Large-Scale SCUBA Maps of Rho Ophiuchi

G. Moriarty-Schieven (NRC/JAC), D. Johnstone (UToronto), C. Wilson (McMasterU), G. Joncas (ULaval), G. Smith (UToronto), E. Gregersen (McMasterU), & M. Fich (UWaterloo)

The process by which stars form out of a molecular cloud is still uncertain. Isolated collapse has been well studied (see Shu, Adams & Lizano (1987, ARA&A, 25, 23 for a review) but has not led to a definitive theory for the distribution of stellar masses. One suggested mechanism invokes energetic feedback from the forming protostar to provide a range of possible stellar masses during the collapse of the clump (Adams & Fatuzzo 1996, ApJ, 464, 256). Under this scenario, the initial physical attributes of the clump do not uniquely determine the stellar mass and there is no requirement that the clump mass distribution have any relationship with the Stellar IMF. A competing mechanism for producing stars relies on fragmentation during the collapse of a large molecular core, which produces a range of coump masses (Myers 1998, ApJ, 507, L157; Klessen et al 1998, ApJ, 501, L205). These clumps eventually form into stars and therefore should have a mass distribution similar to the Stellar IMF. Another method for producing a range of steellar masses, suggested by Bonnell et al (1997, MNRAS, 285, 201), allows the forming stars to accrete material from the molecular cloud, with a range in stellar masses produced by competitive accretion. Rigorous determination of the properties of clumps in molecular clouds provides an important constraint on the validity of each of these mechanisms for producing the stellar IMF. The quality of the data now obtained using submm instruments, coupled with the efficiency with which cold dust in molecular clouds radiates in the submm, allows for unprecedented analysis of the small scale, clumped structure with combined dust and gas masses down the M<0.01 Mo. With such precision, the process of star formation has become directly observable; both the mass and the size of the clumps from which stars form are directly measurable within the reconstructed images.

We have recently completed a survey of the central 700 square arcmin region of the Rho Ophiuchi molecular cloud at 850um using SCUBA. Figure 1 shows the mapped region. Using the Williams clump-finding algorithm, we have identified 55 independent objects and computed the size, flux, and degree of central condensation. Comparison of these clumps with isothermal, pressure-confined, self-gravitating Bonner-Ebert spheres implies that the clumps have internal temperatures of 10-30K and surface pressures (P/k) between 10⁶ and 10⁷, consistent with the expected average pressure in the Rho Ophiuchi central region, log(P/k)~7.3. The clump masses span 0.02-6.3 Mo assuming a dust temperature Td~20K. The distribution of clump masses (Figure 2) is well characterized by a broken power-law N(M) ~ M^-alpha with alpha=1-1.5 for M less than 0.6 Mo, and alpha=0.5 for M less than about 0.6 Mo, although significant incompleteness may affect the slope at the lower mass end. This mass function is in general agreement with the Rho Ophiuchi clump mass function derived at 1.3mm by Motte et al. (1998, A&A, 336, 150), and is similar to the stellar IMF (Salpeter, 1955, ApJ, 121, 161)

This work has been submitted to the Astrophysical Journal (Johnstone et al. 2000b).