Letter to the Editor

Accretion bursts in young stars driven by the cluster environment^*

I. Physikalisches Institut, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany e-mail: pfalzner@ph1.uni-koeln.de

Received: 20 May 2008
Accepted: 25 June 2008

Abstract

Context. The standard picture of accretion is a steady flow of matter from the disc onto the young star – a concept that assumes the star-disc system is completely isolated. However, in a dense cluster environment star-disc systems do interact gravitationally.

Aims. The aim here is to estimate the encounter-induced accretion rate in an ONC-like environment.

Methods. By combining simulations of the cluster dynamics with simulations of the effect of encounters on star-disc systems, we determine the likelihood and degree of encounter-triggered accretion processes.

Results. We show that accretion bursts triggered by encounters of star-disc systems are common in young dense clusters like the ONC, leading in the outburst phase to typical accretion rates of 10^-7-10^-4 /yr. Up to a third of the stars presently in the Trapezium region have accreted at least 1% of their disc mass via this mechanism in the past 1 Myr. Accretion of over 6-7% of the disc material can occur in a single encounter. Despite losing their discs quickly, the total percentage of disc matter accreted per star is greatest for the massive stars.

Conclusions. Supplementing the steady accretion flow are episodic periods of high accretion in dense cluster environments. Because of their high accretion rate, these processes should be observable even now in some of the low-mass stars in the ONC.