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

On the nature of the Be phenomenon

I. The case of 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  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  CMa, ranging from 1 37 to more than 40 years, could be identified.
• The correct mean period of the RV and line-profile changes is 1 371906, not 1 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 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 1 372 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 34 675.
• The O-C deviations of the local epochs of RV maxima from a linear ephemeris for the 1 372 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 34 675 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 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 3719 period - if unrecognized - may be misinterpreted for a multiperiodic variation with several close periods between 1 3 and 1 45. This constitutes a methodological warning for the period analyses of data on some  Cep, Be and "slowly pulsating" B stars.
• The cause(s) of the variations with the 1 37 (and 1 345) period(s) and/or the 35 d cycle remain unexplained. It is obvious, however, that these three periods are not mutually independent. The 34 675 period may be either a real physical period or a beat period between the 1 372 and 1 345 periods. In the former case,  CMa could be a 34 7 binary in an eccentric orbit and the periods twice longer than the two periods near 1 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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Contents

• 1. Introduction
• 2. Observational data used
  • 2.1. RV data
  • 2.2. Photometry
  • 2.3. Balmer line profiles
• 3. Periodic changes with the 1 37 period
  • 3.1. Is there a secularly stable 1 37 period?
  • 3.2. Cyclic changes of a single period or an interference of several short periods?
  • 3.3. Polarimetric and light changes with the 1 372 period
• 4. An overview of photometric and emission-line variations of  CMa
  • 4.1. Photometry
  • 4.2. Balmer line profiles
• 5. The long-term changes
  • 5.1. Their character and phenomenological interpretation
  • 5.2. Are the long-term changes cyclic or periodic?
• 6. Variations on a time scale of a few weeks
• 7. Other short periods?
• 8. Speculations in the absence of firm answers
  • 8.1. Alternative models of  CMa
  • 8.2. What causes the changes with the 1 372 period?
• 9. A methodological remark
• 10. Conclusions
• Acknowledgements
• Appendix A
  • A. Appendix: details on photometric data sets used
  • References

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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