Rotation in the ZAMS: Be and Bn stars

We show that Be stars belong to a high velocity tail of a single B-type star rotational velocity distribution in the MS. This implies that: 1) the number fraction N(Be)/N(Be+B) is independent of the mass; 2) Bn stars having ZAMS rotational velocities higher than a given limit might become Be stars.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazak

Early-type objects in NGC6611 and Eagle Nebula

An important question about Be stars is whether Be stars are born as Be stars or not. It is necessary to observe young clusters to answer this question. Observations of stars in NGC6611 and the star-formation region of Eagle Nebula have been carried out with the ESO-WFI in slitless spectroscopic mode and at the VLT-GIRAFFE. The targets for the GIRAFFE observations were pre-selected from the literature and our catalogue of emission-line stars (ELS) based on the WFI study. GIRAFFE observations allowed us to study accurately the population of the early-type stars with and without emission lines. For this study, we determined the fundamental parameters of OBA stars. We also studied the status of the objects (main sequence or pre-main sequence stars) by using IR data, membership probabilities, and location in HR diagrams. The nature of the early-type ELS in M16 is derived. The slitless observations with the WFI clearly indicate a small number of ELS in M16. We observed with GIRAFFE 101 OBA stars, among them 9 are ELS with circumstellar emission in Halpha. We found that: W080 could be a new He-strong star, like W601. W301 is a possible classical Be star, W503 is a mass-transfer eclipsing binary with an accretion disk, and the other ones are possible Herbig Ae/Be stars. We also found that the rotational velocities of main sequence B stars are 18% lower than those of pre-main sequence B stars, in good agreement with theory about the evolution of rotational velocities. Combining different indications and technics, we found that 27% of the B-type stars are binaries. We also redetermined the age of NGC6611 found equal to 1.2--1.8 Myears in good agreement with the most recent determinations.Comment: Accepted by A&A, english not yet correcte

Be stars and binaries in the field of the SMC open cluster NGC330 with VLT-FLAMES

Observations of hot stars belonging to the young cluster SMC-NGC330 and its surrounding region were obtained with the VLT-GIRAFFE facilities in MEDUSA mode. We investigated the B and Be star properties and proportions in this environment of low metallicity. We also searched for rapid variability in Be stars using photometric databases. With spectroscopic measurements we characterized the emission and properties of Be stars. By cross-correlation with photometric databases such as MACHO and OGLE, we searched for binaries in our sample of hot stars, as well as for short-term variability in Be stars. We report on the global characteristics of the Be star sample (131 objects). We find that the proportion of early Be stars with a large equivalent width of the Halpha emission line is higher in the SMC than in the LMC and MW. We find a slight increase in the proportion of Be stars compared to B-type stars with decreasing metallicity. We also discovered spectroscopic and photometric binaries, and for the latter we give their orbital period. We identify 13 Be stars with short-term variability. We determine their period(s) and find that 9 Be stars are multiperiodic.Comment: english not yet corrected, 23 pages, 4th article about the study in the LMC NGC2004 and SMC NGC33

Study of a Sample of Faint Be Stars in the Exofield of CoRoT. Part III. Global Spectroscopic Characterization and Astrophysical Parameters of the Central Stars

The present work is devoted to the study of faint Be stars observed by CoRoT in the fourth long run (LRA02). The astrophysical parameters were determined from the spectra observed with the VLT/FLAMES instruments at ESO. Spectra were fitted with models of stellar atmospheres using our GIRFIT package. Spectra in the lambda-lambda 6400-7200$ AA domain enabled the confirmation or a first identification of Be star candidates. The apparent parameters (Teff,log g,Vsin i) for a set of 19 B and Be stars were corrected for the effects induced by the rapid rotation. These allowed us to determine: 1) masses that are in agreement with those measured for detached binary systems; 2) distances that agree with the GAIA parallaxes; and 3) centrifugal/gravity equatorial force ratios of ~0.6-0.7, which indicate that our Be stars are subcritical rotators. A study of the Balmer Halpha, Hgamma and Hdelta emission lines produced: extents of the circumstellar disk (CD) emitting regions that agree with the interferometric inferences in other Be stars; R-dependent exponents n(R) of the CD radial density distributions; CD base densities. The Hgamma and Hdelta emission lines are formed in CD layers close to the central star. These lines produced a different value of the exponent n(R) than assumed for Halpha. Further detailed studies of Hgamma and Hdelta emission lines could reveal the physical properties of regions where the viscous transport of angular momentum to the remaining CD regions is likely to originate from. The subcritical rotation of Be stars suggests that their huge discrete mass-ejections and concomitant non-radial pulsations might have a common origin in stellar envelope regions that become unstable to convection due to rotation. The errors induced on the estimated Teff by the possible presence of stripped sub-dwarf O/B companions are not likely to exceed their present uncertainties.Comment: 38 pages, 16 figures, regular paper submitted to Astronomy & Astrophysic

