AAS 204th Meeting, June 2004
Session 15 Diffuse Media Galactic and Intergalactic
Oral, Monday, May 31, 2004, 10:00-11:30am, 601

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[15.05] Electron Temperatures and Abundances for H II Regions in the Spiral Galaxy M51

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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