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Context. IGR J06074+2205 is a poorly studied X-ray source with a Be star companion. It has been proposed to belong to the group of Be/X-ray binaries (BeXBs). In BeXBs, accretion onto the neutron star occurs via the transfer of material from the Be star's circumstellar disk. Thus, in the absence of the disk, no X-ray should be detected. Aims. The main goal of this work is to study the quiescent X-ray emission of IGR J06074+2205 during a disk-loss episode. Methods. We obtained light curves at different energy bands and a spectrum covering the energy range 0.4-12 keV. We used Fourier analysis to study the aperiodic variability and epoch folding methods to study the periodic variability. Model fitting to the energy spectrum allowed us to identify the possible physical processes that generated the X-rays. Results. We show that at the time of the XMM-Newton observation, the decretion disk around the Be star had vanished. Still, accretion appears as the source of energy that powers the high-energy radiation in IGR J06074+2205.We report the discovery of X-ray pulsations with a pulse period of 373.2 s and a pulse fraction of ~50%. The 0.4-12 keV spectrum is well described by an absorbed power law and blackbody components with the best fitting parameters: NH = (6:2 ± 0.5) Ö 1021 cm-2, kTbb = 1.16 ± 0.03 keV, and г = 1.5 ± 0.1. The absorbed X-ray luminosity is LX = 1:4 Ö 1034 erg s-1 assuming a distance of 4.5 kpc. Conclusions. The detection of X-ray pulsations confirms the nature of IGR J06074+2205 as a BeXB. We discuss various scenarios to explain the quiescent X-ray emission of this pulsar. We rule out cooling of the neutron star surface and magnetospheric emission and conclude that accretion is the most likely scenario. The origin of the accreted material remains an open question. [ABSTRACT FROM AUTHOR]

Context. The class of Be stars are the epitome of rapid rotators in the main sequence. These stars are privileged candidates for studying the incidence of rotation on the stellar internal structure and on non-radial pulsations. Pulsations are considered possible mechanisms to trigger mass-ejection phenomena required to build up the circumstellar disks of Be stars. Aims. Time series analyses of the light curves of 15 faint Be stars observed with the CoRoT satellite were performed to obtain the distribution of non-radial pulsation (NRP) frequencies in their power spectra at epochs with and without light outbursts and to discriminate pulsations from rotation-related photometric variations. Methods. Standard Fourier techniques were employed to analyze the CoRoT light curves. Fundamental parameters corrected for rapid-rotation effects were used to study the power spectrum as a function of the stellar location in the instability domains of the Hertzsprung-Russell (H-R) diagram. Results. Frequencies are concentrated in separate groups as predicted for g-modes in rapid B-type rotators, except for the two stars that are outside the H-R instability domain. In five objects the variations in the power spectrum are correlated with the time-dependent outbursts characteristics. Time-frequency analysis showed that during the outbursts the amplitudes of stable main frequencies within 0.03 c d-1 intervals strongly change, while transients and/or frequencies of low amplitude appear separated or not separated from the stellar frequencies. The frequency patterns and activities depend on evolution phases: (i) the average separations between groups of frequencies are larger in the zero-age main sequence (ZAMS) than in the terminal age main sequence (TAMS) and are the largest in the middle of the MS phase; (ii) a poor frequency spectrum with f ≲ 1 cd-1 of low amplitude characterizes the stars beyond the TAMS; and (iii) outbursts are seen in stars hotter than B4 spectral type and in the second half of the MS. Conclusions. The two main frequency groups are separated by δf = (1.24 ± 0.28) × frot in agreement with models of prograde sectoral g-modes (m = -1, -2) of intermediate-mass rapid rotators. The changes of amplitudes of individual frequencies and the presence of transients correlated with the outburst events deserve further studies of physical conditions in the subatmospheric layers to establish the relationship between pulsations and sporadic mass-ejection events. [ABSTRACT FROM AUTHOR]

