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

1. Introduction

Tuc (HR 139, HD 3112, V=6.11, , ) is classified in the Bright Star Catalogue (Hoffleit 1982) as an A7IV star. The variability of Tuc was first noticed by Cousins & Lagerwey (1971) using photometry, and these authors derived a period of the variations of the order of 70-80 minutes. Stobie & Shobbrook (1976) classified Tuc as a Scuti star and noted irregular changes in the frequencies. Therefore, Tuc was thought to be a Scuti star with variable frequencies. On the other hand, Kurtz (1980) determined a set of 8 stable frequencies for Tuc using a dataset spanning 7 years. Beating of these frequencies gives the impression that some frequencies appear and disappear, but this model could be ruled out thanks to the large time span over which the data were taken. Quite a few Scuti stars show a complex spectrum of frequencies, but studies which involve a large amount of observing time now suggest that most of them have a constant frequency behaviour.

A 6-week multisite campaign on Tuc was performed by Paparó et al. (1996). These authors identified 10 highly-stable frequencies. They also noted strict periodicity for the nightly mean variations, a fact that was already noted by Stobie & Shobbrook and by Kurtz. They suggested for the first time that Tuc might be the primary of a binary system with a late F-type companion. Sterken (1997) showed, using Strömgren uvby data, that Tuc is an ellipsoidal binary with a period of 7^d.04 with a low-mass companion. A mass ratio of the order of 0.1-0.15 was determined using these photometric data and making use of the method of analysing the variability of ellipsoidal variable stars described by Morris (1985).

Sterken et al. (1997) determined the physical parameters for Tuc assuming that the contribution of the secondary does not significantly affect the photometric indices. They found an effective temperature of 7575 K, no anomalous metallicity, a value of 3.8 and a mass of the primary not higher than 2.00.1 . The values reported in the literature range from 52 km/s (Bright Star Catalogue, Hoffleit 1982) to 80 km/s (Uesugi & Fukuda 1982).

Until now, few radial velocity measurements of Tuc have been published and it has not been proven yet that the true nature of the nightly mean photometric variations is binarity. In Sect. 2we describe our spectroscopic observations. We present the details of the computation of the elements of the orbit in Sect. 3and of the physical parameters in Sect. 4. Sect. 5is devoted to the analysis of the line-profile variations (LPV) of the primary of Tuc. Finally, we discuss our findings in Sect. 6.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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