the companion WD through a CE event. The initially less massive star becomes the ELM WD as an evolved donor in a Cataclysmic Variable binary. Evolutionary models are constructed using the Modules for Experiments in Stellar Astrophysics (MESA), with ELM WD progenitors in the range $M_d=1.0-1.5\, M_\odot$ and WD companions in the range $M_{\rm a} \simeq 0.4-0.9\, M_\odot$. A prescription to reduce magnetic braking for thin surface convection zones is included. Upon the thinning of the evolved donor’s envelope, the donor star shrinks out of contact and MT ceases, revealing the ELM WD. Systems with small masses have previously been suggested as possible AM CVN’s. Systems with large masses, up to the limit $M \simeq 0.18\, M_\odot$ at which shell flashes occur on the WD cooling track, will tend to expand out to orbital periods $P_{\rm orb} \gtrsim 15\, {\rm hr}$. In between this range, ELM WD may become pulsators both as pre-WD and on the WD cooling track. Brickhill’s criterion for convective mode driving is used to estimate the location of the blue edge of the g-mode instability strip.

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M. Sun and P. Arras
Tue, 7 Mar 17

Comments: 14 pages, 23 figures, submitted to ApJ

X-Shooter spectroscopy of young stellar objects in Lupus. Atmospheric parameters, membership and activity diagnostics [SSA]

A homogeneous determination of basic stellar parameters of young stellar object (YSO) candidates is needed to confirm their evolutionary stage, membership to star forming regions (SFRs), and to get reliable values of the quantities related to chromospheric activity and accretion. We used the code ROTFIT and synthetic BT-Settl spectra for the determination of the atmospheric parameters (Teff and logg), the veiling, the radial (RV) and projected rotational velocity (vsini), from X-Shooter spectra of 102 YSO candidates in the Lupus SFR. We have shown that 13 candidates can be rejected as Lupus members based on their discrepant RV with respect to Lupus and/or the very low logg values. At least 11 of them are background giants. The spectral subtraction of inactive templates enabled us to measure the line fluxes for several diagnostics of both chromospheric activity and accretion. We found that all Class-III sources have H$\alpha$ fluxes compatible with a pure chromospheric activity, while objects with disks lie mostly above the boundary between chromospheres and accretion. YSOs with transitional disks displays both high and low H$\alpha$ fluxes. We found that the line fluxes per unit surface are tightly correlated with the accretion luminosity ($L_{\rm acc}$) derived from the Balmer continuum excess. This rules out that the relationships between $L_{\rm acc}$ and line luminosities found in previous works are simply due to calibration effects. We also found that the CaII-IRT flux ratio, $F_{8542}/F_{8498}$, is always small, indicating an optically thick emission source. The latter can be identified with the accretion shock near the stellar photosphere. The Balmer decrement reaches instead, for several accretors, high values typical of optically thin emission, suggesting that the Balmer emission originates in different parts of the accretion funnels with a smaller optical depth.

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A. Frasca, K. Biazzo, J. Alcala, et. al.
Mon, 6 Mar 17

Comments: 28 pages, 26 figures, accepted by A&A

Search for Polycyclic Aromatic Hydrocarbons in the Outflows from Dust-Producing Wolf-Rayet Stars [SSA]

A combined mid-IR spectrum of five colliding-wind, massive, dust-producing Population I Wolf-Rayet (WR) binaries shows a wealth of absorption and emission details coming from the circumstellar dust envelopes, as well as from the interstellar medium. The prominent absorption features may arise from a mix of interstellar carbonaceous grains formed in high- (e.g., 3.4, 6.8, 7.2 $\mu$m) and low-temperature (3.3, 6.9, 9.3 $\mu$m) environments. The broad emission complexes around $\sim$6.5, 8.0 and 8.8 $\mu$m could arise from ionized, small polycyclic aromatic hydrocarbon (PAH) clusters and/or amorphous carbonaceous grains. As such these PAH emissions may represent the long sought after precursors of amorphous Carbon dust. We also detect a strong $\sim$10.0 $\mu$m emission in the spectra of WR48a and WR112, that we tentatively link to ionized PAHs. Upon examining the available archival spectra of prodigious individual WR dust sources, we notice a surprising lack of 7.7 $\mu$m PAH band in the spectrum of the binary WR19, in contrast to the apparent strength of the 11.2, 12.7 and 16.4 $\mu$m PAH features. Strong PAH emissions are also detected in the $\lambda>$10 $\mu$m spectrum of another dust-producing system, WR118, pointing to the presence of large, neutral, presumably interstellar PAH molecules towards WR19 and WR118.