We present a proper motion investigation of a sample of Be star candidates towards the Magellanic Clouds, which has resulted in the identification of separate populations, in the Galactic foreground and in the Magellanic background. Be stars are broadly speaking B-type stars that have shown emission lines in their spectra. In this work, we studied a sample of 2446 and 1019 Be star candidates towards the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively, taken from the literature and proposed as possible Be stars due to their variability behaviour in the OGLE-II I band. JHKs magnitudes from the InfraRed Survey Facility catalogue and proper motions from the Southern Proper Motion 4 catalogue were obtained for 1188 and 619 LMC and SMC Be stars candidates, respectively. Colour-colour and vector-point diagrams were used to identify different populations amongst the Be star candidates. In the LMC sample, two populations with distinctive infrared colours and kinematics were found, the bluer sample is consistent with being in the LMC and the redder one with belonging to the Milky Way disc. This settles the nature of the redder sample that had been described in previous publications as a possible unknown subclass of stars amongst the Be candidates in the LMC. In the SMC sample, a similar but less evident result was obtained, since this apparent unknown subclass was not seen in this galaxy. We confirm that in the selection of Be stars by their variability, although generally successful, there is a higher risk of contamination by Milky Way objects towards redder B - V and V - I colours. [ABSTRACT FROM AUTHOR]

Context: The structure of the inner parts of Be star disks (≲ 20 stellar radii) is well explained by the viscous decretion disk (VDD) model, which is able to reproduce the observable properties of most of the objects studied so far. The outer parts, on the other hand, are not observationally well-explored, as they are observable only at radio wavelengths. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for several Be stars that were previously detected in the radio, but a convincing physical explanation for this trend has not yet been provided. Aims: We test the VDD model predictions for the extended parts of a sample of six Be disks that have been observed in the radio to address the question of whether the observed turndown in the spectral energy distribution (SED) can be explained in the framework of the VDD model, including recent theoretical development for truncated Be disks in binary systems. Methods: We combine new multi-wavelength radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama Pathfinder Experiment (APEX) with previously published radio data and archival SED measurements at ultraviolet, visual, and infrared wavelengths. The density structure of the disks, including their outer parts, is constrained by radiative transfer modeling of the observed spectrum using VDD model predictions. In the VDD model we include the presumed effects of possible tidal influence from faint binary companions. Results: For 5 out of 6 studied stars, the observed SED shows strong signs of SED turndown between far-IR and radio wavelengths. A VDD model that extends to large distances closely reproduces the observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED. Using a truncated VDD model improves the fit, leading to a successful explanation of the SED turndown observed for the stars in our sample. The slope of the observed SEDs in the radio is however not well reproduced by disks that are simply cut off at a certain distance. Rather, some matter seems to extend beyond the truncation radius, where it still contributes to the observed SEDs, making the spectral slope in the radio shallower. This finding is in agreement with our current understanding of binary truncation from hydrodynamical simulations, in which the disk does extend past the truncation radius. Therefore, the most probable cause for the SED turndown is the presence of binary companions that remain undetected for most of our sources. [ABSTRACT FROM AUTHOR]

We study the effect of density distribution evolution on the global one-armed oscillation modes in low-viscosity discs around isolated and binary Be stars. Observations show that some Be stars exhibit evidence of formation and dissipation of the equatorial disc. In this paper, we first calculate the density evolution in discs around isolated Be stars. To model the formation stage of the disc, we inject mass at a radius just outside the star at a constant rate for 30-50 yr. As the disc develops, the density distribution approaches the form of the steady disc solution. Then, we turn-off the mass injection to model the disc dissipation stage. The innermost part of the disc starts accretion, and a gap forms between the star and the disc. Next, we calculate the one-armed modes at several epochs. We neglect the effect of viscosity because the time-scale of oscillations is much shorter than the disc evolution time-scale for low viscosity. In the disc formation stage, the eigenfrequency increases with time towards the value for the steady state disc. On the other hand, one-armed eigenmodes in dissipating Be discs have significantly higher eigenfrequencies and narrower propagation regions. Observationally, such a change of mode characteristics can be taken as an evidence for gap opening around the star. In binary Be stars, the characteristics of the disc evolution and the eigenmodes are qualitatively the same as in isolated Be stars, but quantitatively, they have shorter evolution time-scales and higher eigenfrequencies, which is in agreement with the observed trend. [ABSTRACT FROM AUTHOR]

