Determining the recurrence timescale 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 digitised photographic plate
surveys provided by SuperCOSMOS with the latest data release from Gaia (DR2).
The long mean baseline of 55 years along with a large sample of class II YSOs
($\simeq$15,000) allows us to study approximately 1 million YSO-years. We find
139 objects with $\Delta R\geq1$~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 hot spots
due to accretion or by variable extinction. There is a tail of YSOs at $\Delta
R\geq3$~mag and they seem to represent a different population. Surprisingly
many objects in this group show high-amplitude irregular variability over
timescales shorter than 10 years, in contrast with the view that high-amplitude
objects always have long outbursts. However, we find 6 objects that are
consistent with undergoing large, long lasting accretion events, 3 of them
previously unknown. This yields an outburst recurrence timescale of 112 kyr,
with a 68\% confidence interval [74 to 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 timescales of $\simeq$100,000 years. In addition, we find that outbursts
in the class II stage are $\simeq$10 times less frequent than during the class
I stage.
