Recent results for classical Be stars are reviewed and links to general astrophysics are presented. Classical Be stars are B-type stars close to the main sequence that exhibit line emission over the photospheric spectrum. The excess is attributed to a circumstellar gaseous component that is commonly accepted to be in the form of an equatorial disk. Since 1988, when the last such review was published, major progress has been made. The geometry and kinematics of the circumstellar environment can be best explained by a rotationally supported relatively thin disk with very little outflow, consistent with interferometric observations. The presence of short-term periodic variability is restricted to the earlier type Be stars. This variation for at least some of these objects has been shown to be due to nonradial pulsation. For at least one star, evidence for a magnetic field has been observed. The mechanisms responsible for the production and dynamics of the circumstellar gas are still not constrained. Observations of nonradial pulsation beating phenomena connected to outbursts point toward a relevance of pulsation, but this mechanism cannot be generalized. Either the evidence that Be stars do not form a homogeneous group with respect to disk formation is growing or the short-term periodic variability is less important than previously thought. The statistics of Be stars investigated in open clusters of the Milky Way and the Magellanic Clouds has reopened the question of the evolutionary status of Be stars. The central B star is a fast rotator, although theoretical developments have revived the question of how high rotational rates are, so the commonly quoted mean value of about 70%-80% of the critical velocity may just be a lower limit. Be stars are in a unique position to make contributions to several important branches of stellar physics, e.g., asymmetric mass-loss processes, stellar angular momentum distribution evolution, astroseismology, and magnetic field evolution.

http://iopscience.iop.org/article/10.1086/378307/pdf

Classical Be Stars

When applied to Be/X-ray binaries, the viscous decretion disc model, which can successfully account for most properties of Be stars, naturally predicts the truncation of the circumstellar disc. The distance at which the circumstellar disc is truncated depends mainly on the orbital parameters and the viscosity. In systems with low eccentricity, the disc is expected to be truncated at the 3:1 resonance radius, for which the gap between the disc outer radius and the critical lobe radius of the Be star is so wide that, under normal conditions, the neutron star cannot accrete enough gas at periastron passage to show periodic X-ray outbursts (type I outbursts). These systems will display only occasional giant X-ray outbursts (type II outbursts). On the other hand, in systems with high orbital eccentricity, the disc truncation occurs at a much higher resonance radius, which is very close to or slightly beyond the critical lobe radius at periastron unless the viscosity is very low. In these systems, disc truncation cannot be efficient, allowing the neutron star to capture gas from the disc at every periastron passage and display type I outbursts regularly. In contrast to the rather robust results for systems with low eccentricity and high eccentricity, the result for systems with moderate eccentricity depends on rather subtle details. Systems in which the disc is truncated in the vicinity of the critical lobe will regularly display type I outbursts, whereas those with the disc significantly smaller than the critical lobe will show only type II outbursts under normal conditions and temporary type I outbursts when the disc is strongly disturbed. In Be/X-ray binaries, material will be accreted via the first Lagrangian point with low velocities relative to the neutron star and carrying high angular momentum. This may result in the temporary formation of accretion discs during type I outbursts, something that seems to be confirmed by observations.

/pdf/a-natural-explanation-for-periodic-x-ray-outbursts-in-be-x-2ghglm8987.pdf

A natural explanation for periodic X-ray outbursts in Be/X-ray binaries

New optical spectroscopy of the high-mass X-ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X-ray/radio outbursts are triggered 2.5-4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ∼65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i ≃ 113 km s -1 .

/pdf/orbital-parameters-of-the-microquasar-ls-i-61-303-4vg3oy72cq.pdf

Orbital parameters of the microquasar LS I +61 303

To probe the distribution and physical characteristics of interstellar gas at temperatures -->T ? 3 ? 105 K in the disk of the Milky Way, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of O VI ?1032 toward 148 early-type stars situated at distances >1 kpc. After subtracting off a mild excess of O VI arising from the Local Bubble, combining our new results with earlier surveys of O VI, and eliminating stars that show conspicuous localized X-ray emission, we find an average O VI midplane density -->n0 = 1.3 ? 10?8 cm?3. The density decreases away from the plane of the Galaxy in a way that is consistent with an exponential scale height of 3.2 kpc at negative latitudes or 4.6 kpc at positive latitudes. Average volume densities of O VI along different sight lines exhibit a dispersion of about 0.26 dex, irrespective of the distances to the target stars. This indicates that O VI does not arise in randomly situated clouds of a fixed size and density, but instead is distributed in regions that have a very broad range of column densities, with the more strongly absorbing clouds having a lower space density. Line widths and centroid velocities are much larger than those expected from differential Galactic rotation, but they are nevertheless correlated with distance and N(O VI), which reinforces our picture of a diverse population of hot plasma regions that are ubiquitous over the entire Galactic disk. The velocity extremes of the O VI profiles show a loose correlation with those of very strong lines of less ionized species, supporting a picture of a turbulent, multiphase medium churned by shock-heated gas from multiple supernova explosions.

The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in the Disk of the Milky Way

The characterization of gas in the inner disks around young stars is of particular interest because of its connection to planet formation. In order to study the gas in inner disks, we have obtained high-resolution K- and M-band spectroscopy of 14 intermediate-mass young stars. In sources that have optically thick inner disks, i.e., E(K - L) > 1, our detection rate of the rovibrational CO transitions is 100%, and the gas is thermally excited. Of the five sources that do not have optically thick inner disks, we only detect the rovibrational CO transitions from HD 141569. In this case, we show that the gas is excited by UV fluorescence and that the inner disk is devoid of gas and dust. We discuss the plausibility of the various scenarios for forming this inner hole. Our modeling of the UV-fluoresced gas suggests an additional method by which to search for and/or place stringent limits on gas in dust-depleted regions in disks around Herbig Ae/Be stars.

/pdf/warm-gas-in-the-inner-disks-around-young-intermediate-mass-3irohbtgrx.pdf

Warm Gas in the Inner Disks around Young Intermediate-Mass Stars

We present an atlas of high-S=N , high-resolution (v 6k m s 1 ) data of Be star emission and shell proles. We have collected proles of H and of Fe ii ,m ostly of the5317 transition. These lines have been selected to provide measures for the overall emission strength and for the velocity eld in these disks. We have collected data for 77 southern and equatorial programme stars, covering the period 1982-1993. This is the most comprehensive overview of prole shapes in Be disks. We propose a three-dimensional scheme in which most observed proles can be classied. The parameters are i) inclination, ii) optical depth, and iii) the pattern of the velocity eld. A search for short-term variability (timescales between ve days and a few minutes) in six stars ended with negative result. Shortest observed timescale for variability is a few days for well-developed disks in binary systems (HR 1910, HR 2142).

/pdf/atlas-of-high-resolution-emission-and-shell-lines-in-be-4947fvndyd.pdf

O. Dietle

Papers

Atlas of high-resolution emission and shell lines in Be stars. Line profiles and short-term variability