A massive disk/envelope in shocked H₂ emission around an UCHII region

M. S. N. Kumar, A. J. L. Fernandes, T. R. Hunter, C. J. Davis, S. Kurtz

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 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*E⁴ L_sun) 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 C¹⁸O 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 C¹⁸O 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 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 A_v ~ 11 mag to the H₂ feature. The line fluxes yield a mass accretion rate of dot {M} ~ 2.6+/-0.9*E^-2 M_sun yr^-1 and a lifetime of ~5360 +/- 1200 yr resulting in a disk/envelope mass of 140 +/- 50 M_sun. The resulting Jeans Mass of 2420 M_sun 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.

Astronomy and Astrophysics
Volume 412, Page 175
December 2003

DOI: 10.1051/0004-6361:20031417