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

5. The long-term changes

5.1. Their character and phenomenological interpretation

As already noted, the secular light changes of  CMa are characterized by occasional larger or smaller brightenings from a certain more or less undisturbed level. There may also be a correlation between the major brightenings and the strength of the Balmer emission (found for several other well-observed Be stars). Note, however, that the maximum brightness corresponds in both so far recorded cases to rather early stages of the new emission-line episode, i.e. that the maximum strength of the emission lags behind the maximum brightness.

This behaviour can be qualitatively well understood if one adopts Harmanec's (1983) concept of an optically thick pseudophotosphere, as recently developed semi-quantitatively by Koubský et al. (1997): The process of the formation of a new envelope starts close to the stellar photosphere. It means that the envelope begins to grow first as an equatorially flattened and optically thick region which mimics the stellar photosphere. Clearly,  CMa is a typical example of the positive correlation between the brightness and emission strength as defined by Harmanec (1983): the object indeed moves from the main towards the giant sequence in the colour-colour diagram when it brightens (see Fig. 9) and this agrees with the view that  CMa ( 80 km s^-1) is a rapidly rotating star seen under a relatively small angle between the line of sight and the rotational axis of the star. Thanks to this geometry, the formation of an extended pseudophotosphere near the equatorial regions effectively increases the apparent radius of the star for an observer on the Earth and the object brightens. Later, as the envelope gets larger and more rarified, it becomes optically thin so that the brightness gradually decreases again while the Balmer emission gets stronger.

Fig. 9. The vs. diagram for  CMa. Individual all-sky observations are shown by crosses, differential ones by black dots. The standard main and supergiant sequences are also shown. It is seen that  CMa moves from the main to supergiant sequence. The same trend can also be seen for the existing uvby observations (not shown here).

I tentatively suggest that also the smaller brightenings (such as the one shown in Fig. 8) are caused by the same process occuring on a smaller and shorter time scale since - for a given star - the character of all such episodes is the same (either brightenings or fadings from a certain light level) which seems to point towards the geometrical interpretation in terms of either more pole-on or equator-on orientation of each particular star (see Harmanec 1983 for details).

A new and exciting result of this study is that also the value of the 1 37 RV period changes cyclically and quite possibly in phase with the major light brightenings, being shortest at the light maxima.

Little can be concluded from Fig. 6 about the changes of the amplitude and mean velocity of the 1 37 RV curve besides the fact that they occur on a much shorter time scale. For instance the mean velocity based on Baade's (1984) Reticon observations secured with the same instrumentation and for the same He I line some 100 d apart (JD 2445245-7 vs. JD 2445352-8) differ by as much as 14 km s^-1.

5.2. Are the long-term changes cyclic or periodic?

In spite of the amount of data presented in Fig. 6 it is impossible at present to make any firm conclusion whether the long-term changes observed are cyclic or truly periodic ones. I verified that the existing data (1 37 period variation, brightness and emission strength) can indeed be reconciled with several long periods, e.g. about 2700 d, 3500 d, 5600 d or 8100 d, but the decision if this is indeed a regular clock can only come from future observations or from an independent piece of evidence.

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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