STAR FORMATION IN THE RHO OPHIUCHI DARK CLOUD

Abstract

New and sensitive millimeter-wave and near-infrared observation have been performed in the central regions of the ρ Ophiuchi dark cloud. High spatial resolution observations of the optically thin C¹⁸O emission lines are shown to be free of self-absorption. They permit the first accurate determination of the distribution of molecular gas and allow us to estimate the mass and visual extinction in the cloud. A completely sampled near-infrared survey of a 105 sq. arcmin area which encloses the region of highest visual extinction has revealed twenty objects (sixteen were previously unknown). We show that these objects are young stars embedded in the cloud. Synthesizing our new observations with existing radio and infrared data, we have made a detailed study of the energetics and star formation process within the ρ Oph cloud. This dissertation concludes that the high temperatures of the molecular gas cannot be due to collisions with warm dust. The feasibility of alternate heat sources such as cosmic-ray ionization, shocks, and the distortion of magnetic fields are discussed. We derive high star formation efficiencies (32-46%) in the centrally condensed core of the ρ Oph cloud which suggests that it is forming a bound open cluster. The most consistent interpretation of our data requires that an efficient burst of star formation has produced a relatively large (with respect to the initial mass function) population of low luminosity stars in ρ Oph within the last 3 million years. At this present rate of star formation, most of the molecular gas will be converted into stars in about 10⁷ yr. yielding a bound cluster. Unless this rate rapidly decreases with time, the duration of star formation in the (rho) Oph cluster (10 million years) will be considerably shorter than that suggested for the Pleiades cluster (175 m. y). The lack of stars in the 3-10 M(⊙) range indicates that subsequent star formation must be dominated by more massive stars if the stellar mass spectrum of the ρ Oph cluster is to resemble that of the conventional initial mass function. The youth of the ρ Oph cluster suggests that it may be the most recent episode of star formation in the Sco-Cen OB association.