Molecular Tracers of Embedded Star Formation in Ophiuchus

Abstract

ABSTRACTIn this paper we analyze nine SCUBA cores in Ophiuchus using the second-lowest rotational transitions of four molecular species (12CO, 13CO, C18O, and C17O) to search for clues to the evolutionary state and star-formation activity within each core. Specifically, we look for evidence of outflows, infall, and CO depletion. The line wings in the CO spectra are used to detect outflows, spectral asymmetries in 13CO are used to determine infall characteristics, and a comparison of the dust emission (from SCUBA observations) and gas emission (from C18O) is used to determine the fractional CO freezeout. Through comparison with Spitzer observations of protostellar sources in Ophiuchus, we discuss the usefulness of CO and its isotopologues as the sole indicators of the evolutionary state of each core. This study is an important pilot project for the JCMT Legacy Survey of the Gould Belt (GBS) and the Galactic Plane (JPS), which intend to complement the SCUBA-2 dust continuum observations with HARP observations of 12CO, 13CO, C18O, and C17O J = 3 → 2 in order to determine whether or not the cold dust clumps detected by SCUBA-2 are protostellar or starless objects. Our classification of the evolutionary state of the cores (based on molecular line maps and SCUBA observations) is in agreement with the Spitzer designation for six or seven of the nine SCUBA cores. However, several important caveats must be noted in the interpretation of these results. First, while these tracers may work well in isolated cores, care must be taken in blindly applying these metrics to crowded regions. Maps of larger areas at higher resolution are required to determine whether the detected outflows originate from the core of interest, or from an adjacent core with an embedded YSO. Second, the infall parameter may not be an accurate tracer of star-formation activity because global motions of the cloud may act to emulate what appears to be the collapse of a single core. Large mapping surveys like the GBS may be able to overcome some of this confusion and disentangle one outflow from another by mapping the full extent of the outflows and allowing us to find the originating object. As well, the higher-energy CO J = 3 → 2 transition used by the GBS has a higher critical density and so will trace the warm dense gas in the outflow rather than the lower density surrounding cloud material. The higher resolution of the GBS observations at 345 GHz (θFWHM ≈ 14″ vs. our 22″) may also provide a clearer picture of activity in crowded fields.