Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........11m&link_type=abstract
Ph.D. Thesis State Univ. of New York, Stony Brook.
Astronomy and Astrophysics
Astronomy
11
Carbon Monoxide, Far Infrared Radiation, Infrared Astronomy, Infrared Sources (Astronomy), Luminosity, Molecular Clouds, Star Formation, H Ii Regions, Heating, Infrared Astronomy Satellite, Interstellar Radiation, Mass To Light Ratios
Scientific paper
In order to study the star formation process, the total far-lR luminosity, CO luminosity, and virial mass were measured for 78 first Galactic quadrant molecular clouds using the Massachusetts-Stony Brook CO Survey and the IRAS Galactic plane images. Fifty-one of the clouds are distinct IR sources (IR-strong) associated with H II regions. The remaining 27 clouds have low-IR surface brightnesses (IR-quiet), most of which contain weak radio continuum sources that are not cataloged H II regions. Estimates of the far-IR flux due to external heating of the clouds enabled a determination of the radial distribution of the Galactic interstellar radiation field, which has an approximate exponential scale length of 1.5-2.0 kpc, and a separation of the external and internal contributions to the clouds total far-IR luminosity. The total and internal far-IR luminosities of both IR-types are nearly proportional to the CO luminosity. IR-strong clouds are dominantly heated internally by embedded high-mass, ionizing stars of spectral types earlier than approximately B2, and IR-quiet clouds are mostly heated externally with internal intermediate-mass stars of spectral types later than approximately B3. An average equivalent to 1/3 of the total far-IR luminosity of IR-strong clouds, but nearly 3/4 of that of IR-quiet clouds, is due to external heating. The ratio of the far-IR luminosity to cloud mass -- a measure of the high-mass star formation activity of the clouds -- ranges over several orders of magnitude with a moderate maximum of approximately 20 (solar luminosity/solar mass), and is independent of cloud mass for both IR-types. High-mass star formation occurring in IR-strong clouds is equally efficient or inefficient across a wide range of cloud mass. The independence of high-mass star formation activity on cloud mass, and the long gas depletion times (approximately greater than 109 years) expected from the modest average activity of 2-3 (solar luminosity/solar mass), exclude efficient self-induced, or cloud-cloud collision initiated star formation mechanisms. The evolutionary state of molecular clouds can be inferred from their IR/CO morphologies, far-IR properties, and observed internal populations. Driven primarily by high-mass star formation, molecular clouds evolve from recently formed massive cold clouds, through a cloud-destructive high-mass star formation phase, to post-OB star formation cloud remnants. IR-quiet clouds are most likely a mixture of old and young molecular clouds, with IR-strong clouds as the transition between these two IR-quiet phases.
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