X-ray shadows of the Draco nebula. A new method to determine total hydrogen column densities

Details X-ray shadows of the Draco nebula. A new method to determine total hydrogen column densities X-ray shadows of the Draco nebula. A new method to determine total hydrogen column densities

Aug 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...336..682m&link_type=abstract

Astronomy and Astrophysics, v.336, p.682-696 (1998)

Astronomy and Astrophysics

Astrophysics

11

Ism: Atoms, Ism: Clouds, Dust, Extinction, Ism: Individual Objects: Draco Nebula, Ism: Molecules, X-Rays: Ism

Scientific paper

We have used the ROSAT (1)/(4) keV 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 = -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 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 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 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 la25 %. We also find an approximately constant FIR-emissivity per hydrogen nucleon (H I + 2H_2) of about 1.0 10(-20) MJy sr(-1) cm(2) . This is close 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 strongly across the nebula. The xWCO values (equiv N(H_2)/W((12) CO)) found in the Draco nebula are typically in the range 0.34 < xWCO < 0.52 10(20) cm(-2) (K km s(-1) )(-1) , similar to other cirrus clouds. We find a very low xWCO 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 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(6.3) K and 10(5.85) K respectively. Since 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 extended coronal hot spot. The intensity of this coronal emission is constant over the observed 7degr -field within the uncertainties of our analysis (< 15%).

Affiliated with

Also associated with

No associations

Rating

X-ray shadows of the Draco nebula. A new method to determine total hydrogen column densities does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with X-ray shadows of the Draco nebula. A new method to determine total hydrogen column densities, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and X-ray shadows of the Draco nebula. A new method to determine total hydrogen column densities will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-1574171