2. Program stars, observations and data reduction
The 10 observed Praesepe stars are in the hottest part of the cluster color-magnitude diagram (Fig. 1). Owing to the weakness of the Li i doublet, they include all the A stars with presumably projected rotational velocities v sin i less than about 60 km s-1 from McGee et al. (1967) and Abt (1986). Two stars with higher v sin i (100 for Cnc and 85 for HD 73818) were added: their double-lined spectroscopic binary (SB2) character may have influenced the v sin i determination. Six out of ten stars lie in the sequence turnoff. Another, 40 Cnc, is a blue straggler. We note for the two coolest stars of our sample that the first one, the SB1 HD 73045 (Bolte 1991), lies above the main sequence in the "second-sequence" composed of photometric binary candidates when the other, the SB2 HD 73818 (Dickens et al. 1968), lies within the main sequence. The cluster SB2 stars are not always located in the color-magnitude diagram owing to their (now known) spectroscopic binarity (This was, too, noted for Pleiades SB2 stars observed by Burkhart & Coupry (1997)).
Some stellar characteristics and observational data are collected in Table 1. Cnc, the brightest star in Praesepe, is the only star that has not been previously always considered as bona fide member. In the late high-precision study of proper motions and membership in Praesepe (Wang et al. 1995), this star appears as a member with high probability; this agrees with HIPPARCOS results. Memberships are, so, confirmed for all the observed stars. The normal A or Am character is evaluated from the classification of Gray & Garrison (1987, 1989) in col. 2 and/or the line ratio Ca i-6717/Fe i-6678 (Burkhart & Coupry 1991) in col.3. There is a general agreement between both proceedings (and with classifications by Bidelman (1956) and Abt (1986)). Our sample is composed exclusively of Am stars. These are the slowly rotating A stars. As the discrimination of the observed A stars in the cluster was made only against large projected rotation rates, seemingly no normal A star is seen nearly pole-on. The many binary (or triple) systems agree with the general properties of Am stars. For SB2 stars, the different classifications in col. 2 were done with the system seen as a single-lined star, but from the Ca/Fe line ratio the Am character of each component can be found in favorable circumstances.
The equivalent widths are given in Table 2. Each of the Al, Li, Ca, and S lines is the main line of a blend with a weak Fe line; the equivalent width value found in Table 2 is, then, that of the entire blend and "bd" is put for the satellite line. For Eu, the equivalent width value is that of the blend. Different cases arose with the SB2 systems. Only in the spectrum of Cnc, we observed both line systems; both components are Am stars; some lines could be separately measured. From comparison with our other spectra it visually appears that v sin i of each component is about 50 km s-1 and not 100 km s-1 as previously given. For HD 73618, the two line systems are not separated but visible in the line profiles, matching similar Am stars. The spectrum of HD 73818 has a relatively low signal-to-noise ratio (about 130), no SB2 feature could be detected. Further, the line profiles of HD 73711, a single star up to now, are asymmetrical. They are all similar except the Ca profile. This likely excludes a non-radial pulsator signature and favors an SB2 explanation with 2 Am stars from their Ca/Fe ratios. The spectra of the other 3 SB2 stars were processed as single-lined stars.
© European Southern Observatory (ESO) 1998
Online publication: September 17, 1998