THE FORM OF ABUNDANCE GRADIENTS IN 3 NEARBY SPIRAL GALAXIES - M33, M81, AND M101

Available emission-line data have been compiled for giant H II regions located in three nearby, well-observed spirals M33, M81, and M101 in order to study the form of interstellar abundance patterns in the disk s of these galaxies. Numerous line strength ratios are shown to vary s ystematically with galactocentric distance. Gradients for stellar effe ctive temperature, ionization parameter, and the abundance ratios O/H, N/O, and S/O were determined using a sequential photoionization model analysis procedure. The observed behavior of line ratios across the d isks of these galaxies is found to be consistent with abundance gradie nts which are exponential in form and have constant slopes, thereby ch allenging recent suggestions that gradients flatten in the outer disk. Additionally, the N/O gradient in M101 is found to be markedly steepe r than in M33 and M81, suggesting that in M101 secondary nitrogen prod uction is relatively more efficient than primary nitrogen production.