The giant spiral galaxy M 101. VIII - Star formation in H I-H II associations

Details The giant spiral galaxy M 101. VIII - Star formation in H I-H II associations The giant spiral galaxy M 101. VIII - Star formation in H I-H II associations

Nov 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982a%26a...115..373v&link_type=abstract

Astronomy and Astrophysics, vol. 115, no. 2, Nov. 1982, p. 373-387.

Astronomy and Astrophysics

Astrophysics

54

Hydrogen Clouds, Interstellar Matter, Spiral Galaxies, Star Formation, Balmer Series, Cosmic Dust, Gravitational Collapse, H Lines, Interstellar Extinction, Lyman Spectra, Magellanic Clouds, Milky Way Galaxy, Molecular Gases

Scientific paper

In Paper VII of this series, we have discovered several correlations among the physical properties of the giant H I-H II associations in M 10l. The present paper deals with the interpretation of these correlations in terms of models for the collapse of the interstellar medium into massive H I clouds and the subsequent formation of stars.
The major results are:
1. The observed physical properties of the associations (gas mass, volume density, size) can be. consistently described by a model of gravitational collapse in the spiral arms. The derived collapse time scales are about 108 yr with shock velocities of about 25km s-1 (Table 1). The model also accounts for the observed variation in morphology of the complexes with galactocentric radius.
2. A new correlation of the observed dust-to-gas ratio with the optical excitation [O III]/Hβ has been discovered for these giant complexes (Fig. 1). The higher-excitation objects have a lower relative dust content.
3. The observed correlations of the star formation rate and the dust-to-gas ratio with the optical excitation provide a parametrization of the star formation rate in terms of the physical properties of the associations [Eq. (16)].
4. An evolutionary picture is developed which provides a framework for understanding the observed correlations among physical properties of the H I-H II associations. The evidence confirms that these spectacular objects are very short-lived on a time scale of one revolution (∼5 108 yr) of the galaxy. The collapse phase takes about one quarter of a revolution, but the luminous star-forming phase in a given H I-H II association is probably not longer than a few percent of a revolution. Stars are currently being formed at such a high rate in these associations (several solar masses per year) that the lifetimes of the complexes in this phase must be less than about 107 yr.

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