A new method to determine total hydrogen column densities

We have used the ROSAT 1 keV all-sky survey to- gether with H I observations to derive the total column den- sity of hydrogen nuclei, N (H), of the Draco nebula (= G91+38 (vLSR= 21 km s 1 )), which casts a deep 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 Draco nebula, the other in front of all the absorbing material (the so-called Local Hot Bubble, LHB), we fit the parameters of the radiation transport equation to the observed X-ray count rates. The optical depth in 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 ob- tained by this approach are biased since H I column densities underestimate the absorption in regions where molecular hy- drogen 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 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 ob- served H I column densities. At the edge towards low galactic latitudes and longitudes, up to 70% of the hydrogen is in molec- ular form. In other parts of the nebula the molecular abundance is < 25%. We also find an approximately constant FIR-emissivity per hydrogen nucleon (H I +2 H2) of about 1.0 10 20 MJy sr 1 cm 2 .