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Astron. Astrophys. 364, 205-216 (2000)


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Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images * **

IV. The northern hemisphere sample

G. Cutispoto 1, L. Pastori 2, A. Guerrero 2, G. Tagliaferri 2, S. Messina 1, M. Rodonò 1,3 and J.R. de Medeiros 4

1 Catania Astrophysical Observatory, via S. Sofia 78, 95125 Catania, Italy
2 Brera Astronomical Observatory, Via Bianchi 46, 22055 Merate, Italy
3 Catania University, Department of Physics and Astronomy, via S. Sofia 78, 95125 Catania, Italy
4 University Federal of Rio Grande do Norte, Department of Physics, 59072-970 Natal, R.N., Brazil

Received 20 July 2000 / Accepted 2 October 2000

Abstract

We present high-precision photometry, high- and medium-resolution spectroscopy for a sample of 32 stars likely to be the optical counterparts of X-ray sources serendipitously detected by the EXOSAT satellite. Using also recent results from the Hipparcos satellite, we infer spectral types, compute X-ray luminosities and Li abundances and investigate the single or binary nature of the sample stars. We found eleven new variable stars, whose photometric periods fall in the 1.2-27.5 day range, for most of which the optical variability is consistent with the presence of photospheric cool spots. For our sample of X-ray selected stars we confirm the existence of a strong correlation between the stellar rotation rate and the level of activity, and also between the X-ray and bolometric luminosities. Two stars in our sample are likely to be pre-main sequence objects, one is likely to be a previously unknown M-type star within 25 pc.

Key words: techniques: photometric – techniques: spectroscopic – stars: abundances – stars: late-type – stars: variables: general – X-rays: stars

* based on data collected at Catania Astrophysical Observatory (Mt. Etna, Italy), at Observatorio Astronomico Nacional (UNAM, S. Pedro Martir, BC, Mexico) and at Kitt Peak National Observatory (Kitt Peak, AZ, USA)
** Table 1 is available here in the on-line version only; Table 3 and the complete data set are available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/264/205 or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/364/205

Send offprint requests to: G. Cutispoto (gcutispoto@alpha4.ct.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000

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