The "True" Column Density Distribution in Star-Forming Molecular Clouds

Creators: Goodman, Alyssa A.; Pineda, Jaime E.; Schnee, Scott L.

Abstract

We use the COMPLETE Survey's observations of the Perseus star-forming region to assess and intercompare the three methods used for measuring column density in molecular clouds: near-infrared (NIR) extinction mapping; thermal emission mapping in the far-IR; and mapping the intensity of CO isotopologues. Overall, the structures shown by all three tracers are morphologically similar, but important differences exist among the tracers. We find that the dust-based measures (NIR extinction and thermal emission) give similar, log-normal, distributions for the full (~20 pc scale) Perseus region, once careful calibration corrections are made. We also compare dust- and gas-based column density distributions for physically meaningful subregions of Perseus, and we find significant variations in the distributions for those (smaller, ~few pc scale) regions. Even though we have used ^(12)CO data to estimate excitation temperatures, and we have corrected for opacity, the ^(13)CO maps seem unable to give column distributions that consistently resemble those from dust measures. We have edited out the effects of the shell around the B-star HD 278942 from the column density distribution comparisons. In that shell's interior and in the parts where it overlaps the molecular cloud, there appears to be a dearth of 13CO, which is likely due either to ^(13)CO not yet having had time to form in this young structure and/or destruction of ^(13)CO in the molecular cloud by the HD 278942's wind and/or radiation. We conclude that the use of either dust or gas measures of column density without extreme attention to calibration (e.g., of thermal emission zero-levels) and artifacts (e.g., the shell) is more perilous than even experts might normally admit. And, the use of ^(13)CO data to trace total column density in detail, even after proper calibration, is unavoidably limited in utility due to threshold, depletion, and opacity effects. If one's main aim is to map column density (rather than temperature or kinematics), then dust extinction seems the best probe, up to a limiting extinction caused by a dearth of sufficient background sources. Linear fits among all three tracers' estimates of column density are given, allowing us to quantify the inherent uncertainties in using one tracer, in comparison with the others.

Additional Information

© 2009 The American Astronomical Society. Received 2008 June 19; accepted 2008 October 14; published 2009 February 13. This paper was originally inspired by a workshop at the Aspen Center for Physics in the Summer of 2004 on "Star Formation in Galaxies," where it seemed that the assembled audience of experts could not agree on the least biased way to measure the "initial conditions" for stars to form from molecular gas. A conversation with Eve Ostriker at that meeting, about how observations and simulations of star-forming molecular gas might best be compared, was particularly important. The quest to offer the most bias- and error-free column density distributions we could publish based on the COMPLETE data in this paper took nearly four years, and it spawned several other papers by our group (Schnee et al. 2006, 2008; Ridge et al. 2006a; Pineda et al. 2008; Foster et al. 2008) The "2007" version of these distributions and their implications were discussed intensively at the KITP Santa Barbara Workshop on "Star Formation Near and Far," and we deeply thank Eve Ostriker, Paolo Padoan and Enrique Vazquez-Semadeni for their comments both at and since that meeting. We thank Jo˜ao Alves, Michelle Borkin, Paola Caselli, Jonathan Foster, Jens Kauffmann, Di Li, Marco Lombardi, and Naomi Ridge for their important contributions to the data and results presented in this work. This material is based upon work supported by the National Science Foundation under Grant No.AST-0407172. JEP is supported by the National Science Foundation through grant #AF002 from the Association of Universities for Research in Astronomy, Inc., under NSF cooperative agreement AST-9613615 and by Fundaci´on Andes under project No. C-13442. Scott Schnee acknowledges support from the Owens Valley Radio Observatory, which is supported by the National Science Foundation through grant AST 05- 40399.

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