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Astron. Astrophys. 331, 171-178 (1998)

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1. Introduction

20 CVn is a bright and luminous low amplitude [FORMULA]  Sct type star with V=4.m 73, [FORMULA] V=0.m 02, P=0.d 1217 and spectral type F3III (Rodríguez et al. 1994). Its variability was first reported independently by Wehlau et al. (1966) and Danziger & Dickens (1966) during the course of photometric observations. Danziger & Dickens (1967) list this star as a new member of the [FORMULA]  Sct group. New photometric observations were collected by Penfold (1971) using the filter V of the Johnson photometric system, Shaw (1976) in BV, Peña & González-Bedolla (1981) in V, Bossi et al. (1983) in V, Chun et al. (1983) in BV and Nishimura et al. (1983) in V. However, the majority of these data sets are too short and scanty to analyse the pulsational behaviour of 20 CVn. The two best data sets are those from Shaw (1976) (with about 783 and 262 measurements in B and V, respectively) and Peña & González-Bedolla (1981) (with about 198 measurements in V). They seem to be good enough to gain some insight into the frequency content. In both cases, the authors suggest that 20 CVn is monoperiodic with a period of P=0.d 1217. However, Bossi et al. (1983) (from their own data) suspect the presence of a second pulsational mode of 6.99 cd-1 (P2 =0.d 143).

In addition, some spectroscopic data were also obtained by different authors: Penfold (1971), Smith (1982), Nishimura et al. (1983), Yang & Walker (1986) and Mathias & Aerts (1996). From his data, Smith (1982) suggests the existence of a beating effect (of about 2 days) caused by a second period modulating the primary one. This leads to a secondary frequency of about 8.7 cd-1 (P2 =0.d 115). Moreover, Smith (1982) and Yang & Walker (1986) obtained low values for the term 2K/ [FORMULA] mv (from 42 to 39 Km s-1 mag-1, respectively). On this basis, they suggest that the primary pulsation is nonradial. Finally, Mathias & Aerts (1996), from only one cycle of spectroscopic observations and using the moment method, find this star pulsating nonradially with l=3 or 2. Moreover, they suggest the existence of a second mode. In this way, the main aim of this investigation is to clarify the mono or multiperiodic nature of this star on the basis of new and high quality photometric observations.

On the other hand, very few [FORMULA]  Sct stars showing both low amplitudes and a single pulsation frequency are known. Three well studied examples are: [FORMULA]  Peg (Breger 1991), [FORMULA]  Cas (Rodríguez et al. 1992) and 28 And (Rodríguez et al. 1993). In all the three cases nonradial pulsation has been suggested on the basis of phase shifts and amplitude ratios between observed light and colour variations (Watson 1988, Garrido et al. 1990). This is quite different to that occurring in the known high amplitude [FORMULA]  Sct stars (Rodríguez et al. 1996) where radial pulsation is always found. Then, 20 CVn promises to be a good object to study. In order to confirm or not the nonradial nature of this star, we have carried out simultaneous uvby photometry.

The observations and light curves are described in Sect. 2. In Sect. 3.1 and 3.2, the periodicity of this star is analysed using the Discrete Fourier Transform and classical O-C methods. Revisions about possible long term luminosity amplitude variations and about the stellar parameters of this star are given in Sect. 3.3 and 3.4, respectively. In Sect. 3.5, the nature of radial or nonradial pulsation in 20 CVn is examined on the basis of the phase shifts and amplitude ratios between different colours. Finally, some conclusions are given in Sect. 4.

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998