A massive disk/envelope in shocked H2emission around an UCHII region

Abstract

A multi-wavelength study of IRAS 07427-2400 in line and continuum emission was conducted to investigate the nature of a H2 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 µm 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 (β ∼ 0.66) indicative of a circumstellar disk/envelope. Our C 18 O map shows a dense core centered on the continuum source, with the major axis roughly aligned with the H2 feature. A position-velocity diagram of the C 18 Oc ore obtained along the major axis shows rotation with a velocity gradient of ∼0.1 km s −1 arcsec −1 .N ew COJ = 3-2 maps of the region are presented which reveal a massive molecular outflow from the IRAS source. We argue that the H2 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 H2 features that shows several ro-vibrational emission lines of H2 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 Av ∼ 11 mag to the H2 feature. The line fluxes yield a mass accretion rate of u M ∼ 2.6 ± 0.9 × 10 −2 Myr −1 and a lifetime of ∼5360 ± 1200 yr resulting in a disk/envelope mass of 140 ± 50 M� . The resulting Jeans Mass of 2420 Mindicates 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.