Abstract Digest #5, 10.09.2014

We are sorry that due to other newsletter related tasks this acquired a bit backlog.

Today's abstracts:

  • [A&A] "Early-time spectra of supernovae and their precursor winds: the luminous blue variable/yellow hypergiant progenitor of SN 2013cu" by Jose H. Groh
  • [Conference proc.] "Hot Stars With Cool Companions" by Kevin Gullikson, Adam Kraus, Sarah Dodson-Robinson
  • [IAUS307] "Non-LTE Abundances in OB stars: Preliminary Results for 5 Stars in the Outer Galactic Disk" by G. A. Bragança et al.
  • [Conference proc.] "Rotation of the Mass Donors in High-mass X-ray Binaries and Symbiotic Stars" by Kiril Stoyanov, Radoslav Zamanov
  • [MNRAS Letter] "Discovery of a new Galactic bona fide luminous blue variable with Spitzer" by V.V. Gvaramadze et al.
  • [Research in Astronomy and Astrophysics] "Optical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars" by Blesson Mathew et al.
  • [ApJ] "NIR Spectroscopy of the HAeBe Star HD 100546: III. Further Evidence of an Orbiting Companion?" by Sean D. Brittain et al.
  • [IAUS307] "The BinaMIcS project: understanding the origin of magnetic fields in massive stars through close binary systems" by E. Alecian et al.
  • [A&A] "Improving the surface-brightness color relation for early-type stars using optical interferometry" by M. Challouf et al.
  • [A&A Research Note] "The applicability of the wind compression model" by Zuzana Cariková, Augustin Skopal
  • [ApJ Letter] "An ionized outflow from AB Aur, a Herbig Ae Star with a transitional disk" by Luis F. Rodríguez et al.
  • [A&A] "The blue-edge problem of the V1093 Her instability strip revisited using evolutionary models with atomic diffusion" by S. Bloemen et al.
  • [ApJ] "X-ray emission from magnetic massive stars" by Yael Naze et al.
  • [IAUS 307] "Asteroseismology of Massive Stars : Some Words of Caution" by A. Noels et al.
  • [A&A] "KIC7668647: a 14 day beaming sdB+WD binary with a pulsating subdwarf" by J.H. Telting et al.
  • [IAUS307] "Are the stars of a new class of variability detected in NGC 3766 fast rotating SPB stars?" by S.J.A.J. Salmon et al.
  • [IAUS307] "Asteroseismology of OB stars with hundreds of single snapshot spectra (and a few time-series of selected targets)" by S. Simón-Díaz
  • [A&A] "Scattering line polarization in rotating, optically thick disks" by Ivan Milic, Marianne Faurobert

Early-time spectra of supernovae and their precursor winds: the luminous blue variable/yellow hypergiant progenitor of SN 2013cu

Jose H. Groh

We present the first quantitative spectroscopic modeling of an early-time supernova that interacts with its progenitor wind. Using the radiative transfer code CMFGEN, we investigate the recently-reported 15.5 h post-explosion spectrum of the type IIb SN 2013cu. For the first time, we are able to directly measure the chemical abundances of a SN progenitor and find a relatively H-rich wind, with H and He abundances (by mass) of X=0.46 +- 0.2 and Y=0.52 +- 0.2, respectively. The wind is enhanced in N and depleted in C relative to solar values (mass fractions of 8.2e-3 and 1e-5). We obtain that a dense wind/circumstellar medium, with a mass-loss rate of Mdot= 3e-3 Msun/yr and wind velocity vwind=100 km/s, surrounds the star at the pre-SN stage. These values are lower than previous analytical estimates, although we find Mdot/vinf consistent with previous work. We also compute a CMFGEN model to constrain the progenitor spectral type and find that the high Mdot and low vwind imply that the star had an effective temperature of ~8000 K immediately before the SN explosion. Our models suggest that the progenitor was either an unstable luminous blue variable or a yellow hypergiant undergoing an eruptive phase, and rule out a WR star. We classify the post-explosion spectra at 15.5 h as XWN5(h) and advocate for the use of the prefix `X' (eXplosion) to avoid confusion between post-explosion, non-stellar spectra with those of massive stars. We show that the progenitor spectral type is significantly different than the early post-explosion spectral type owing to the huge differences in the ionization structure before and after the SN event. We find the following temporal evolution: LBV/YHG -> XWN5(h) -> SN IIb. Future early-time spectroscopy in the UV will give access to additional spectroscopic diagnostics and further constrain the properties of SN precursors, such as their metallicities.

