Heating and Cooling in the Translucent ISM: PAHs and [C II] in High Galactic Latitude Clouds

Details Heating and Cooling in the Translucent ISM: PAHs and [C II] in High Galactic Latitude Clouds Heating and Cooling in the Translucent ISM: PAHs and [C II] in High Galactic Latitude Clouds

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sptz.prop40207i&link_type=abstract

Spitzer Proposal ID #40207

Physics

Scientific paper

The systematic study of the emission from unidentified infrared bands (UIBs) and [C II] in translucent clouds can provide the missing key empirical information needed to understand the energetics of translucent gas in the Milky Way. Current consensus ascribes the UIB emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 microns to fluorescent emission from PAH molecules excited by local FUV photons. Thus observation of the UIBs in high Galactic latitude molecular clouds (HLCs) coupled with existing measurements of [C II] and FIR emission, allow us to make a direct assessment of the relative importance of heating by both large and small (PAH) grains. In our GO-2 Spitzer program 20093, we detected the faintest PAH emission spectra in Galactic clouds to date, demonstrating the unique capabilities of the Spitzer IRS to spectrally image low-extinction regions. The derived PAH band strength in our HLC sample is linearly correlated with [C II ] integrated intensity. The 7.7/11.3 micron intensity ratio, which is related to the PAH ionization fraction, appears to be inversely proportional to [C II]. Unfortunately the large scatter and paucity of data points in key regions of [C II]-space makes this potentially groundbreaking result difficult to establish reliably. The proposed IRS Short-Low observations will triple the sample size of HLC positions with both UIB and [C II] measurements from 11 to 33 lines of sight, with only a 1.4-fold increase in observatory time over our GO-2 program. It will fill the gaps in [C II] coverage in our UIB sample and greatly enhance the statistical significance of our results. Our ultimate goal is to clarify the relationship between PAH ionization fraction, which crucially affects gas heating, and [C II] intensity, which measures gas cooling. This research can dramatically improve our understanding of the most important processes that govern the thermal properties of ISM gas in low-extinction environments.

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