  To constrain theoretical models of high-mass star formation, observational
signatures of mass accretion in O-type forming stars are desirable. Using the
JVLA, we have mapped the hot and dense molecular gas in the hot core NGC7538
IRS1, with 0.2'' angular resolution, in seven metastable (J=K) inversion
transitions of ammonia: (J,K)=(6,6), (7,7), (9,9), (10,10), (12,12), (13,13),
and (14,14). These lines arise from energy levels between ~400 K and ~1950 K
above the ground state, and are observed in absorption against the HC-HII
region associated with NGC7538 IRS1. With a 500 AU linear resolution, we
resolve the elongated North-South ammonia structure into two compact
components: the main core and a southernmost component. Previous observations
of the radio continuum with a 0.08'' (or 200 AU) resolution, resolved in turn
the compact core in two (northern and southern) components. These features
correspond to a triple system of high-mass YSOs IRS1a, IRS1b, and IRS1c
identified with VLBI measurements of methanol masers. The velocity maps of the
compact core show a clear velocity gradient in all lines, which is indicative
of rotation in a (circumbinary) envelope, containing ~40 solar masses
(dynamical mass). In addition, we derived physical conditions of the molecular
gas: rotational temperatures ~280 K, ammonia column densities ~1.4-2.5 x 10^19
cm-2, H_2 volume densities ~3.5-6.2 x 10^10 cm-3, and a total gas mass in the
range of 19-34 solar masses, for the main core. We conclude that NGC7538 IRS1
is the densest hot molecular core known, containing a rotating envelope which
hosts a multiple system of high-mass YSOs, possibly surrounded by accretion
disks. Future JVLA observations in the A-configuration are needed to resolve
the binary system in the core and may allow to study the gas kinematics in the
accretion disks associated with individual binary members.
