From T Tauri stars to protostars: Circumstellar material and young stellar objects in the rho Ophiuchi cloud

Abstract

We present the results of a 1.3 mm continuum survey for cold circumstellar dust, conducted with the IRAM 30 m telescope on a sample of over 100 young stellar objects (YSOs) in or near the rho Ophiuchi molecular cloud. To correlate the millimeter results with other source properties, we have used the IR classification of Wilking, Lada, & Young, but revising it critically to take into account factors such as heavy extinction. We find a sharp threshold in millimeter flux density at an infrared spectral index alphaIR(2.2-10 microns) is approximately - 1.5, which is also visible in the IRAM 30 m survey of Taurus-Auriga T Tauri stars by Beckwith and coworkers. We show that this threshold is well correlated with a disk opacity transition at wavelength is approxmately 10 microns, and can be used to set a physical boundary between Class III and Class II IR sources. At a detection sensitivity of approximately 20-30 mJy/beam (3 sigma at 1.3 mm, less than 15% of the Class III IR sources, but as much as 60% of the Class II sources and 70%-90% of the Class I sources, are detected. Statistical studies show that the peak 1.3 mm fluxes of deeply embedded Class I sources, currently referred to a 'protostars,' and of 'classical' T Tauri stars (Class II sources) are comparable within a factor of 2 at the angular resolution of the telescope (12 sec Full Width at half Maximum (FWHM) or a linear diameter approximately 2000 AU). Maps of the millimeter emission are consistent with the presence of unresolved disks around Class II sources and of resolved, extended envelopes around Class I sources. Therefore, the difference between Class I and Class II YSOs lies mainly in the spatial distribution of their circumstellar dust. Converting the integrated millimeter fluxes derived from our maps into masses, we find that (1) approximately 30% of the Class II sources have masses larger than the 'minimum-mass solar nebula' (approximately 0.01 solar mass); (2) the envelopes of Class I sources contain more circumstellar material than Class II disks, consistent with Class I sources being younger than Class II sources; but (3) their total circumstellar masses are not large (is less than or approximately 0.1 solar mass). This suggests that the central object has already accumulated most of its final stellar mass at the Class I stage. In contrast, a very strong 1.3 mm emission is found toward two deeply embedded outflow sources (IRAS 16293 and VLA 1623) which remain undetected shortward of 25 microns. These latter sources belong to a new class of YSOs (Class O) introduced by Andre, Ward-Thompson, & Barsony, which are surrounded by significantly larger amounts of circumstellar material (approximately 0.5 solar mass or more), still to be accreted by central protostellar core. Class O YSOs appear to be significantly younger, and therefore at an earlier protostar stage, than Class I sources.