On the Rotation of Supermassive Stars

Abstract

Supermassive stars born from pristine gas in atomically-cooled haloes are thought to be the progenitors of supermassive black holes at high redshifts. However, the way they accrete their mass is still an unsolved problem. In particular, for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here, we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution model of a supermassive Population III star including simultaneously accretion and rotation. We find that, for supermassive stars to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angular momentum. This tight constraint comes from the ΩΓ-limit, at which the combination of radiation pressure and centrifugal force cancels gravity. It implies that supermassive stars are slow rotators, with a surface velocity less than 10 -20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, the deformation of the star due to rotation is negligible.

Formation of massive seed black holes via collisions and accretion

“…Models 2 and 4 give consistent results, which are the position dependent accretion models where a dominant object is formed in the core. For the other models we observe a statistical difference due to the more conservative radii of Haemmerlé et al (2017). This difference does not change the conclusion that a single massive object is formed due to a high collision rate.…”

“…2). For the higher accretion rates, the models of Haemmerlé et al (2017) produce somewhat smaller radii, but they show the same behaviour at large masses. We use both models to test the sensitivity of the formation of massive objects to the underlying mass-radius parametrization.…”

“…Its rapid growth causes many collisions to occur irrespective of the detailed mass-radius parametrization. For models 3, 5 and 6 we observe that the models based on Haemmerlé et al (2017) produce somewhat fewer collisions and less massive objects. These models are sensitive to the more conservative radii of the Haemmerlé et al (2017) Figure 7.…”

“…The large accretion rates of 0.1 M yr −1 or more found in the simulations strongly favor protostellar models with cool atmospheres and highly extended envelopes (Hosokawa et al 2013;Schleicher et al 2013;Woods et al 2017;Haemmerlé et al 2017), implying that feedback is rather inefficient. Under such conditions, final black hole masses of 10 5 M can be reached based on the results of cosmological simulations (Latif et al 2013b;Umeda et al 2016).…”

“…We compare our parametrization based on Hosokawa et al (2012) to that of Haemmerlé et al (2017). We use the standard set of parameters defined in Sec.…”