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The accretion histories of embedded protostars are an integral part ofdescriptions of their physical and chemical evolution. In particular, are theaccretion rates smoothly declining from the earlier toward later stages or infact characterized by variations such as intermittent bursts? We aim tocharacterize the impact of possible accretion variations in a sample ofembedded protostars by measuring the size of the inner regions of theirenvelopes where CO is sublimated and relate those to their temperature profilesdictated by their current luminosities. Using observations from theSubmillimeter Array we measure the extents of the emission from the C18Oisotopologue toward 16 deeply embedded protostars. We compare thesemeasurements to the predicted extent of the emission given the currentluminosities of the sources through dust and line radiative transfercalculations. Eight out of sixteen sources show more extended C18O emissionthan predicted by the models. The modeling shows that the likely culprit forthese signatures is sublimation due to increases in luminosities of the sourcesby about a factor five or more during the recent 10,000 years - the time ittakes for CO to freeze-out again on dust grains. For four of those sources theincrease would have had to have been a factor 10 or more. The compact emissionseen toward the other half of the sample suggests that C18O only sublimateswhen the temperature exceeds 30 K - as one would expect if CO is mixed with H2Oin the grain ice-mantles. The small-number statistics from this survey suggestthat protostars undergo significant bursts about once every 20,000 years. Thisalso illustrates the importance of taking the physical evolutionary historiesinto account for descriptions of the chemical structures of embeddedprotostars.

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