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Newell' has calculated the spherically symmetrical contribution 0~to the magnetic shielding constant for H2. Extrapolating his result to HD through considerations based on the relative internuclear distances for the two molecules combined with an average of Eqs. (82) and (83) yields the following average shielding constant for the HD molecule: HD (~) (ss/3m') HD (Q rr) HD (~hf) = (3.216&0.010)X 10 '-(0.594&0.030) X 10 ' = (2.622&0.032) X 10 '.

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We have mapped the [C II] 158 km line over in the Magellanic irregular galaxy IC 10, thus 8@ .5 ] 6@ .5 presenting the Ðrst complete [C II] map of an entire low-metallicity galaxy. The total luminosity in the [C II] line in IC 10 is 1.5 ] 106We discuss the origin of the [C II] emission toward di †erent regions L _ . in the galaxy. Overall, about 10% of the [C II] emission can originate in standard H I clouds (n D 80, T D 100 K), while up to about 10% of the emission can originate in ionized gas, either the low-density warm gas or the denser H II regions. For the two brightest regions, most of the [C II] emission is associated with dense photodissociation regions (PDRs). For several regions, however, the [C II] emission may not be explained by standard PDR models. For these regions, emission solely from the atomic medium can also be precluded because the cooling rate per hydrogen atom would be much greater than the heating rate provided by photoelectric UV heating. We speculate that in these regions the presence of an additional column density of 5 times that observed in H I, is required to explain the [C II] emission. H 2 , The ambient UV Ðelds present in these regions, combined with the low metallicity, create a situation where small CO cores exist surrounded by a relatively large [C II]-emitting envelope where molecular hydrogen is self-shielded. This additional molecular mass is equivalent to at least 100 times the mass in the CO core that one would derive from the CO integrated intensity alone using the standard CO-to-H 2 conversion factor. These [C II] observations may, therefore, make a more reliable inventory of the gas reservoir in dwarf irregular galaxies where use of CO alone may signiÐcantly underestimate the molecular mass.