New weak-line T Tauri stars in Orion from the ROSAT all-sky survey ^*

¹ Max-Planck-Institut für Extraterrestrische Physik, D-85740 Garching, Germany
² Instituto Nacional de Astrofísica Optica y Electrónica, A.P. 51 y 216 C.P. 72000, Puebla, México
³ Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy
⁴ Landessternwarte-Königstuhl, D-69117 Heidelberg, Germany
⁵ Max-Planck-Institut für Astronomie Königstuhl, D-69117 Heidelberg, Germany
⁶ Instituto de Astronomía, Ensenada B.C., México CP 22860
⁷ Lowell Observatory, 1400 West Mars Hill Road. Flagstaff, Az 86001, U.S.A.

Send offprint request to: J.M. Alcalá

Received: 20 September 1995
Accepted: 26 January 1996

Abstract

We present results of the spectroscopic and photometric follow-up observations of the ROSAT all-sky survey in the direction of the Orion cloud complex. The main goal of these observations is the search for X-ray emitting pre-main sequence stars. 820 X-ray sources were detected with high confidence in about 450 square degrees. The mean density of X-ray sources in this region is a factor of about two higher than that of the whole RASS. 5% of the RASS sources in this region are identified with previously known and likely pre-main sequence stars. We have investigated spectroscopically 181 new RASS sources widely distributed over the entire cloud complex. On the basis of the presence of strong Li I λ6707 absorption, spectral type later than F0 and chromospheric emission, 112 new weak-line T Tauri stars could be found. We present coordinates, X-ray count-rates and finding charts of the new PMS. Optical _KC, near-infrared JHKLM and uvby-β photometry for the new WTTS is also provided. In addition 24 dKe-dMe stars were also found on the basis of the RASS data.