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Avery, L.W., and S. Green, 1989: Collisional rate coefficients of C3H2 and the determination of physical conditions in molecular clouds. Astrophys. J., 337, 306-317, doi:10.1086/167104.
Collisional excitation rates for C3H2, calculated using the coupled states approximation, are presented. These rates are used in statistical equilibrium calculations to evaluate the potential of C3H2 rotational spectral lines as diagnostic probes of physical conditions in molecular clouds. C3H2 produces a number of spectral line pairs, whose members are close together in frequency but arise from levels with different excitation energies. The brightness temperature ratio of such a line pair is independent of systematic observational errors but is sensitive to variations in density and radiative trapping in molecular clouds. The calculated dependence of brightness temperature ratio upon H2 density and C3H2 abundance is presented for a number of line pairs for a simple, homogeneous cloud under the large velocity gradient radiative transfer model. Among the more useful pairs for diagnostic purposes are 1(10)-1(01), 2(20)-2(11) (18343 MHz, 21587 MHz), 2(12)-1(01), 3(12)-3(03) (85339 MHz, 82966 MHz), 2(12)-1(01), 2(02)-1(11) (85339 MHz, 82094 MHz), and 4(14)-3(03), 4(04)-3(13) (150852 MHz, 150821 MHz).
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