Accurate Density Measurements of Molecular Cores Using K-Doublet Formaldehyde Transitions

Details Accurate Density Measurements of Molecular Cores Using K-Doublet Formaldehyde Transitions Accurate Density Measurements of Molecular Cores Using K-Doublet Formaldehyde Transitions

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21541417m&link_type=abstract

American Astronomical Society, AAS Meeting #215, #414.17; Bulletin of the American Astronomical Society, Vol. 42, p.256

Astronomy and Astrophysics

Astronomy

Scientific paper

The densities of protostellar cores are poorly constrained. The current techniques come with substantial uncertainty, and the molecular tracers traditionally used are subject to radiative transfer effects at densities approaching and greater than 106cm-3. Formaldehyde (H2CO) is a proven tracer of the high-density environs in molecular clouds. It is ubiquitous: H2CO is associated with 80% of the HII regions surveyed by Downes et al. (1980) and because of its low relative abundance, H2CO transitions rarely become optically thick. H2CO is a slightly asymmetric rotor molecule, so each rotational energy level is split into two, leading to two basic types of transitions: the higher frequency, "P-branch” transitions (ΔJ=1, ΔK-1=0, ΔK+1=1) and the lower frequency, "Q-branch” or "K-doublet” transitions (ΔJ=0, ΔK-1=0, ΔK+1=+1), which are uniquely sensitive to spatial density. For n(H2)<105cm-3 the lower energy states (J<5) become overpopulated due to a collisional selection effect. This cools them to excitation temperatures lower than that of the CMB, causing the K-doublet transitions to appear in absorption. For n(H2)>105.5cm-3, the collisional pump is quenched and the lines are seen in emission. An LVG model prediction of the ratio of two of these transitions can further constrain the density with a high degree of accuracy.
We present GBT observations of the J=3 and J=4 K-doublets toward 22 galactic, star-forming sites. Adopting kinetic temperatures from the literature, we have used an LVG model to derive the average density within a 16” beam toward each source. Spatial densities between 1-2 x106cm-3 and column densities between 1-9 x1013cm-2/km s-1 are found for most of the sources. The statistical uncertainty from our measurement uncertainty alone is 10%. The ease with which these transitions are detected along with their unique sensitivity to spatial density make them excellent monitors of density in molecular cores for this and future experiments.

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