X-RAY SHADOWS OF THE DRACO NEBULA - A NEW METHOD TO DETERMINE TOTAL HYDROGEN COLUMN DENSITIES

We have used the ROSAT 1/4 keV all-sky survey together with HI observa tions to derive the total column density of hydrogen nuclei, N(H), of the Draco nebula [= G91+38 (v(LSR) = -21 km s(-1))], which casts a dee p shadow in the soft X-ray background. Adopting a two-component model for the X-ray plasma in which one component is located behind the Drac o nebula, the other in front of all the absorbing material (the so-cal led Local Hot Bubble, LHB), we fit the parameters of the radiation tra nsport equation to the observed X-ray count rates. The optical depth i n this equation is derived from H I column densities obtained with the 100-m telescope and the appropriate X-ray absorption cross sections. The solutions obtained by this approach are biased since HI column den sities underestimate the absorption in regions where molecular hydroge n is abundant. The bias is avoided by excluding regions with strong X- ray shadowing from the fit and by comparing fits which are obtained on the basis of hydrogen column densities derived from IRAS 100(-) mu m data. We find that the absorbing column densities at the deepest X-ray shadows are up to about 3 10(20) cm(-2) larger than the observed 1-1 I column densities. At the edge towards low galactic latitudes and lon gitudes, up to 70% of the hydrogen is in molecular form. In other part s of the nebula the molecular abundance is less than or similar to 25% . We also find an approximately constant FIR-emissivity per hydrogen n ucleon (H I + 2H(2)) of about 1.0 10(-20) MJy sr(-1) cm(2) This is clo se to the mean value for the galactic cirrus (0.86 10(-20) MJy sr(-1) cm(2)). In contrast, the FIR-emissivity per H I atom is varying strong ly across the nebula. The x(WCO) values (= N(H-2)/W((CO)-C-12)) found in the Draco nebula are typically in the range 0.36 < x(WCO) < 0.52 10 (20) cm(-2) (K km s(-1))(-1), similar to other cirrus clouds. We find a very low x(WCO) ratio of 0.17 cm(-2) (K km s(-1))(-1) at the edge of the Draco nebula towards low galactic coordinates where the CO abunda nce could be altered in a low-velocity shack. Finally, the X-ray emiss ion measure for the distant component of the X-ray emitting plasma is found to be about 5 times larger than that for the LHB, assuming const ant plasma temperatures of 10(6.3) K and 10(5.85) K respectively. Sinc e the Draco nebula(distance > 300 pc) is located outside the galactic gas layer, this is evidence of a bright Galactic X-ray corona or an ex tended coronal hot spot. The intensity of this coronal emission is con stant over the observed 7 degrees-field within the uncertainties of ou r analysis (< 15%).