The Composition Gradient in M101 Revisited. I. H II Region Spectra and Excitation Properties

We present emission-line spectrophotometry in the 3700-9700 Å region for 41 H II regions in the nearby spiral galaxy M101. In this paper, we describe the observations and use the large homogeneous data set to examine the nature of the H ii region excitation and ionization sequences, and to examine the form and dispersion of the abundance distribution in M 101, based on the empirical R₂₃ index of Edmunds & Pagel. We find that the shape of the abundance distribution is very sensitive to the choice of calibration of the R₂₃ parameter, with some calibrations producing an excellent fit to a single exponential distribution, and others producing a distinct break in the gradient. The dispersion in abundance at fixed radius can be attributed partly to variations in nebular ionization which correlate with residuals in R₂₃. This implies that the actual dispersion in abundance is considerably less than the observed dispersion of ±0.1-±0.2 dex. The abundance residuals also show a marked spatial asymmetry, which suggests the possibility of a nonaxisymmetric distribution, perhaps the result of a recent tidal interaction in M101. However, more data are needed to test whether this asymmetry is real.