[CII] as a star formation rate tracer in M31: Is [CII]/TIR the best photoelectric heating efficiency proxy?

The [CII] 158$\mu$m line is typically the brightest far-IR emission line from star-forming galaxies. To use this line as a tracer of star-formation and a diagnostic of the ISM conditions, we must understand details of the heating mechanisms generating it and contributing to the total observed emission, especially the photoelectric (PE) heating mechanism.