Properties and nature of Be stars

Abstract

Abstract.We present an analysis of new spectroscopic observations of the bright Be star κ Dra obtained at the Ondřejov observatory during 1992−2003 and UBV photometric observations secured at several observatories. General characteristics and a line identification of the spectrum of κ Dra are obtained in the regions 3730−5650 Å and 5850−7800 Å by a comparison with the theoretical spectrum. The fundamental stellar parameters have been obtained from a comparison with a grid of NLTE model atmospheres. The best fit was found for T eff = 14 000 K, log g = 3.5, and v sin i = 170 km s −1 . These values together with a Hipparcos parallax lead to a stellar mass M = 4.8 ± 0.8 M and radius R = 6.4 ± 0.5 R . It is encouraging to see that these values agree well with the expected evolutionary mass and radius for the effective temperature we derived. Long-term variations of κ Dra were analysed using measurements of equivalent widths, central intensities, peak intensities of emission lines and emission peak velocity differences for Hα, Hβ, Hγ, Hδ, and some helium, silicon, and iron lines. The previously reported period of 23 years in the variation of the emission strength is probably a cyclic, not a strictly periodic phenomenon. An attempt to find out a period from all available records of the Hβ emission strength led to a value of (8044 ± 167) days (22.0 years) but the phase plots show that each cycle has a different shape and length. The maximum strength of the emission lags behind the brightness maximum. This is a behaviour usually observed for long-term changes of Be stars with a positive correlation between the brightness and emission strength. Since there are obviously no published speckle observations of the star, we suggest these should be carried out. They could help to deny or confirm the possibility that the emission episodes are triggered by a periastron passage of a putative binary companion moving in an eccentric orbit with a 8044-d period, as it seems to be the case for some Be binaries. For the moment, the nature and origin of the disk around κ Dra remains unknown. From the comparison of the electronic spectra obtained at different phases of the long-term cycle and synthetic spectra it appears that there are no detectable changes in the photospheric part of the Balmer lines related to variations in the Balmer emission strength which could be attributed to an extended photosphere corresponding to inner parts of the disk, optically thick in continuum.

CHARA ArrayK′‐Band Measurements of the Angular Dimensions of Be Star Disks

“…We find that the H emission strength is approximately that expected for the epoch of our observations and the long term variation (see Fig. 4 of Saad et al 2004). Once again the H profile (and perhaps the H profile) appear to be broader than predicted by the Keplerian model.…”

“…The fact that our model H line depth is shallower than the observed depth probably indicates that the outer parts of the disk are cooler than assumed in our isothermal model. Juza et al (1991) and Saad et al (2005) show that Dra is a low-amplitude, single-lined spectroscopic binary with a period of 65.6 days. Saad et al (2004) describe a long-term cyclic variation of about 22 years that is present in the emission lines, and they suggest that this period may correspond to the disk's modulation due to an interaction with a third star in an eccentric orbit.…”

“…Juza et al (1991) and Saad et al (2005) show that Dra is a low-amplitude, single-lined spectroscopic binary with a period of 65.6 days. Saad et al (2004) describe a long-term cyclic variation of about 22 years that is present in the emission lines, and they suggest that this period may correspond to the disk's modulation due to an interaction with a third star in an eccentric orbit. The semimajor axis of the third star would be on the order of 100 mas for the adopted distance, but we find no obvious evidence (separated fringe packets) for such a companion even at the shortest baselines.…”

“…References.-(1) Harmanec et al 2000; (2) Miroshnichenko et al 2002; (3) Gies et al 1998;(4) Harmanec 1984;(5) Yang et al 1990;(6) Kaye & Gies 1997;(7) Saad et al 2005. instrumentation, and data-reduction procedures are described in detail by ten Brummelaar et al (2005). The CHARA Array consists of six 1 m telescopes in a Y-configuration with pairwise baselines ranging from 34 to 330 m in length.…”

The early-type semidetached binary system V716 Centaurus

“…An effective way of investigating the circumstellar matter around interacting binary stars is to study the variations and/or emission structures of Hα in the stellar spectra (cf. Albright & Richards 1995; Koubskỳ, Richards & Šimon 1998; Richards & Albright 1999; Harmanec et al 2002; Saad et al 2004). Therefore, 25 high‐resolution (λ/δλ= 41 000) Hα line profiles were studied in the present paper.…”