Sub-milliarcsecond precision spectro-astrometry of Be stars

The origin of the disks around Be stars is still not known. Further progress requires a proper parametrization of their structure, both spatially and kinematically. This is challenging as the disks are very small. Here we assess whether a novel method is capable of providing these data. We obtained spectro astrometry around the Pa beta line of two bright Be stars, alpha Col and zeta Tau, to search for disk signatures. The data, with a pixel to pixel precision of the centroid position of 0.3..0.4 milliarcsecond is the most accurate such data to date. Artefacts at the 0.85 mas level are present in the data, but these are readily identified as they were non-repeatable in our redundant datasets. This does illustrate the need of taking multiple data to avoid spurious detections. The data are compared with simple model simulations of the spectro astrometric signatures due to rotating disks around Be stars. The upper limits we find for the disk radii correspond to disk sizes of a few dozen stellar radii if they rotate Keplerian. This is very close to observationally measured and theoretically expected disk sizes, and this paper therefore demonstrates that spectro-astrometry, of which we present the first such attempt, has the potential to resolve the disks around Be stars.Comment: 6 pages, A&A accepte

A cyclic bipolar wind in the interacting binary V393 Scorpii

V393 Scorpii is a Double Periodic Variable characterized by a relatively stable non-orbital photometric cycle of 253 days. Mennickent et al. argue for the presence of a massive optically thick disc around the more massive B-type component and describe the evolutionary stage of the system. In this paper we analyze the behavior of the main spectroscopic optical lines during the long non-orbital photometric cycle. We study the radial velocity of the donor determining their orbital elements and find a small but significant orbital eccentricity (e = 0.04). The donor spectral features are modeled and removed from the spectrum at every observing epoch using the light-curve model given by Mennickent et al. We find that the line emission is larger during eclipses and mostly comes from a bipolar wind. We find that the long cycle is explained in terms of a modulation of the wind strength; the wind has a larger line and continuum emissivity on the high state. We report the discovery of highly variable chromospheric emission in the donor, as revealed by Doppler maps of the emission lines MgII 4481 and CI 6588. We discuss notable and some novel spectroscopic features like discrete absorption components, especially visible at blue-depressed OI 7773 absorption wings during the second half-cycle, Balmer double emission with V/R-curves showing "Z-type" and "S-type" excursions around secondary and main eclipse, respectively, and H_beta emission wings extending up to +- 2000 km/s. We discuss possible causes for these phenomena and for their modulations with the long cycle.Comment: 19 pages, 22 figures, accepted for publication in MNRA

An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars

We present an atlas of spectra of O- and B-type stars, obtained with the Short Wavelength Spectrometer (SWS) during the Post-Helium program of the Infrared Space Observatory (ISO). This program is aimed at extending the Morgan & Keenan classification scheme into the near-infrared. Later type stars will be discussed in a seperate publication. The observations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra acquired during the nominal mission with a similar observational setting. For B-type stars, this sample provides ample spectral converage in terms of subtype and luminosity class. For O-type stars,the ISO sample is coarse and therefore is complemented with 8 UKIRT L'-band observations. In terms of the presence of diagnostic lines, the L'-band is likely the most promising of the near-infrared atmospheric windows for the study of the physical properties of B stars. Specifically, this wavelength interval contains the Brackett alpha, Pfund gamma, and other Pfund lines which are probes of spectral type, luminosity class and mass loss. Here, we present simple empirical methods based on the lines present in the 2.4 to 4.1 microns interval that allow the determination of: the spectral type of B dwarfs and giants to within two subtypes; the luminosity class of B stars to within two classes; the mass-loss rate of O stars and B supergiants to within 0.25 dex.Comment: 19 pages, 11 Postscript figures, accepted by A&

The X-ray emission of the gamma Cassiopeiae stars

Long considered as the "odd man out" among X-ray emitting Be stars, \gamma Cas (B0.5e IV) is now recognized as the prototype of a class of stars that emit hard thermal X-rays. Our classification differs from the historical use of the term "gamma Cas stars" defined from optical properties alone. The luminosity output of this class contributes significantly to the hard X-ray production in massive stars in the Galaxy. The gamma Cas stars have light curves showing variability on a few broadly-defined timescales and spectra indicative of an optically thin plasma consisting of one or more hot thermal components. By now 9--13 Galactic \approx B0-1.5e main sequence stars are judged to be members or candidate members of the \gamma Cas class. Conservative criteria for this designation are for a \approxB0-1.5e III-V star to have an X-ray luminosity of 10^{32}--10^{33} ergs s^{-1}, a hot thermal spectrum containing the short wavelength Ly \alpha FeXXV and FeXXVI lines and the fluorescence FeK feature all in emission. If thermality cannot be demonstrated, for example from either the presence of these Ly \alpha lines or curvature of the hard continuum; these are the gamma Cas candidates. We discuss the history of the discovery of the complicated characteristics of the variability in the optical, UV, and X-ray domains, leading to suggestions for the physical cause of the production of hard X-rays. These include scenarios in which matter from the Be star accretes onto a degenerate secondary star and interactions between magnetic fields on the Be star and its decretion disk. The greatest aid to the choice of the causal mechanism is the temporal correlations of X-ray light curves and spectra with diagnostics in the optical and UV wavebands. We show why the magnetic star-disk interaction scenario is the most tenable explanation for the creation of hard X-rays on these stars.Comment: Review paper for "X-ray Emissions from Hot Stars and their Winds" compendium to be published by Advances in Space Research in mid-2016. Paper is comprised of 66 pages, 15 figure

