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D. R. Garnett, R. C. Kennicutt (University of Arizona), F. Bresolin (Institute for Astronomy)
We present the first direct measurements of electron temperatures and element abundances for two H~II regions in the spiral galaxy M51. H~II regions in M51 have very low excitation, which led previous studies to infer that the abundances in this galaxy were quite high (log O/H = -2.9 to -2.6) compared to the solar value, based on photoionization models or extrapolation of empirical relations of abundances vs. strong emission line intensities. We have detected the [N~II] \lambda5755 line for the first time in the regions M51-CCM72 and M51-CCM10, which allows us to derive electron temperatures from the [N~II] \lambda6583/\lambda5755 intensity ratio. We find T(N~II) = 6000±300 K for CCM72 and 7400(+1000,-600) K for CCM10. Based on these temperature measurements, we derive log O/H = -3.2±0.1 for CCM72 and -3.5±0.2 for CCM10. These abundances are a factor of 2-3 smaller than values derived previously from photoionization models or extrapolation of strong-line calibrations. If confirmed, the new results imply that the inner parts of spirals are not as metal-rich as previously thought, and that the excitation of metal-rich H~II regions is relatively insensitive to abundances. Lower abundances in spiral disks could affect the derivation of quantities that may have metallicity dependence, e.g. the Cepheid P-L relation, or the mass of molecular gas derived from CO observations.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.