We investigate the continuum emission of viscous decretion discs around Be stars in this paper. The results obtained from non-LTE (local thermodynamic equilibrium) radiative transfer models show two regimes in the disc surface brightness profile: an inner optically thick region, which behaves as a pseudo-photosphere with a wavelength-dependent size, and an optically thin tenuous outer part, which contributes with about a third of the total flux. The isophotal shape of the surface brightness is well described by elliptical contours with an axial ratio b/a = cos i for inclinations i < 75°. Based on these properties, a semi-analytical model was developed to describe the continuum emission of gaseous discs. It provides fluxes and spectral slopes at the infrared within an accuracy of 10 and 5 per cent, respectively, when compared to the numerical results. The model indicates that the infrared spectral slope is mainly determined by both the density radial slope and the disc flaring exponent, being practically independent of disc inclination and base density. As a first application, the density structure of 15 Be stars was investigated, based on the infrared flux excess, and the results compared to previous determinations in the literature. Our results indicate that the decretion rates are in the range of 10-12-10-9M⊙ yr-1, which is at least two orders of magnitude smaller than the previous outflowing disc model predictions. [ABSTRACT FROM AUTHOR]

Context. The spectral features of HD 94509 are highly unusual, adding an extreme to the zoo of Be and shell stars. The shell dominates the spectrum, showing lines typical for spectral types mid-A to early-F, while the presence of a late/mid B-type central star is indicated by photospheric hydrogen line wings and helium lines. Numerous metallic absorption lines have broad wings but taper to narrow cores. They cannot be fit by Voigt profiles. Aims. We describe and illustrate unusual spectral features of this star, and make rough calculations to estimate physical conditions and abundances in the shell. Furthermore, the central star is characterized. Methods. We assume mean conditions for the shell. An electron density estimate is made from the Inglis-Teller formula. Excitation temperatures and column densities for Fe I and Fe II are derived from curves of growth. The neutral H column density is estimated from high Paschen members. The column densities are compared with calculations made with the photoionization code CLOUDY. Atmospheric parameters of the central star are constrained employing non-LTE spectrum synthesis. Results. Overall chemical abundances are close to solar. Column densities of the dominant ions of several elements, as well as excitation temperatures and the mean electron density are well accounted for by a simple model. Several features, including the degree of ionization, are less well described. Conclusions. HD 94509 is a Be star with a stable shell, close to the terminal-age main sequence. The dynamical state of the shell and the unusually shaped, but symmetric line profiles, require a separate study. [ABSTRACT FROM AUTHOR]

Reliable determination of the basic physical properties of hot emission-line binaries with Roche-lobe filling secondaries is important for developing the theory of mass exchange in binaries. It is a very hard task, however, which is complicated by the presence of circumstellar matter in these systems. So far, only a small number of systems with accurate values of component masses, radii, and other properties are known. Here, we report the first detailed study of a new representative of this class of binaries, BR CMi, based on the analysis of radial velocities and multichannel photometry from several observatories, and compare its physical properties with those for other well-studied systems. BR CMi is an ellipsoidal variable seen under an intermediate orbital inclination of ~51?, and it has an orbital period of 12.d 919059(15) and a circular orbit. We used the disentangled component spectra to estimate the effective temperatures 9500(200) K and 4655(50) K by comparing them with model spectra. They correspond to spectral types B9.5e and G8III. We also used the disentangled spectra of both binary components as templates for the 2D cross-correlation to obtain accurate radial velocities and a reliable orbital solution. Some evidence of a secular period increase at a rate of (1.1±0.5) s per year was found. This, together with a very low mass ratio of 0.06 and a normal mass and radius of the mass gaining component, indicates that BR CMi is in a slow phase of the mass exchange after the mass-ratio reversal. It thus belongs to a still poorly populated subgroup of Be stars for which the origin of Balmer emission lines is safely explained as a consequence of mass transfer between the binary components. [ABSTRACT FROM AUTHOR]