On the presence and absence of disks around O-type stars

(abridged) As the favoured progenitors of long-duration gamma-ray bursts, massive stars may represent our best signposts of individual objects in the early Universe, but special conditions seem required to make these bursters, which might originate from the progenitor's rapid rotation and associated asymmetry. To obtain empirical constraints on the interplay between stellar rotation and wind asymmetry, we perform linear Halpha spectropolarimetry on a sample of 18 spectroscopically peculiar massive O stars, including OVz, Of?p, Oe, and Onfp stars, supplemented by an earlier sample of 20 O supergiants. Despite their rapid rotation (with vsin(i) up to 400 km/s) most O-type stars are found to be spherically symmetric, but with notable exceptions amongst specific object classes. We divide the peculiar O stars into four distinct categories: Groups III and IV include the Oe stars and Onfp stars, which are on the high-end tail of the O star rotation distribution and have in the past been claimed to be embedded in disks. Here we report the detection of a classical depolarization ``line effect'' in the Oe star HD 45314, but the overall incidence of line effects amongst Oe stars is significantly lower (1 out of 6) than amongst Be stars. The chance that the Oe and Be datasets are drawn from the same parent population is negligible (with 95% confidence). This implies there is as yet no evidence for a disk hypothesis in Oe stars, providing relevant constraints on the physical mechanism that is responsible for the Be phenomenon. Finally, we find that 3 out of 4 of the group IV Onfp stars show evidence for complex polarization effects, which are likely related to rapid rotation, and we speculate on the evolutionary links to B[e] stars.Comment: Astronomy & Astrophyics (in press) 12 page

First direct detection of a Keplerian rotating disk around the Be star α\alpha Arae using the VLTI/AMBER instrument

Aims. We aim to study the geometry and kinematics of the disk around the Be star $\alpha$ Arae as a function of wavelength, especially across the Br$\gamma$ emission line. The main purpose of this paper is to answer the question about the nature of the disk rotation around Be stars. Methods. We use the VLTI/AMBER instrument operating in the K band which provides a gain by a factor 5 in spatial resolution compared to previous VLTI/MIDI observations. Moreover, it is possible to combine the high angular resolution provided with the (medium) spectral resolution of AMBER to study the kinematics of the inner part of the disk and to infer its rotation law. Results. We obtain for the first time the direct evidence that the disk is in keplerian rotation, answering a question that occurs since the discovery of the first Be star $\gamma$ Cas by father Secchi in 1866. We also present the global geometry of the disk showing that it is compatible with a thin disk + polar enhanced winds modeled with the SIMECA code. We found that the disk around $\alpha$ Arae is compatible with a dense equatorial matter confined in the central region whereas a polar wind is contributing along the rotational axis of the central star. Between these two regions the density must be low enough to reproduce the large visibility modulus (small extension) obtained for two of the four VLTI baselines. Moreover, we obtain that $\alpha$ Arae is rotating very close to its critical rotation. This scenario is also compatible with the previous MIDI measurements.Comment: 15 page

Probing the circumstellar structure of pre-main sequence stars

Peer reviewe

Observations of o andromedae. III. A model

We present a model for the Be star o And which attempts to account for the observed spectroscopic, photometric, and polarimetric changes during the recent shell episode. The model suggests that the waning of the shell was a two-phased process. The initial phase was one in which the density in the inner regions of the circumstellar envelope decreased resulting in a decrease in polarization. Subsequently, the density in the outer regions decreased resulting in the fading of the shell absorption lines. A simple model in which the expansion velocity is independent of time and the mass-loss rate decreases, resulting in a density decrease which propagates outward, fails to satisfy the observed changes in o And. A more complex scenario which does fit the data is proposed.NRC publication: Ye

On the alignment between the circumstellar disks and orbital planes of Herbig Ae/Be binary systems

Original article can be found at: http://www.aanda.org/ Copyright The European Southern ObservatoryContext : The majority of the intermediate mass, pre-main-sequence Herbig Ae/Be stars reside in binary systems. As these systems are young, their properties may contain an imprint of the star formation process at intermediate masses (2-15M). However, these systems are generally spatially unresolved, making it difficult to probe their circumstellar environment to search for manifestations of their formation process, such as accretion disks. Aims. Here we investigate the formation mechanism of Herbig Ae/Be (HAe/Be) binary systems by studying the relative orientation of their binary orbits and circumstellar disks. Methods : We present linear spectropolarimetric observations of HAe/Be stars over the Hα line, which are used to determine the orientation of their circumstellar disks. In conjunction with data from the literature, we obtain a sample of 20 binaries with known disk position angles (PAs). We subsequently compare our disk PA data to a model to investigate whether HAe/Be binary systems and their disks are co-planar. Moreover, in the light of a relatively recent suggestion that some HAe/Be star spectropolarimetric signatures may not necessarily be related to circumstellar disks, we re-assess the relationship between spectropolarimetric signatures and disk PAs. We do this by comparing spectropolarimetric and high spatial resolution observations of young stellar objects (both HAe/Be and T Tauri stars). Results : We find that spectropolarimetric observations of pre-main-sequence stars do indeed trace circumstellar disks. This finding is significant above the 3σ level. In addition, our data are entirely consistent with the situation in which HAe/Be binary systems and circumstellar disks are co-planar, while random orientations can be rejected at the 2.2σ level. Conclusions : The conclusive alignment (at more than 3σ) between the disk PAs derived from linear spectropolarimetry and high spatial resolution observations indicates that linear spectropolarimetry traces disks. This in turn allows us to conclude that the orbital planes of HAe/Be binary systems and the disks around the primaries are likely to be co-planar, which is consistent with the notion that these systems form via monolithic collapse and subsequent disk fragmentation.Peer reviewe