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Register a new userA representative sample of Be Stars I: Sample Selection, Spectral Classification and Rotational Velocities
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We present a sample of 58 Be stars containing objects of spectral types O9 to B8.5 and luminosity classes III to V. We have obtained 3670 - 5070 Angstrom spectra of the sample which are used to derive spectral types and rotational velocities. We discuss the distribution of spectral types and rotational velocities obtained and conclude that there are no significant selection effects in our sample.
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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.
We present H band (1.53 - 1.69 micron) spectra of 57 isolated Be stars of spectral types O9-B9 and luminosity classes III,IV & V. The HI Brackett (n-4) series is seen in emission from Br 11-18, and FeII emission is also apparent for a subset of those stars with HI emission. No emission from species with a higher excitation temperature, such as He II or CIII is seen, and no forbidden line emission is present. A subset of 12 stars show no evidence for emission from any species; these stars appear indistinguishable from normal B stars of a comparable spectral type. In general the line ratios constructed from the transitions in the range Br 11-18 do not fit case B recombination theory particularly well. Strong correlations between the line ratios with Br-gamma and spectral type are found. These results most likely represent systematic variations in the temperature and ionization of the circumstellar disc with spectral type. Weak correlations between the line widths and projected rotational velocity of the stars are observed; however no systematic trend for increasing line width through the Brackett series is observed.
Context. The study of rotational velocity distributions for normal stars requires an accurate spectral characterization of the objects in order to avoid polluting the results with undetected binary or peculiar stars. This piece of information is a key issue in the understanding of the link between rotation and the presence of chemical peculiarities. Aims. A sample of 47 low v sin i A0-A1 stars (v sin i < 65km/s), initially selected as main-sequence normal stars, are investigated with high-resolution and high signal-to-noise spectroscopic data. The aim is to detect spectroscopic binaries and chemically peculiar stars, and eventually establish a list of confirmed normal stars. Methods. A detailed abundance analysis and spectral synthesis is performed to derive abundances for 14 chemical species. A hierarchical classification, taking measurement errors into account, is applied to the abundance space and splits the sample into two different groups, identified as the chemically peculiar stars and the normal stars. Results. We show that about one third of the sample is actually composed of spectroscopic binaries (12 double-lined and five single-lined spectroscopic binaries). The hierarchical classification breaks down the remaining sample into 13 chemically peculiar stars (or uncertain) and 17 normal stars.
High resolution optical spectra of 57 Galactic B-type supergiant stars have been analyzed to determine their rotational and macroturbulent velocities. In addition, their atmospheric parameters (effective temperature, surface gravity and microturbulent velocity) and surface nitrogen abundances have been estimated using a non-LTE grid of model atmospheres. Comparisons of the projected rotational velocities have been made with the predictions of stellar evolutionary models and in general good agreement was found. However for a small number of targets, their observed rotational velocities were significantly larger than predicted, although their nitrogen abundances were consistent with the rest of the sample. We conclude that binarity may have played a role in generating their large rotational velocities. No correlation was found between nitrogen abundances and the current projected rotational velocities. However a correlation was found with the inferred projected rotational velocities of the main sequence precursors of our supergiant sample. This correlation is again in agreement with the predictions of single star evolutionary models that incorporate rotational mixing. The origin of the macroturbulent and microturbulent velocity fields is discussed and our results support previous theoretical studies that link the former to sub-photospheric convection and the latter to non-radial gravity mode oscillations. In addition, we have attempted to identify differential rotation in our most rapidly rotating targets.
We present a well-defined and characterized all-sky sample of classical Cepheids in the Milky Way, obtained by combining two time-domain all-sky surveys: Gaia DR2 (Gaia Collaboration et al. 2018) and ASAS-SN (Shappee et al. 2014). We first use parallax and variability information from Gaia to select ~30,000 bright (G<17) Cepheid candidates with M_K<-1. We then analyze their ASAS-SN V-band lightcurves, determining periods, and classifying the lightcurves using their Fourier parameters. This results in ~1900 likely Galactic Cepheids, which we estimate to be >90% complete and pure within our adopted selection criteria. This is the largest all-sky sample of Milky Way Cepheids that has such a well-characterized selection function, needed for population modeling and for systematic spectroscopic follow-up foreseen with SDSS-V. About 130 of these Cepheids have not been documented in the literature even as possible candidates.
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