AN INFRARED SPECTROSCOPIC SURVEY OF THE RHO-OPHIUCHI YOUNG STELLAR CLUSTER - MASSES AND AGES FROM THE H-R DIAGRAM

We have conducted near-IR spectroscopic observations of young stellar objects (YSOs) in the rho Ophiuchi dark cloud (L1688), many of which a re optically invisible. We present equivalent widths of K-band spectra l features for 34 YSOs, and we determine their spectral types, stellar luminosities, and extinctions. A subset of 19 YSOs with small near-IR excesses are placed in the H-R diagram, and we estimate their masses and ages via comparisons to pre-main-sequence (PMS) stellar models. Th ese objects are apparently very young, low-mass, PMS stars. The median stellar age is about 3 x 10(5) yr according to modern PMS models. Mos t stars in the cloud core are certainly less than 3 x 10(6) yr old and probably less than 10(6) yr old. The sample spans a mass range 0.1-2. 5 M., with a median mass of 0.4-0.5 M.. Combining these results with p revious photometric studies, we find the distribution of masses in the rho Oph cloud core to be consistent with the initial mass function of field stars in the solar neighborhood. In addition, we find that this IR population is significantly younger and more coeval than the optic ally visible weak-line T Tauri stars found in a much larger region enc ompassing the core. This study quantitatively confirms previous result s that suggested that a very young, low-mass cluster is forming in the rho Oph cloud core. The technique of IR spectroscopy has proven valua ble in compiling a detailed picture of star formation in this deeply e mbedded cluster of young stars.