Hot Ammonia around O-type Young Stars. I. JVLA imaging of Ammonia (6,6) to (14,14) in NGC7538 IRS1

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.