Abstract

Massive primordial halos exposed to moderate UV backgrounds are the potential birthplaces of supermassive black holes. In these halos, an initially isothermal collapse will occur, leading to high accretion rates of \~{}0.1 M$_{⊙}$ yr$^{-1}$. During the collapse, the gas in the interior will turn into a molecular state, and will form accretion disk in order to conserve angular momentum. We consider here the structure of such an accretion disk and the role of viscous heating in the presence of high accretion rates for a central star of 10, 100, and 10$^{4}$ M$_{⊙}$. Our results show that the temperature in the disk increases considerably due to viscous heating, leading to a transition from the molecular to the atomic cooling phase. We found that the atomic cooling regime may extend out to several 100 AU for a 10$^{4}$ M$_{⊙}$ central star and that it provides substantial support to stabilize the disk. It therefore favors the formation of a massive central object. The comparison of clump migration and contraction time scales shows that stellar feedback from these clumps may occur during the later stages of the evolution. Overall, viscous heating provides an important pathway to obtain an atomic gas phase within the center of the halo, and helps in the formation of very massive objects. The massive object may collapse to form a massive black hole of about {\ge}10$^{4}$ M$_{⊙}$.