The Galactic Census of High- and Medium-mass Protostars. II. Luminosities and Evolutionary States of a Complete Sample of Dense Gas Clumps

TL;DR

This study uses a large-scale survey of Galactic molecular clumps to analyze their luminosities and evolutionary states, revealing correlations with star formation efficiency indicators and extending known luminosity relations across scales.

Contribution

It provides the first comprehensive analysis of dense gas clumps' luminosities and evolution, establishing new correlations and extending luminosity relations from individual clumps to galactic scales.

Findings

Clumps have luminosities from 10 Lsun to 10^6.5 Lsun.

L/M ratio correlates with warm/cold flux ratios and surface brightness.

Star formation efficiency per free-fall time is less than 0.2.

Abstract

(Abridged) The Census of High- and Medium-mass Protostars (CHaMP) is the first large-scale (280 degree<l<300 degree, -4 degree<b<2 degree), unbiased, sub-parsec resolution survey of Galactic molecular clumps and their embedded stars. Barnes et al. (2011) presented the source catalog of ~300 clumps based on HCO+(1-0) emission, used to estimate masses M. Here we use archival mid-infrared to mm continuum data to construct spectral energy distributions. Fitting two-temperature grey-body models, we derive bolometric luminosities, L. We find the clumps have 10Lsun<L<1E6.5Lsun and 0.1<L/M<1E3, consistent with theoretical expectations of a clump population that spans a range of instantaneous star formation efficiencies from 0 to ~50%. We thus expect L/M to be a useful, strongly-varying indicator of clump evolution during the star cluster formation process. We find correlations of the ratio of warm to cold component fluxes and of cold component temperature with L/M. We also find a near linear relation between L/M and Spitzer-IRAC specific intensity (surface brightness), which may thus also be useful as a star formation efficiency indicator. The lower bound of the clump $L/M$ distribution suggests the star formation efficiency per free-fall time is epsilon<0.2. We do not find strong correlations of L/M with mass surface density, velocity dispersion or virial parameter. We find a linear relation between L and L_{HCO+(1-0}}, although with large scatter for any given individual clump. Fitting together with extragalactic systems, the linear relation still holds, extending over 10 orders of magnitude in luminosity. The complete nature of the CHaMP survey over a several kiloparsec-scale region allows us to derive a measurement at an intermediate scale bridging those of individual clumps and whole galaxies.