Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...464..247w&link_type=abstract
Astrophysical Journal v.464, p.247
Astronomy and Astrophysics
Astronomy
27
Ism: H Ii Regions, Ism: Molecules, Ism: Clouds, Galaxy: Open Clusters And Associations: General, Stars: Formation
Scientific paper
Observations are presented of an atomic cloud that we contend links a non-star-forming giant molecular cloud discovered by Maddalena & Thaddeus to two separate regions of OB star formation each over 50 pc away. The maps suggest that the cloud is a part of a larger molecular complex that has been partially photodissociated by these stars. The association of the cloud with stars of known spectral type allows a more definitive estimate of its heliocentric distance, 2.3 kpc. The precise amount of H I that is associated with the molecular gas is highly uncertain because of the ubiquity of emission along the line of sight, but it is argued that it may possess a large envelope (as seen around other molecular clouds) of mass ˜106 Msun, and that the mass of photodissociated gas due to the nearby OB stars is only a small fraction of this, ˜105 Msun. The molecular cloud itself is gravitationally bound, but the atomic cloud (whatever its mass) is unbound and probably evaporating into the general interstellar medium. The two sites of star formation that are most likely to have caused the photodissociation are S287 (an H II region dominated by an O9.5 V star) and a second, unidentified H II region with a infrared luminosity of 1.1 x 104 Lsun. However, the incident flux on the molecular cloud due to these H II regions is extremely weak and appears to be no more than the average Habing value, i.e., G0 ≃ 1, implying that the current rate of dissociation is small. The implications of these observations for diverse arguments concerning the evolutionary state of this cloud is discussed.
Maddalena Ronald J.
Williams Jonathan P.
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