Molecule sublimation as a tracer of protostellar accretion - Evidence for accretion bursts from high angular resolution C18O images

摘要

Context. The accretion histories of embedded protostars are an integral part of descriptions of their physical and chemical evolution. In particular, we want to know whether the accretion rates smoothly decline from earlier to later stages or whether they are in fact characterized by variations such as intermittent bursts. Aims. We aim to characterize the impact of possible accretion variations for a sample of embedded protostars by measuring the sizes of the inner regions of their envelopes where CO is sublimated and relate these extents to the temperature profiles dictated by the current luminosities of the protostars. Methods. Using observations from the Submillimeter Array we measure the extent of the emission from the C18O isotopologue toward 16 deeply embedded protostars. We compare these measurements to the predicted extent of the emission given the current luminosities of the sources through dust and line radiative transfer calculations. Results. Eight out of sixteen sources show more extended C18O emission than predicted by the models. The modeling shows that the likely culprit for these signatures is sublimation due to increases in luminosities of the sources by about a factor of five or more during the recent 10?000 yr, i.e., the time it takes for CO to freeze-out again on dust grains. For four of these sources the increase must have been a factor of 10 or more. The compact emission seen toward the other half of the sample suggests that C18O only sublimates when the temperature exceeds 30 K – as expected if CO is mixed with H2O in the grain ice-mantles. Conclusions. The results from this survey suggest that protostars undergo significant bursts about once every 20?000 yr, although the statistics suffer from the small sample size. The results illustrate the importance of taking the physical evolutionary histories into account for descriptions of the chemical structures of embedded protostars.