The Composition Gradient in M101 Revisited:\\ H II Region Spectra and Excitation Properties
Session 49 -- Star Clusters in the Milky Way and Other Galaxies
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

## [49.09] The Composition Gradient in M101 Revisited:\\ H II Region Spectra and Excitation Properties

D. R. Garnett (University of Minnesota), R. C. Kennicutt, Jr. (University of Arizona)

We present the results of spectrophotometry, covering 3700-9700\AA, of 40 H II regions in the spiral galaxy M101. We are using the data to study the excitation and composition gradients in M101, and to examine the form and dispersion of the composition gradient. In addition, the large set of spectra for H II regions within a single galaxy allows us to examine the nature of H II region excitation and ionization sequences in a homogeneous way. We find that the shape of the abundance distribution, determined from the line intensity ratio R$_{23}$ $\equiv$ ([O II] + [O III])/H$\beta$, is sensitive to the choice of calibration of R$_{23}$ vs O/H. The dispersion in abundance at fixed radius can be attributed partly to variations in nebular ionization which correlate with residuals in R$_{23}$; this implies that the actual dispersion is smaller than the observed dispersion of $\pm$0.1-0.2 dex. The abundance residuals also show a marked spatial asymmetry, which suggests the possibility of a nonaxisymmetric abundance distribution. The residuals appear to show no correlation with either ionization parameter or ionizing radiation field hardness; these results strengthen the hypothesis that we are seeing a real difference in the abundance pattern. We speculate that the deviation from axisymmetry may be the result of a recent tidal interaction, perhaps related to the very large high-velocity H I structure detected by van der Hulst \& Sancisi (1988) and Kamphuis (1993).