Multiwavelength observations of collisional ring galaxies. III. Oxygen/nitrogen abundances and star formation properties of ring knots

This is the third paper in a series examining the multiwavelength propertie s of a sample of northern ring galaxies, in which we present optical long-s lit spectra and broadband BVR and JHK colors of individual star-forming kno ts embedded in the ring. We present the oxygen and nitrogen abundances of t he starburst knots and compare the reddening-corrected colors of individual star clusters with recent models of stellar evolution. We also present kin ematic data for one system, LT 41. The star-forming knots in the rings have very similar colors, which implies similar ages in the range 4-80 Myr. The se ages are less than the dynamical ages of the rings, which are typically 100-300 Myr. The fact that the implied ages of knots within individual ring galaxies cluster around the same age argues that their formation is recent ly triggered by a coherent event all around the ring. This strongly support s the triggering of the star formation as being due to the propagation of a radial density wave, as in the classical picture. The ring galaxies are fo und to have subsolar metallicities in the range one-half to one-fifth solar in [O/H] and [N/H] ratios. There is a suggestion of an increase in the mea n nitrogen abundance for the rings of larger linear size, but oxygen abunda nces show no trend with ring diameter. The uniformity of the oxygen abundan ces over the sample is consistent with their rather narrow range of K-band luminosities, which suggests that even in strongly disturbed galaxies, when prompt local enrichment is expected to be important, the primary factor go verning oxygen metallicity is total luminosity.