An analysis of the far-ultraviolet spectra of the pole-on Be stars Upsilon Cygni and MU Centauri

Astronomy and Astrophysics – Astrophysics

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B Stars, Binary Stars, Far Ultraviolet Radiation, Spectrum Analysis, Stellar Envelopes, Ultraviolet Spectra, Abundance, Graphs (Charts), Ground Truth, Line Spectra, Mass Transfer, Oao 3, Radial Velocity, Stellar Rotation, Tables (Data)

Scientific paper

The technique of spectrum synthesis has been employed to interpret Copernicus U2 spectra (0.2 Å resolution) and Lick Observatory coudé spectra of the pole-on Be stars υ Cyg and μ Cen. Features which are analyzed include C II λλ1335 and 4267; C III λλ1176 and 1247; N II λλ1085 and 3995; Si II λλ1265 and 1309; Si III λλ1110, 1297, and 4568; and Si IV λ1128. An initial set of physical parameters, Teff, log g, and υ sin i, are obtained from the ground-based spectra. These parameters are then used to compute theoretical profiles for the selected UV features. In general, there is no difficulty in fitting the observed UV features with profiles computed from the same model atmosphere, abundances, and υ sin i as are suggested by the visual spectral features. The analysis suggests an effective temperature of 24,000 K and a log g of 3.8 for υ Cyg. Teff and log g adopted for υ Cen are 20,500 K and 3.7, respectively. Upsilon Cygni, a star suspected of being a mass-transfer binary, appears to have a low abundance of carbon but normal abundances of nitrogen and silicon. The abundances of nitrogen and silicon in υ Cen are normal; the carbon results are inconclusive. Although υ Cyg has a quiescent emission-line envelope in contrast to the oscillatory one of υ Cen, the photospheric parameters for both stars are comparable.

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