from prestellar cores to protostars - The University of Arizona Press

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´ 68 P. ANDRE ET AL. 4 3 is 10 cm corresponds to time t1, and the stage at which the central 5 density is 10 cm 3 corresponds to time t2. The time at which a proto- stellar object forms is effectively t6. In this model the time taken togofrom t 6 1 t o t2 ( 2 10 yr) is six times longer than the time taken to go from t2 t o t6. Some discrepancy could perhaps be accounted for by the statistical errors associated with our source number counting technique, but the ratio between the two timescales should be fairly robust. The model predicts that SCUBA should only have detected one-seventh of the cores, whereas it detected half of the sample. We are left with the conclusion that cores at central densities 5 10 cm 3 evolve more slowly than ambipolar diffusion models predict, a trend opposite tothat noted at low densities. These departures from models involving only a static magnetic field are perhaps due to turbulence, which generates nonstatic fields and modifies core support (e.g., Gammie and Ostriker 1996; Nakano 1998; Balsara et al. 1998). E. Prestellar Condensations in Star-Forming Clusters In regions of multiple star formation, submillimeter dust continuum mapping has revealed a wealth of small-scale cloud fragments, sometimes or- ganized along filaments [e.g., Mezger et al. 1992; Andre´ et al. 1993; Casali et al. 1993; Launhardt et al. 1996; Chini et al. 1997 b; Johnstone and Bally 1999]. Such fragmentation along filaments has not been observed in Taurus, but examples do exist in young embedded clusters forming primarily low-mass stars like Ophiuchi (Motte et al. 1998). The individual fragments, which are denser ( 10 –10 cm n 6 7 3) and more compact (a few thousand AU in size) than the isolated prestellar cores discussed above, of- ten remained totally undetected (in emission) by IRAS or ISO in the midtofar-IR. Since molecules tend tofreeze out ontodust grains at low temperatures and high densities, (sub)millimeter dust emission may be the most effective tracer of such condensations (e.g., Mauersberger et al. 1992). The most centrally condensed of these starless fragments have been claimed tobe isothermal protostars: collapsing condensations with nocen- tral hydrostatic object (see section I) (e.g., Mezger et al. 1992; Launhardt et al. 1996; Motte et al. 1998). Good examples are FIR 3 and FIR 4 in NGC 2024, OphA-SM1 and OphE-MM3 in Oph, LBS 17-SM in Orion B, and MMS1 and MMS4 in OMC-3. This isothermal protostar interpretation remains to be confirmed, however, by observations of appro- priate spectral line signatures (cf. the chapter by Myers et al. this volume). Furthermore, the advent of large-format bolometer arrays now makes possible systematic studies of the genetic link between prestellar cloud fragments and young stars. Color Plate 3 is a 1.3-mm continuum widefield mosaic of the Oph cloud (Motte et al. 1998), showing a total of 100 structures with characteristic angular scales of 15 –30 (i.e., 2000–4000 AU), which are associated with 59 starless condensations (undetected by ISO in the mid-IR) and 41 embedded young stellar objects (YSOs) (detected at IR or radio continuum wavelengths).
FROM PRESTELLAR CORES TO PROTOSTARS 69 Comparison of the masses derived from the 1.3 mm continuum (from 0. 05 M to3 M ) with Jeans masses suggests that most of the 59 starless fragments are close togravitational virial equilibrium, with MM / vir 0. 3–0.5, and will form stars in the near future. These prestellar condensations generally have flat inner density profiles, like isolated prestellar cores, but are distinguished by compact, finite sizes of a few thousand AU. The typical fragmentation lengthscale derived from the average pro- jected separation between condensations is 6000 AU in Oph. This is 5 times smaller than the radial extent of isolated dense cores in the Taurus cloud (see Go´ mez et al. 1993). Figure 5 shows the mass distribution of the 59 Oph prestellar fragments. It follows approximately N/ M M 15 . below 0. 5 M , which is similar tothe clump mass spectrum found by large-scale molecular line studies (e.g., Blitz 1993 and the chapter by Williams et al., this volume). The novel feature, however, is that the fragment mass spectrum found at 1.3 mm in Oph appears tosteepen t oN/ M M 25 . above 0. 5 M . A similarly steep mass spectrum above 0. 5 M was obtained by Testi and Sargent (1998) for compact 3-mm starless condensations in the Serpens core. These prestellar mass spectra resemble the shape of the stellar initial mass function (IMF), which is known to approach N/ M 27 M . for 1M M 12 10M and N/ M M . for01M . M 1M (e.g., Kroupa et al. 1993; Tinney 1993, 1995; Scalo 1998; see also the chapter by Meyer et al., this volume). Given the factor of 2 uncertainty on the measured prestellar masses, such a resemblance is remarkable and Figure 5. Mass spectrum of the 59 prestellar fragments extracted from the Oph 1.3 mm continuum mosaic shown in Color Plate 3 (from Motte et al. 1998). For comparison, dotted and long-dashed lines show power laws of the form 15 . 25 . N/ M M and N/ M M , respectively. This prestellar mass spectrum is remarkable in that it resembles the shape of the IMF.
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