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تسجيل مستخدم جديدStellar parameters and H$alpha$ line profile variability of Be stars in the BeSOS survey
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The Be phenomenon is present in about 20$%$ of the B-type stars. Be stars show variability on a broad range of timescales, which in most cases is related to the presence of a circumstellar disk of variable size and structure. For this reason a time resolved survey is highly desirable in order to understand the mechanisms of disk formation which are still poorly understood. In addition, a complete observational sample would improve the statistical significance of the study of the stellar and disk parameters. The Be Stars Observation Survey (BeSOS) is a survey containing reduced spectra obtained using the echelle spectrograph PUCHEROS with a spectral resolution of 17000 in the range of 4260-7300 $text{AA}$. BeSOSs main objective is to offer consistent spectroscopic and time resolved data obtained with one instrument. The user can download or plot the data and get the stellar parameters directly from the website. We also provide a star-by-star analysis based on photometric, spectroscopic and interferometric data as well as general information about the whole BeSOS sample. Recently, BeSOS led to the discovery of a new Be star HD 42167 and facilitated study of the V/R variation of HD 35165 and HD 120324, the steady disk of HD 110335 and the Be shell status of HD 127972. Optical spectra used in this work, as well as the derived stellar parameters are available online in url{http://besos.ifa.uv.cl}.
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Aims. We attempt to determine if a dependency on spectral subtype or vsini exists for stars undergoing phase-changes between B and Be states, as well as for those stars exhibiting variability in H{alpha} emission. Methods. We analyse the changes in H
{alpha} line strength for a sample of 55 Be stars of varying spectral types and luminosity classes using five epochs of observations taken over a ten year period between 1998 and 2010. Results. We find i) that the typical timescale between which full phase transitions occur is most likely of the order of centuries, although no dependency on spectral subtype or vsini could be determined due to the low frequency of phase-changing events observed in our sample, ii) that stars with earlier spectral types and larger values of vsini show a greater degree of variability in H{alpha} emission over the timescales probed in this study, and iii) a trend of increasing variability between the shortest and longest baselines for stars of later spectral types and with smaller values of vsini.
The circumstellar disk density distributions for a sample of 63 Be southern stars from the BeSOS survey were found by modelling their H$alpha$ emission line profiles. These disk densities were used to compute disk masses and disk angular momenta for
the sample. Average values for the disk mass are 3.4$times$10$^{-9}$ and 9.5$times$10$^{-10}$ $M_{star}$ for early (B0-B3) and late (B4-B9) spectral types, respectively. We also find that the range of disk angular momentum relative to the star are between 150-200 and 100-150 $J_{star}/M_{star}$, again for early and late-type Be stars respectively. The distributions of the disk mass and disk angular momentum are different between early and late-type Be stars at a 1% level of significance. Finally, we construct the disk mass distribution for the BeSOS sample as a function of spectral type and compare it to the predictions of stellar evolutionary models with rapid rotation. The observed disk masses are typically larger than the theoretical predictions, although the observed spread in disk masses is typically large.
We present the results of high precision, high resolution (R~68000) optical observations of the short-period (4d) eccentric binary system Alpha Virginis (Spica) showing the photospheric line-profile variability that in this system can be attributed t
o non-radial pulsations driven by tidal effects. Although scant in orbital phase coverage, the data provide S/N>2000 line profiles at full spectral resolution in the wavelength range delta-lambda = 4000--8500 Angstroms, allowing a detailed study of the night-to-night variability as well as changes that occur on ~2 hr timescale. Using an ab initio theoretical calculation, we show that the line-profile variability can arise as a natural consequence of surface flows that are induced by the tidal interaction.
Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS telescope, we have obtained a large number of high precision spectropolarimetrc observations (284) of Herbig AeBe stars collected over 53 nights totaling more than 300 hours of obse
rving. Our sample of five HAeBe stars: AB Aurigae, MWC480, MWC120, MWC158 and HD58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the H-alpha line. In all our stars, the H-alpha line profiles show evidence of an intervening disk or outflowing wind, evidenced by strong emission with an absorptive component. The linear polarization varies by 0.2% to 1.5% with the change typically centered in the absorptive part of the line profile. These observations are inconsistent with a simple disk-scattering model or a depolarization model which produce polarization changes centered on the emmissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.
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xtension of the Be phenomenon into early A stars. Methods. An adapted version of the BCD method is used, utilizing the Balmer discontinuity parameters to determine effective temperature and surface gravity. This method is optimally suited for late B stars. The projected rotational velocity was obtained by profile fitting to the Mg ii lines of the targets, and the spectra were inspected visually for the presence of peculiar features such as the infrared Ca ii triplet or the presence of a double Balmer discontinuity. The Balmer line equivalent widths were measured, but due to uncertainties in determining the photospheric contribution are useful only in a subsample of Be stars for determining the pure emission contribution. Results. A total of 78 Be stars, mostly late type ones, were identified in the X-shooter telluric standard star archive, out of which 48 had not been reported before. The general trend of late type Be stars having more tenuous disks and being less variable than early type ones is confirmed. The relatively large number (48) of relatively bright (V > 8.5) additional Be stars casts some doubt on the statistics of late type Be stars; they are more common than currently thought: The Be/B star fraction may not strongly depend on spectral subtype.
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