Complex Organic Molecules Towards Embedded Low-Mass Protostars

Abstract

Complex organic molecules (COMs) have been observed towards several low-mass young stellar objects (LYSOs). Small and heterogeneous samples have so far precluded conclusions on typical COM abundances, as well as the origin(s) of abundance variations between sources. We present observations towards 16 deeply embedded (Class 0/I) low-mass protostars using the IRAM 30m telescope. We detect CH${}_2$CO, CH${}_3$CHO, CH${}_3$OCH${}_3$, CH${}_3$OCHO, CH${}_3$CN, HNCO, and HC${}_3$N towards 67%, 37%, 13%, 13%, 44%, 81%, and 75% of sources respectively. Median column densities derived using survival analysis range between 6.0x10${}^{10}$ cm${}^{-2}$ (CH${}_3$CN) and 2.4x10${}^{12}$ cm${}^{-2}$ (CH${}_3$OCH${}_3$) and median abundances range between 0.48% (CH${}_3$CN) and 16% (HNCO) with respect to CH${}_3$OH. Column densities for each molecule vary by about one order of magnitude across the sample. Abundances with respect to CH${}_3$OH are more narrowly distributed, especially for oxygen-bearing species. We compare observed median abundances with a chemical model for low-mass protostars and find fair agreement, although some modeling work remains to bring abundances higher with respect to CH${}_3$OH. Median abundances with respect to CH${}_3$OH in LYSOs are also found to be generally comparable to observed abundances in hot cores, hot corinos, and massive young stellar objects. Compared with comets, our sample is comparable for all molecules except HC${}_3$N and CH${}_2$CO, which likely become depleted at later evolutionary stages.

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