Properties and nature of Be stars

Abstract

Radial-velocity variations of the Hα emission measured on the steep wings of the Hα line, prewhitened for the long-time changes, vary periodically with a period of 218. d 025 ±0. d 022, confirming the suspected binary nature of the bright Be star BU Tau, a member of the Pleiades cluster. The orbit seems to have a high eccentricity over 0.7, but we also briefly discuss the possibility that the true orbit is circular and that the eccentricity is spurious owing to the phase-dependent effects of the circumstellar matter. The projected angular separation of the spectroscopic orbit is large enough to allow the detection of the binary with large optical interferometers, provided the magnitude difference primary − secondary is not too large. Since our data cover the onset of a new shell phase up to development of a metallic shell spectrum, we also briefly discuss the recent long-term changes. We confirm the formation of a new envelope, coexisting with the previous one, at the onset of the new shell phase. We find that the full width at half maximum of the Hα profile has been decreasing with time for both envelopes. In this connection, we briefly discuss Hirata's hypothesis of precessing gaseous disk and possible alternative scenarios of the observed long-term changes.

The discovery of a shell-like event in the O-type star HD 120678

“…Thus, B stars show an A/F-type shell spectrum, whereas O stars show a B-type shell spectrum. Pleione has recently been confirmed to be a binary with a period of 218 days (Nemravová et al 2010).…”

Observations of variables

“…Our data confirms the period of 455 days. V720 Cas (also known as IRAS 00422+5310, TAV 0042+53, CGCS 105, GSC 3655.01254, 2MASS J00450706+5326473, WISE J004507.06+532647.7) was initially classified as a carbon star from low resolution IRAS spectra (Olnon et al 1986) in the 8-22µm mid-infrared band by Little-Marenin et al (1987). A carbon star is a late-type bright and red AGB giant star whose atmosphere contains more carbon than oxygen, those combining in the upper layers of the star forming CO and consuming all the oxygen in the atmosphere.…”

“…In 2014 the variation was of 2 magnitudes from 13.72 ± 0.01 to 11.72 + 0.02. The period analysis of our 2013-2014 total light curve is performed using the LombScargle periodogram (LSP) technique (Lomb 1976, Scargle 1982. Our analysis results in dominating periodic signal peak (Fig.…”

“…The period analysis was performed using Period04 software (Lenz & Breger 2005) for obtaining exact results. The analysis of the original dataset confirmed the large contribution of the average value of f 1 term, i.e., 8.2144 d −1 on two filters and the period of 0.121737 d was obtained.…”

Methods of the Long-term Radial-Velocity Variation Removal and their Application to Detect Duplicity of Several Be Stars

“…The second method resulted in an orbital RV curve with significantly lower rms for BU Tauri (Nemravová et al 2010) and γ Cassiopeiae (in preparation) than smoothing with the first method. The main reason is that the systemic velocities computed with SPEL properly model the orbital RV curve of the given subset.…”