Probing the methanol and CO snow lines in young protostars

Abstract

“Snow lines”, marking regions where abundant volatiles freeze out onto the surface of dust grains, play an important role for planet growth and bulk composition in protoplanetary disks. However, they can already be observed in the envelopes of the much younger, low-mass Class 0 protostars that are still in their early phase of heavy accretion. The information on the sublimation regions of different kinds of ices can be used to understand the chemistry of the envelope, its temperature and density structure, and may even hint at the history of the accretion process. As part of the CALYPSO Large Program, we have obtained observations of C 18 O, N 2 H + and CH 3 OH towards the nearest low-luminosity Class 0 protostars with the IRAM Plateau de Bure interferometer at sub-arcsecond resolution. We observe an anti-correlation of C 18 O and N 2 H + in four of these sources, with N 2 H + forming a ring (perturbed by the outflow) around the centrally peaked C 18 O emission. This reveals the CO snow line in these protostellar envelopes with unprecedented resolution. In addition, we observe compact methanol emission towards three of the sources. We have modeled the emission using a chemical model coupled with a radiative transfer module, using the temperature and density profiles self-consistently determined by Kristensen et al. ([4]). We find that for all four sources the CO snow line appears further inwards than expected from the binding energy of pure CO ices. This may hint at CO being frozen out on H 2 O surfaces or in mixed ices. Our observations can thereby yield clues on the widely unknown composition of interstellar ices, being the initial seeds of complex organic chemistry.