SPECTROSCOPIC CLASSIFICATION OF X-RAY SELECTED STARS IN THE RHO-OPHIUCHI STAR-FORMING REGION AND VICINITY

We present intermediate-resolution (FWHM similar to 1.5-2.6 Angstrom) optical spectra of 106 candidate optical counterparts of 77 X-ray sour ces detected in four pointed (t(exp) greater than or equal to 8700 s) ROSATPSPC observations of the rho Ophiuchi star-forming region and vic inity. Using the spectral types and equivalent widths of Ha and Li I l ambda 670.8 nm obtained from our spectra, we applied spectroscopic cri teria in order to classify our sample in different pre-main-sequence s ubtypes: classical' T Tauri stars (CTTS); 'weak' T Tauri stars (WTTS), and 'post' T Tauri stars (PTTS). A total of 10 CTTS, 43 WTTS and 6 PT TS were found among the PSPC-selected stars, Our results more than dou ble the number of pre-main-sequence stars spectroscopically identified in the rho Ophiuchi region. We considered regions with different mole cular cloud properties: the central core and the outer ring of the rho Ophiuchi dark cloud (L1688); the 'streamers' (L1709); the R7 clump; a nd the 'smoke rings'. In the inner field of L1688, the ratio of WTTS o ver CTTS is similar to 1:1, significantly smaller than in the other re gions (4:1 in the outer ring of L1688 and 5:1 in the smoke rings). The WTTS/CTTS ratio in the R7 field is the highest of our survey (10:1). We argue that this could be a result of the UV radiation from the near by massive binary rho Oph AB, and/or of winds from the Upper Sco OB as sociation, which might shorten the lifetime of the circumstellar discs of the low-mass stars. We find no PTTS in the inner field of our L168 8 PSPC image, and only 3 PTTS in the outer ring despite the high sensi tivity of our X-ray observations in this region. This result confirms that the central region of L1688 is extremely young (age < 5 Myr), as suggested by near-infrared surveys. The presence of a small number of PTTS scattered around the rho Ophiuchi molecular clouds suggests that star formation may have been going on for 10 to 30 Myr, very slowly at first, but at a much higher rate for the last similar to 10 Myr. We p rovide rough estimates of the star formation rate for the main rho Oph iuchi molecular cloud complex.