Small-area molecular clouds without shielding

Abstract

Using the IRAM 30m telescope two molecular structures have been detected which cover very small areas, FWHM<1'. The clouds have velocities of v_lsr~5kms and linewidth of dv~0.8km/s; thus they belong most likely to the Milky Way. Applying standard conversion factors one finds that even at the upper distance limit of 2200pc the structures are low mass objects (M=(1-6)x10^{-4}(d/100pc)^2Msun) which are not gravitationally virialized. HI 21cm line data towards the clouds show no prominent HI clouds. The total HI column densities for both structures are below N(HI)<2.1x10^{20}cm^{-2}, corresponding to A_V<0.2mag, assuming a standard gas-to-dust ratio. IRAS 100micron data towards the structures show also only low emission, consistent with low extinction. Unless there is unseen cold dust associated with the structures this shielding is too low for the structures to survive the interstellar radiation field for a long time. The detection of 2 such structures in a rather limited sample of observations suggests that they could be a rather common feature in the interstellar medium, however, so far not recognized as such due to the weakness of their lines and their small extent.