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Astron. Astrophys. 353, 186-202 (2000)


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High-resolution spectroscopy of ROSAT low-mass pre-main sequence stars in Orion *

J.M. Alcalá 1,5, E. Covino 1, G. Torres 2, M.F. Sterzik 3, M.J. Pfeiffer 4 and R. Neuhäuser 6

1 Osservatorio Astronomico di Capodimonte, 80131 Napoli, Italy
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 European Southern Observatory, Casilla 19001, Santiago 19, Chile
4 Institut für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, 81679 München, Germany
5 Instituto Nacional de Astrofísica, Optica y Electrónica, A.P. 51 y 216 C.P. 72000, Puebla, México
6 Max-Planck-Institut für Extraterrestrische Physik, 85740 Garching, Germany

Received 8 March 1999 / Accepted 18 October 1999

Abstract

High-resolution spectroscopic observations of the lithium-rich stars found on the basis of the ROSAT All-Sky Survey (RASS) in the general direction of the Orion star forming region are presented. Different properties are derived from the spectra and analyzed: i) the equivalent widths of the lithium 6708 Å absorption line have been measured and a revision of spectral types has been performed; ii) radial velocities (RV) and projected rotational velocities, [FORMULA], have been derived by application of cross-correlation techniques. A relatively large number of spectroscopic binaries and of suspected spectroscopic binary and multiple systems are found among the stars in the sample. Based on the strength of the lithium line with respect to young open cluster ZAMS stars of the same spectral type, the pre-main sequence nature is confirmed for more than 70% of the stars in the sample. The interrelation of the derived observational properties, such as kinematics, lithium abundance, age and projected rotational velocity of the stars in the sample are analyzed also in connection with the spatial location of the objects. In particular, the comparison between the kinematics of the stars and that of the gas, provided by the CO and CS molecular emission observations, reveals different degrees of clustering of the stars with respect to the cloud material and different kinematical groups can be distinguished. The sample of RASS lithium-rich stars found in the general direction of Orion appears to be a mixture of true Orion stars and, possibly, stars belonging to the Gould Belt.

Key words: stars: low-mass, brown dwarfs – stars: pre-main sequence – stars: rotation – techniques: radial velocities – techniques: spectroscopic – X-rays: stars

* Based on observations carried out at the Calar Alto observatory, the Multiple Mirror Telescope (a joint facility of the Smithsonian Institution and the University of Arizona) and the European Southern Observatory, La Silla, Chile under proposal number 60.C-0170

Send offprint requests to: J.M. Alcalá

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999

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