Available at: arXiv:1408.5397

Hot Stars With Cool Companions

Kevin Gullikson, Adam Kraus, Sarah Dodson-Robinson

Young intermediate-mass stars have become high-priority targets for direct-imaging planet searches following the recent discoveries of planets orbiting e.g. HR 8799 and Beta Pictoris. Close stellar companions to these stars can affect the formation and orbital evolution of any planets, and so a census of the multiplicity properties of nearby intermediate mass stars is needed. Additionally, the multiplicity can help constrain the important binary star formation physics. We report initial results from a spectroscopic survey of 400 nearby A- and B-type stars. We search for companions by cross-correlating high resolution and high signal-to-noise ratio echelle spectra of the targets stars against model spectra for F- to M-type stars. We have so far found 18 new candidate companions, and have detected the spectral lines of the secondary in 4 known spectroscopic binary systems. We present the distribution of mass-ratios for close companions, and find that it differs from the distribution for wide (a

Available at: arXiv:1408.5411

Non-LTE Abundances in OB stars: Preliminary Results for 5 Stars in the Outer Galactic Disk

G. A. Bragança et al.

The aim of this study is to analyse and determine elemental abundances for a large sample of distant B stars in the outer Galactic disk in order to constrain the chemical distribution of the Galactic disk and models of chemical evolution of the Galaxy. Here, we present preliminary results on a few stars along with the adopted methodology based on securing simultaneous O and Si ionization equilibria with consistent NLTE model atmospheres.

Available at: arXiv:1408.5807

Rotation of the Mass Donors in High-mass X-ray Binaries and Symbiotic Stars

Kiril Stoyanov, Radoslav Zamanov

Our aim is to investigate the tidal interaction in High-mass X-ray Binaries and Symbiotic stars in order to determine in which objects the rotation of the mass donors is synchronized or pseudosynchronized with the orbital motion of the compact companion. We find that the Be/X-ray binaries are not synchronized and the orbital periods of the systems are greater than the rotational periods of the mass donors. The giant and supergiant High-mass X-ray binaries and symbiotic stars are close to synchronization. We compare the rotation of mass donors in symbiotics with the projected rotational velocities of field giants and find that the M giants in S-type symbiotics rotate on average 1.5 times faster than the field M giants. We find that the projected rotational velocity of the red giant in symbiotic star MWC 560 is v sin i = 8.2 +/- 1.5 km/s, and estimate its rotational period to be Prot = 144 - 306 days. Using the theoretical predictions of tidal interaction and pseudosynchronization, we estimate the orbital eccentricity e = 0.68 - 0.82.

Available at: arXiv:1408.6050

Discovery of a new Galactic bona fide luminous blue variable with Spitzer

V.V. Gvaramadze et al.

We report the discovery of a circular mid-infrared shell around the emission-line star Wray 16-137 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of Wray 16-137 with the Southern African Large Telescope revealed a rich emission spectrum typical of the classical luminous blue variables (LBVs) like P Cygni. Subsequent spectroscopic and photometric observations showed drastic changes in the spectrum and brightness during the last three years, meaning that Wray 16-137 currently undergoes an S Dor-like outburst. Namely, we found that the star has brightened by \approx 1 mag in the V and I_c bands, while its spectrum became dominated by Fe ii lines. Taken together, our observations unambiguously show that Wray 16-137 is a new member of the family of Galactic bona fide LBVs.

Available at: arXiv:1408.6232

Optical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars

Blesson Mathew et al.

We present optical and infrared photometric and spectroscopic studies of two Be stars in the 70--80-Myr-old open cluster NGC 6834. NGC 6834(1) has been reported as a binary from speckle interferometric studies whereas NGC 6834(2) may possibly be a gamma Cas-like variable. Infrared photometry and spectroscopy from the United Kingdom Infrared Telescope (UKIRT), and optical data from various facilities are combined with archival data to understand the nature of these candidates. High signal-to-noise near-IR spectra obtained from UKIRT have enabled us to study the optical depth effects in the hydrogen emission lines of these stars. We have explored the spectral classification scheme based on the intensity of emission lines in the H and K bands and contrasted it with the conventional classification based on the intensity of hydrogen and helium absorption lines. This work also presents hitherto unavailable UBV CCD photometry of NGC 6834, from which the evolutionary state of the Be stars is identified.

