Molecular Clouds and Star Formation

Author: Sargent, Anneila Isabel

Year: 1978

Degree: Dissertation (Ph.D.)

Advisor: Goldreich, Peter Martin

Committee Member: Unknown, Unknown

Option: Astronomy

DOI: 10.7907/XZSK-2H71

Abstract

Observations of the J = 1 → 0 transition of 12CO were made in and around the region occupied by the young OB association Cepheus OB3 to determine the connection between newly formed stars and molecular clouds. An extended (20 pc x 60 pc) molecular cloud was detected and mapped, and additional observations of 13CO and H2CO were made at selected positions. The total mass of the cloud is 5 x 103 M☉. A range of velocities is seen over the cloud complex. This is comparable with the stellar velocities in the association.

Within the molecular cloud are found three regions in which different stages of star formation are identified. In one an embedded star may be present. Another appears to be collapsing on a time scale of ~3 x 105 years to form a new subgroup of the OB association. The mass of this region, 500 M☉, is sufficient to produce only the number of O and B stars typically found in association subgroups, suggesting that lower mass stars must form under different conditions. The third displays signs of enhanced density and is probably the precursor of a star forming region. All three regions are situated in that part of the molecular cloud which is closest to the association stars.

The membership and ages of the subgroups of the Cepheus OB3 association have been re-examined, and their places of origin determined. Ages of 1-3 x 105 years and 5-7 x 105 years are found for younger and older subgroups respectively. Their birthplaces are not coincident but are situated at that end of the cloud where star formation still appears to be taking place. The younger subgroup appears to have formed closer to the active regions than did the older subgroup, so that there is a well-defined age sequence across the association which continues into the cloud.

There is currently little direct interaction between the stars and the molecular cloud, although a few younger subgroup stars still lie close to or within the complex. The absence of primeval gas in the environs of the older subgroup is attributed to the unusually high velocity of separation between these stars and the cloud.

It appears that the younger and still-forming subgroups of Cepheus OB3 are created in a different way from the older subgroup. However, the way in which star formation is initiated in this association remains uncertain.

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