We present a catalogue of 247 photometrically and spectroscopically confirmed fainter classical Be stars (13 ≲ r ≲ 16) in the direction of the Perseus Arm of the Milky Way (−1° < b < +4°, 120° < ℓ < 140°). The catalogue consists of 181 IPHAS-selected new classical Be stars, in addition to 66 objects that we studied in our previous work more closely, and three stars identified as classical Be stars in earlier work. This study more than doubles the number known in the region. Photometry spanning 0.6–5 μm, spectral types, and interstellar reddenings are given for each object. The spectral types were determined from low-resolution spectra (λ/Δλ ≈ 800–2000), to a precision of 1–3 subtypes. The interstellar reddenings are derived from the (r − i) colour, using a method that corrects for circumstellar disc emission. The colour excesses obtained range from E(B − V) = 0.3 up to 1.6 – a distribution that modestly extends the range reported in the literature for Perseus-Arm open clusters. For around half the sample, the reddenings obtained are compatible with measures of the total sightline Galactic extinction. Many of these are likely to lie well beyond the Perseus Arm. [ABSTRACT FROM AUTHOR]

Aims. We present a multiwavelength study of the Be/X-ray binary system SAXJ2103.5+4545 with the goal of better characterizing the transient behaviour of this source. Methods. SAXJ2103.5+4545 was observed by Swift/XRT four times in 2007 from April 25 to May 5, and during quiescence in 2012 August 31. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain), and Sierra Nevada (Granada, Spain) since 2011 August, and from the TÜB˙ITAK National Observatory (Antalya, Turkey) since 2009 June. We performed spectral and photometric temporal analyses to investigate the different states exhibited by SAXJ2103.5+4545. Results. In X-rays, an absorbed power-law model provided the best fit for all the XRT spectra. An iron-line feature at ∼6.42 keV was present in all the observations except for that taken during quiescence in 2012. The photon indexes are consistent with previous studies of SAXJ2103.5+4545 in high/low-luminosity states. Pulsations were found in all the XRT data from 2007 (2.839(2) mHz; MJD 54 222.02), but not during quiescence. The two optical outbursts in 2010 and 2012 lasted for about eight or nine months (as the one in 2007 probably did and the current one in 2014 might do) and were most probably caused by mass-ejection events from the Be star that eventually fed the circumstellar disc. All of these outbursts started about three months before the triggering of the X-ray activity, and at about the same period before the maximum of the Hα line equivalent width (in emission) was reached at only ∼–5Å. The global correlation between the BV variability and the X-ray intensity was also observed at longer wavelengths in the IR domain. [ABSTRACT FROM AUTHOR]

In the past decade, a consensus has emerged regarding the nature of classical Be stars: They are very rapidly rotating main sequence B stars, which, through a still unknown, but increasingly constrained process, form an outwardly diffusing gaseous, dust-free Keplerian disk. In this work, first the definition of Be stars is contrasted to similar classes, and common observables obtained for Be stars are introduced and the respective formation mechanisms explained. We then review the current state of knowledge concerning the central stars as non-radially pulsating objects and non-magnetic stars, as far as it concerns large-scale, i.e., mostly dipolar, global fields. Localized, weak magnetic fields remain possible, but are as of yet unproven. The Be-phenomenon, linked with one or more mass-ejection processes, acts on top of a rotation rate of about 75 % of critical or above. The properties of the process can be well constrained, leaving only few options, most importantly, but not exclusively, non-radial pulsation and small-scale magnetic fields. Of these, it is well possible that all are realized: In different stars, different processes may be acting. Once the material has been lifted into Keplerian orbit, memory of the details of the ejection process is lost, and the material is governed by viscosity. The disks are fairly well understood in the theoretical framework of the viscous decretion disk model. This is not only true for the disk structure, but as well for its variability, both cyclic and secular. Be binaries are reviewed under the aspect of the various types of interactions a companion can have with the circumstellar disk. Finally, extragalactic Be stars, at lower metallicities, seem more common and more rapidly rotating. [ABSTRACT FROM AUTHOR]

