Vol. 577
In section 6. Interstellar and circumstellar matter

Chemical tracers of episodic accretion in low-mass protostars

It is well known that young stellar objects (YSOs) have occasional outbursts where, for a short time, the luminosity can amount to as much as 100 times the quiescent value (the FU0r and EXor outbursts). There is, however, a long-standing debate about whether the rapid accretion associated with such outbursts can explain the discrepancies between protostar luminosities and the estimated timescales for YSO evolution. The resolution of this question depends on the timescale between bursts which is very poorly known. In the article highlighted in this issue, Visser et al. show that a partial answer to this question can be obtained from the study of the chemical makeup of the protostellar envelope because the ices evaporated in an outburst take a considerable time (10^3-10^4 yrs) to recondense and thus thus the time since the last big outburst can, in principle, be estimated. Thus gas phase abundances in protostellar envelopes depend sensitively on the past history of luminosity variations. In practice, exploiting this effect will require high angular resolution observations, which is precisely what ALMA is going to provide.