Astron. Astrophys. 324, 566-572 (1997)

1. Introduction

The Doradus variables are a newly discovered class of multiperiodic variable stars, found mainly among main-sequence stars at or near the cool border of the Scuti star instability strip. In the literature they have also been called slowly variable early F-type stars. These stars show periodic light variations with periods of many hours to days. Such periods are too long to be attributed to radial or p-mode pulsation.

Evidence for a new class of pulsating stars was presented by Balona, Krisciunas & Cousins (1994), who photometrically examined the star Doradus and found it to be a slowly variable multiperiodic star. Two attractive hypotheses to explain the variability of this new class of variable stars are spots rotating with the star and pulsation. The spot hypothesis can explain some multiperiodic behavior, e. g. two close periods could be caused by differential stellar rotation. However, for the star Dor, Balona, Krisciunas & Cousins found a period of 0.678 d in addition to the 0.757 and 0.733 d periods. The relative large difference between the values of the periods is not compatible with the spot hypothesis and therefore supports the interpretation in terms of nonradial gravity mode pulsation. Furthermore, for 9 Aur, Krisciunas et al. (1995a) found periods of 1.26 and 2.90 d. Aerts & Krisciunas (1996) interpret these two periods with a = 3, m = 1 spheroidal mode and its toroidal corrections due to the rotation of the star.

The pulsational excitation of Doradus stars is caused by the mechanism operating in the HeII ionization zone, the same as for the Scuti stars, according to model calculations by Moskalik (private communication). These models predict that among A and F stars, there would be a separation in temperature between p-mode ( Scuti) and g-mode ( Doradus) pulsators. This separation would be similar to that found between the Cep and SPB variables among the B stars. For the latter, we refer to model calculations by Gautschy & Saio (1993) and Moskalik (1995).

At the present time, less than thirty Doradus stars are known (e.g. see Krisciunas & Handler 1995). It is important to detect additional members of the class in order to understand the physical properties involved. Such searches are being undertaken. Examples are the systematic examinations of stars in the Hyades cluster (Krisciunas et al. 1995b) and NGC 2516 (Zerbi et al. 1997).

The star HD 108100 is a candidate for Doradus-type variability because of its spectral type, F2, and its main-sequence position in the H-R Diagram near the cool border of the Scuti star instability strip.

An additional motivation to test HD 108100 for variability arises from two unusually low frequency values found among the multiple frequencies of the Scuti variable, 4 CVn. These two frequencies, 1.327 and 1.402 c/d, appear in extensive series of unpublished photometric data (see Fitch 1980). If true, 4 CVn would be one the few stars exhibiting both short-period Scuti-type and long-period Doradus-type variability (Breger & Beichbuchner 1996). However, only a single comparison star, viz. HD 108100, was used in the Fitch study. An origin of the low-frequency variability in the comparison star is, therefore, possible.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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