Monitoring accretion rate variability in the Orion Nebula Cluster with the Wendelstein Wide Field Imager

TL;DR

This study monitored accretion rate variability in Orion Nebula Cluster stars over several years to assess its impact on the correlation between X-ray activity and accretion, finding minimal influence on observed relationships.

Contribution

It provides the first long-term analysis of accretion variability and its effect on X-ray and accretion rate correlations in young stars.

Findings

Accretion rate variability has an interquartile range of ~0.3 dex over weeks to 2 years.

Variability does not significantly affect the correlation between X-ray luminosity and accretion rate.

The observed anticorrelation is likely not due to observational timing biases.

Abstract

The understanding of the accretion process has a central role in the understanding of star and planet formation. We aim to test how accretion variability influences previous correlation analyses of the relation between X-ray activity and accretion rates, which is important for understanding the evolution of circumstellar disks and disk photoevaporation. We monitored accreting stars in the Orion Nebula Cluster from November 24, 2014, until February 17, 2019, for 42 epochs with the Wendelstein Wide Field Imager in the Sloan Digital Sky Survey u'g'r' filters on the 2 m Fraunhofer Telescope on Mount Wendelstein. Mass accretion rates were determined from the measured ultraviolet excess. The influence of the mass accretion rate variability on the relation between X-ray luminosities and mass accretion rates was analyzed statistically. We find a typical interquartile range of ~ 0.3 dex for the mass accretion rate variability on timescales from weeks to ~ 2 years. The variability has likely no significant influence on a correlation analysis of the X-ray luminosity and the mass accretion rate observed at different times when the sample size is large enough. The observed anticorrelation between the X-ray luminosity and the mass accretion rate predicted by models of photoevaporation-starved accretion is likely not due to a bias introduced by different observing times.