ABSTRACT Photometric and spectroscopic results are presented for the Be star X Per/HD 24534 from near-infrared (near-IR) monitoring in 2010-2011. The star is one of a sample of selected Be/X-ray binaries being monitored by us in the near-IR to study correlations between their X-ray and near-IR behaviour. Comparison of the star's present near-IR magnitudes with earlier records shows the star to be currently in a prominently bright state with mean J, H, K magnitudes of 5.49, 5.33 and 5.06, respectively. The JHK spectra are dominated by emission lines of He i and Paschen and Brackett lines of H i. Lines of O i 1.1287 and 1.3165 μm are also present and their relative strength indicates, since O i 1.1287 is the stronger among the two lines, that Lyman β fluorescence plays an important role in their excitation. Recombination analysis of the H i lines is done, which shows that the Paschen and Brackett line strengths deviate considerably from case B predictions. These deviations are attributed to the lines being optically thick, and this supposition is verified by calculating the line centre optical depths predicted by recombination theory. Similar calculations indicate that the Pfund and Humphrey series lines should also be expected to be optically thick, which is found to be consistent with observations reported in other studies. The spectral energy distribution of the star is constructed and shown to have an IR excess. Based on the magnitude of the IR excess, which is modelled using a free-free contribution from the disc, the electron density in the disc is estimated and shown to be within the range of values expected in Be star discs. [ABSTRACT FROM AUTHOR]

ABSTRACT Mennickent et al. and Sabogal et al. identified a large number of classical Be (CBe) candidates (∼3500) in the Large and Small Magellanic Clouds (LMC and SMC) based on their photometric variability using the OGLE II data base. They classified these stars into four different groups based on the appearance of their variability. In order to refine and understand the nature of this large number of stars, we studied the infrared properties of the sample and the spectroscopic properties of a subsample. We cross-correlated the optical sample with the IRSF-MCPS catalogue to obtain the J, H, Ks magnitudes of all the four types of stars (∼2500) in the LMC and SMC. Spectra of 120 stars belonging to the types 1, 2 and 3 were analysed to study their spectral properties. Among the four types, the type 4 stars are the dominant group, with ∼60 and ∼65 per cent of the total sample in the LMC and SMC, respectively. The near-infrared (NIR) colour-colour diagrams suggest that the type 4 stars in the LMC have a subclass, which is not found in our Galaxy or in the SMC. This subclass is ∼18 per cent of the type 4 sample. The main type 4 sample which is ∼49 per cent of the total sample has NIR properties similar to the Galactic CBe stars and the SMC type 4 stars. Though the new subclass of type 4 stars have high E( B− V) ∼ 0.75, they are not located close to regions with high reddening. The type 3 stars (∼6 per cent and 7.3 per cent in the LMC and SMC) are found to have large Hα equivalent width (EW) in the SMC and some are found to have large NIR excess. This small fraction of stars are unlikely to be CBe stars. Three stars among the type 3 stars in the LMC are found to be double periodic variables. The type 2 stars are found in larger fraction in the SMC (∼14.5 per cent), when compared to the LMC (∼6 per cent). The spectroscopic and the NIR properties suggest that these could be CBe stars. The type 1 stars are relatively more in the LMC (∼24 per cent) when compared to the SMC (∼13 per cent). The SMC type 1 stars have relatively large Hα EW and this class has properties similar to CBe stars. The spectroscopic sample of type 1 stars which show Hα in emission and are confirmed as CBe stars are more abundant in the SMC by a factor of 2.6. If the effect of metallicity is to cause more CBe stars in the SMC, when compared to the LMC, then type 1, type 2 and type 4 stars follow this rule, with an enhancement of 2.6, 2.4 and 1.3, respectively. [ABSTRACT FROM AUTHOR]