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S. Marchenko and A. Moffat
Mon, 6 Mar 17

Comments: Accepted for publication in Monthly Notices of the Royal Astronomical Society, Main Journal; 14pp., 12 figures

The behavior of the spotless active regions during the solar minimum 23-24 [SSA]

In this work, we analysed the physical parameters of the spotless actives regions observed during solar minimum 23 – 24 (2007 – 2010). The study was based on radio maps at 17~GHz obtained by the Nobeyama Radioheliograph (NoRH) and magnetograms provided by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). The results shows that the spotless active regions presents the same radio characteristics of a ordinary one, they can live in the solar surface for long periods (>10 days), and also can present small flares.

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A. Silva and C. Selhorst
Mon, 6 Mar 17

Comments: 3 pages, 3 figures, IAUS 328: Living around active stars

Seismic inference of 57 stars using full-length Kepler data sets [SSA]

We present stellar properties (mass, age, radius, distances) of 57 stars from a seismic inference using full-length data sets from Kepler. These stars comprise active stars, planet-hosts, solar-analogs, and binary systems. We validate the distances derived from the astrometric Gaia-Tycho solution. Ensemble analysis of the stellar properties reveals a trend of mixing-length parameter with the surface gravity and effective temperature. We derive a linear relationship with the seismic quantity $\langle r_{02} \rangle$ to estimate the stellar age. Finally, we define the stellar regimes where the Kjeldsen et al (2008) empirical surface correction for 1D model frequencies is valid.

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O. Creevey, T. Metcalfe, D. Salabert, et. al.
Mon, 6 Mar 17

Comments: 4-page proceedings from Seismology of the Sun and the Distant Stars 2016, TASC/KASC, Azores, Portugal

Robust detection of quasi-periodic variability: A HAWKI mini survey of late T dwarfs [SSA]

We present HAWK-I J-band light curves of five late-type T dwarfs (T6.5-T7.5) with a typical duration of four hours, and investigate the evidence for quasi-periodic photometric variability on intra-night timescales. Our photometry reaches precisions in the range 7-20 mmag, after removing instrumental systematics that correlate with sky background, seeing and airmass. Based upon a Lomb-Scargle periodogram analysis, the latest object in the sample – ULAS J2321 (T7.5) – appears to show quasi-periodic variability with a period of 1.64 hours and an amplitude of 3 mmag.
Given the low amplitude of variability and presence of systematics in our lightcurves, we discuss a Bayesian approach to robustly determine if quasi-periodic variability is present in a lightcurve affected by red noise. Using this approach, we conclude that the evidence for quasi-periodic variability in ULAS J2321 is not significant. As a result, we suggest that studies which identify quasi-periodic variables using the false alarm probability from a Lomb-Scargle periodogram are likely to over-estimate the number of variable objects, even if field stars are used to set a higher false alarm probability threshold. Instead we argue that a hybrid approach combining a false alarm probability cut, followed by Bayesian model selection, is necessary for robust identification of quasi-periodic variability in lightcurves with red noise.

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S. Littlefair, B. Burningham and C. Helling
Mon, 6 Mar 17

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

Gaia Data Release 1. Open cluster astrometry: performance, limitations, and future prospects [SSA]

Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information. Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters. Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed. Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier Hipparcos-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters. Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the Hipparcos data, with clearly increased luminosities for older A and F dwarfs.

