5. On the origin of variations
The periodicity of the mid-term variability, the large value of the period, 395 d, and the fact that the star is redder when brighter (see Sect. 4) suggests for HR 1960 a model similar to the one proposed by Carrier et al. (1999) for the Be star HR 2968, i.e.:
Is HR 1960 member of a binary system? Three radial velocity measurements have been found in literature (Andersen & Nordström, 1983): 14.9 1.3 km s-1 (Sep 25, 1974), 15.6 2.8 km s-1 (Sep 29, 1974) and 12.9 1.3 km s-1 (Nov 24, 1974). According to the authors, these values do not indicate a variability of the radial velocity. However, it can be noted that the third measurement has a value lower by 2.0 and 2.7 km s-1 than the first two measurements obtained respectively 60 and 56 d earlier. We can only postulate that these values are not in contradiction with an orbital motion of period 395 d, but, of course, a radial velocity survey must be organized in order to confirm the model proposed.
The observational constraints imposed to this model from the optical light variations are summarized in Fig. 4 which shows that:
In the case of HR 1960, the very small and constant semi-amplitude observed between February 1987 and March 1993, 3 millimags, indicates that the Be star was not very active during this period and/or that the disk was of low importance and stable in shape. Note here that the observed rotational velocity, = 175-250 km s-1 (Andersen & Nordström, 1983), indicates that the vue angle of the Be star is far from the pole-on case (i = 0o). Indeed, by adopting for the equatorial rotational velocity v a value typical for the Be stars, i.e. (Porter, 1996), where is the critical rotational velocity ( km s-1), the vue angle is in the interval 30o - 60o.
The long-term variation described in Sect. 3 can be tentatively also attributed to the Be phenomenon. Such variations with characteristic times from months to decades, are a general property of Be stars (e.g. Moujtahid et al., 1998). In the case of HR 1960, the amplitude of this long-term variation is very small, 5 millimags in 10 y (see Fig. 1). This is an additional indication that this Be star is a relatively quiet one.
© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999