We present UBVRI CCD photometry in the region of the open clusters NGC 637 and 957. The radii are found to be 4.2 and 4.3 arcmin. Their reddenings are and mag and their distances, from main-sequence fitting are and . Comparison with isochrones leads to an age of for both clusters. Combining our photometry with Two Micron All Sky Survey (2MASS) JHK shows the reddening law in these directions to be normal. Mass function slopes of and are derived for the clusters, both of which are found to be dynamically relaxed. Spectral and photometric characteristics of three Be stars, two in NGC 957 and one (newly discovered) in NGC 637, indicate them to be of Classical Be type. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Papers published in recent years have contributed to resolve the enigma of the hypothetical Be nature of the hot pulsating star β Cephei. This star shows variable emission in the Hα line, typical for Be stars, but its projected rotational velocity is very much lower than the critical limit, contrary to what is expected for a typical Be star. The emission has been attributed to the secondary component of the β Cephei spectroscopic binary system. In this paper, using both our and archived spectra, we attempt to recover the Hα profile of the secondary component and to analyse its behaviour with time for a long period. To accomplish this task, we first derive the atmospheric parameters of the primary, and , and then we use these values to compute its synthetic Hα profile, and finally we reconstruct the secondary's profile disentangling the observed one. The secondary's Hα profile shows the typical two-peak emission of a Be star with a strong variability. We also analysed the behaviour versus time of some linewidth parameters: equivalent width, ratio of blue to red peak intensities, full width at half-maximum, peak separation and radial velocity of the central depression. The projected rotational velocity of the secondary and the dimension of the equatorial surrounding disc have also been estimated. [ABSTRACT FROM AUTHOR]

We have constructed models of axisymmetric, circumstellar envelopes for Be star discs by successfully combining two numerical codes: a non-local thermodynamic equilibrium (non-LTE) radiative transfer code which calculates the level populations and disc temperature distribution self-consistently, and a hydrodynamical code. The output of one code is used as input to the other code, and hence evolving density and thermal structures may be examined. The temperatures, disc density and velocity distribution are used to investigate the outflowing viscous disc model for Be stars. We find that these simulations place constraints on the power-law density decrease in the disc with increasing distance from the star. We find that the power-law index for the line-forming region of the disc lies between 3 and 3.5 with a small dispersion. [ABSTRACT FROM AUTHOR]

We investigate the optical counterparts of recently discovered Be/X-ray binaries in the Small Magellanic Cloud (SMC). In total four sources, SXP101, SXP700, SXP348 and SXP65.8 were detected during the Chandra survey of the wing of the SMC. SXP700 and SXP65.8 were previously unknown. Many optical ground-based telescopes have been utilized in the optical follow-up, providing coverage in both the red and blue bands. This has led to the classification of all of the counterparts as Be stars and confirms that three lie within the Galactic spectral distribution of known Be/X-ray binaries. SXP101 lies outside this distribution and is the latest spectral type known. Monitoring of the Hα emission line suggests that all the sources barring SXP700 have highly variable circumstellar discs, possibly a result of their comparatively short orbital periods. Phase-resolved X-ray spectroscopy has also been performed on SXP65.8, revealing that the emission is indeed harder during the passage of the X-ray beam through the line of sight. [ABSTRACT FROM AUTHOR]

