Witnessing the birth of a supermassive protostar
  The detection of $\rm z>6$ quasars reveals the existence of supermassive
black holes of a few $\rm 10^9~M_{\odot}$. One of the potential pathways to
explain their formation in the infant universe is the so-called direct collapse
model which provides massive seeds of $\rm 10^5-10^6~M_{\odot}$. An isothermal
direct collapse mandates that halos should be of a primordial composition and
the formation of molecular hydrogen remains suppressed in the presence of a
strong Lyman Werner flux. In this study, we perform high resolution
cosmological simulations for two massive primordial halos employing a detailed
chemical model which includes $\rm H^-$ cooling as well as realistic opacities
for both the bound-free $\rm H^-$ emission and the Rayleigh scattering of
hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high
densities of $\rm \sim 10^{-3}~g/cm^3$ and to scales of about $\rm 10^{-4}$ AU.
Our results show that the gas cools down to $\rm \sim $ 5000 K in the presence
of $\rm H^-$ cooling, and induces fragmentation at scales of about 8000 AU in
one of the two simulated halos, which may lead to the formation of a binary. In
addition, fragmentation also occurs on the AU scale in one of the halos but the
clumps are expected to merge on short time scales. Our results confirm that
$\rm H^-$ cooling does not prevent the formation of a supermassive star and the
trapping of cooling radiation stabilises the collapse on small scales.
