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A SCUBA survey of the NGC 2068/2071 protoclusters
We have used SCUBA on JCMT to conduct a submillimeter dust continuum survey of the protoclusters NGC 2068 and NGC 2071 in Orion B (Motte, André, Ward-Thompson, & Bontemps 2001, A&A, 372, L41). The region mapped at both 850 um and 450 um is ~32'x18' in size (~4x2pc) and consists of filamentary dense cores which break up into small-scale (~5000AU) fragments, including 70 starless condensations and 5 circumstellar envelopes/disks (see Fig. 1). The starless condensations appear to be virialized and pre-stellar in nature. Their mass spectrum, ranging from ~0.3Mo to ~5Mo, is reminiscent of the stellar initial mass function (IMF). In agreement with earlier studies by Motte, André, & Neri (1998), Testi & Sargent (1998), and Johnstone et al. (2000), this result strongly suggests that pre-collapse cloud fragmentation plays a major role in shaping the IMF (see also review by Motte & André 2001, in From Darkness to Light, ASP Conf. Ser., vol. 243, p. 301, astro-ph/0102376). The pre-stellar condensations of NGC 2068/71, as well as those identified at 1.3 mm by Motte et al. (1998) in the rho Oph protocluster, follow a M ~ R mass-size relation which is consistent with that of self-gravitating isothermal structures (such as critical Bonnor-Ebert spheres, cf. Fig. 2). Note that this differs from the M ~ R2 mass-size relation of the clumps found by Johnstone et al. (2000) in a related 850 um SCUBA survey of rho Oph. The latter is similar to the mass-size relation of CO clumps and is consistent with the fractal, turbulent nature of molecular clouds. The difference arises from the use of different clump-finding algorithms. While Johnstone et al. used the Clumpfind algorithm (Williams et al. 1994) which does not discriminate between spatial scales, we employed a multi-resolution wavelet analysis to select only gravitationally-bound starless fragments seen on the same scales as protostellar envelopes (diameter < 10000 AU in NGC 2068/71). We believe that our method is more appropriate to identify the direct progenitors of stars, i.e., the structures within which individual protostellar collapse is initiated.
Figure 1: Dust continuum mosaic of the NGC 2068 and NGC 2071 protoclusters at 850um. Starless condensations are denoted by crosses and young embedded stars by star markers.
Figure 2: Mass-size relation of the starless submm condensations identified by Motte et al. (1998) and Motte et al. (2001) in the rho Oph and NGC 2068/2071 protoclusters (open and filled squares, respectively). The thick solid line is the mass-size relation expected for critical Bonnor-Ebert spheres (T = 15K and Ps ~ 105-107 kB cm-3 K). The two dashed curves show the 5sigma detection threshold as a function of size in the two protoclusters.
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