A double shock wave atmospheric model for the beta Cephei star sigma
Scorpii?
P. Mathias
- D. Gillet
- R. Crowe
Received May 29; accepted July 24, 1991
Abstract:
High spectral and time resolution observations of high signal-to-noise ratio
(above 200), carried out at Halpha and the Si III 4553, 4567 Å lines of
the
beta Cephei star sigma Sco, are presented. The observations were done at the
European Southern Observatory (ESO) 1.4-m CAT (La Silla, Chile), using the
Coudé Echelle Spectrograph. On a few spectra, the line of the 33-day
companion of sigma Sco is visible for the first time. These new data were
used to obtain new orbital parameters. After removing the component of the
companion, we have determined heliocentric radial velocity curves which show
a nice stillstand between two discontinuities. These discontinuities are
interpreted as being due to the propagation of two shock waves, separated
by about 1h 30min. The first and main shock is confirmed by a clearly visible
line doubling phenomenon which is predicted by the mechanism of Schwarzschild
(1952). These waves produce an acceleration ratio of 4:1 for Si III and 6:1 for
Halpha, implying a weaker second shock wave compared to the first one. This is
also indicated by the presence of a stronger Van Hoof effect at the second
acceleration phase, which is interpreted as due to the propagation time of the
wave through the atmosphere. In this view, the stillstand is the result of
the transition between the end of the infalling motion and the beginning of
the new outward motion. The physical origin of both shocks is still not well
understood. We suggest that the first and strongest one is the consequence of
the sudden iron opacity increase mechanism recently predicted by Cox &
Morgan
(1991), while the second one could be the consequence of the CNO opacity
bump.
Keywords:
lines:profiles -- shock waves -- stars: Variables: other -- stars:
binaries: spectroscopic -- stars: individual: sigma Sco
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