Astronomy and Astrophysics, volume 356, 913-928 (2000/4-3)
Properties and nature of Be stars. XIX. Spectral and light variability of 60 Cygni.
KOUBSKY P., HARMANEC P., HUBERT A.M., FLOQUET M., KUBAT J., BALLEREAU D., CHAUVILLE J., BOZIC H., HOLMGREN D., YANG S., CAO H., EENENS P., HUANG L. and PERCY J.R.
Abstract (from CDS):
An analysis of electronic spectra secured between 1992 and 1999 at the Haute Provence, Ondrejov and Dominion Astrophysical Observatories and of differential UBV measurements of 60 Cygobtained between 1984 and 1999 at Hvar, San Pedro Martir, Toronto and Xinglong Observatories, the all-sky Hipparcos satellite Hp photometry transformed to Johnson V and B magnitudes, and all-sky UBV observations published by several authors and dating back to fifties, led to the following findings: 1. 60 Cygexhibits pronounced long-term spectral variations characterized by the B -> Be -> B phase transitions. These long-term spectral changes of 60 Cygare also accompanied by corresponding, though rather mild, secular light and colour variations. The character of these variations is indicative of a positive correlation between the brightness and emission-line strength. 2. NLTE model atmosphere analysis of spectra secured during the quiescence state (B phase) of 60 Cyg shows that the star has overabundance of helium. Best results were obtained for NHe/NH= 0.2. 3. The presence of periodic medium-term changes, with a period of 146.6d±0.6d was found in the radial-velocity of the Hα and He i 6678 Å lines. If confirmed by future observations, these variations could indicate that 60 Cygis a spectroscopic binary. 4. There are clear rapid periodic line-profile changes of (a) overall line asymmetry, and (b) weak sub-features passing across the line profiles every about 0.1d. The radial velocity and asymmetry of He I lines vary with a period of 1.0647d and a double-wave curve. There is no evidence of this period in photometry, however. 5. The rapid light variations of 60 Cygare dominated by rapid changes with a full amplitude of almost 0.1m. A period analysis of V magnitude data prewhitened for the long-term changes indicates a period of 0.2997029d, reported earlier. The most interesting finding is that also all recorded series of moving sub-features in the line profiles can be reconciled with this period: the sub-features reappear at the same phase intervals of the 0.2997d period in the line profiles over an interval of several years. Considering the acceleration of these sub-features, 1900km/s/d, it is conceivable that the true physical (super) period of these changes is either 0.8991d or 1.1988d. 6. The findings mentioned in points 4 and 5 represent a challenge for the NRP scenario since the light changes would be dominated by a high-order mode instead of a low-order one.
stars: early-type - stars: emission-line, Be - stars: individual: 60 Cyg - stars: oscillations - stars: variables: general