OBSERVATIONS OF 11 PROTOSTELLAR SOURCES IN TAURUS WITH NOBEYAMA MILLIMETER ARRAY - GROWTH OF CIRCUMSTELLAR DISKS

The CS (J = 2 - 1 ) line and 98 GHz continuum observations of 11 protostellar IRAS sources have been made with high spatial resolution of 2".8-8".8 using the Nobeyama millimeter array (NMA). The observed IRAS sources are low-mass protostellar objects associated with the Taurus molecular cloud, and consist of six embedded sources and five T Tauri stars. Spatially extended CS emission is detected around all the embedded sources and the two T Tauri stars DG Tau and HL Tau, while four T Tauri stars and one embedded source L1551-IRS5 have detectable 98 GHz continuum emission from unresolved circumstellar dust disks of mass approximately 0.1 M.. Continuum detectability depends on the total mass within the NMA beam size of approximately 500 AU in radius, and the sources without detectable continuum emission have upper limits of approximately 0.03 M. to the total mass within this radius. Continuum emission tends to be detected mainly toward the T Tauri stars, while CS emission tends to be detected toward the embedded sources among the observed sources, although the number of sample is small. This tendency may suggest that the T Tauri stars have circumstellar mass distribution more concentrated as disks compared to the embedded sources. If we assume that the trend is the result of the evolution from embedded sources into T Tauri stars, compact and highly dense disks are formed through dynamical accretion in the course of the evolution. Estimation of the mass accretion rate onto such disks indicates that for embedded sources with average luminosity (approximately 3 L.) a significant amount of matter must be accreted onto disks not only in the embedded phase (approximately 1O(5) yr) but also in the very early (less-than-or-similar-to 3 X 10(5) yr) T Tauri phase. In the case of L1551-IRS5, its high luminosity (approximately 33 L.) naturally explains that the detectable disk has already been formed in the embedded phase.