Astron. Astrophys. 354, 881-891 (2000)

1. Introduction

Am stars are A- or early F-type stars, whose effective temperature lies between 7000 K and 9000 K. They are the coolest chemically peculiar stars on the main sequence (excluding barium or carbon dwarfs, which owe their peculiarity to binary evolution). Their main characteristics are an underabundance of calcium and scandium (about 5 to 10 times lower than in the Sun) and a slight overabundance of iron-peak elements. Three spectral types are identified on the basis of the intensity of the K, Balmer and metallic lines, respectively.

The chemical anomalies of Am stars are usually explained by the radiative diffusion theory developed by Michaud et al. (1983). This theory predicts that, in a slowly rotating star where the large-scale meridional circulation is weak enough, helium is no longer sustained and flows inside the star, gradually disappearing from the atmosphere. The diffusion process could therefore take place just below the thin H I convective zone where the diffusion time is short with respect to the stellar lifetime; as a first approximation, the chemical elements whose radiative acceleration is larger than gravity become overabundant and, in the opposite case, underabundant.

Most Am stars show slow rotational velocities () and a high rate of spectroscopic binaries with periods shorter than 100 days (Abt & Levy 1985). However, few long-term studies (see for instance Conti & Barker 1973or Abt & Willmarth 1999) have been undertaken to detect longer period Am stars or to prove that some are not variable in radial velocity. In addition, several questions have not received a satisfactory solution. Among them the nature of the secondary seems of primary importance in understanding the history of Am stars. Carquillat et al. (1997) studied a sample of 33 Am stars and detected a late-type companion for 22 systems from red spectra. We are interested in knowing more precisely what is the frequency of double-lined Am binaries (SB2) and what are the distributions of secondary masses or mass functions in single-lined Am binaries (SB1). Are these distributions similar to those observed for other mass intervals along the main sequence or do we have any evidence for an excess of low-mass companions or white dwarfs?

Therefore, radial velocity observations with the CORAVEL scanner (Baranne et al. 1979) have been obtained for Am stars in nearby open clusters (this paper) and in the field (Ginestet & Carquillat 1998, North et al. 1998, Debernardi 2000). Due to their low rotation and enhanced metallic lines, about half of the Am stars can be measured with the CORAVEL scanner and produce quite good correlation functions. These observations were intended to improve known orbits, search for evidences of secondaries in the correlation functions, monitor stars apparently stable and determine rotational velocities.

Several spectroscopic binaries were already kown in the Hyades and orbits had been determined (Abt 1961, 1985; Conti 1969). We obtained a new orbit for vB 83. However, in Praesepe, one orbit of Am star was known for a long time, the double-lined binary KW 229 (Sanford 1931). When we submitted the paper, Abt & Willmarth (1999) published a radial-velocity study of Praesepe, including orbital elements for the triple system KW 40 and KW 279. Both the short and long periods of KW 40 have been determined, independently of Abt & Willmarth's (1999) value, from our observations. The long period is twice as large as Abt & Willmarth's (1999) determination. We have determined new orbital elements for one binary: KW 538.

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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