Available at: arXiv:1408.6658

NIR Spectroscopy of the HAeBe Star HD 100546: III. Further Evidence of an Orbiting Companion?

Sean D. Brittain et al.

We report high resolution NIR spectroscopy of CO and OH emission from the Herbig Be star HD100546. We discuss how our results bear striking resemblance to several theoretically predicted signposts of giant planet formation. The properties of the CO and OH emission lines are consistent with our earlier interpretation that these diagnostics provide indirect evidence for a companion that orbits the star close to the disk wall (at ~13au). The asymmetry of the OH spectral line profiles and their lack of time variability are consistent with emission from gas in an eccentric orbit at the disk wall that is approximately stationary in the inertial frame. The time variable spectroastrometric properties of the CO v=1-0 emission line point to an orbiting source of CO emission with an emitting area similar to that expected for a circumplanetary disk (~0.1au^2) assuming the CO emission is optically thick. We also consider a counterhypothesis to this interpretation, namely that the variable CO emission arises from a bright spot on the disk wall. We conclude with a brief suggestion of further work that can distinguish between these scenarios.

Available at: ApJ 791, 136

The BinaMIcS project: understanding the origin of magnetic fields in massive stars through close binary systems

E. Alecian et al.

It is now well established that a fraction of the massive (M>8 Msun) star population hosts strong, organised magnetic fields, most likely of fossil origin. The details of the generation and evolution of these fields are still poorly understood. The BinaMIcS project takes an important step towards the understanding of the interplay between binarity and magnetism during the stellar formation and evolution, and in particular the genesis of fossil fields, by studying the magnetic properties of close binary systems. The components of such systems are most likely formed together, at the same time and in the same environment, and can therefore help us to disentangle the role of initial conditions on the magnetic properties of the massive stars from other competing effects such as age or rotation. We present here the main scientific objectives of the BinaMIcS project, as well as preliminary results from the first year of observations from the associated ESPaDOnS and Narval spectropolarimetric surveys.

Available at: arXiv:1409.1094

Improving the surface-brightness color relation for early-type stars using optical interferometry

M. Challouf et al.

The aim of this work is to improve the SBC relation for early-type stars in the $-1 \leq V-K \leq 0$ color domain, using optical interferometry. Observations of eight B- and A-type stars were secured with the VEGA/CHARA instrument in the visible. The derived uniform disc angular diameters were converted into limb darkened angular diameters and included in a larger sample of twenty four stars, already observed by interferometry, in order to derive a revised empirical relation for O, B, A spectral type stars with a V-K color index ranging from -1 to 0. We also take the opportunity to check the consistency of the SBC relation up to $V-K \simeq 4$ using 100 additional measurements. We determined the uniform disc angular diameter for the eight following stars: $\gamma$ Ori, $\zeta$ Per, $8$ Cyg, $\iota$ Her, $\lambda$ Aql, $\zeta$ Peg, $\gamma$ Lyr and $\delta$ Cyg with V-K color ranging from -0.70 to 0.02 and typical precision of about $1.5\%$. Using our total sample of 132 stars with $V-K$ colors index ranging from about $-1$ to $4$, we provide a revised SBC relation. For late-type stars ($0 \leq V-K \leq 4$), the results are consistent with previous studies. For early-type stars ($-1 \leq V-K \leq 0$), our new VEGA/CHARA measurements combined with a careful selection of the stars (rejecting stars with environment or stars with a strong variability), allows us to reach an unprecedented precision of about 0.16 magnitude or $\simeq 7\%$ in term of angular diameter.

Available at: arXiv:1409.1351

The applicability of the wind compression model

Zuzana Cariková, Augustin Skopal

Compression of the stellar winds from rapidly rotating hot stars is described by the wind compression model. However, it was also shown that rapid rotation leads to rotational distortion of the stellar surface, resulting in the appearance of non-radial forces acting against the wind compression. In this note we justify the wind compression model for moderately rotating white dwarfs and slowly rotating giants. The former could be conducive to understanding density/ionization structure of the mass outflow from symbiotic stars and novae, while the latter can represent an effective mass-transfer mode in the wide interacting binaries.

Available at: A&A Forthcoming paper

An ionized outflow from AB Aur, a Herbig Ae Star with a transitional disk

Luis F. Rodríguez et al.