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Gaia. Collaboration, F. Leeuwen, A. Vallenari, et. al.
Mon, 6 Mar 17

Comments: Accepted for publication by A&A. 21 pages main text plus 46 pages appendices. 34 figures main text, 38 figures appendices. 8 table in main text, 19 tables in appendices

The M33 Synoptic Stellar Survey. II. Mira Variables [SSA]

We present the discovery of 1847 Mira candidates in the Local Group galaxy M33 using a novel semi-parametric periodogram technique coupled with a Random Forest classifier. The algorithms were applied to ~2.4×10^5 I-band light curves previously obtained by the M33 Synoptic Stellar Survey. We derive preliminary Period-Luminosity relations at optical, near- & mid-infrared wavelengths and compare them to the corresponding relations in the Large Magellanic Cloud.

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W. Yuan, S. He, L. Macri, et. al.
Mon, 6 Mar 17

Comments: Accepted for publication in the Astronomical Journal

Pulsating white dwarf stars and asteroseismology [SSA]

At present, a large number of pulsating white dwarf (WD) stars is being discovered either from Earth-based surveys such as the Sloan Digital Sky Survey, or through observations from space (e.g., the Kepler mission). The asteroseismological techniques allow us to infer details of internal chemical stratification, the total mass, and even the stellar rotation profile. In this paper, we first describe the basic properties of WD stars and their pulsations, as well as the different sub-types of these variables known so far. Subsequently, we describe some recent findings about pulsating low-mass WDs.

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A. Corsico
Mon, 6 Mar 17

Comments: 10 pages, 4 figures. To be published in the proceedings of the “THIRD CONFERENCE ON STELLAR ASTROPHYSICS” to honor Prof. Dr. Juan J. Clari\’a, June 21 st to 24rd, 2016, C\’ordoba, Argentina, AAA Workshop Series, Vol. 9, 2016

Project 1640 Observations of Brown Dwarf GJ 758 B [SSA]

The nearby Sun-like star GJ 758 hosts a cold substellar companion, GJ 758 B, at a projected separation of $\lesssim$30 AU, previously detected in high-contrast multi-band photometric observations. In order to better constrain the companion’s physical characteristics, we acquired the first low-resolution ($R \sim 50$) near-infrared spectrum of it using the high-contrast hyperspectral imaging instrument Project 1640 on Palomar Observatory’s 5-m Hale telescope. We obtained simultaneous images in 32 wavelength channels covering the $Y$, $J$, and $H$ bands ($\sim$952-1770 nm), and used data processing techniques based on principal component analysis to efficiently subtract chromatic background speckle-noise. GJ 758 B was detected in four epochs during 2013 and 2014. Basic astrometric measurements confirm its apparent northwest trajectory relative to the primary star, with no clear signs of orbital curvature. Spectra of SpeX/IRTF observed T dwarfs were compared to the combined spectrum of GJ 758 B, with ${\chi}^2$ minimization suggesting a best fit for spectral type {T7.0$\pm$1.0}, but with a shallow minimum over T5-T8. Fitting of synthetic spectra from the BT-Settl13 model atmospheres gives an effective temperature {$T_{\text{eff}}=741 \pm 25$ K} and surface gravity {$\log g = 4.3 \pm 0.5$ dex} (cgs). Our derived best-fit spectral type and effective temperature from modeling of the low-resolution spectrum suggest a slightly earlier and hotter companion than previous findings from photometric data, but do not rule out current results, and confirm GJ 758 B as one of the coolest sub-stellar companions to a Sun-like star to date.