Critical study of the distribution of rotational velocities of Be stars; II: Differential rotation and some hidden effects interfering with the interpretation of the Vsin i parameter

We assume that stars may undergo surface differential rotation to study its impact on the interpretation of $V\!\sin i$ and on the observed distribution $\Phi(u)$ of ratios of true rotational velocities u=V/V_\rm c (V_\rm c is the equatorial critical velocity). We discuss some phenomena affecting the formation of spectral lines and their broadening, which can obliterate the information carried by $V\!\sin i$ concerning the actual stellar rotation. We studied the line broadening produced by several differential rotational laws, but adopted Maunder's expression $\Omega(\theta)=\Omega_o(1+\alpha\cos^2\theta)$ as an attempt to account for all of these laws with the lowest possible number of free parameters. We studied the effect of the differential rotation parameter $\alpha$ on the measured $V\!\sin i$ parameter and on the distribution $\Phi(u)$ of ratios u=V/V_\rm c. We conclude that the inferred $V\!\sin i$ is smaller than implied by the actual equatorial linear rotation velocity V_\rm eq if the stars rotate with $\alpha0$. For a given $|\alpha|$ the deviations of $V\!\sin i$ are larger when $\alpha<0$. If the studied Be stars have on average $\alpha<0$, the number of rotators with V_\rm eq\simeq0.9V_\rm c is larger than expected from the observed distribution $\Phi(u)$; if these stars have on average $\alpha>0$, this number is lower than expected. We discuss seven phenomena that contribute either to narrow or broaden spectral lines, which blur the information on the rotation carried by $V\!\sin i$ and, in particular, to decide whether the Be phenomenon mostly rely on the critical rotation. We show that two-dimensional radiation transfer calculations are needed in rapid rotators to diagnose the stellar rotation more reliably.Comment: To appear in A&

Effects of metallicity, star-formation conditions, and evolution in B and Be stars. II: Small Magellanic Cloud, field of NGC 330

We search for effects of metallicity on B and Be stars in the Small and Large Magellanic Clouds (SMC and LMC) and in the Milky Way (MW). We extend our previous analysis of B and Be stars populations in the LMC to the SMC. The rotational velocities of massive stars and the evolutionary status of Be stars are examined with respect to their environments. Spectroscopic observations of hot stars belonging to the young cluster SMC-NGC 330 and its surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA mode. We determine fundamental parameters for B and Be stars with the GIRFIT code, taking into account the effect of fast rotation, and the age of observed clusters. We compare the mean vsini obtained by spectral type- and mass-selection for field and cluster B and Be stars in the SMC with the one in the LMC and MW. We find that (i) B and Be stars rotate faster in the SMC than in the LMC, and in the LMC than in the MW; (ii) at a given metallicity, Be stars begin their main sequence life with a higher initial rotational velocity than B stars. Consequently, only a fraction of B stars that reach the ZAMS with a sufficiently high initial rotational velocity can become Be stars; (iii) the distributions of initial rotational velocities at the ZAMS for Be stars in the SMC, LMC and MW are mass- and metallicity-dependent; (iv) the angular velocities of B and Be stars are higher in the SMC than in the LMC and MW; (v) in the SMC and LMC, massive Be stars appear in the second part of the main sequence, contrary to massive Be stars in the MW