The continuum re-processing efficiency of Be‐star circumstellar discs

We have calculated the total flux emitted in H α , P α and Br α by the circumstellar envelope of both an early Be star, γ Cas, and a late Be star, 1 Delphini, assuming the central star is the only source of energy input into the circumstellar envelope. These estimates are based on the Be-star models of Millar & Marlborough which have self-consistent temperature distributions determined by equating the local rates of energy gain and energy loss in the envelopes. We find that an additional source of ionizing photons, as argued by Apparao, is not necessary to account for the observed emission.     

Probing the circumstellar structure of Herbig Ae/Be stars

We present Hα spectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarization across Hα is detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disc-like) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be stars, with characteristics changing from depolarization across Hα in the Herbig Be stars, to line polarizations in the Ae group. The frequency of depolarizations detected in the Herbig Be stars (seven out of 12) is particularly interesting as, by analogy with classical Be stars, it may be the best evidence to date that the higher-mass Herbig stars are surrounded by flattened structures. For the Herbig Ae stars, nine out of 11 show a line polarization effect that can be understood in terms of a compact Hα emission that is itself polarized by a rotating disc-like circumstellar medium. The spectropolarimetric difference between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung–Russell diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. Alternatively, the interior polarized line emission apparent in the Ae stars may be masked in the Herbig Be stars owing to their higher levels of Hα emission.     

Circumstellar disc geometry constrained by infrared line fluxes

Lenorzer et al. introduce ratios of hydrogen infrared recombination lines as a diagnostic tool to constrain the spatial distribution and physical condition of circumstellar material around hot massive stars. They demonstrate that the observed line flux ratios Hu14/Brα and Hu14/Pfγ from different types of objects associated with circumstellar material, such as Be stars, B[e] stars, and Luminous Blue Variable (LBV) stars are well separated in a diagnostic diagram. In this paper, we investigate this diagnostic tool using a non-LTE disc code developed by Sigut & Jones focusing on Be discs. We find good agreement between the empirical and predicted locations of Be stars in the Hu14/Brα versus Hu14/Pfγ diagram and show that indeed this diagnostic tool can be used to constrain basic properties of the discs of these stars.     

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory （ISO） Short-Wavelength Spectrometer （SWS） spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Brα/HI（14-6） line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot be used to judge whether a star is a Be star or not.     

Be phenomenon in open clusters: results from a survey of emission-line stars in young open clusters

Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour–magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.     

Radio observations of IRAS-selected Southern hemisphere classical Be stars

We present the first radio observations of a sample of 13 optically and IR-bright Southern hemisphere classical Be stars made from the Australian Telescope Compact Array at 3.5 and 6.3 cm simultaneously. One star, δ Cen, was detected at 3.5 cm, and a second, μ Cen, was also thought to have been detected; further observations of this source are required to confirm this detection. No sources were detected at 6.3 cm, although δ Cen was previously detected at this wavelength by other observers at a higher flux than our detection limit. The radio observations show that the spectral energy distribution undergoes a turnover between the far-IR and radio wavelengths, as was seen in previous studies. Likewise we find no simple correlation between far-IR and radio flux. Lower limits to the outer disc radius were found to be of the order of a few hundred solar radii i.e. of the order of those found previously by Taylor et al.     

Evidence for weak magnetic fields in early-type emission stars

We report the results of our study of magnetic fields in a sample of 15 Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four stars, HD56014, HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary status is not obvious due to the fact that it is a binary system currently observed in the initial rapid phase of mass exchange between the two components. Further, we notify the possible presence of distinct circular polarisation features in the circumstellar components of Ca II H&K in three stars, HD58011, HD117357, and HD181615, hinting at a probable presence of magnetic fields in the circumstellar mass loss disks of these stars. We emphasize the need for future spectropolarimetric observations of Be stars with detected magnetic fields to study the temporal evolution of their magnetic fields and the correlation of magnetic field properties with dynamical phenomena taking place in the gaseous circumstellar disks of these stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

3D eccentric discs around Be stars

One-armed oscillation modes in the circumstellar discs of Be stars may explain the cyclical variations in their emission lines. We show that a 3D effect, involving vertical motion and neglected in previous treatments, profoundly influences the dynamics. Using a secular theory of eccentric discs that reduces the problem to a second-order differential equation, we show that confined prograde modes are obtained for all reasonable disc temperatures and stellar rotation rates. We confirm these results using a numerical analysis of the full set of linearized equations for 3D isothermal discs including viscous terms that couple the horizontal motions at different altitudes. In order to make these modes grow, viscous damping must be overcome by an excitation mechanism such as viscous overstability.     

IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery

We report on the discovery of over 50 strong Hα emitting objects towards the large OB association Cyg OB2 and the H  ii region DR 15 on its southern periphery. This was achieved using the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r ',  i ' and H α photometry from IPHAS, and near-infrared J ,  H and K photometry from Two Micron All Sky Survey. The position of the objects in the ( J − H ) versus ( H − K ) diagram strongly suggests most of them are young. Many show Ca  ii infrared triplet emission indicating that they are in a pre-main-sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the H  ii region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable. The emission-line objects towards Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main-sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.     

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Brc/HI(14-6) line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot he used to judge whether a star is a Be star or not.     

Protodiscs around hot magnetic rotator stars

We develop equations and obtain solutions for the structure and evolution of a protodisc region that is initially formed with no radial motion and super-Keplerian rotation speed when wind material from a hot rotating star is channelled towards its equatorial plane by a dipole-type magnetic field. Its temperature is around 10⁷ K because of shock heating and the inflow of wind material causes its equatorial density to increase with time. The centrifugal force and thermal pressure increase relative to the magnetic force and material escapes at its outer edge. The protodisc region of a uniformly rotating star has almost uniform rotation and will shrink radially unless some instability intervenes. In a star with angular velocity increasing along its surface towards the equator, the angular velocity of the protodisc region decreases radially outwards and magnetorotational instability (MRI) can occur within a few hours or days. Viscosity resulting from MRI will readjust the angular velocity distribution of the protodisc material and may assist in the formation of a quasi-steady disc. Thus, the centrifugal breakout found in numerical simulations for uniformly rotating stars does not imply that quasi-steady discs with slow outflow cannot form around magnetic rotator stars with solar-type differential rotation.     

Excitation and visibility of slow modes in rotating B-type stars

We use the traditional approximation to describe oscillations with frequencies comparable to the angular rotation rate. Validity of this approximation in application to main-sequence B stars is discussed. Numerical results regarding mode stability and visibility are presented for a model of the Be star HD 163868. For this object, Walker et al. detected a record number of mode frequencies using data from the small space telescope MOST . Our interpretation of these data differs from that of Walker et al. In particular, we interpret peaks in the lowest frequency range as retrograde g modes. We find instability in a large number of modes that remain undetectable because of unfavourable aspect and/or effect of cancellation. There is no clear preference to excitation of prograde modes.