  HCN is considered a good tracer of the dense molecular gas that serves as
fuel for star formation. However, recent large-scale surveys of giant molecular
clouds (GMCs) have detected extended HCN line emission. Such observations often
resolve the HCN J=1-0 hyperfine structure (HFS). A precise determination of the
physical conditions of the gas requires treating the HFS line overlap effects.
Here, we study the HCN HFS excitation and line emission using nonlocal
radiative transfer models that include line overlaps and new HFS-resolved
collisional rate coefficients for inelastic collisions of HCN with both para-H2
and ortho-H2 (computed via the scaled-IOS approximation up to Tk=500 K). In
addition, we account for the role of electron collisions in the HFS level
excitation. We find that line overlap and opacity effects frequently produce
anomalous HCN J=1-0 HFS line intensity ratios (inconsistent with the common
assumption of the same Tex for all HFS lines) as well as anomalous HFS line
width ratios. Line overlap and electron collisions also enhance the excitation
of the higher J rotational lines. Electron excitation becomes important for
molecular gas with H2 densities below a few 10^5 cm-3 and electron abundances
above ~10^-5. In particular, electron excitation can produce
low-surface-brightness HCN emission from very extended but low-density gas in
GMCs. The existence of such a widespread HCN emission component may affect the
interpretation of the extragalactic relationship HCN luminosity versus
star-formation rate. Alternatively, extended HCN emission may arise from dense
star-forming cores and become resonantly scattered by large envelopes of lower
density gas. There are two scenarios - namely, electron-assisted (weakly)
collisionally excited versus scattering - that lead to different HCN J=1-0 HFS
intensity ratios, which can be tested on the basis of observations.
