# Determining the recurrence timescale of long-lasting YSO outbursts

## TL;DR

This study estimates the recurrence timescale of large accretion outbursts in young stellar objects, revealing they occur roughly every 112,000 years, with fewer events in the class II stage compared to class I.

## Contribution

First robust measurement of outburst rate in class II YSOs, showing large accretion events are less frequent than in earlier stages.

## Key findings

- Outburst recurrence timescale is approximately 112 kyr.
- Large accretion events are about 10 times less frequent in class II than in class I YSOs.
- Identified 3 previously unknown long-lasting accretion events.

## Abstract

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.