|Institution:||University of Hawaii – Manoa|
|Keywords:||O stars; B stars; Infrared astronomy|
|Full text PDF:||http://hdl.handle.net/10125/9340|
Photocopy of typescript. Bibliography: leaves 127-138. Microfiche. x, 138 leaves ill., maps 29 cm The dense, hot cores of 24 molecular clouds were examined at infrared wavelengths between 2.2 and 25 µm in search of embedded, luminous objects thought to massive 0 and B stars at an early stage of their evolution. 19 of the clouds were examined at 20 µm for high sensitivity to sources of low color temperature between 80 and 200 K. The clouds IC 1848 A, S 235, S 255, S 269, IR 12.4 + 0.5, S 140 and Cepheus A contain a total of 12 embedded sources of radiation more luminous than 6 x 10^3 L⊙ of which 7 were discovered during the course of this research. The 20 µm searches were particularly fruitful, yielding discoveries of 4 objects too red to be discovered in previous searches at 2.2 and 10 µm. In all cases the infrared sources are found projected against the densest part of the molecular cloud where the 13CO column density exceeds 2 x 1016 cm-2 On the other hand, no compact infrared sources were found in clouds such as Cepheus B which has high gas temperature, about 30 K, but low gas density less than 3 x 10^3 cm^-3 . The absence of embedded sources in some cases suggests that heating by stars external to the cloud can result in elevated temperatures in rarified clouds. The success of the 20 µm searches points out two biases inherent in studies made at shorter wavelengths; First, the effect of intra-cloud extinction is to obscure deeply embedded objects at wavelengths shorter than 20 µm. A reliance on counts of sources discovered at 2.2 and 10 µm will result in a serious underestimate of the rate of formation of massive stars. Second, short wavelength studies favor the discovery of objects which lie close to the front of a cloud where the intra-cloud extinction is low, thereby creating an artificial need to explain the preferential location of these objects close to the cloud boundaries. The importance of these prejudices to theories of star formation is discussed. This work has led to the realization that infrared sources in molecular clouds tend to form in clusters whose characteristic size is very similar to that found in trapezium-like groups of 0 and B stars. It seems likely that molecular clouds fragment as their densities reach 10^4 cm^-3 into independently collapsing nuclei separated by about 0.1 pc and each capable of producing one (or more) star(s). Some embedded infrared sources are surrounded by a small amount of ionized material. The strength of the radio emission seen toward objects brighter than 10^4 L⊙ such as Cepheus A and S 140 is consistent with these objects being pre-main sequence 0 and B stars.