The luminosity of Population III star clusters

Abstract

We analyze the time evolution of the luminosity of a cluster of Population III protostars formed in the early universe. We argue from the Jeans criterion that primordial gas can collapse to form a cluster of first stars that evolve relatively independently of one another (i.e., with negligible gravitational interaction). We model the collapse of individual protostellar clumps using 2+1D nonaxisymmetric numerical hydrodynamics simulations. Each collapse produces a protostar surrounded by a massive disk (i.e., M disk /M * 0.1), whose evolution we follow for a further 30-40 kyr. Gravitational instabilities result in the fragmentation and the formation of gravitationally bound clumps within the disk. The accretion of these fragments by the host protostar produces accretion and luminosity bursts on the order of 10 6 L ⊙ . Within the cluster, we show that a simultaneity of such events across several protostellar cluster members can elevate the cluster luminosity to 5-10× greater than expected, and that the cluster spends ∼ 15% of its star-forming history at these levels. This enhanced luminosity effect is particularly enabled in clusters of modest size with ≃ 10-20 members. In one such instance, we identify a confluence of burst events that raise the luminosity to nearly 1000× greater than the cluster mean luminosity, resulting in L > 10 8 L ⊙ . This phenomenon arises solely through the gravitational-instability-driven episodic fragmentation and accretion that characterizes this early stage of protostellar evolution.

Formation of Massive Primordial Stars: Intermittent Uv Feedback With Episodic Mass Accretion

“…This is driven by the inward migration of the fragments due to strong gravitational torques caused by the asymmetric disk structure (e.g., spiral arms). As a result, accretion histories normally become highly variable, resulting in short accretion bursts followed by relatively long quiescent phases (e.g., Smith et al 2012;Vorobyov et al 2013;DeSouza & Basu 2015).…”

On the existence of accretion-driven bursts in massive star formation

“…However, recent numerical hydrodynamics simulations of primordial disc formation around the first very massive stars have also revealed the presence of accretion bursts caused by disc gravitational fragmentation followed by rapid migration of the fragments onto the protostar (Stacy et al 2010;Greif et al 2012;Smith et al 2012;Vorobyov et al 2013;Hosokawa et al 2016). These studies have revealed highly variable protostellar accretion with multiple bursts, exceeding in numbers their present-day counterparts (DeSouza & Basu 2015). The same process of bursts driven by disc fragmentation operates around primordial super-massive stars, relaxing the ultraviolet photon output and enabling the stellar growth to the limit where general-relativistic instability results in the formation of super-massive black holes (Sakurai et al 2016).…”

A search for non-thermal radio emission from jets of massive young stellar objects

“…The shocks are therefore traced by the non-thermal radio knots and Herbig-Haro (HH) objects. Some of the theories put forward to explain how shocks are formed in YSOs include episodic accretion (DeSouza & Basu 2015) and interaction between jet material with the ambient medium and/or internal working surfaces within the jet (Rubini et al 2007). Star formation models that incorporate disks can explain how the shocks are formed in MYSOs.…”