A massive disk/envelope in shocked H 2emission around an UCHII region

A multi-wavelength study of IRAS 07427-2400 in line and continuum emission was conducted to investigate the nature of a H 2 v=1-0 S(1) line emitting feature around this ultra-compact HII region. High resolution 3.6 cm continuum observations from the Very Large Array and 350 (mu)m continuum observations from the Caltech Submillimeter Observatory, combined with archival far-infrared data of IRAS 07427-2400 show a flux density distribution indicating a luminous ( L= 5.6 * 10^4 L…) point source associated with an ultra-compact HII region. A Grey body model fit to the flux density distribution yields a dust emissivity index ( (beta) ~ 0.66) indicative of a circumstellar disk/envelope. Our C 18O map shows a dense core centered on the continuum source, with the major axis roughly aligned with the H 2 feature. A position-velocity diagram of the C 18O core obtained along the major axis shows rotation with a velocity gradient of ~0.1 km s -1 arcsec -1. New CO  J=3-2 maps of the region are presented which reveal a massive molecular outflow from the IRAS source. We argue that the H 2 feature arises in a disk/envelope around IRAS 07427-2400 and not in an outflow. We present a near-infrared HK band spectrum of the H 2 features that shows several ro-vibrational emission lines of H 2 and [FeII]. Analysis of the line ratios indicates that the line emission is shock-excited and not due to fluorescence. We estimate an excitation temperature of ~1600 K and an average extinction of A ~ 11 mag to the H 2 feature. The line fluxes yield a mass accretion rate of …yr -1 and a lifetime of ~5360 +-1200 yr resulting in a disk/envelope mass of 140 +-50  ….. The resulting Jeans Mass of 2420  …indicates that the disk/envelope will not undergo fragmentation. IRAS 07427-2400 represents one of the most massive YSOs known to date forming by means of accretion.