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

Abstract

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 for any PN produced by the cluster. From HST ’s Space Telescope Imaging Spectrograph observations and C loudy 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 ⊙ .