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

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Binarity of Am stars in Praesepe and Hyades *  **

Y. Debernardi 1, J.-C. Mermilliod 1, J.-M. Carquillat 2 and N. Ginestet 2

1 Institut d'Astronomie de l'Université de Lausanne, 1290 Chavannes-des-Bois, Switzerland
2 Observatoire de Toulouse, 1 avenue Camille Flammarion, 31500 Toulouse, France

Received 24 August 1999 / Accepted 5 January 2000


CORAVEL radial-velocity observations of Am stars in the Hyades and Praesepe have allowed the determination of orbital elements for 10 spectroscopic binaries, among which 3 are first determinations. One Am star (KW 40) is found to be a well hierarchised triple system. KW 538 has a rather long period (435 days) for an Am star. Orbits of systems with periods shorter than 8.5 days are circularized, or present eccentricities smaller than 0.04. For 19 Am stars, the number of quadruple-, triple-, double-, single systems is 1:2:14(10+4?):(2?).

The Am stars in a ([FORMULA], [FORMULA]) diagram clearly stand away by 0.03 mag from the sequence defined by normal main-sequence stars. This diagram could be a powerful method to identify Am stars in more distant open clusters, provided there is no differential reddening. In the colour-magnitude diagram ([FORMULA], [FORMULA]), double-lined binaries are 0.6 - 0.7 mag above the ZAMS as expected, while most single-lined are close to or on the ZAMS because the secondary does not contribute much light. The absence of X-ray detection of 4 systems in the Hyades is an argument for the presence of a white dwarf secondary.

Key words: Galaxy: open clusters and associations: individual: Praesepe – Galaxy: open clusters and associations: individual: Hyades – stars: chemically peculiar – stars: binaries: spectroscopic

* based on observations collected at the Haute-Provence Observatory (France)
** Table 2 is available only in electronic form at CDS by ftp at ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/354/881 ( or on the Web at http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/354/881

Send offprint requests to: Y. Debernardi

Correspondence to: yves.debernardi@obs.unige.ch

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000