Astrophys. J., Suppl. Ser., 256, 3-3 (2021/September-0)
Census of High- and Medium-mass Protostars. V. CO abundance and the galactic XCO factor.
PITTS R.L. and BARNES P.J.
Abstract (from CDS):
We present the second dust continuum data release in the Census of High- and Medium-mass Protostars (CHaMP), expanding the methodology trialed in Pitts et al. to the entire CHaMP survey area (280° < l < 300°, - 4° < b < + 2°). This release includes maps of dust temperature (Td), H2 column density (NH2_), gas-phase CO abundance, and temperature-density plots for every prestellar clump with Herschel coverage, showing no evidence of internal heating for most clumps in our sample. We show that CO abundance is a strong function of Td and can be fit with a second-order polynomial in log-space, with a typical dispersion of a factor of 2-3. The CO abundance peaks at 20.0–1.0+0.4 K with a value of 7.4–0.3+0.2×10–5 per H2; the low Td at which this maximal abundance occurs relative to laboratory results is likely due to interstellar UV bombardment in the largest survey fields. Finally, we show that, as predicted by theoretical literature and hinted at in previous studies of individual clouds, the conversion factor from integrated 12CO line intensity (I_12CO) to NH2_, the XCO factor, varies as a broken power law in I_12CO with a transition zone between 70 and 90 K km s–1. The XCO function we propose has NH2_∝I_12CO0.51 for I_12CO≲70 K km s–1 and NH2_∝I_12CO2.3 for I_12CO≳90 K km s–1. The high-I_12CO side should be generalizable with known adjustments for metallicity, but the influence of interstellar UV fields on the low-I_12CO side may be sample specific. We discuss how these results expand on previous works in the CHaMP series and help tie together observational, theoretical, and laboratory studies on CO over the past decade.
© 2021. The American Astronomical Society. All rights reserved.
Dust continuum emission - CO line emission - Interstellar medium - Milky Way disk - Protostars - Young stellar objects - Circumstellar dust - Circumstellar matter - Star formation - Star-forming regions
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