Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...334..558h&link_type=abstract
Astronomy and Astrophysics, v.334, p.558-570 (1998)
Astronomy and Astrophysics
Astrophysics
18
Stars: Emission-Line, Be, Binaries: Eclipsing, Stars: Individual: Omega Cma, Hr 2733
Scientific paper
The main purpose of this paper is to demonstrate the extreme complexity of the observed variations of Be stars on the example of a well-observed bright Be star omega CMa. A detailed analysis of all published radial velocities and a representative set of photometric and spectral observations of this star led to the following firm conclusions: \begin{itemize} At least three and possibly four different time scales of variability of omega CMa, ranging from 1\D37 to more than 40 years, could be identified. The correct mean period of the RV and line-profile changes is 1\D371906, not 1\D3667 as derived earlier. The brightness of the object and the strength of the Balmer emission vary in an apparent cycle of several thousands of days. The long-term brightness and emission-line changes can be understood as consequences of the formation and gradual dispersal of a gaseous envelope which is flattened and seen more face-on than equator-on. During each episode, the envelope grows from an optically thick pseudophotosphere to a more extended and optically thin envelope. Existence of much smaller episodes of light brightening which can have the same cause (though on a more limited scale) has clearly been demonstrated. The amplitude of the 1\D37 RV curve varies on a time scale somewhere between 10 and 300 d. The following conclusions are less certain and represent possible alternatives to be tested by future, systematic and homogeneous observations: \begin{itemize} Some evidence is presented that the amplitude of the 1\D372 RV variations, local mean RV and brightness of the object, prewhitened for the long-term changes, all vary on a time scale of about 35 d, possibly with a period of 34\D675. The O-C deviations of the local epochs of RV maxima from a linear ephemeris for the 1\D372 period seem to be undergoing a slow and probably cyclic variation in time, being shortest at times when the star is brightest and when a new Be envelope begins to grow. However, the same O-C deviations can also be reconciled with the 34\D675 period. Whatever the true timescale of the O-C deviations is, their behaviour can also be simulated as an interference of several periods, the second most significant period being close to 1\D35. Several reasons are given why the explanation in terms of one variable period appears more probable. With the help of both, real and artificial data it is demonstrated that the slow variation of the 1\D3719 period -- if unrecognized -- may be misinterpreted for a multiperiodic variation with several close periods between 1\D3 and 1\D45. This constitutes a methodological warning for the period analyses of data on some \bn, Be and ``slowly pulsating" B stars. The cause(s) of the variations with the 1\D37 (and 1\D345) period(s) and/or the 35 d cycle remain unexplained. It is obvious, however, that these three periods are not mutually independent. The 34\D675 period may be either a real physical period or a beat period between the 1\D372 and 1\D345 periods. In the former case, omega CMa could be a 34\D7 binary in an eccentric orbit and the periods twice longer than the two periods near 1\D4 would represent the sidereal and synodic rotational periods of the Be primary. Finally, some speculations are offered in terms of a hierarchical multiple system of three or even four stars.
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