Astron. Astrophys. 336, 527-534 (1998)

6. Discussion

High-spectral-resolution and high-S/N spectroscopy of  Tuc allowed us to confirm the suspicion that the star is a binary system and to derive the orbital elements of this binary. Lines of both components were visible in the spectra which allowed us to determine the mass ratio q with much more accuracy than photometrically determined. Thanks to the availability of the photometric light curve, we were able to determine an estimate of the physical parameters of both components. The cooler, less massive, secondary is probably the remnant of a former loser in an Algol system.

We detected rapid variations of the profiles of the lines of the main component and determined some of the frequencies responsible for these variations which in some part are consistent with the variations found in photometry.

We were able to determine not only the main pulsation frequency but we also showed that the pulsation mode solution is radial. This is rather surprising, because the light curve indicates that Tuc is a non-eclipsing binary with ellipsoidal variations. We must note here that the method used is only tested for single stars. No method for the determination of pulsation modes exists for stars in binary systems where one must include the effect of tidal interactions. We were not able to investigate all the modes present in the primary component of Tuc, due to the limited number of spectra and the long integration time compared to the variations, but we can conclude that some modes must be non-axisymmetric pulsation modes.

A further step in the investigation of this star is the detection of the new frequency as the one which is only visible in the spectra and, therefore, must be due to a high-degree pulsation mode. We need more high-signal to noise spectra to refine the value of this pulsation period and to identify the pulsation parameters.

Also in the case of the p-mode pulsations in this Scuti star, it remains a problem for future research to what extent the binary nature is related to the observed line-profile variations. By all means, Tuc is a Scuti star which deserves intensive further study. Full identification of the extremely rich pulsation-mode spectrum of this star should be confronted with theoretical computations of its pulsation frequencies. Such a study would put important constraints on structural models for this star, as it is now being performed in asteroseismological studies of Scuti stars such as FG Vir (Breger 1995) and CD-24.7599 (Handler et al. 1997). In the specific case of Tuc one is confronted with the exceptional opportunity to perform asteroseismology on a post-mass-transfer Scuti star, the history of which is severely constrained by the study of the orbit and the detectability of the companion. This object then offers a unique opportunity to put tight observational constraints on close-binary evolution, and on the structure of the mass gainer in particular.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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