TRACING THE MOLECULAR-HYDROGEN CONTENT OF THE DRACO NEBULA - VERY LOWN(H-2) W(CO(C-12) RATIOS OR VARYING FIR-EMISSIVITIES/

We have analysed three complete data sets, the IRAS 100 mum intensity, I100, the H I column density, N(H i), and the integral of the J = 1 - -> 0 line of the (CO)-C-12 molecule, W((CO))-C-12, for the Draco nebul a [= IVC G091.0 +38.0 (VLSR = -21)] to trace the total column density of hydrogen nuclei, N(H), and to derive the extinction, A(V), and the total gas mass of this nebula. Adopting the relations between these qu antities which were set up by Heithausen & Mebold we have analysed the [I100, N(H i), W((CO))-C-12]-data cube by the method of the principal component transformation. The results are tested by the statistical F - and t-tests. We derive the FIR-emissivity per H I nucleon at 100 mum , a100, and the ratio x(WCO) = N(H-2)/W((CO))-C-12 for different regio ns of the Draco nebula. A difference of the FIR-emissivities for the p redominantly molecular cloud cores and their predominantly atomic halo s is established and taken into account for the determination of x(WCO ). The FIR-emissivities found in the Draco nebula are larger than the mean value for the galactic cirrus of 1 10(-20) MJy ster-1 cm2. They d epend on the molecular content as well as on the 60 mum/100 mum colour temperature. This dependence is probably due to screening of the inte rstellar radiation field, variations of the dust grain size, and/or a non-constant gas-to-dust ratio. The x(WCO) values in the Draco nebula are typically in the range 0.02 10(20) cm < x(WCO) < 0.26 10(20) (K km s-2)-1, i.e. more than a factor of 10 smaller than in the galactic pl ane and smaller than in most other cirrus clouds. The lowest values ar e found in regions where we suspect interaction with high velocity clo uds. The low x(WCO) values are only partly due to optical depth effect s of the (CO)-C-12 J = 1 --> 0 lines or excitation conditions of the C O molecules by hydrogen atoms. They result probably from the combined effects of high temperatures and an increase of the CO abundance in th e shocks of the interaction zone. Our results are used to derive the a tomic and the molecular content [M(H-2) = 45M.D2 (500 pC)-2] and the t otal mass of the Draco nebula [M = 630M.D2 (500 pc)-2] and its differe nt molecular clumps. The consequences for the visual extinction and th e distance, D, are discussed.