Mathematics – Probability
Scientific paper
Dec 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........2h&link_type=abstract
PhD Thesis, Vanderbilt University, Nashville, TN, USA, 2000.
Mathematics
Probability
1
Scientific paper
We have mapped the submillimeter emission from protostars that are forming within small molecular clouds known as Bok globules. From these maps, which reveal the spatial distribution of thermal dust emission around the protostars, we have partitioned the total observed flux into two components: flux from protostellar dust and flux from dust associated with the molecular cloud. Assuming that both the protostar and cloud emit as greybodies with dust opacities given by κν propto νβ, we have constrained the values of the ``emissivity index'' β using our separate protostellar and cloud fluxes supplemented with previous flux observations at other wavelengths. Our analysis of the protostar B 335 SMM1, for which we have the best data, demonstrates that the likely range for the protostellar dust emissivity index is 0.4 <~βp <~1.0, while the likely range for the cloud dust emissivity index is 1.0 <~βc <~1.8. A similar analysis for two other protostars, CB 68 SMM1 and CB 230 SMM1, produce results consistent with those for B 335 SMM1. We then show that the protostellar dust emissivities are less than and inconsistent with emissivities predicted by standard models for interstellar dust grains that include grains only as large as ~1 micron. The addition of larger grains is known to yield smaller emissivities. For this reason, we have constructed a protostellar dust model with grains composed of a presumed realistic mixture of constituents: amorphous carbon, silicates, and H2O-ice. We demonstrate that, for such a protostellar dust population, a distribution of grain sizes following a power law in grain radius proportional to r-(3.6 +/- 0.3) is most consistent with the protostellar dust emissivities determined in this study. Furthermore, we show that this protostellar dust population must include grains at least as large as ~200 microns, more than two orders of magnitude larger than standard interstellar grains. Since protostars form by collapse of fragments of molecular clouds while molecular clouds, in turn, form from material within the diffuse interstellar medium, the observed protostellar dust must once have been dust associated with molecular clouds. Furthermore, this cloud dust was originally interstellar dust. Since the typical density of these environments increases from the diffuse interstellar medium to molecular clouds to protostars, dust grain growth may be facilitated by increased probability of coagulation of the smaller grains. In this scenario, we might expect to see a progression in the typical grain sizes, from submicron-sized interstellar grains to intermediate-sized cloud grains to perhaps millimeter-sized protostellar grains. These millimeter-sized protostellar grains would represent the seeds from which planetesimals and eventually planets might grow in the disks around protostars. Our analysis does not detect any difference in the sizes of interstellar and cloud dust grains, but this result may be due to a lack of sensitivity on the part of our method to distinguish these intermediate grains from the small interstellar grains. However, the results of our analysis for the protostars B 335 SMM1, CB 68 SMM1, and CB 230 SMM1, as described above, indicate that significant grain growth has occurred from interstellar grains to the dust grains surrounding these protostars. Given the abundance of extrasolar planets that have been detected recently, the findings of this study suggest that extensive dust grain growth likely occurs around protostars, in general. http://www.hep.vanderbilt.edu/~huard/index.html
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