7. Conclusions and discussion
We analysed spectroscopic and photometric data of 60 Cygni and found changes on long, medium and rapid time-scales. These variations could be understood as follows:
The long-term variations, both spectroscopic and photometric, are indicative of a gradual formation and dispersal of the Be envelope around 60 Cyg. They represent a typical case of the so called positive correlation between the brightness and emission strength (Harmanec 1983, 1994), which implies that the disk of 60 Cyg is not observed equator-on.
The medium-term changes: The RV of the emission and absorption profile of H and also RV of the He I 6678 Å absorption vary with a 1466 period. The V/R ratio of H, however, seems to vary with a somewhat longer period of about 152-154 d. The interpretation of the RV changes in terms of orbital motion in a binary system leads to binary properties quite similar to those of known Be binaries. It seems premature to speculate about possible revolution of an asymmetric envelope as the explanation of the longer V/R period.
Rapid changes of 60 Cyg: Investigation of the character of rapid variability was in fact the original motivation for this study. The variations of helium profiles obtained in nightly series were analysed with three different methods (CLEAN, least-squares sinusoid fitting and PDM analysis of the measurements of local RV at specific levels). These analyses indicated a single period of 10647 (frequency of 0.939 c d-1) with a double-wave phase curve or its roughly sinusoidal alias with a frequency of 1.88 c d-1. Using the measured acceleration km s-1 d-1, observed v sin i and the well-known formula
one can also estimate the recurrence period with which a particular moving subfeature reappears in the line centre to be 1058. The close agreement of this value with the value of the period of line-profile changes may be fortuitous, considering the uncertainties involved. Even so, one can conclude however that the 0532 alias seems to be safely excluded as the true physical period of the changes observed.
No convincing evidence of the 1065 periodicity was found in the photometric data, prewhitened for the long-term changes. The light of the object seems to vary with a much shorter period of 02997029, which has no obvious relation to the 10647 period. However, all the observed acceleration curves of the moving sub-features in the He I 4471 Å line measured in the spectra obtained in 1994, 1996 and 1997 can be folded with the 0.2997-day period in such a way that the emission and absorption components define parts of overlapping sinusoids at always the same phase intervals of the 02997 period. Several harmonics of the 02997 period, with more complicated phase curves, were also detected in the photometric data - cf. Table 8.
The last possibly relevant value is the expected period of Keplerian rotation at the stellar surface which amounts to 039 (028-062).
In summary, there seems to be two independent types of rapid variations: (1) The first one is associated with the 1064 period. It is detected in the profile variations. However, it seems to be absent in the light changes. (2) The second one has a period of 02997 or its integer multiples. It dominates the rapid light changes and very probably also the recurrence of the traveling sub-features moving across the line profiles.
Our results - if confirmed by more extensive series of observations - seem to set stringent limits on any quantitative model of rapid line-profile variations of 60 Cyg:
If the detected variability is due to non-radial pulsations, then the first period of 1065 would be probably identified with a low m=2 mode. As Figs. 16, 17, and 18 indicate, 3-4 different sub-features are simultaneously seen in the line profiles which would indicate that the traveling sub-features should be identified with a modal number of 6-8. This is in fact corroborated by the slope of the phase variation across the profile of He I 4388 Å (cf. Fig. 9). It is constructed for the 053 period and indicates m=3, therefore m=6 for the twice longer true period of 1065. Considering also Fig. 19 where three different sub-features are seen for the 02997 period, one can conclude that the true physical superperiod of the moving sub-features must be 08991 or 11988, i.e. three or four times longer than the 02997 period. The obvious challenge for the NRP scenario is to explain why the light variations are dominated by a high-order, not a low-order mode .
If the observed changes are due to some corotating structures, one has to assume that the 1065 period is the rotational period of the star. The light variations and moving sub-features would have to be associated with some corotating structures above the stellar photosphere, somehow associated with the inner parts of the circumstellar disk. Their probable corotation period would be either 0899 or 1199. We note that two different periods with a similar frequency difference were also reported for CMa (Stefl et al. 1999). For this Be star, too, the 137 period dominates the RV variations while another one, 148, is mainly seen in photometry and also in spectral lines affected by a contribution from the circumstellar disk.
The observations of lpv described in Sects. 5.2.2 and 5.2.3 suggest that the amplitude of the lpv was diminishing from 1994 to 1997, when the Balmer emission of 60 Cyg has strongly increased. Such a correlation between the character of lpv and development of a new Be envelope was suggested by several authors (cf., e.g. Osaki, 1999), but the observational verification of it was contested (see the consecutive studies of Eri by Penrod 1986, Bolton & Stefl 1990, Smith 1989 and Kambe et al. 1993). It appears that 60 Cyg could be a good candidate for a study of the possible relations between lpv and the strength of emission. The lpv's are relatively strong and always present while the strength of the Balmer emission varies, sometimes even on a time scale of months.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000