IRAS Observations of Dark Globules

Details IRAS Observations of Dark Globules IRAS Observations of Dark Globules

Jun 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991jkas...24...55l&link_type=abstract

Journal of the Korean Astronomical Society, vol. 24, no. 1, p. 55-70

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Scientific paper

Infrared emission maps are constructed at 12.5, 25, 60 and 100 micrometer for dark globules B5, B34, B133, B134, B361, L134 and L1523 by using Infrared Astronomical Satellite data base. These clouds are selected on the basis of their appearance in Palomar print as dark obscuring objects with angular sizes in the range of 3 to 30 arcminutes. The short wavelength(12.5 and 25 micrometer) maps show the embedded infrared sources. We found many such sources only in B5, B361, and B34 regions. Diffuse component at 12.5 and 25 micrometer, possibly arising from the stochastically heated very small dust grains(a < 0.01 micrometer) by interstellar radiation field, is found in B361 and L1523 regions. Such emission is characterized by the limb brightening. and it is confirmed in L1523 and in B361. Infrared emissions at the long wavelengths (60 and 100 micrometer) are due to colder dusts with temperature less than 20K. The distribution of color index determined by the ratio 60 to 100 micrometer intensities shows monotonic decrease of dust temperature toward the center. The black body temperature determined from these ratios is found to lie between 16 and 23K. Such temperature is possible for small(i.e., a <= 0.01 micrometer) graphite grains if the grains are mainly heated by interstellar radiation field. Thus IRAS 100 and 60 micrometer emissions are arising from small grains in the cloud. The distribution of such dust grains implied from the emissivity distributions at 100 and 60 micrometer resembles that of isothermal sphere. This contrasts to earlier findings of much steeper distribution of dusts contributing visible extinction. These dust grains are mainly larger ones(i.e., a ~ 0.1 micrometer). Therefore, we conclude that the average grain size increase, toward the cloud center.

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