AB Aur is a Herbig Ae star with a transitional disk. Transitional disks present substantial dust clearing in their inner regions, most probably because of the formation of one or more planets, although other explanations are still viable. In transitional objects, accretion is found to be about an order of magnitude smaller than in classical full disks. Since accretion is believed to be correlated with outflow activity, centimeter free-free jets are expected to be present in association with these systems, at weaker levels than in classical protoplanetary (full) systems. We present new observations of the centimeter radio emission associated with the inner regions of AB Aur and conclude that the morphology, orientation, spectral index, and lack of temporal variability of the centimeter source imply the presence of a collimated, ionized outflow. The radio luminosity of this radio jet is, however, about 20 times smaller than that expected for a classical system of similar bolometric luminosity. We conclude that centimeter continuum emission is present in association with stars with transitional disks, but at levels than are becoming detectable only with the upgraded radio arrays. On the other hand, assuming that the jet velocity is 300 km s–1, we find that the ratio of mass loss rate to accretion rate in AB Aur is ~0.1, similar to that found for less evolved systems.

Available at: ApJ 793, L21

The blue-edge problem of the V1093 Her instability strip revisited using evolutionary models with atomic diffusion

S. Bloemen et al.

We have computed a new grid of evolutionary subdwarf B star (sdB) models from the start of central He burning, taking into account atomic diffusion due to radiative levitation, gravitational settling, concentration diffusion, and thermal diffusion. We have computed the non-adiabatic pulsation properties of the models and present the predicted p-mode and g-mode instability strips. In previous studies of the sdB instability strips, artificial abundance enhancements of Fe and Ni were introduced in the pulsation driving layers. In our models, the abundance enhancements of Fe and Ni occur naturally, eradicating the need to use artificial enhancements. We find that the abundance increases of Fe and Ni were previously underestimated and show that the instability strip predicted by our simulations solves the so-called blue edge problem of the subdwarf B star g-mode instability strip. The hottest known g-mode pulsator, KIC 10139564, now resides well within the instability strip {even when only modes with low spherical degrees (l<=2) are considered.

Available at: arXiv:1409.1604

X-ray emission from magnetic massive stars

Yael Naze et al.

Magnetically confined winds of early-type stars are expected to be sources of bright and hard X-rays. To clarify the systematics of the observed X-ray properties, we have analyzed a large series of Chandra and XMM observations, corresponding to all available exposures of known massive magnetic stars (over 100 exposures covering ~60% of stars compiled in the catalog of Petit et al. 2013). We show that the X-ray luminosity is strongly correlated with the stellar wind mass-loss-rate, with a power-law form that is slightly steeper than linear for the majority of the less luminous, lower-Mdot B stars and flattens for the more luminous, higher-Mdot O stars. As the winds are radiatively driven, these scalings can be equivalently written as relations with the bolometric luminosity. The observed X-ray luminosities, and their trend with mass-loss rates, are well reproduced by new MHD models, although a few overluminous stars (mostly rapidly rotating objects) exist. No relation is found between other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary to expectations (e.g. higher temperature for stronger mass-loss rate). This suggests that the main driver for the plasma properties is different from the main determinant of the X-ray luminosity. Finally, variations of the X-ray hardnesses and luminosities, in phase with the stellar rotation period, are detected for some objects and they suggest some temperature stratification to exist in massive stars' magnetospheres.

Available at: arXiv:1409.1690

Asteroseismology of Massive Stars : Some Words of Caution

A. Noels et al.

Although playing a key role in the understanding of the supernova phenomenon, the evolution of massive stars still suffers from uncertainties in their structure, even during their "quiet" main sequence phase and later on during their subgiant and helium burning phases. What is the extent of the mixed central region? In the local mixing length theory (LMLT) frame, are there structural differences using Schwarzschild or Ledoux convection criterion? Where are located the convective zone boundaries? Are there intermediate convection zones during MS and post-MS phase, and what is their extent and location? We discuss these points and show how asteroseismology could bring some light on these questions.

Available at: arXiv:1409.1765

KIC7668647: a 14 day beaming sdB+WD binary with a pulsating subdwarf

J.H. Telting et al.

