AAS 196th Meeting, June 2000
Session 35. High Resolution Spectroscopy at Visible and Ultraviolet Wavelengths
Topical Session Oral, Wednesday, June 7, 2000, 8:30-10:00am, 10:45am-12:30pm, 2:30-4:00pm, 4:15-6:00pm, Highland A/K

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[35.18] STIS Observations of Interstellar C I

E.B. Jenkins (Princeton U.)

In the interstellar medium, the excited fine-structure levels in the ground electronic state of neutral carbon can be populated by collisions with other atoms and electrons. In turn, the levels are depopulated by spontaneous radiative decays. The balance between these processes allows us to solve for the physical conditions in the C~I-bearing clouds by measuring the relative populations in the three levels. For the most part, the thermal pressure of the gas p/k is the primary parameter that establishes the level populations.

In an effort to sample p/k in different places, a survey of the UV spectra of 21 stars is underway using the highest resolution mode of the STIS instrument on HST. Most of the target stars are in the HST Continuous Viewing Zone (CVZ), which means that galactic longitudes 120±20 and 300± 20 degrees are most heavily sampled. For each star, nine multiplets of C~I between 1189 and 1656Å\ are recorded at a resolving power \lambda/\Delta\lambda of about 200,000, revealing the details of the narrow C~I velocity components in the absorption profiles. A special analysis technique developed for this survey can be used to unscramble the overlapping features within the multiplets (a different pattern in each case), so that the velocity profiles for the three levels can be derived without implementing tedious, trial-and-error model fitting procedures. Also, we can derive the relative strengths of the different multiplets and the lines within each multiplet. The survey is about half complete, and the conclusions derived so far about the distribution of thermal pressures will be presented.

This research is supported by NASA contract NAG5-30110.

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