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

摘要

We have conducted near-IR spectroscopic observations of young stellar objects (YSOs) in the ρ Ophiuchi dark cloud (LI 688), many of which are optically invisible. We present equivalent widths of K-band spectral 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. These 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. Most 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 solar mass, with a median mass of 0.4-0.5 solar mass. Combining these results with previous photometric studies, we find the distribution of masses in the ρ 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 optically visible weak-line T Tauri stars found in a much larger region encompassing the core. This study quantitatively confirms previous results that suggested that a very young, low-mass cluster is forming in the ρ Oph cloud core. The technique of IR spectroscopy has proven valuable in compiling a detailed picture of star formation in this deeply embedded cluster of young stars.