The recently discovered subdwarf B (sdB) pulsator KIC7668647 is one of the 18 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. We use new ground-based low-resolution spectroscopy, and the near-continuous 2.88 year Kepler lightcurve, to reveal that KIC7668647 consists of a subdwarf B star with an unseen white-dwarf companion with an orbital period of 14.2d. An orbit with a radial-velocity amplitude of 39km/s is consistently determined from the spectra, from the orbital Doppler beaming seen by Kepler at 163ppm, and from measuring the orbital light-travel delay of 27 by timing of the many pulsations seen in the Kepler lightcurve. The white dwarf has a minimum mass of 0.40 M_sun. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, and find that nitrogen and iron have abundances close to solar values, while helium, carbon, oxygen and silicon are underabundant relative to the solar mixture. We use the full Kepler Q06--Q17 lightcurve to extract 132 significant pulsation frequencies. Period-spacing relations and multiplet splittings allow us to identify the modal degree L for the majority of the modes. Using the g-mode multiplet splittings we constrain the internal rotation period at the base of the envelope to 46-48d as a first seismic result for this star. The few p-mode splittings may point at a slightly longer rotation period further out in the envelope of the star. From mode-visibility considerations we derive that the inclination of the rotation axis of the sdB in KIC7668647 must be around ~60 degrees. Furthermore, we find strong evidence for a few multiplets indicative of degree 3 <= L <= 8, which is another novelty in sdB-star observations made possible by Kepler.

Available at: arXiv:1409.2345

Are the stars of a new class of variability detected in NGC~3766 fast rotating SPB stars?

S.J.A.J. Salmon et al.

A recent photometric survey in the NGC 3766 cluster led to the detection of stars presenting an unexpected variability. They lie in a region of the Hertzsprung-Russell (HR) diagram where no pulsation are theoretically expected, in between the delta Scuti and slowly pulsating B (SPB) star instability domains. Their variability periods, between ~0.1--0.7~d, are outside the expected domains of these well-known pulsators. The NCG 3766 cluster is known to host fast rotating stars. Rotation can significantly affect the pulsation properties of stars and alter their apparent luminosity through gravity darkening. Therefore we inspect if the new variable stars could correspond to fast rotating SPB stars. We carry out instability and visibility analysis of SPB pulsation modes within the frame of the traditional approximation. The effects of gravity darkening on typical SPB models are next studied. We find that at the red border of the SPB instability strip, prograde sectoral (PS) modes are preferentially excited, with periods shifted in the 0.2-0.5d range due to the Coriolis effect. These modes are best seen when the star is seen equator-on. For such inclinations, low-mass SPB models can appear fainter due to gravity darkening and as if they were located between the delta Scuti and SPB instability strips.

Available at: arXiv:1409.2402

Asteroseismology of OB stars with hundreds of single snapshot spectra (and a few time-series of selected targets)

S. Simón-Díaz

Imagine we could do asteroseismology of large samples of OB-type stars by using just one spectrum per target. That would be great! But this is probably a crazy and stupid idea. Or maybe not. Maybe we have the possibility to open a new window to investigate stellar oscillations in massive stars that has been in front of us for many years, but has not attracted very much our attention: the characterization and understanding of the so-called macroturbulent broadening in OB-type stars.

Available at: arXiv:1409.2416

Scattering line polarization in rotating, optically thick disks

Ivan Milic, Marianne Faurobert

To interpret observations of astrophysical disks it is essential to understand the formation process of the emitted light. If the disk is optically thick, scattering dominated and permeated by a Keplerian velocity field, Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to compute the emergent spectrum from a given disk model. We investigate Non-local thermodynamic equilibrium polarized line formation in different simple disk models and aim to demonstrate the importance of both radiative transfer effects and scattering as well as the effects of velocity fields. We self-consistently solve the coupled equations of radiative transfer and statistical equilibrium for a two level atom model by means of Jacobi iteration. We compute scattering polarization, that is Q/I and U/I line profiles. The degree of scattering polarization is significantly influenced by the inclination of the disk with respect to observer, but also by the optical thickness of the disk and the presence of rotation. Stokes U shows double-lobed profiles with amplitude which increases with the disk rotation. Our results suggest that the line profiles, especially the polarized ones, emerging from gaseous disks differ significantly from the profiles predicted by simple approximations. The profiles are diverse in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A clear indicator of disk rotation is the presence of Stokes U, which might prove to be a useful diagnostic tool. We also demonstrate that, for moderate rotational velocities, an approximate treatment can be used, where non-local thermodynamic equilibrium radiative transfer is done in the velocity field-free approximation and Doppler shift is applied in the process of spatial integration over the whole emitting surface.

Available at: arXiv:1409.2654


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