Characterizing the line emission from molecular clouds. II. A comparative study of California, Perseus, and Orion A

Abstract

$Aims.$ We characterize the molecular-line emission of three clouds whose star-formation rates span one order of magnitude: California, Perseus, and Orion A. $Methods.$ We use stratified random sampling to select positions representing the different column density regimes of each cloud and observe them with the IRAM-30m telescope. We cover the 3 mm wavelength band and focus our analysis on CO, HCN, CS, HCO+, HNC, and N2H+. $Results.$ We find that the line intensities depend most strongly on the H2 column density. A secondary effect, especially visible in Orion A, is a dependence of the line intensities on the gas temperature. We explored a method that corrects for temperature variations and show that, when it is applied, the emission from the three clouds behaves very similarly. CO intensities vary weakly with column density, while the intensity of traditional dense-gas tracers such as HCN, CS, and HCO+ varies almost linearly with column density. N2H+ differs from all other species in that it traces only cold dense gas. The intensity of the rare HCN and CS isotopologs reveals additional temperature-dependent abundance variations. Overall, the clouds have similar chemical compositions that, as the depth increases, are sequentially dominated by photodissociation, gas-phase reactions, molecular freeze-out, and stellar feedback in the densest parts of Orion A. Our observations also allowed us to calculate line luminosities for each cloud, and a comparison with literature values shows good agreement. We used our HCN data to explore the behavior of the HCN conversion factor, finding that it is dominated by the emission from the outermost cloud layers. It also depends strongly on the gas kinetic temperature. Finally, we show that the HCN/CO ratio provides a gas volume density estimate, and that its correlation with the column density resembles that found in extragalactic observations.

36 pages, 19 figures, accepted for publication in A&A

References in corpus (32)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• The COMPLETE Survey of Star-Forming Regions: Phase I Data
• A Large Catalog of Accurate Distances to Local Molecular Clouds: The Gaia DR2 Edition
• An Ammonia Spectral Atlas of Dense Cores in Perseus
• Herschel-Planck dust optical-depth and column-density maps: I. Method description and results for Orion
• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• Revealing the physical properties of molecular gas in Orion with a large scale survey in J=2-1 lines of 12CO, 13CO and C18O
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• Molecular clouds have power-law probability distribution functions
• The HNC/HCN Ratio in Star-Forming Regions
• HCN/HNC intensity ratio: a new chemical thermometer for the molecular ISM
• Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions
• The CARMA-NRO Orion Survey: The filamentary structure as seen in C${}^{18}$O emission
• Dissecting the molecular structure of the Orion B cloud: Insight from Principal Component Analysis
• LEGO II: A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way Disc
• Characterizing the line emission from molecular clouds. Stratified random sampling of the Perseus cloud
• Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?
• Slow Star Formation in the Milky Way: Theory Meets Observations
• HP2 survey: III The California Molecular Cloud--A Sleeping Giant Revisited
• Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band
• 3D shape explains star formation mystery of California and Orion A
• A Complete HCN Survey of the Perseus Molecular Cloud
• TRAO Survey of Nearby Filamentary Molecular clouds, the Universal Nursery of Stars (TRAO FUNS) I. Dynamics and Chemistry of L1478 in the California Molecular Cloud
• A wide-field CO survey towards the California Molecular Filament
• HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI
• Tracers of Dense Gas in the Outer Galaxy
• TIMES I: a Systematic Observation in Multiple Molecular Lines Toward the Orion A and Ophiuchus Clouds
• Do simulated molecular clouds look like real ones?

Cited by in corpus (1)

• Characterizing the line emission from molecular clouds. Stratified random sampling of the Perseus cloud