Physical conditions in the warped accretion disk of a massive star. 349
  GHz ALMA observations of G023.01$-$00.41
  Young massive stars warm up the large amount of gas and dust which condenses
in their vicinity, exciting a forest of lines from different molecular species.
Their line brightness is a diagnostic tool of the gas physical conditions
locally, which we use to set constraints on the environment where massive stars
form. We made use of the Atacama Large Millimeter/submillimeter Array at
frequencies near 349 GHz, with an angular resolution of $0.1''$, to observe the
methyl cyanide (CH$_3$CN) emission which arises from the accretion disk of a
young massive star. We sample the disk midplane with twelve distinct beams,
where we get an independent measure of the gas (and dust) physical conditions.
The accretion disk extends above the midplane showing a double-armed spiral
morphology projected onto the plane of the sky, which we sample with ten
additional beams: along these apparent spiral features, gas undergoes velocity
gradients of about $\rm 1 km s^{-1}$ per 2000 au. The gas temperature (T) rises
symmetrically along each side of the disk, from about 98 K at 3000 au to 289 K
at 250 au, following a power law with radius, R$^{-0.43}$. The CH$_3$CN column
density (N) increases from $\rm 9.2\times10^{15} cm^{-2}$ to $\rm
8.7\times10^{17} cm^{-2}$ at the same radii, following a power law with radius,
R$^{-1.8}$. In the framework of a circular gaseous disk observed approximately
edge-on, we infer an H$_2$ volume density in excess of $\rm 4.8\times10^9
cm^{-3}$ at a distance of 250 au from the star. We study the disk stability
against fragmentation following the methodology by Kratter et al. (2010),
appropriate under rapid accretion, and we show that the disk is marginally
prone to fragmentation along its whole extent.
