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We present a survey of variable stars detected in K2 Campaign 13 within themassive intermediate age ($\sim1$ Gyr) open cluster NGC 1817. We identify acomplete sample of 44 red clump stars in the cluster, and have measuredasteroseismic quantities ($\nu_{\rm max}$ and/or $\Delta \nu$) for 29 of them.Five stars showed suppressed dipole modes, and the occurrence rates indicatethat mode suppression is unaffected by evolution through core helium burning. Asubset of the giants in NGC 1817 (and in the similarly aged cluster NGC 6811)have $\nu_{\rm max}$ and $\Delta \nu$ values at or near the maximum observedfor core helium burning stars, indicating they have core masses near theminimum for fully non-degenerate helium ignition. Further asteroseismic studyof these stars can constrain the minimum helium core mass in red clump starsand the physics that determines this limit.  Two giant stars show photometric variations on timescales similar topreviously measured spectroscopic orbits. Thirteen systems in the field showeclipses, but only five are probable cluster members.  We identify 32 $\delta$ Sct pulsators, 27 $\gamma$ Dor candidates, and 7hybrids that are probable cluster members, with most new detections. We usedthe ensemble properties of the $\delta$ Sct stars to identify stars withpossible radial pulsation modes. Among the oddities we have uncovered are: aneccentric orbit for a short-period binary containing a $\delta$ Sct pulsatingstar; a rare subgiant within the Hertzsprung gap showing $\delta$ Sctpulsations; and two hot $\gamma$ Dor pulsating star candidates.

the astronomical journal

Variability in the Massive Open Cluster NGC 1817 from K2: A Rich  Population of Asteroseismic Red Clump, Eclipsing Binary, and Main Sequence  Pulsating Stars