Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud

R. Neuhäuser¹ - F.M. Walter² - E. Covino³ - J.M. Alcalá³ - S.J. Wolk⁴ - S. Frink⁵ - P. Guillout⁶ - M.F. Sterzik⁷ - F. Comerón⁸

1 - Max-Planck-Institut für extraterrestrische Physik, D-85740 Garching, Germany
e-mail: rne@mpe.mpg.de
2 - Department of Physics and Astronomy, SUNY, Stony Brook, NY 11794-3800, U.S.A.
3 - Osservatorio Astronomico di Capodimonte, I-80131 Napoli, Italy
4 - Harvard-Smithonian Center for Astrophysics, Cambridge, MA 02138, U.S.A.
5 - University of California San Diego, La Jolla, CA 92093, U.S.A.
6 - Observatoire Astronomique, CNRS UMR 7550, F-67000 Strasbourg, France
7 - European Southern Observatory, Casilla 19001, Santiago 19, Chile
8 - European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching, Germany

Abstract:

We present the ROSAT All-Sky Survey data in a 126 deg² area in and around the CrA star forming region. With low-resolution spectroscopy of unidentified ROSAT sources we could find 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be $\sim 10$ Myrs old, by comparing their location in the H-R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new off-cloud weak-line T Tauri stars may have formed in similar cloudlets, which have dispersed recently. High-resolution spectra of our new T Tauri stars show that they have significantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion - south relative to the galatic plane - of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.

Key words: stars: formation -- stars: luminosiy function, mass function -- stars: pre-main sequence -- X-rays: stars