The Evolution of the Circumstellar Environment of Embedded Young Stars from Observations of Rare Species of Carbon Monoxide

Abstract

Studies of the properties of the circumstellar material around young embedded sources can provide important insights into the relative importance of infall and outflow in clearing the circumstellar environment. In this paper we report one such study of 20 of the most deeply embedded young stars in the Taurus molecular cloud using the J=1-->0, J=2-->1, and J=3-->2 transitions of C18O and C17O. The profiles comprise a narrow component, with a line width of ~0.7 km s-1 plus in many cases a second broader component that contains a similar mass of material. The broad-component line width decreases with increasing source age as measured by the source bolometric temperature, consistent with what would be expected if the broad component traces the interaction between a stellar wind and ambient material. The momentum flux of the broad-component emission in the immediate vicinity of the forming star is comparable to that in the spatially extended outflows from these sources and also decreases with increasing source age. Comparison with models of this interaction provide a reasonable match to the behavior of the broad component with source age. The derived total column density toward the sources shows a systematic decrease with increasing bolometric temperature and implies a circumstellar mass-loss rate that varies from ~10-4 Msolar yr-1 for the youngest sources to ~10-8 Msolar yr-1 for sources in the late Class I phase. The broad components are sufficiently massive and energetic to clear the circumstellar environment on timescales of ~105 yr, and we argue that the broad component of the C18O line profiles is the dense, low-velocity inner part of the outflow from the sources and may be the material being directly swept up by the stellar wind. The widths of the broad and narrow components are correlated, indicating that the narrow-component material is already being stirred by the stellar wind, even in the youngest sources.