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Young massive stars warm up the large amount of gas and dust which condensesin their vicinity, exciting a forest of lines from different molecular species.Their line brightness is a diagnostic tool of the gas physical conditionslocally, which we use to set constraints on the environment where massive starsform. We made use of the Atacama Large Millimeter/submillimeter Array atfrequencies near 349 GHz, with an angular resolution of $0.1''$, to observe themethyl cyanide (CH$_3$CN) emission which arises from the accretion disk of ayoung 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 spiralmorphology projected onto the plane of the sky, which we sample with tenadditional beams: along these apparent spiral features, gas undergoes velocitygradients of about $\rm 1 km s^{-1}$ per 2000 au. The gas temperature (T) risessymmetrically along each side of the disk, from about 98 K at 3000 au to 289 Kat 250 au, following a power law with radius, R$^{-0.43}$. The CH$_3$CN columndensity (N) increases from $\rm 9.2\times10^{15} cm^{-2}$ to $\rm8.7\times10^{17} cm^{-2}$ at the same radii, following a power law with radius,R$^{-1.8}$. In the framework of a circular gaseous disk observed approximatelyedge-on, we infer an H$_2$ volume density in excess of $\rm 4.8\times10^9cm^{-3}$ at a distance of 250 au from the star. We study the disk stabilityagainst fragmentation following the methodology by Kratter et al. (2010),appropriate under rapid accretion, and we show that the disk is marginallyprone to fragmentation along its whole extent.

Physical conditions in the warped accretion disk of a massive star. 349 GHz ALMA observations of G023.01$-$00.41