Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000pasp..112...31b&link_type=abstract
The Publications of the Astronomical Society of the Pacific, Volume 112, Issue 767, pp. 31-49.
Astronomy and Astrophysics
Astronomy
6
Stars: Binaries: Eclipsing, Stars: Circumstellar Matter, Stars: Emission-Line, Be, Stars: Individual (Vv Cephei), Ultraviolet Emission
Scientific paper
The International Ultraviolet Explorer (IUE) satellite observed VV Cephei (M2 Iab+B?) over nearly a full 20.3 year orbital period, but all the observations were obtained outside total eclipse. Egress of the chromospheric eclipse was the only portion of the eclipse observed by IUE. We have examined all 124 high-resolution IUE spectra of VV Cephei and describe the spectrum and its variations. Contrary to initial expectations, the spectrum remained complex throughout the entire orbit, and five line components have been identified: (1) emission lines; (2) neutral, low-excitation (e.g., Fe I) lines, which disappear as the hot component emerges from eclipse; (3) broad, high-ionization (e.g., Si IV and Fe III) lines arising from the hot component or nearby circumstellar material; (4) narrow absorption cores from the circumstellar envelope of the M supergiant; and (5) ``shell'' absorption lines with variable profiles which do not absorb below a minimum depth. Mg II h and k emission broadens and strengthens when the system is near periastron and secondary eclipse. Most of the variation in the other emission features appears to be stochastic rather than phase dependent. The continuum shows rapid variations on timescales as short as 2 weeks. At any one time, the continuum flux between 1400 and 3000 Å is fairly constant but varies, apparently stochastically, in the range (2-7)x10-12 ergs s-1 cm-2 Å-1 for observations obtained outside of chromospheric eclipse. The dereddened continuum fluxes vary as ~λ-0.8, implying that the source is not purely stellar. Radial velocity measurements of the circumstellar and shell lines show that neither follows the radial velocity curve of the two stars. Circumstellar line velocities range from the systemic velocity to ~15 km s-1 more negative, consistent with their formation in an extended circumstellar envelope of the M supergiant which is large compared to the scale of the orbit. The shell lines are redshifted from the systemic velocity by ~30 km s-1. The last IUE observations (near phase 0.9) showed shell lines with greater redshifts and deeper absorption than around the rest of the cycle. The ultraviolet spectrum of VV Cep is compared to spectra of stars showing similar features: the Be shell star 28 Tauri, the B[e] star HD 45677, and the M supergiant+B binary systems α Scorpii and KQ Puppis. A table detailing the appearance and behavior of numerous line multiplets is presented.
Bennett Philip D.
Hagen Bauer Wendy
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