The Arizona Radio Observatory CO Mapping Survey of Galactic Molecular Clouds. VI. The Cep OB3 Cloud (Cepheus B and C) in CO J = 2–1, ¹³CO J = 2–1, and CO J = 3–2

Abstract

We present (1) new fully sampled maps of CO and CO 13 J=2-1 emission and CO J=3-2 emission toward the molecular clouds Cep B and C, associated with the Cep OB3 association; (2) a map of extinction, A V , derived from IR colors of background stars; and (3) the distribution of young stellar objects (YSOs) over the same field as the molecular maps. An LTE analysis of the CO and CO 13 maps yields the distribution of molecular column densities and temperatures. Substantial variations are evident across the clouds; smaller subregions show correlations between molecular properties and dust extinction, consistent with a picture of outer photodissociation regions with a layer of CO-dark molecular gas, a CO self-shielded interior, and an inner cold dense region where CO is largely depleted onto grains. Comparing the distribution of YSOs with molecular gas surface density shows a power-law relation very similar in slope to that for the giant molecular cloud associated with the H II region Sh2-235 from a previous paper in this series that employed the same methodology. We note the presence of several compact, isolated CO emission sources in the J=3-2 maps. The gas temperature and CO 13 velocity dispersion yield a map of the sonic Mach number, which varies across the cloud but always exceeds unity, confirming the pervasiveness of supersonic turbulence over length scales 0.1 pc (the map resolution). We also compute a J=2-1 CO X-factor that varies with position but is, on average, within20% of the Galactic average derived from CO J=1-0 observations.

The Star Formation–Gas Density Relation in Four Galactic GMCs: Effects of Stellar Feedback

“…In our previous studies of Sh2-235 (Bieging et al 2016) and Cep B/C (Bieging et al 2018) GMCs, we also observed the J = 3-2 transition of CO with resolution comparable to the J = 2-1 maps over the same regions. In both cases, we found that the peak CO brightness temperatures in the two lines followed very closely the expected relation for an LTE population of the rotational levels, with the J = 3 level slightly subthermally excited, at ∼90% of the kinetic temperature.…”

“…The present study is a continuation of a series of papers reporting our mapping observations of CO isotopologues in the J = 2-1 and J = 3-2 rotational transitions toward a selection of Galactic molecular clouds. Regions previously reported include the Sh2-254-258 group of H II regions (Bieging et al 2009); W51 (Bieging et al 2010); W3 (Bieging & Peters 2011); Serpens Main (Burleigh et al 2013); NGC 1333 (Bieging et al 2014); Sh2-235 (Bieging et al 2016); and Cep B and C, adjacent to the Cep OB3 association (Bieging et al 2018). The last two studies compared maps of the molecular gas column density derived from CO observations, with the distribution of young stellar objects (YSOs); in both cases, we found that the surface density of YSOs showed a power-law dependence on the molecular gas column density.…”

“…The molecular clouds associated with Cep OB3 also showed a power-law dependence of YSO surface density on molecular gas column density (Bieging et al 2018). The number of YSOs was smaller than for Sh2-235, however, so the statistical errors were somewhat larger for Cep OB3.…”

Bird’s eye view of molecular clouds in the Milky Way

“…For most clouds in our sample, we used extinction maps derived using the NICEST near-infrared dust extinction mapping technique (Lombardi 2009) and data from 2MASS (Skrutskie et al 2006). For most nearby clouds within ∼0.7 kpc distance, these maps are available from our earlier works (Kainulainen et al 2009(Kainulainen et al , 2014Kainulainen & Federrath 2017;Bieging et al 2018). New maps were adopted for clouds for which those studies provide no data, either by applying NICEST or by cropping a map from Juvela & Montillaud (2016) (wich also uses NICEST).…”

Bird's eye view of molecular clouds in the Milky Way: I. Column density and star formation from sub-pc to kpc scales

“…For most clouds in our sample, we used extinction maps derived using the NICEST near-infrared dust extinction mapping technique (Lombardi 2009) and data from 2MASS (Skrutskie et al 2006). For most nearby clouds within ∼0.7 kpc distance, these maps are available from our earlier works (Kainulainen et al 2009(Kainulainen et al , 2014Kainulainen & Federrath 2017;Bieging et al 2018). New maps were adopted for clouds for which those studies provide no data, either by applying NICEST or by cropping a Juvela & Montillaud (2016) used a background colour determination that is well suited for automated full-sky work and no dedicated foreground star removal process; in case of some individual clouds, we considered that adjustments in these result in more accurate maps, and we therefore used our own implemen-tation of NICEST for these clouds.…”