THE CONTRIBUTION OF DISKS AND ENVELOPES TO THE MILLIMETER CONTINUUM EMISSION FROM VERY YOUNG LOW-MASS STARS

We investigate the question of disk formation during the protostar pha se. We build on the results of Keene & Masson (1990) whose analysis of L1551 IRS 5 showed the millimeter continuum emission comes from both an unresolved circumstellar component, i.e., disk, and a more extended envelope/dense core. We model the dust continuum emission from the de nse cloud core using the cloud-collapse models of Terebey, Shu, & Cass en (1984) and show that dust emission from the dense core is important when measured with large single-dish telescopes at 1.3 mm, but nearly negligible with interferometers at 2.7 mm. Combining new 2.7 mm Owens Valley Interferometer data of IRAS-Dense cores with data from the lit erature, we conclude that massive disks are also seen toward a number of other sources including L1448 IRS 3 (IRAS 03225+3034), whose disk m ass is estimated to be 0.5 M., (assuming T(D) = 40 K, beta = 1.5, and optically thin emission). However 1.3 mm data from the IRAM 30 m teles cope for a larger sample shows that massive disks are relatively rare, occurring around perhaps 5% of young embedded stars. This implies tha t either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. The median 1.3 mm flux density of IRAS-Dense cores in our sample is nearly the same as T Tau ri stars in the sample of Beckwith et al. (1990). We conclude that the typical disk mass is not significantly higher during the embedded pha se than during the later T Tauri phase.