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Issue A&A
Volume 412, Number 1, December II 2003
Page(s) 175 - 184
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20031417



A&A 412, 175-184 (2003)
DOI: 10.1051/0004-6361:20031417

A massive disk/envelope in shocked H $\mathsf{_2}$ emission around an UCHII region

M. S. N. Kumar1, A. J. L. Fernandes1, 2, T. R. Hunter3, C. J. Davis4 and S. Kurtz5

1  Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 7150-462 Porto, Portugal
2  Instituto Superior da Maia, Av. Carlos Oliveira Campos, 4475-690 Avioso S.Pedro, Portugal
3  Harvard Smithsonian Center for Astrophysics, 60 Garden Street, MS-78 Cambridge, MA 02138, USA
4  Joint Astronomy Center, 660 N. A'ohoku Place, University Park, Hilo, HI 96720, USA
5  Instituto de Astronomia, UNAM-Morelia, Apartado postal 3-72, CP 58090 Morelia, Michoacan, Mexico

(Received 23 April 2003 / Accepted 2 September 2003 )

Abstract
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 ( $L=5.6\times10^4~L_{\odot}$) point source associated with an ultra-compact HII region. A Grey body model fit to the flux density distribution yields a dust emissivity index ( $\beta\sim0.66$) 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 $A_v\sim11$ mag to the H 2 feature. The line fluxes yield a mass accretion rate of $\dot{M}\sim 2.6\pm0.9\times10^{-2}~M_\odot$ yr -1 and a lifetime of ~5360 $\,\pm\,$1200 yr resulting in a disk/envelope mass of 140 $\,\pm\,$50  $M_\odot$. The resulting Jeans Mass of 2420  $M_\odot$ 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.


Key words: stars: formation -- accretion: accretion disks -- interstellar medium: jets and outflows -- ISM: HII regions

Offprint request: M. S. N. Kumar, nanda@astro.up.pt

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© ESO 2003


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