1998A&A...336..682M

We have used the ROSAT 1/4keV all-sky survey together with H I observations to derive the total column density of hydrogen nuclei, N(H), of the Draco nebula [= G91+38 (v_LSR=-21km/s)], 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 obtained by this approach are biased since H I column densities underestimate the absorption in regions where molecular hydrogen 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 100µm data. We find that the absorbing column densities at the deepest X-ray shadows are up to about 3x10²⁰cm^–2 larger than the observed H I column densities. At the edge towards low galactic latitudes and longitudes, up to 70% of the hydrogen is in molecular 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 + 2H₂) of about 1.0x10^–20MJy/sr.cm². This is close to the mean value for the galactic cirrus (0.86x10^–20MJy/sr.cm²). In contrast, the FIR-emissivity per H I atom is varying strongly across the nebula. The x_WCO values (=N(H₂)/W(¹²CO)) found in the Draco nebula are typically in the range 0.34<x_WCO<0.52x10²⁰cm^–2.(K.km/s)^–1, similar to other cirrus clouds. We find a very low x_WCO ratio of 0.17cm^–2.(K.km/s)^–1 at the edge of the Draco nebula towards low galactic coordinates where the CO abundance could be altered in a low-velocity shock. Finally, the X-ray emission 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 constant plasma temperatures of 10^6.3K and 10^5.85K respectively. Since the Draco nebula (distance >300pc) is located outside the galactic gas layer, this is evidence of a bright Galactic X-ray corona or an extended coronal hot spot. The intensity of this coronal emission is constant over the observed 7°-field within the uncertainties of our analysis (<15%).