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Fine structure lines are powerful tracers of ISM, especially regions mechanically and radiatively affected by massive star formation. [CII] and [OI] generally trace star formation both in Galactic sources and external galaxies but there are important caveats emerging from detailed studies of velocity-resolved spectra. [CII] absorption by diffuse ISM can corrupt results for emission regions when observed with inadequate velocity resolution and optical depths may be significant in large PDRs. For [CII] observation of the [13CII] is a powerful tool to allow for correction for optical depth. For [OI], there are extensive regions of low-excitation atomic oxygen that significantly absorb the emission from hot PDR gas adjacent to HII region. There is no useful isotopic line, so that we are highly dependent upon modeling of the emission to determine the optical depth. The input from astrochemical models is also valuable and more work to have them include realistic geometries would be valuable. Understanding these issues will require velocity-resolved fine structure line images of extended regions, which will be produced by SOFIA and upcoming balloon missions GUSTO and ASTHROS.