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Astron. Astrophys. 363, 958-969 (2000)

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Optical follow-up of ROSAT discovered candidate members of the open cluster Coma Berenices *

R.J. García López 1,2, S. Randich 3, M.R. Zapatero Osorio 1 and R. Pallavicini 4

1 Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain (rgl@ll.iac.es; mosorio@ll.iac.es)
2 Departamento de Astrofísica, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez s/n, 38071 La Laguna, Tenerife, Spain
3 Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy (randich@arcetri.astro.it)
4 Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy (pallavic@oapa.astropa.unipa.it)

Received 24 March 2000 / Accepted 12 October 2000


We have carried out an optical follow-up of twelve ROSAT discovered candidate members of the Coma Berenices open cluster. VRI photometry and low resolution ([FORMULA] 3 Å) spectroscopy in the range 3800-7000 Å were performed to obtain colour-magnitude diagrams, proper motions, spectral types, chromospheric activity levels and radial velocities that allow us to establish whether or not these candidates are reliable cluster members. Only four of these objects show optical photometry marginally compatible with the main sequence delineated by known cluster stars. They also exhibit spectral types corresponding to late-K and early-M, and radial velocities which are not inconsistent with membership in Coma Berenices. The proper motions of these candidates are, however, very high and incompatible with the small value associated with the cluster. Thus, none of the ROSAT candidates can be considered members of Coma Berenices. This result tends to favour the hypothesis that the mass of the cluster and its average mass density are lower than what is required for stability, allowing the cluster dissolution by the escape of the less massive stars.

We discuss the probable nature of these objects on the basis of the available information. From our radial velocity observations we infer that five of them could be formed by multiple stars. In particular, we have discovered one W-type W UMa contact binary system (K6 spectral type) which is, in addition, a visual companion to a cooler star.

Key words: stars: activity – stars: binaries: eclipsing – stars: fundamental parameters – stars: late-type – Galaxy: open clusters and associations: individual: Coma Berenices – X-rays: stars

* Based on observations made with the Isaac Newton telescope, operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos, with the IAC80 telescope and with the European Space Agency OGS telescope operated on the island of Tenerife by the Instituto de Astrofísica de Canarias in the Spanish Observatorio del Teide of the Instituto de Astrofsica de Canarias.

Send offprint requests to: R.J. García López

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000