DUST EMISSION FROM STAR-FORMING REGIONS .3. THE RHO OPHIUCHUS CLOUD - WHERE ARE THE LOW-MASS PROTOSTARS

We have surveyed the dust emission of the densest part of the Rho Oph cloud as outlined by (CO)-O-18 and DCO+ line emission (Fig. 1a,b) and have found three structureless cores of densities n(H) approximately 4 10(5) - 10(6) cm-3 and masses M(H) approximately 15, 3 and 1m. (Rho Oph B1, B2 and F1; Figs. 3, 4 and 5) and another core of approximately 15m. which contains four high-density (approximately 10(8)cm-3) condensations with masses ranging from approximately 0.3-3m., (SM1; Fig. 2a-c). These could be the low-mass counterparts of the more massive isothermal protostars detected previously in the cloud core associated with NGC2024 (Mezger et al. 1988, 1992). One of the condensations (FIR 4) is associated with a highly collimated gas flow and an ultra-compact HII region (Andre et al. 1990a,b). We have also observed IRAS16293-2422 (Figs. 6a,b and 7) which appears to represent a somewhat more evolved pre-stellar object with inner disk and outer circular shell rather clearly separated. This source, too is associated with gas outflow and compact HII regions. Spectroscopic gas masses of this object - obtained with standard molecular abundances - are by factors of ten lower than masses derived from dust emission. In the Rho Oph cloud Wilking et al. (1989) have identified 78 NIR/MIR sources as young stellar object (YSO) members of a cluster of low-mass stars, half of which appear to be dust embedded stars and the other half T-Tauri stars. If star formation continued at the same rate for approximately 10(6) yr, and assuming that our survey detected all isothermal protostars, their lifetime would be about one tenth of the lifetime of T-Tauri stars (which is approximately 4 10(5) yr according to Wilking et al.). This lifetime of 4-10(4)yr is comparable to the free-fall time of a fragment with an initial volume density of n(H) approximately 3 10(5)cm-3. The following picture of low-to-medium mass star formation emerges: Structureless static cores like B1, B2 and F1 with densities n(H) approximately 4 10(5) - 10(6)cm-3, which could be stabilized against collapse by magnetic fields, form the first evolutionary stage. When collapse occurs, about one third of the core mass goes into isothermal condensations which contract on free-fall time scales of approximately 4 10(4) yr and become outflow sources before a central stellar core has formed. The short lifetime explains, why isothermal protostars are such rare objects.