Organic chemistry of low-mass star-forming cores I: 7 mm spectroscopy of Chamaeleon MMS1

Abstract

Observations are presented of emission lines from organic molecules at frequencies 32 - 50 GHz in the vicinity of Chamaeleon MMS1. This chemically-rich dense cloud core habours an extremely young, very low-luminosity protostellar object and is a candidate first hydrostatic core. Column densities are derived and emission maps are presented for species including polyynes, cyanopolyynes, sulphuretted carbon-chains and methanol. The polyyne emission peak lies about 5000 AU from the protostar, whereas methanol peaks about 15,000 AU away. Averaged over the telescope beam, the molecular hydrogen number density is calculated to be 10^6 cm^-3 and the gas kinetic temperature is in the range 5 - 7 K. The abundances of long carbon chains are very large, and are indicative of a non-equilibrium carbon chemistry; C6H and HC7N column densities are 5.9 (+2.9 -1.3) \times 10^11 cm^-2 and 3.3 (+8.0 -1.5) \times 10^12 cm^-2, respectively, which are similar to the values found in the most carbon-chain-rich protostars and prestellar cores known, and are unusually large for star-forming gas. Column density upper limits were obtained for the carbon-chain anions C4H- and C6H-, with anion-to-neutral ratios [C4H-]/[C4H] < 0.02% and [C6H-]/[C6H] < 10%, consistent with previous observations in interstellar clouds and low-mass protostars. Deuterated HC3N and c-C3H2 were detected. The [DC3N]/[HC3N] ratio of approximately 4% is consistent with the value typically found in cold interstellar gas.

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