Infrared studies of star formation in the rho Ophiuchi dark cloud.

Abstract

We present a near-infrared study of the stars forming in the ρ Ophiuchi dark cloud and a mid- to far-infrared study of their environment. We determine that the total cloud luminosity matches that of the known embedded sources, ruling out the existence of a numerous but faint low mass embedded population. IRAS and optically thin C¹⁸O column density data are used to evaluate dust grain sizes and compositions via competing grain models. Radiative modleing shows that a standard power law distribution of graphite and silicate grains is responsible for IRAS 60 and 100 μm band emissions. These grains are heated to about one tenth of the cloud's depth in the core region. Their optical depths closely follow molecular column density structure, but these grains are considerably colder than the molecular gas. We detect 481 sources in the J,H, or K bands in a 0.184 deg.² survey region in the cloud. Approximately 79% of the embedded 3 band (JHK) detected sources have near-infrared color indices greater than the local background population, suggesting that many of these objects are in pre-main-sequence evolutionary phases. The reddest of these sources are grouped in a high column and spatial density area within the survey region. Sources in this area have a normal power-law K luminosity function which is consistent with a theoretical model of a standard mass function and an age of 10⁵ yr. Sources exterior to this area have a luminosity function with an excess of intermediate luminosity sources that significantly differs from the luminosity function of the interior region. We interpret this non-standard luminosity function and the wide range of source reddenings in this peripheral region to be indicators of a considerable age (10⁶ yr) or age spread among sources there. We estimate that the cloud's star formation efficiency is currently greater than or equal to 25%. These newly discovered young stellar sources provide a statistically significant sample for studies of the cloud's embedded population and support established ideas of bound cluster formation and star formation bursts within the cloud.