The Physical Properties of the Midcourse Space Experiment Galactic Infrared-dark Clouds

The SPIRIT III infrared telescope on the Midcourse Space Experiment (MSX) satellite has provided an unprecedented view of the mid-infrared emission (8-25 μm) of the Galactic plane. An initial analysis of images from MSX Galactic plane survey data reveals dark clouds seen in silhouette against the bright emission from the Galactic plane (Egan et al.). These clouds have mid-infrared extinctions in excess of 2 mag at 8 μm. We probed the physical properties of 10 of these MSX dark clouds using millimeter-wave molecular rotational lines as an indicator of dense molecular gas. All 10 clouds were detected in millimeter spectral lines of H2CO, which confirms the presence of dense gas. The distances to these clouds range from 1 to 8 kiloparsecs and their diameters from 0.4 to 15.0 pc. Excitation analysis of the observed lines indicates that the clouds are cold (T < 20 K) and dense [n(H2) > 105 cm-3]. Some of the clouds have nearby H II regions, H2O masers, and other tracers of star formation at comparable spectral line velocities; however, only one cloud contains embedded centimeter or infrared sources. The lack of mid- to far-infrared emission associated with these clouds suggests that they are not currently forming high-mass stars. If star formation is present in these clouds, it is clearly protostellar class 0 or earlier.

[1]  Martin G. Cohen,et al.  A Population of Cold Cores in the Galactic Plane , 1998 .

[2]  S. Charnley.  Chemical models of interstellar gas-grain processes — III. Molecular depletion in NGC 2024 , 1997 .

[3]  F. Clark,et al.  All-Sky 4.85 GHz Flux Measurements of H II Regions , 1997 .

[4]  C. Beichman,et al.  The High-Resolution IRAS Galaxy Atlas , 1997, astro-ph/9705147.

[5]  A. Bolatto,et al.  HCN and CO in the Central 630 Parsecs of the Galaxy , 1996 .

[6]  R. Hurt,et al.  Potential Protostars in Cloud Cores: H 2 CO Observations of Serpens , 1996 .

[7]  J. Bernard,et al.  Molecular clouds in Cygnus. 1: A large-scale (13)CO survey , 1994 .

[8]  O. Manuel Uy,et al.  Midcourse Space Experiment: Introduction to the Spacecraft, Instruments, and Scientific Objectives , 1994 .

[9]  J. Carlstrom,et al.  SUBMILLIMETER CONTINUUM SURVEY OF THE GALACTIC CENTER , 1994 .

[10]  K. Menten,et al.  Star formation in the galactic center dust ridge , 1994 .

[11]  J. Mangum,et al.  Formaldehyde as a probe of physical conditions in dense molecular clouds , 1993 .

[12]  S. Green.  Collisional excitation of formaldehyde in 'hot' interstellar molecular regions , 1991 .

[13]  R. Laureijs,et al.  The temperature of large dust grains in molecular clouds , 1991 .

[14]  A. Penzias,et al.  C-12/C-13 isotope ratio across the Galaxy from observations of C-13/O-18 in molecular clouds , 1990 .

[15]  P. Myers,et al.  A survey for dense cores in dark clouds , 1989 .

[16]  J. Mathis,et al.  Composite interstellar grains , 1989 .

[17]  R. Wilson,et al.  Galactic center molecular clouds. II. Distribution and kinematics , 1988 .

[18]  L. Mundy,et al.  Models of molecular cloud cores. III. A mulitransition study of H2CO , 1987 .

[19]  Francis J. Lovas,et al.  NIST Recommended Rest Frequencies for Observed Interstellar Molecular Microwave Transitions -- 2002 Revision , 1986 .

[20]  A. Wootten,et al.  High-excitation lines of deuterated formaldehyde (HDCO) in the Orion molecular cloud , 1985 .

[21]  D. Clemens.  Massachusetts-Stony Brook Galactic plane CO survey: the galactic disk rotation curve. , 1985 .

[22]  H. M. Lee,et al.  Optical properties of interstellar graphite and silicate grains , 1984 .

[23]  B. L. Ulich,et al.  Recommendations for calibration of millimeter-wavelength spectral line data. , 1981 .

[24]  N. Evans,et al.  Models of molecular clouds and the abundances of H2CO and HCO , 1980 .

[25]  P. Goldsmith,et al.  Molecular cooling and thermal balance of dense interstellar clouds , 1978 .

[26]  C. Leung.  Radiation transport in dense interstellar dust clouds. I. Grain temperature , 1975 .

[27]  E. Barnard.  On the dark markings of the sky, with a catalogue of 182 such objects. , 1919 .