The star CD-42°11721 is a curious B[e] star sometimes pointed as an evolved B[e] supergiant and sometimes as a young HAeBe star, due to very uncertain or even unknown stellar parameters, especially the distance. In this paper, we present new data gained from high-resolution optical spectroscopy and a detailed description of infrared (IR) data of this star. We present a qualitative study of the numerous emission lines in our optical spectra and the classification of their line profiles, which indicate a non-spherically symmetric circumstellar environment. The first real detection of numerous [Fe ii] emission lines and of many other permitted and forbidden emission lines is reported. From our optical data, we derive an effective temperature of , a radius of , as well as a luminosity of . We advocate that CD-42°11721 might be a post-main-sequence object, even though a pre-main-sequence nature cannot be ruled out due to the uncertain distance. We further found that the spectral energy distribution in the optical and IR can best be fitted with an outflowing disc-forming wind scenario rather than with a spherical symmetric envelope or with a flared disc, supporting our tentative classification as a B[e] supergiant. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a photometric and spectroscopic study of the young open cluster NGC 7419, which is known to host a large number of classical Be stars for reasons not well understood. Based on CCD photometric observations of 327 stars in UBV passbands, we estimated the cluster parameters as, reddening and . The turn-off age of the cluster was estimated as using isochrone fits. UBV data of the stars were combined with the JHK data from Two-Micron All-Sky Survey (2MASS) and were used to create the near-infrared (NIR) ( J− H) versus ( H− K) colour–colour diagram. A large fraction of stars (42 per cent) was found to have NIR excess and their location in the diagram was used to identify them as intermediate mass pre-main-sequence (MS) stars. The isochrone fits to pre-MS stars in the optical colour–magnitude diagram showed that the turn-on age of the cluster is 0.3–3 Myr. This indicates that there has been a recent episode of star formation in the vicinity of the cluster. Slitless spectra were used to identify 27 stars which showed Hα in emission in the field of the cluster, of which six are new identifications. All these stars were found to show NIR excess and are located closer to the region populated by Herbig Ae/Be stars in the ( J− H) versus ( H− K) diagram. Slit spectra of 25 stars were obtained in the region 3700–9000 Å. The spectral features were found to be very similar to those of Herbig Be stars. These stars were found to be more reddened than the main-sequence stars by 0.4 mag, on an average. Thus, the emission-line stars found in this cluster are more similar to the Herbig Be-type stars where the circumstellar material is the remnant of the accretion disc. We conclude that the second episode of star formation has led to the formation of a large number of Herbig Be stars as well as intermediate mass pre-MS stars in the field of NGC 7419, thus explaining the presence of emission-line stars in this cluster. This could be one of the young open clusters with the largest number of Herbig Be stars. [ABSTRACT FROM AUTHOR]

I examine the stability of retrograde mixed modes in rotating B-type stars. These modes can be regarded as a hybrid between the Rossby modes that arise from conservation of vorticity, and the Poincaré modes that are gravity waves modified by the Coriolis force. Using a non-adiabatic pulsation code based around the traditional approximation, I find that the modes are unstable in mid- to late-B-type stars, as a result of the same iron-bump opacity mechanism that is usually associated with SPB and β Cep stars. At one-half of the critical rotation rate, the instability for the modes with azimuthal orders spans the spectral types B4 to A0. Inertial-frame periods of the unstable modes range from 100 days down to a fraction of a day, while normalized growth rates can reach in excess of 10−5. I discuss the relevance of these findings to SPB and pulsating Be stars, and to the putative Maia class of variable star. I also outline some of the questions raised by this discovery of a wholly new class of pulsational instability in early-type stars. [ABSTRACT FROM AUTHOR]

We argue that, in general, observational studies of Be-star rotation have paid insufficient attention to the effects of equatorial gravity darkening. We present new line-profile calculations that emphasize the insensitivity of line width to rotation for fast rotators. Coupled with a critical review of observational procedures, these calculations suggest that the observational parameter may systematically underestimate the true projected equatorial rotation velocity, , by some tens of per cent for rapid rotators. A crucial implication of this work is that Be stars may be rotating much closer to their critical velocities than is generally supposed, bringing a range of new processes into contention for the elusive physical mechanism responsible for the circumstellar disc thought to be central to the Be phenomenon. [ABSTRACT FROM AUTHOR]

