CO Isotopologues in the Perseus Molecular Cloud Complex: the X-factor and Regional Variations

By J. Pineda, P. Caselli, A. Goodman. 2008

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We use data gathered by the COMPLETE survey of star-forming regions to find new calibrations of the “X-factor” and 13CO abundance within the Perseus molecular cloud. We divide Perseus into six subregions, using groupings in a dust temperature vs. LSR velocity plot. The standard X-factor, X ≡ N(H2)/W(12CO) , is derived both for the whole Perseus complex and for each of the six subregions with values consistent with previous estimates. However, the X-factor is heavily affected by the saturation of the emission above AV ∼ 4 mag, and variations are also found between regions. Linear fits to relate W(12CO) and AV using only points below 4 mag of extinction yield a better estimate of the AV than the X-factor. Linear relations of W(13CO) , N(13CO) , and W(C18O) with AV are derived. The extinction thresholds above which 13CO(1-0) and C18O(1-0) are detected are about 1 mag larger than previous estimates, so that a more efficient shielding is needed for the formation of CO than previously thought. The 12CO and 13CO lines saturate above 4 and 5 mag, respectively, whereas C18O(1-0) never saturates in the whole AV range probed by our study (up to 10 mag). Approximately 60% of the positions with 12CO(1-0) emission have subthermally excited lines, and almost all positions have excitation temperatures below the dust temperature. PDR models, using the Meudon code, can explain the 12CO(1-0) and 13CO(1-0) emission with densities ranging between 103 and 104 cm−3. In general, local variations in the volume density and nonthermal motions (linked to different star formation activity) can explain the observations. Higher densities are needed to reproduce CO data toward active star-forming sites, such as NGC 1333, where the larger internal motions driven by the young protostars allow more photons from the embedded high-density cores to escape the cloud. In the most quiescent region, B5, the 12CO and 13CO emission appears to arise from an almost uniform thin layer of molecular material at densities around 104 cm−3, and in this region the integrated intensities of the two CO isotopologues are the lowest in the whole complex.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/2dc5dfee7ac47ba8454ca5a0293ed22efffd6899

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Citations in the corpus

CO-dark gas and molecular filaments in Milky Way-type galaxies (2014, methodology): “Finally, once we have τ 10, we compute WCO using the curve-ofgrowth method of Pineda et al. (2008), which relates the integral of the photon escape probability β(τ ) to the integrated CO intensity: WCO = 2TCO v ∫ τ10 0 β(τ )dτ.”
HOW MANY INFRARED DARK CLOUDS CAN FORM MASSIVE STARS AND CLUSTERS? (2010, background): “Similarly, Pineda et al. (2008) suggests 13CO-to-mass conversion factors lower than used by Simon et al. (2006).”
Planck 2013 results. XI. All-sky model of thermal dust emission (2013, background): “Now we concentrate on the link between the dust emission and the interstellar gas column density, following on many detailed studies in environments from the diffuse ISM (Boulanger & Pérault 1988; Boulanger et al. 1996) to molecular clouds (Pineda et al. 2008; Goodman et al. 2009).”
Molecular Clouds in the Milky Way (2015, background): “2–7 magnitudes ( J.E. Pineda et al. 2008, J.L. Pineda et al. 2010, Ripple et al. 2013)—bounded at the low end by rapidly decreasing CO abundances relative to H2 and low excitation and at the high end by high opacity and decreasing abundance as CO depletes onto cold grain surfaces.”
LIFTING THE DUSTY VEIL WITH NEAR- AND MID-INFRARED PHOTOMETRY. I. DESCRIPTION AND APPLICATIONS OF THE RAYLEIGH–JEANS COLOR EXCESS METHOD (2011, methodology): “The relatively low abundance and higher excitation threshold of 13CO make this molecule optically thin even in dense molecular clouds, and hence more effective at tracing the total cloud mass (e.g., Pineda et al. 2008).”
DIRECT OBSERVATION OF A SHARP TRANSITION TO COHERENCE IN DENSE CORES (2010, background): “Extinction maps trace the total column density along the line of sight, and they are useful to study the large scale structure in MCs (e.g., Lada et al. 1994; Alves et al. 1998; Lombardi & Alves 2001; Pineda et al. 2008; Goodman et al. 2009) and to identify dense cores (e.g., Alves et al. 2007).”
Modelling CO emission – I. CO as a column density tracer and the X factor in molecular clouds (2010, background): “Indeed, saturation at high CO intensities has been observed in MCs in the solar neighborhood (e.g. Lombardi et al. 2006; Pineda et al. 2008).”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, methodology): “The extinction maps used in this study probe to higher AV (up to 40 mag) than some previous studies (e.g., Pineda et al. (2008); Lombardi et al. (2008, 2010)) due to the inclusion of both 2MASS and mid–IR Spitzer data.”
THE CO-TO-H2 CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP (2011, background): “To the contrary comparisons of dust and CO emission imply dramatic variations in the CO-to-H2 ratio within individual clouds (Pineda et al. 2008).”
THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS). I. A CLOUD-SCALE/MULTI-WAVELENGTH VIEW OF THE INTERSTELLAR MEDIUM IN A GRAND-DESIGN SPIRAL GALAXY (2013, background): “…measures of AV from individual stars with NIR colors typically find that the 12CO(1-0) emission has an almost linear relation to AV for 2mag ≤ AV ≤ (4−6) mag while for large extinction values the 12CO emission appears to saturate (Lombardi et al. 2006; Pineda et al. 2008; Heiderman et al. 2010).”
Comparing the statistics of interstellar turbulence in simulations and observations - Solenoidal versus compressive turbulence forcing (2009, methodology): “This could potentially explain the apparent lack of correlation between turbulent density and velocity dispersions inferred from observations (e.g., Goodman et al. 2009; Pineda et al. 2008).”
RE-EXAMINING LARSON'S SCALING RELATIONSHIPS IN GALACTIC MOLECULAR CLOUDS (2008, result): “A similar result is obtained by Pineda et al. (2008) and Goodman et al. (2008), who used a simple curve of growth analysis to demonstrate that the 13CO integrated emission increases more rapidly with extinction in the low column density regime.”