Astron. Astrophys. 350, 566-570 (1999)

4. Discussion

The character of light and spectral changes of Be stars and their mutual correlation has been the subject of a number of statistical investigations. Nordth & Olofson (1977) and Hirata (1981) showed that for the majority of Be variables the light increase is connected with the reddening of index and blueing of . Dachs (1982) found a correlation between the light changes and the intensity of emission and shell lines in the sense that the light maxima correspond to larger equivalent widths, while the light minima are connected with the intensity-maxima of the shell lines.

The concept of a positive and inverse correlation was formulated by Harmanec (1983) and Harmanec (1994). The positive correlation is characterised by simultaneous increase of both the Balmer emission and the brightness, while in the colour diagram the star is changing its luminosity but not the spectral class. A light decrease along with the strengthening of the Balmer emission and, at the same time, a change of spectral but not luminosity class is the characteristic of inverse correlation. Harmanec (1983) interpreted the existence of the two types of correlation as the aspect effect, the inverse correlation being observed for stars seen roughly equator-on. This hypothesis was supported by Hubert & Floquet (1998) who have shown that the brightenings are characteristic for Be stars having small v sin i values while the fadings are associated with the rapid rotators.

In Fig. 4 we present the diagram for the seasonal means of OT Gem. It is seen that during the first four seasons of Hvar measurements, the star is becoming redder in and slightly bluer in . From 1992 to 1998 (HJD 2448705-2450195) OT Gem passed through at least three major brightenings with an amplitude up to 0. 4 in V. It is seen from Fig. 4 that the star in the active phases moved toward the supergiant sequence of the diagram. Such behaviour is typical for the positive correlation in Harmanec's scheme. The lack of spectroscopic data at the time of outbursts does not allow us to search for the correlation between the emission and the position of OT Gem in the colour diagram.

Fig. 4. The U-B vs. B-V diagram for OT Gem.The following symbols are used: solid boxes... Hvar, solid diamond... Mendoza (1958), open circle... Dachs et al. (1988), open triangle... Schuster & Guichard (1984), open diamond... Crawford et al. (1971)

According to Hubert & Floquet (1998) sudden brightenings in Be stars are more frequent in early Be stars and there are three categories of their manifestation: i) reccurent short-lived outbursts, ii) long lived outbursts and iii) outbursts closely linked to a temporary Be phase or strong emission line variations. OT Gem should be classified as a member of the second class that is characterized by amplitude variations from 0.15 to 0.35 in and by the duration of an outburst that exceed several hundreds days.

There are several other Be stars that exhibit behaviour similar as OT Gem. Pavlovski et al. (1997) have found that 2 out of 48 objects from their set of well observed Be stars show occasional sudden brightenings, o Cas Horn et al. (1985) and QR Vul Pavlovski et al. (1983). The measurements secured at Hvar Observatory indicate a positive correlation in the diagram for these stars.

The well known rapid light and spectroscopic Be variable  CMa shows outbursts with a maximum amplitude of 0. 4 as well as a positive correlation between the emission strength and the brightness - see Harmanec (1998a) and Stefl et al. (1998). Mennickent et al. (1994) announced that at the active phase the light changes of the star showed quasi-periodic oscillations up to 0. 1 with a mean period of 25 days. Using a larger data sets Harmanec (1998a) was able to uncover the 34. 67 period both from spectroscopic and photometric data. He has also pointed out the possibility that those variations could be explained by the orbital motion in a binary system with a highly eccentic orbit.

Recently, Carrier et al. (1999) have discovered the new Be variable HR 2968 in the the open cluster NGC 2451. They showed that the luminosity of the star started to increase in 1990 after a longer period of constancy. The increase of brightness was accompanied with oscillations around the mean light curve with a period of 371 days and an amplitude of 0. 08 to 0. 10. In the colour-colour diagram the star is moving from the main sequence toward the supergiant sequence indicating again a positive correlation.

There are convincing arguments that OT Gem is observed under an inclination far from . An estimate made by Ruusalep (1989) put the value of the inclination in the range from to . Slettebak (1994) derived a rather low rotational velocity v sin i = 130 km s^-1 for OT Gem. Hanuschik (1996) classified OT Gem as a non-shell star and concluded that a significant part of the disk is projected against the sky. The variation of the dimension and/or density of the inner disk may then act as an apparent change of the radius of the object causing both the light and colour variations (see Harmanec 1983). Harmanec (1998a) describes the long-term variability and the outbursts of  CMa as a manifestation of the formation and gradual dispersal of the circumstellar gaseous envelope. Variations of OT Gem are in many aspects similar to that of  CMa and we believe that they can be explained by the same physical process. Such an idea was also suggested by Koubsky et al. (1997) for the explanation of the spectral and photometric variations of the Be binary 4 Her, though for the case of an inverse correlation. In that scheme, the new optically-thick envelope that is created near the photospere in the equatorial region can simulate the stellar photosphere. Consequently, the apparent radius of the object grows and the brightness increases. At the same time, the star moves toward the supergiant sequence in the diagram. As the envelope expands and rarifies, it becomes optically thin and brightness fades while in the diagram the star goes back to the main sequence.

Such a scenario is in good agreement with the light and colour changes of OT Gem. The precursor to the quiet phase was the sudden light increase observed by Figer (1981) in 1980-81. Hubert-Delplace et al. (1982) reported that the Balmer discontinuity in 1979 had been seen in absorption but in November 1980 it was in emission. According to Divan et al. (1982), Be stars have two Balmer discontinuities, one belonging to the underlying star and another one corresponding to a low density plasma in the envelope. The variations occur only in the later. A rise of the Balmer discontinuity may be an indication of the formation of the new envelope.

It should be mentioned that in the concept described above, the emission reaches a maximum after the light maximum as is certainly observed in the case of  CMa- see, e.g., Harmanec (1998a). In the later stages of the envelope evolution when the disk is rarified, the brightness and emission both decrease. This closely resembles the spectral and photometric behaviour of OT Gem during the quiet phase from HJD 2444977 to 2446159. The previous episode of a large brightening of the star (not shown in the diagram, but documented by visual and photographic observations by Figer (1981) and Berthold (1983)) was obviously followed, with some delay, by a steep increase and a gradual decrease of the H emission (see Fig. 1). In the diagram, the star was moving towards the main sequence changing its photometric luminosity class with remaining nearly constant.

Notably, a binary interpretation of the cyclic light variations was put forward in all above-mentioned cases. Ferro et al. (1998) investigated the possibility that the cyclic light variations of OT Gem during the active phase with a period between 70 and 80 days are caused by tidal effects of the unseen companion in an eccentric orbit but they were not able to find any evidence of binarity. As an explanation of the 34. 67 period in  CMa Harmanec (1998a) has suggested that the object could be a binary in an eccentric orbit. To explain the 371-d photometric period of the Be star HR 2968 Carrier et al. (1999) proposed also a binary model in which the secondary in a highly eccentric orbit interacts gravitationaly and radiatively with the disk around the primary. At the moment, because of the lack of spectral observations, we were unable to check on the possible duplicity of OT Gem. Such a possibility should, however, be tested by dedicated observations of all above-mentioned objects.

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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