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Astron. Astrophys. 334, 558-570 (1998)

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On the nature of the Be phenomenon

I. The case of [FORMULA] Canis Majoris

P. Harmanec

Astronomical Institute, Academy of Sciences, CZ-251 65 Ond ejov, Czech Republic (hec@sunstel.asu.cas.cz, hec@mbox.cesnet.cz)

Received 6 August 1997 / Accepted 30 January 1998

Abstract

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 [FORMULA]  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:

  • At least three and possibly four different time scales of variability of [FORMULA]  CMa, ranging from 1 [FORMULA] 37 to more than 40 years, could be identified.
  • The correct mean period of the RV and line-profile changes is 1 [FORMULA] 371906, not 1 [FORMULA] 3667 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 [FORMULA] 37 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:

  • Some evidence is presented that the amplitude of the 1 [FORMULA] 372 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 [FORMULA] 675.
  • The O-C deviations of the local epochs of RV maxima from a linear ephemeris for the 1 [FORMULA] 372 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 [FORMULA] 675 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 [FORMULA] 35. 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 [FORMULA] 3719 period - if unrecognized - may be misinterpreted for a multiperiodic variation with several close periods between 1 [FORMULA] 3 and 1 [FORMULA] 45. This constitutes a methodological warning for the period analyses of data on some [FORMULA]  Cep, Be and "slowly pulsating" B stars.
  • The cause(s) of the variations with the 1 [FORMULA] 37 (and 1 [FORMULA] 345) period(s) and/or the 35 d cycle remain unexplained. It is obvious, however, that these three periods are not mutually independent. The 34 [FORMULA] 675 period may be either a real physical period or a beat period between the 1 [FORMULA] 372 and 1 [FORMULA] 345 periods. In the former case, [FORMULA]  CMa could be a 34 [FORMULA] 7 binary in an eccentric orbit and the periods twice longer than the two periods near 1 [FORMULA] 4 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.

Key words: stars: emission-line, Be – binaries: eclipsing – stars: individual: ! CMa, HR 2733

Send offprint requests to: P. Harmanec

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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