The Young Ages of 70 μm-dark Clumps Inferred from Carbon Chain Chemistry

Abstract

The physical conditions of the earliest environment of high-mass star formation are currently poorly understood. To that end, we present observations of the carbon chain molecules HC${}_5$N , CCS, and HC${}_7$N in the 22-25 GHz band towards 12 high-mass 70 micron-dark clumps (SMDC) with the Jansky Very Large Array (VLA). We detect HC${}_5$N and CCS towards 11 of these SMDC sources. We calculate column densities and abundances relative to H${}_2$ for HC${}_5$N and CCS. We do not find any clear HC${}_7$N detections in the 11 sources individually, but by stacking the HC${}_7$N spectra, we do detect HC${}_7$N on average in these sources. We also calculate the ratio of the column densities of HC${}_5$N to HC${}_7$N using the stacked spectra of both species. We compare our measured abundances of HC${}_5$N and our measured ratio of HC${}_5$N to HC${}_7$N to the UMIST dark cloud chemistry models to constrain an age for the gas assuming a fixed volume density and temperature. The chemical models favor a chemical evolutionary age less than 1 Myr at densities of n(H2) = 2 x 10${}^4$ cm${}^{-3}$. The consistent carbon-chain detections and young model-derived ages support the conclusion that these 11 70 micron-dark clumps lack high mass protostars because they are young and not because they are inefficient and incapable of high mass star formation.

Accepted for publication in ApJ. 10 Figures