Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986a%26a...159..276d&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 159, no. 1-2, April 1986, p. 276-290.
Astronomy and Astrophysics
Astrophysics
90
B Stars, H Alpha Line, Stellar Envelopes, Stellar Spectra, Variable Stars, Balmer Series, Emission Spectra, Spectral Line Width
Scientific paper
In order to establish a quantitative classification of Be star envelopes, measured Hα emission line profiles are intercompared and statistically analyzed for a large sample of bright early-Be type stars in various states of their activity cycles. It is shown that for the vast majority of Hα profiles, the coarse shapes of emission lines (i. e., single-peak or double-peak structure) and Hα emission line half-widths (FWHM) are essentially determined by only two parameters, the equivalent width of the emission component of the profile, W(α), and the projected rotational velocity of the underlying star, v sin i. As a rule, Hα emission line half-widths, H(α), are linearly related to v sin i for constant equivalent width, and at the same time are inversely related to equivalent width at constant v sin i. The combination of these average relations is quantitatively expressed by Eq. (7). The direct correlation between Hα line half-width and rotational velocity, v sin i, can be regarded as convincing evidence supporting the Struve-Kogure model for cool Balmer line emitting regions of Be star envelopes, which consists of a differentially rotating disk surrounding the equatorial belt of the star. The relation between H(α), W(α) and v sin i is most easily explained quantitatively by the assumption that Hα emitting envelopes are Keplerian disks, and that equivalent width of Hα emission in Be star spectra approximately scales with the visible surface of the disks, i. e., with the square of disk radius times a projection factor. Actual radii of Hα emitting disks, as computed earlier by different authors for individual Be and Be-shell stars on the basis of various methods and several variants of the disk model, are found to be fairly well correlated with equivalent widths of Hα emission obtained for the same stars at epochs close to those valid for the radius determinations. This mean correlation is used to determine typical disk radii in terms of observed values of W(α). In addition to rotational broadening, Hα emission-line half- widths contain a small contribution depending on effective temperature of the central star, being largest for Oe to B0e stars and barely significant for B 5-7e stars. Delimitation between singly peaked and doubly peaked Hα emission line profiles in the W vs. v sin i diagram is discussed as well as the role played by stellar obscuration of the rear part of small equatorial disks in producing duplicity of Hα emission lines. Only one Be star is known to have developed recently, during a relatively short stage of envelope fading, extremely narrow Hα emission-line profiles of variable shape showing a positive correlation between half-width and equivalent width. This is μ Cen between 1973 and 1976 as observed by Peters (1979). The exceptional behaviour of this star during this period can be understood as a result of increasing disk diameter at continuously decreasing gas density during envelope fading and dispersal.
Dachs Joachim
Hanuschik Reinhard
Kaiser David
Rohe Daniela
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