Institution(s): 1. Laboratoire d'Astrophysique de Marseille, 2. Princeton University
The local cloud surrounding the Sun turns out to be a great laboratory to explore the details of the warm diffuse medium without the confusion from piling up absorptions from different regions. UV absorption lines observed by HST in the spectra of nearby stars permit us to construct a comprehensive picture of a diffuse interstellar cloud and reveals that it departs from homogeneity in a number of aspects and physical properties:
- This local cloud undergoes a deformation related to a compression in the direction of motion and an expansion in perpendicular directions, much like a squashed balloon.
- The cloud average HI density varies from 0.03 to 0.1 cm-3. The cloud outer boundary inferred from the average density and column densities is very irregular with an average distance away from the Sun of 9 +/- 7 pc.
- The metal abundances decrease steadily from the rear to the head of the cloud, and this phenomenon appears not to be related to ionization effects but rather to changes in the amounts of depletion due to dust formation.
- Most of the sight-lines on one hemisphere evidence the presence of gas at a uniform differential velocity of -7 km/s relative to the cloud, which may be the manifestation of a shock progressing toward the cloud interior on one half of the cloud, possibly created by a surge in the thermal pressure in the much lower density medium that surrounds the cloud.