The Relationship between the Dust and Gas-Phase CO across the California Molecular Cloud

Author(s)

S. Kong, C. J. Lada, E. A. Lada, C. Román-Zúñiga, J. H. Bieging, M. Lombardi, J. Forbrich, J. F. Alves

Abstract

We present results of an extinction-CO line survey of the southeastern part of the California molecular cloud (CMC). Deep, wide-field, near-infrared images were used to construct a sensitive, relatively high resolution (∼0.5 arcmin) (NICEST) extinction map of the region. The same region was also surveyed in the

¹²CO(2-1),

¹³CO(2-1), and C

¹⁸O(2-1) emission lines at the same angular resolution. These data were used to investigate the relation between the molecular gas, traced by CO emission lines, and the dust column density, traced by extinction, on spatial scales of 0.04 pc across the cloud. We found strong spatial variations in the abundances of

¹³CO and C

¹⁸O that were correlated with variations in gas temperature, consistent with temperature-dependent CO depletion/desorption on dust grains. The

¹³CO-to-C

¹⁸O abundance ratio was found to increase with decreasing extinction, suggesting selective photodissociation of C

¹⁸O by the ambient UV radiation field. The effect is particularly pronounced in the vicinity of an embedded cluster where the UV radiation appears to have penetrated deeply (i.e., ≲ 15 mag) into the cloud. We derived the cloud-averaged X-factor to be X = 2.53 × 10

²⁰ , a value somewhat higher than the Milky Way average. On sub-parsec scales we find there is no single empirical value of the

¹²CO X-factor that can characterize the molecular gas in cold (T ≲ 15 K) cloud regions, with X ∝ for 3 mag. However, in regions containing relatively hot (T 25 K) molecular gas we find a clear correlation between W(

¹²CO) and over a large (3 ≲ ≲ 25 mag) range of extinction. This results in a constant X = 1.5 × 10

²⁰ for the hot gas, a lower value than either the average for the CMC or the Milky Way. Overall we find an (inverse) correlation between X and T in the cloud with X ∝ T. This correlation suggests that the global X-factor of a giant molecular cloud may depend on the relative amounts of hot gas contained within the cloud.

Organisation(s)

Department of Astrophysics

External organisation(s)

University of Florida, Gainesville, Harvard-Smithsonian Center for Astrophysics, Universidad Nacional Autonoma de Mexico, University of Arizona, Università degli Studi di Milano-Bicocca

Journal

The Astrophysical Journal

Volume

805

No. of pages

ISSN

0004-637X

DOI

https://doi.org/10.1088/0004-637X/805/1/58

Publication date

05-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Keywords

ASJC Scopus subject areas

Astronomy and Astrophysics, Space and Planetary Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/fd909b02-9cf2-432f-96ba-081356e91a22