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R. Nilsson, A. Veicht, P. Godfrey, et. al.
Mon, 6 Mar 17

Comments: Accepted for publication in ApJ, 13 pages, 11 figures

Convective-core overshoot and suppression of oscillations: Constraints from red giants in NGC6811 [SSA]

Using data from the NASA spacecraft Kepler, we study solar-like oscillations in red-giant stars in the open cluster NGC6811. We determine oscillation frequencies, frequency separations, period spacings of mixed modes and mode visibilities for eight cluster giants. The oscillation parameters show that these stars are helium-core-burning red giants. The eight stars form two groups with very different oscillation power spectra; the four stars with lowest Delta_nu-values display rich sets of mixed l=1 modes, while this is not the case for the four stars with higher Delta_nu. For the four stars with lowest Delta_nu, we determine the asymptotic period spacing of the mixed modes, DeltaP, which together with the masses we derive for all eight stars suggest that they belong to the so-called secondary clump. Based on the global oscillation parameters, we present initial theoretical stellar modeling which indicate that we can constrain convective-core overshoot on the main sequence and in the helium-burning phase for these ~2M_sun stars. Finally, our results indicate less mode suppression than predicted by recent theories for magnetic suppression of certain oscillation modes in red giants.

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T. Arentoft, K. Brogaard, J. Jessen-Hansen, et. al.
Mon, 6 Mar 17

Comments: 29 pages, 12 figures. Accepted for publication in ApJ

The astrophysics of visible-light orbital phase curves in the space age [SSA]

The field of visible-light continuous time series photometry is now at its golden age, manifested by the continuum of past (CoRoT, Kepler), present (K2), and future (TESS, PLATO) space-based surveys delivering high precision data with a long baseline for a large number of stars. The availability of the high quality data has enabled astrophysical studies not possible before, including for example detailed asteroseismic investigations and the study of the exoplanet census including small planets. This has also allowed to study the minute photometric variability following the orbital motion in stellar binaries and star-planet systems which is the subject of this review. We focus on systems with a main sequence primary and a low-mass secondary, from a small star to a massive planet. The orbital modulations are induced by a combination of gravitational and atmospheric processes, including the beaming effect, tidal ellipsoidal distortion, reflected light, and thermal emission. Therefore, the phase curve shape contains information about the companion’s mass and atmospheric characteristics, making phase curves a useful astrophysical tool. For example, phase curves can be used to detect and measure the mass of short-period low-mass companions orbiting hot fast-rotating stars, out of reach of other detection methods. Another interesting application of phase curves is using the orbital phase modulations to look for non-transiting systems, which comprise the majority of stellar binary and star-planet systems. We discuss the science done with phase curves, the first results obtained so far, and the current difficulties and open questions related to this young and evolving subfield.

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A. Shporer
Fri, 3 Mar 17

Comments: Invited Review submitted to PASP. Posted on the arXiv for comments from the community

First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds [SSA]

Massive O-type stars play a dominant role in our Universe, but many of their properties remain poorly constrained. In the last decade magnetic fields have been detected in all Galactic members of the distinctive Of?p class, opening the door to a better knowledge of all O-type stars. With the aim of extending the study of magnetic massive stars to nearby galaxies, to better understand the role of metallicity in the formation of their magnetic fields and magnetospheres, and to broaden our knowledge of the role of magnetic fields in massive star evolution, we have carried out spectropolarimetry of five extra-Galactic Of?p stars, as well as a couple of dozen neighbouring stars. We have been able to measure magnetic fields with typical error bars from 0.2 to 1.0 kG, depending on the apparent magnitude and on weather conditions. No magnetic field has been firmly detected in any of our measurements, but we have been able to estimate upper limits to the field values of our target stars. One of our targets, 2dFS 936, exhibited an unexpected strengthening of emission lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period with two photometric peaks and one spectroscopic maximum. The observed strengthening of the emission lines of 2dFS 936, and the lack of detection of a strong magnetic field in a star with such strong emission lines is at odd with expectations. Together with the unusual periodic behaviour of BI 57, it represents a challenge for the current models of Of?p stars. The limited precision that we obtained in our field measurements (in most cases as a consequence of poor weather) has led to field-strength upper limits that are substantially larger than those typically measured in Galactic magnetic O stars. Further higher precision observations and monitoring are clearly required.

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S. Bagnulo, Y. Naze, I. Howarth, et. al.
Fri, 3 Mar 17

Comments: Accepted by A&A