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

TL;DR

This study uses high-resolution C18O imaging to identify episodic accretion bursts in protostars, revealing that many experience significant luminosity increases every 20,000 years, impacting their chemical and physical evolution.

Contribution

It provides observational evidence linking C18O sublimation extents to accretion bursts, highlighting the importance of episodic luminosity increases in protostellar evolution.

Findings

Eight of sixteen protostars show extended C18O emission indicating recent accretion bursts.

Protostars likely undergo significant luminosity bursts approximately every 20,000 years.

C18O sublimates when temperature exceeds 30 K, consistent with ice-mantle composition.

Abstract

The accretion histories of embedded protostars are an integral part of descriptions of their physical and chemical evolution. In particular, are the accretion rates smoothly declining from the earlier toward later stages or in fact characterized by variations such as intermittent bursts? We aim to characterize the impact of possible accretion variations in a sample of embedded protostars by measuring the size of the inner regions of their envelopes where CO is sublimated and relate those to their temperature profiles dictated by their current luminosities. Using observations from the Submillimeter Array we measure the extents 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. 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 five or more during the recent 10,000 years - the time it takes for CO to freeze-out again on dust grains. For four of those sources the increase would have had to have been a factor 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 one would expect if CO is mixed with H2O in the grain ice-mantles. The small-number statistics from this survey suggest that protostars undergo significant bursts about once every 20,000 years. This also illustrates the importance of taking the physical evolutionary histories into account for descriptions of the chemical structures of embedded protostars.