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T. Freeman (Bevill State College), G. Byrd, R. Buta (U. Alabama)
Rings of star formation are a common phenomenon of early-type barred spiral galaxies. These rings are thought to form by gas accumulation at resonances, under the continuous action of gravity torques from a bar pattern. The bright stars follow the original periodic orbits of their parent clouds, tracing out the rings. We study star formation at high resolution in the inner ring of NGC 3081, based on HST WFPC2 observations with filters F255W (mid-UV), F336W (U), F439W (B), F555W (V), and F814W (I). This ring is likely connected with an inner 4:1 ultraharmonic resonance in the disk plane. Our goals were to examine how star formation is distributed around this highly elliptical feature (b/a = 0.7), and whether we could identify clusters over a range of ages at a given position angle along the ring. We use colors, reddening-free parameters, and number counts to show that star formation in this ring is intimately connected with the dynamics of the ring. HST resolves the ring into several hundred slightly resolved or unresolved sources which clump strongly within +/-60 deg of the bar axis. This clumping is likely due to the slower speeds of gas clouds near the apocentra of their periodic orbits, and suggests that the ring is confined entirely within corotation. This work has been supported by NASA/STScI Grant GO 8707 and by NSF Grant RUI 9802918.