"PAHs in the Diffuse ISM"
Our understanding of the IR emission features and their carriers (PAHs) is almost exclusively based upon spectral studies of bright HII regions and planetary nebulae. In contrast, while PAHs spend most of their lives cycling between the diffuse ISM and dark clouds, very little is known about the comparatively weak IR emission spectrum of the diffuse ISM. One of the major discoveries of IRAS was the widespread presence of cirrus emission at mid-IR wavelengths throughout the diffuse ISM, but IRAS was unable to characterize this emission further, and more recent studies using the IRTS and ISOPHOT did not produce spectra of sufficient quality for meaningful comparisons with other types of sources (Onaka et al. 1996; Mattila et al. 1996). We propose to measure the IR emission spectrum of the diffuse ISM along well studied lines of sight towards diffuse translucent clouds, of bright cirrus clumps, and of reflection nebulae. This will provide a census of low to mid density and UV field intensities that can be compared to existing Spitzer/ISO data of regions with high density and high UV field intensity (i.e. HII regions and planetary nebulae). The questions we will address are:
1. What is the mid-IR spectrum of the diffuse ISM?
2. How does the spectrum of the mid-IR emission spectrum depend on the local physical conditions (density, UV field) ?
3. How do the spectra of the diffuse ISM (low density, low UV field) compare to the spectra of reflection nebulae (intermediate conditions) and to those of HII regions and reflection nebulae (high density, high UV field)?
The proposed observations will provide direct information on the abundance and ionization state of PAHs in the diffuse ISM.