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Session 60 - Multiple Stars.
Display session, Wednesday, June 12
VV Cephei is an eclipsing binary consisting of an M2Iab supergiant and a Be companion, with a 20-year period. The M supergiant fills its Roche lobe at periastron. In the optical and infrared, the spectrum is dominated by the enormous red supergiant. However, in the ultraviolet, the hot component dominates the spectrum. Observations with the IUE satellite began as the hot component emerged from eclipse in 1978. VV Cep has now been observed throughout almost the entire out-of-eclipse portion of its orbit, and a rich absorption spectrum persists, arising mostly from singly ionized metals.
This absorption line spectrum is more extensive than that observed during deep chromospheric eclipse for Zeta Aurigae binaries such as 31 Cyg (K4 Ib + B4 V). However, a nearly complete match to the absorption features seen in VV Cep outside eclipse is observed in the Be star Pleione (28 Tau) during a strong shell phase. Among the absorption features seen in VV Cep and 28 Tau but not 31 Cyg are lines due to Cr II and Mg II whose lower energy levels lie around 4 eV, higher excitation than is seen in 31 Cyg. The line widths in VV Cep are significantly greater than those seen in 28 Tau. Unlike 28 Tau, the absorption spectrum in VV Cephei is veiled by a continuous source. The only lines whose central depths penetrate below this continuum are those which would be expected from the extensive circumstellar envelope of the M supergiant primary, e.g. both zero-volt and excited lines of Fe II (UV 1).
While the line profiles in this shell spectrum are sometimes symmetric, more often additional red-shifted, and occasionally blue-shifted, absorption features are seen. The existence and strength of these additional features, and the strength of the continuum can vary significantly over time scales as short as two weeks. Similar additional components have been seen in the spectrum of Beta Pic, in which red-shifted events are also much more frequent than blue- shifted events (Vidal-Madjar et al. 1994, AA 290, 245). However, in VV Cephei, nearly all shell lines show this behavior, while in Beta Pic, only the strongest ones do. (Lagrange-Henri et al. 1988, AA 190, 275).
VV Cephei also shows a number of emission lines in the UV spectrum, most of which are due to Fe II. The profiles of these emission lines are also seen to vary significantly over time scales as short as two weeks. Although UV emission lines are rare in Be stars (Slettebak and Carpenter 1983, ApJS 53, 869), the Be star HD 45677 shows emission in nearly all the same lines as does VV Cep. HD 45677 has also recently been shown to have variable additional red-shifted absorption features (Grady et al. 1993, ApJ 415, L42).
Program listing for Wednesday