A Sample of Intermediate-Mass Star-Forming Regions: Making Stars at Mass Column Densities <1 g/cm^2

Abstract

In an effort to understand the factors that govern the transition from low- to high-mass star formation, we identify for the first time a sample of intermediate-mass star-forming regions (IM SFRs) where stars up to - but not exceeding - 8 solar masses are being produced. We use IRAS colors and Spitzer Space Telescope mid-IR images, in conjunction with millimeter continuum and CO maps, to compile a sample of 50 IM SFRs in the inner Galaxy. These are likely to be precursors to Herbig AeBe stars and their associated clusters of low-mass stars. IM SFRs constitute embedded clusters at an early evolutionary stage akin to compact HII regions, but they lack the massive ionizing central star(s). The photodissociation regions that demarcate IM SFRs have typical diameters of ~1 pc and luminosities of ~10^4 solar luminosities, making them an order of magnitude less luminous than (ultra)compact HII regions. IM SFRs coincide with molecular clumps of mass ~10^3 solar masses which, in turn, lie within larger molecular clouds spanning the lower end of the giant molecular cloud mass range, 10^4-10^5 solar masses. The IR luminosity and associated molecular mass of IM SFRs are correlated, consistent with the known luminosity-mass relationship of compact HII regions. Peak mass column densities within IM SFRs are ~0.1-0.5 g/cm^2, a factor of several lower than ultra-compact HII regions, supporting the proposition that there is a threshold for massive star formation at ~1 g/cm^2.

61 pages, 6 tables, 20 figures. Accepted for publication in the Astronomical Journal

References in corpus (10)

• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• MAMBO Mapping of Spitzer c2d Small Clouds and Cores
• A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
• The Bolocam Galactic Plane Survey -- II. Catalog of the Image Data
• Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey
• Molecular clouds and clumps in the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The Milky Way Rotation Curve and its Vertical Derivatives Inside the Solar Circle
• The Arecibo Methanol Maser Galactic Plane Survey - III: Distances and Luminosities
• The Arecibo Methanol Maser Galactic Plane Survey - II: Statistical and Multi-wavelength Counterpart Analysis

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• The Co-ordinated Radio and Infrared Survey for High-Mass Star Formation - II. Source Catalogue
• The Bolocam Galactic Plane Survey IX: Data Release 2 and Outer Galaxy Extension
• Observing Extended Sources with the \Herschel SPIRE Fourier Transform Spectrometer
• An All-Sky Sample of Intermediate-Mass Star-Forming Regions
• The Interstellar Bubbles of G38.9-0.4 and the Impact of Stellar Feedback on Star Formation
• IRAS03063+5735: A Bowshock Nebula Powered by an Early B Star
• Near-infrared and Millimeter-wavelength Observations of Mol 160: A Massive Young Protostellar Core
• The intermediate-mass star-forming region Lynds 1340. An optical view
• Infrared spectroscopy of intermediate mass young stellar objects
• The young stellar population of Lynds 1340. An infrared view
• Submillimeter and molecular views of three Galactic ring-like HII regions
• Massive Star Formation, Outflows, and Anomalous H2 Emission in Mol 121 (IRAS 20188+3928)