Abstract Digest #1, 17.7.2014

The BeSN is introducing a new model of distributing abstracts. Instead of relying on abstracts being sent directly by the authors, everyone is invited to submit abstracts and short bits of news of interest, which then will be published regularly in a digest. Today's abstracts:

  • "Irradiation Instability at the Inner Edges of Accretion Disks" by Jeffrey Fung and Pawel Artymowicz
  • "HD 161306: a radiatively interacting Be binary?" by P.Koubský et al.
  • "A K-band spectral mini-survey of Galactic B[e] stars" by A. Liermann et al.
  • "A search for weak or complex magnetic fields in the the B3V star iota Herculis" by G.A. Wade et al.
  • "KIC 10526294: a slowly rotating B star with rotationally split quasi-equally spaced gravity modes" by P.I. Pápics et al
  • "UVMag: stellar formation, evolution, structure and environment with space UV and visible spectropolarimetry" C. Neiner et al.

Irradiation Instability at the Inner Edges of Accretion Disks

Jeffrey Fung and Pawel Artymowicz

An instability can potentially operate in highly irradiated disks where the disk sharply transitions from being radially transparent to opaque (the "transition region"). Such conditions may exist at the inner edges of transitional disks around T Tauri stars and accretion disks around active galactic nuclei. We derive the criterion for this instability, which we term the "irradiation instability," or IRI. We also present the linear growth rate as a function of beta, the ratio between radiation force and gravity, and c s, the sound speed of the disk, obtained using two methods: a semi-analytic analysis of the linearized equations and a numerical simulation using the GPU-accelerated hydrodynamical code PEnGUIn. In particular, we find that IRI occurs at beta ~ 0.1 if the transition region extends as wide as ~0.05r, and at higher beta values if it is wider. This threshold value applies to c s ranging from 3% of the Keplerian orbital speed to 5%, and becomes higher if c s is lower. Furthermore, in the nonlinear evolution of the instability, disks with a large beta and small c s exhibit "clumping," extreme local surface density enhancements that can reach over 10 times the initial disk surface density.

Available at: ApJ 790, 78


HD 161306: a radiatively interacting Be binary?

P.Koubský et al.

The spectrum of the Be star HD 161306 is shown to vary periodically with a period of ~100 days. The radial velocity of the HeI 6678 Å emission peak varying in antiphase to the radial velocity of the Halpha emission wings component suggests that the star is a binary similar to phi Per, 59 Cyg, or FY CMa, i.e. a radiatively interacting Be binary – a rare case among Be stars. This type of object is also called a phi Per-type binary or Be + sdO binaries. The range of radial-velocity variations of the strong emission peak in the helium line observed in HD 161306 is about 180 km s-1, similar to what is observed for these systems. We therefore conclude that HD 161306 may represent another case of a Be star with a hot subdwarf companion.

Available at: A&A 567, A57


A K-band spectral mini-survey of Galactic B[e] stars

A. Liermann et al.

We present a mini-survey of Galactic B[e] stars mainly undertaken with the Large Binocular Telescope (LBT). B[e] stars show morphological features with hydrogen emission lines and an infrared excess, attributed to warm circumstellar dust. In general, these features are assumed to arise from dense, non-spherical, disk-forming circumstellar material in which molecules and dust can condensate. Due to the lack of reliable luminosities, the class of Galactic B[e] stars contains stars at very different stellar evolutionary phases like Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit K-band spectra for a sample of Galactic B[e] stars with the LBT-Luci I. Prominent spectral features, such as the Brackett gamma line and CO band heads are identified in the spectra. The analysis shows that the stars can be characterized as evolved objects. Among others we find one LBV candidate (MWC314), one supergiant B[e] candidate with 13CO (MWC137) and in two cases (MWC623 and AS 381) indications for the existence of a late-type binary companion, complementary to previous studies. For MWC84, IR spectra were taken at different epochs with LBT-Luci I and the GNIRS spectrograph at the Gemini North telescope. The new data show the disappearance of the circumstellar CO emission around this star, previously detectable over decades. Also no signs of a recent prominent eruption leading to the formation of new CO disk emission are found during 2010 and 2013.

Available at: arxiv.org:1407.3900


A search for weak or complex magnetic fields in the the B3V star iota Herculis

G.A. Wade et al.

We obtained 128 high signal-to-noise ratio Stokes V spectra of the B3V star iota Her on 5 consecutive nights in 2012 with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, with the aim of searching for the presence of weak and/or complex magnetic fields. Least-Squares Deconvolution mean profiles were computed from individual spectra, averaged over individual nights and over the entire run. No Zeeman signatures are detected in any of the profiles. The longitudinal magnetic field in the grand average profile was measured to be -0.24±0.32 G, as compared to -0.22±0.32 G in the null profile. Our observations therefore provide no evidence for the presence of Zeeman signatures analogous to those observed in the A0V star Vega by Lignieres et al. (2009). We interpret these observations in three ways. First, we compare the LSD profiles with synthetic Stokes V profiles corresponding to organised (dipolar) magnetic fields, for which we find an upper limit of about 8~G on the polar strength of any surface dipole present. Secondly, we compare the grand average profile with calculations corresponding to the random magnetic spot topologies of Kochukhov & Sudnik (2013), inferring that spots, if present, of 2 degrees radius with strengths of 2-4 G and a filling factor of 50% should have been detected in our data. Finally, we compare the observations with synthetic V profiles corresponding to the surface magnetic maps of Vega (Petit et al. 2010) computed for the spectral characteristics of iota Her. We conclude that while it is unlikely we would have detected a magnetic field identical to Vega's, we would have likely detected one with a peak strength of about 30 G, i.e. approximately four times as strong as that of Vega.

Available at: arXiv:1407.3991


KIC 10526294: a slowly rotating B star with rotationally split quasi-equally spaced gravity modes

P.I. Pápics et al

Massive stars are important for the chemical enrichment of the universe. Since internal mixing processes influence their life, it is of high importance to place constraints on the corresponding physical parameters, such as core overshooting and the internal rotation profile, to calibrate their stellar structure and evolution models. Although asteroseismology was shown to be able to deliver the most precise constraints so far, the number of detailed seismic studies delivering quantitative results is limited. Our goal is to extend this limited sample with an in-depth case study and provide a well constrained set of asteroseismic parameters, contributing to the ongoing mapping efforts of the instability strips of the beta Cep and SPB stars. We derived fundamental parameters from high-resolution spectra using spectral synthesis techniques. We used custom masks to obtain optimal light curves from the original pixel level data from the Kepler satellite. We used standard time-series analysis tools to construct a set of significant pulsation modes which provide the basis for the seismic analysis carried out afterwards. We find that KIC 10526294 is a cool SPB star, one of the slowest rotators ever found. Despite this fact, the length of Kepler observations is sufficient to resolve narrow rotationally split multiplets for each of its nineteen quasi-equally spaced dipole modes. The number of detected consecutive (in radial order) dipole modes in this series is higher than ever before. The observed amount of splitting shows an increasing trend towards longer periods, which - largely independent of the seismically calibrated stellar models - points towards a non-rigid internal rotation profile. From the average splitting we deduce a rotation period of 188 d. From seismic modelling we find that the star is young with a central hydrogen mass fraction X_c>0.64; it has a core overshooting alpha_ov<=0.15.

Available at: arXiv:1407.2986


UVMag: stellar formation, evolution, structure and environment with space UV and visible spectropolarimetry

C. Neiner et al.

Important insights into the formation, structure, evolution and environment of all types of stars can be obtained through the measurement of their winds and possible magnetospheres. However, this has hardly been done up to now mainly because of the lack of UV instrumentation available for long periods of time. To reach this aim, we have designed UVMag, an M-size space mission equipped with a high-resolution spectropolarimeter working in the UV and visible spectral range. The UV domain is crucial in stellar physics as it is very rich in atomic and molecular lines and contains most of the flux of hot stars. Moreover, covering the UV and visible spectral domains at the same time will allow us to study the star and its environment simultaneously. Adding polarimetric power to the spectrograph will multiply tenfold the capabilities of extracting information on stellar magnetospheres, winds, disks, and magnetic fields. Examples of science objectives that can be reached with UVMag are presented for pre-main sequence, main sequence and evolved stars. They will cast new light onto stellar physics by addressing many exciting and important questions. UVMag is currently undergoing a Research and Technology study and will be proposed at the forthcoming ESA call for M-size missions. This spectropolarimeter could also be installed on a large UV and visible observatory (e.g. NASA's LUVOIR project) within a suite of instruments.

Available at: arXiv:1407.2868

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