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We present coupled stellar evolution (SE) and 3D radiation-hydrodynamic (RHD)simulations of the evolution of primordial protostars, their immediateenvironment, and the dynamic accretion history under the influence of stellarionizing and dissociating UV feedback. Our coupled SE-RHD calculations resultin a wide diversity of final stellar masses covering 10 Msun $\lesssim M_*\lesssim$ 1000 Msun. The formation of very massive ($\gtrsim$ 250 Msun) starsis possible under weak UV feedback, whereas ordinary massive (a few x 10 Msun)stars form when UV feedback can efficiently halt the accretion. This mayexplain the peculiar abundance pattern of a Galactic metal-poor star recentlyreported by Aoki et al. (2014), possibly the observational signature of verymassive precursor primordial stars. Weak UV feedback occurs in cases ofvariable accretion, in particular when repeated short accretion burststemporarily exceed 0.01 Msun/yr, causing the protostar to inflate. In thebloated state, the protostar has low surface temperature and UV feedback issuppressed until the star eventually contracts, on a thermal adjustmenttimescale, to create an HII region. If the delay time between successiveaccretion bursts is sufficiently short, the protostar remains bloated forextended periods, initiating at most only short periods of UV feedback. Diskfragmentation does not necessarily reduce the final stellar mass. Quite thecontrary, we find that disk fragmentation enhances episodic accretion as manyfragments migrate inward and are accreted onto the star, thus allowingcontinued stellar mass growth under conditions of intermittent UV feedback.This trend becomes more prominent as we improve the resolution of oursimulations. We argue that simulations with significantly higher resolutionthan reported previously are needed to derive accurate gas mass accretion ratesonto primordial protostars.

Formation of Massive Primordial Stars: Intermittent UV Feedback with Episodic Mass Accretion