We present three seasons of photometric observations and one season of intensive high-dispersion spectroscopic observations of the Be star λ Eridani. We show that only one period, P=0.70173 d , is present in the photometry, although there are large light amplitude variations from season to season. We confirm a suspicion that light outbursts repeat at intervals of about 475 d. A total of 348 echelle spectra of the star were obtained over a 2-week observing run. We show that the periodic variations are present in the emission wings of the helium lines, in the emission wings of the Hα line and in the absorption cores of Hβ and Hγ. Together with the fact that the periodic variations appear outside the projected rotational velocity limit, this indicates that they are associated with circumstellar material immediately above the photosphere and supports the idea of corotating gas clouds. We present evidence in support of a true rotational period of 2P=1.40346 d and suggest that the mass loss in Be stars is caused by centrifugal magnetic acceleration. [ABSTRACT FROM AUTHOR]

Absolute visual magnitudes of OB stars are determined from Hipparcos parallaxes using a sample of 1207 unreddened and 441 reddened stars of luminosity classes V–IV, III, Ia and Iab–Ib–II. The value of total-to-selective extinction R for reddened stars was calculated from near-infrared photometry. The new M[sub V] calibration is compared with existing calibrations. The visual absolute magnitudes of Be stars were obtained from a sample of 90 unreddened and 25 reddened stars. Generally, Be stars are brighter than B stars of the corresponding spectral types, in agreement with earlier results. [ABSTRACT FROM AUTHOR]

ABSTRACTWe analyse the periodic variability of the Be star 28 (ω) CMa. The data consist of 275 new He I 6678 line profiles of high spectral and temporal resolution, and new Strömgren and Geneva photometry. The photometric analysis has been extended by including previously published data to cover an interval of 16 yr. The principal spectroscopic period of 1.37 d is confirmed by analysis of He I 6678 modes and moments. There is evidence to show that a single period is insufficient to explain the variation in the line wings. In particular, the variation of the second moment, which samples the line wings, is best described when a second period is included. The photometric data have been divided into 10 sets in which the time is sufficiently contiguous for period analysis. These sets have been smoothed to remove as well as possible the long-term irregular variations which are always present in Be stars. The residuals have then been analysed for coherent periodicities using several techniques. Evidence for one or the other of the two known spectroscopic periods can be found in individual data sets. By an analysis in which two periods are assumed, it is possible to recover both the principal spectroscopic period and the satellite period of 1.46 d. Unlike in the He I 6678 line profiles, the latter period seems to be most often present in the light variations. The amplitude of the periodic components of the light variation is only a few millimagnitudes, in contrast to the very large radial velocity amplitude. We find that the photometric amplitudes of the two periodic components, and their ratio, vary smoothly on a time-scale of years. The amplitudes are larger in seasons when the mean brightness is higher. [ABSTRACT FROM AUTHOR]

ABSTRACTWe analyse a series of high-resolution line profiles of He I 6678 in the periodic Be star 28 CMa. A simple zeroth-order approximation to the eigenfunctions is used to model the line profile variations in terms of non-radial pulsation. In addition, we use the best available theory for the eigenfunction in a rotating star. In all cases the calculated fits to the observed profiles are poor. We conclude that non-radial pulsation is almost certainly not responsible for the line profile variations in this star. In an attempt to understand the very large line profile changes, but negligible light variation, we consider a `patch' model. This consists of a circular area on the photosphere having the same temperature, but a different intrinsic line width. A simple model of this kind produces a good fit to the line profile variations and to the photometry. We conclude that modulation of the profile by a patch on, or close to, the photosphere may offer a plausible starting point for understanding the periodic variations in some Be stars. [ABSTRACT FROM AUTHOR]