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

Abstract

$Context.$ The traditional approach to characterize the structure of molecular clouds is to map their line emission. $Aims.$ We aim to test and apply a stratified random sampling technique that can characterize the line emission from molecular clouds more efficiently than mapping. $Methods.$ We sampled the molecular emission from the Perseus cloud using the H2 column density as a proxy. We divided the cloud into ten logarithmically spaced column density bins, and we randomly selected ten positions from each bin. The resulting 100 cloud positions were observed with the IRAM 30m telescope, covering the 3mm-wavelength band and parts of the 2 and 1mm bands. $Results.$ We focus our analysis on 11 molecular species detected toward most column density bins. In all cases, the line intensity is tightly correlated with the H2 column density. For the CO isotopologs, the trend is relatively flat, while for high-dipole moment species such as HCN, CS, and HCO+ the trend is approximately linear. We reproduce this behavior with a cloud model in which the gas density increases with column density, and where most species have abundance profiles characterized by an outer photodissociation edge and an inner freeze-out drop. The intensity behavior of the high-dipole moment species arises from a combination of excitation effects and molecular freeze out, with some modulation from optical depth. This quasi-linear dependence with the H2 column density makes the gas at low column densities dominate the cloud-integrated emission. It also makes the emission from most high-dipole moment species proportional to the cloud mass inside the photodissociation edge. $Conclusions.$ Stratified random sampling is an efficient technique for characterizing the emission from whole molecular clouds. It shows that despite the complex appearance of Perseus, its molecular emission follows a relatively simple pattern.

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

References in corpus (28)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• Structural Analysis of Molecular Clouds: Dendrograms
• Spitzer Observations of NGC 1333: A Study of Structure and Evolution in a Nearby Embedded Cluster
• 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
• The Large and Small Scale Structures of Dust in the Star-Forming Perseus Molecular Cloud
• 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
• FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results
• On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
• Dynamics of Dense Cores in the Perseus Molecular Cloud
• Current Star Formation in the Perseus Molecular Cloud: Constraints from Unbiased Submillimeter and Mid-Infrared Surveys
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• 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
• Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B
• Gas phase Elemental abundances in Molecular cloudS (GEMS). II. On the quest for the sulphur reservoir in molecular clouds: the $H_{2}S$ case
• 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
• Dust Emission from the Perseus Molecular Cloud
• Physical properties of the ambient medium and of dense cores in the Perseus star-forming region derived from Herschel Gould Belt Survey observations
• Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?
• Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band
• Molecular-Cloud-Scale Chemical Composition II: Mapping Spectral Line Survey toward W3(OH) in the 3 mm Band
• A dynamically young, gravitationally stable network of filaments in Orion B
• The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans