Hubble Space Telescope Spectroscopy of a Planetary Nebula in an M31 Open Cluster: Hot-bottom Burning at 3.4 M _o

We use imaging and spectroscopy from the Hubble Space Telescope (HST) to examine the properties of a bright planetary nebula (PN) projected within M31's young open cluster B477-D075. We show that the probability of a chance superposition of the PN on the cluster is small, ≲2%. Moreover, the radial velocity of the PN is the same as that of the cluster within the measurement error of ∼10 km s^-1. Given the expected ∼70 km s^-1 velocity dispersion in this region, ∼8 kpc from M31's nucleus, the velocity data again make it extremely likely that the PN belongs to the cluster. Applying isochrone fitting to archival color-magnitude photometric data from the HST Advanced Camera for Surveys, we determine the cluster age and metallicity to be 290 Myr and Z = 0.0071, respectively, implying an initial mass of 3.38-0.02+0.03 M for any PN produced by the cluster. From HST's Space Telescope Imaging Spectrograph observations and Cloudy photoionization modeling, we find that the PN is likely a Type I planetary, with a nitrogen abundance that is enhanced by ∼5-6 times over the solar value scaled to the cluster metallicity. If the PN is indeed a cluster member, these data present strong empirical evidence that hot-bottom burning occurs in asymptotic giant branch stars with initial masses as low as 3.4 M _o.