Determining the recurrence time-scale of long-lasting YSO outbursts

We have determined the rate of large accretion events in Class I and II young stellar objects (YSOs) by comparing the all-sky digitized photographic plate surveys provided by SuperCOSMOS with the latest data release from Gaia (DR2). The long mean baseline of 55 yr along with a large sample of Class II YSOs (≃15 000) allows us to study approximately 1 million YSO-years. We find 139 objects with ΔR ≥ 1 mag, most of which are found at amplitudes between 1 and 3 mag. The majority of YSOs in this group show irregular variability or long-lasting fading events, which is best explained as hotspots due to accretion or by variable extinction. There is a tail of YSOs at ΔR ≥ 3 mag and they seem to represent a different population. Surprisingly many objects in this group show high-amplitude irregular variability over time-scales shorter than 10 yr, in contrast with the view that high-amplitude objects always have long outbursts. However, we find six objects that are consistent with undergoing large, long-lasting accretion events, three of them previously unknown. This yields an outburst recurrence time-scale of 112 kyr, with a 68 per cent confidence interval [74-180] kyr. This represents the first robust determination of the outburst rate in Class II YSOs and shows that YSOs in their planet-forming stage do in fact undergo large accretion events, and with time-scales of ≃100 000 yr. In addition, we find that outbursts in the Class II stage are ≃10 times less frequent than during the Class I stage.