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.