Institution(s): 1. Saint Mary's University, 2. Wellesley College
VV Cephei is an eclipsing binary system with a red supergiant primary with a spectral type of M2 Iab, very similar to α Orionis. The companion is most likely an early B main-sequence star, although it orbits so deeply within the M star wind that even far from eclipse, the ultraviolet spectrum resembles that of a Be-shell star. The orbital period is 20.3 years, and as the hot star goes into or emerges from eclipse, its orbital motion can be used a probe of the structure of the extended supergiant atmosphere.
VV Cephei was observed with the STIS spectrograph on the HST at 21 epochs spanning total eclipse through first quadrature as the system emerged from its last eclipse, and we report on using these observations to model the M supergiant chromosphere. During total eclipse, a rich emission line spectrum is observed, most of which arises from singly ionized elements in the iron group. Sharp absorption features from strong low-excitation transitions appear at about -20 km s-1, which are due to the expanding M supergiant wind (and which remain throughout the entire orbit).
When the hot component emerges from eclipse, the nature of the spectrum changes to one dominated by absorption features, the majority of which persist past first quadrature. However, lines of easily-ionized neutral species such as Fe I and some singly ionized species such as V II weaken and disappear by the time the hot component has moved to about 1.5 M-star radii. We have used the observations during this “chromospheric” eclipse to model the extended M star atmosphere.
The chromospheric absorption lines have typical breadths of ~25 km s-1, and were resolved by STIS into two (and sometimes three) components. The relative strength of the components depends on the level of ionization and excitation. This doubled structure remains consistent throughout chromospheric eclipse. The column density and velocity structure of gas along the line of sight can be reconstructed from measurements of lines with different optical depths. In addition, broad damping wings from the strongest aborption features (the h and k lines of Mg II, Lyman α, and the strongest Fe II lines) have also provided column density information.