Gradients in the physical conditions of M101 and the pregalactic helium abundance

Photoelectric spectrophotometry of five H II regions and the nucleus of the nearby Sc galaxy M101 has been conducted in order to study gradients in galactic physical conditions and abundances and the pregalactic helium abundance. Observations were carried out in the range 3400-7400 A with the 2.1-m telescope at KPNO and an intensified image dissector scanner. Radial abundance gradients are found for the elements He, N, O, Ne, S and Ar, with the O and N gradients about half as great as those measured in the solar vicinity. An electron temperature gradient is also derived which is similar to that found in Galactic H II regions at galactocentric distances from 4 to 14 kpc. The N/O ratios in M101 are, however, considerably smaller than those for the Galaxy, an observation explained by a greater proportion of low mass and therefore relatively younger stars in M101. Estimations of the helium abundance gradient with Z content in the metal-poor regions of M101 yield a value of 0.216 plus or minus 0.010 for the pregalactic helium abundance.