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

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 HC₅N, CCS, and HC₇N in the 22-25 GHz band toward 12 high-mass 70 μm dark clumps (SMDC) with the Karl G. Jansky Very Large Array. We detect HC₅N and CCS toward 11 of these SMDC sources. We calculate column densities and abundances relative to H₂ for HC₅N and CCS. We do not find any clear HC₇N detections in the 11 sources individually, but by stacking the HC₇N spectra, we do detect HC₇N on average in these sources. We also calculate the ratio of the column densities of HC₅N to HC₇N using the stacked spectra of both species. We compare our measured abundances of HC₅N and our measured ratio of HC₅N to HC₇N 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 <1 Myr at densities of n(H₂) ≈ 2 × 10⁴ cm⁻³. The consistent carbon-chain detections and young model-derived ages support the conclusion that these 11 70 μm dark clumps lack high-mass protostars because they are young and not because they are inefficient and incapable of high-mass star formation.