Hubble Space Telescope Spectroscopy of a Planetary Nebula in an M31 Open Cluster: Hot-Bottom Burning at $3.4M_{\odot }$

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, $\lesssim 2\%$. Moreover, the radial velocity of the PN is the same as that of the cluster within the measurement error of $\sim 10$ km s${}^{-1}$. Given the expected $\sim 70$ km s${}^{-1}$ velocity dispersion in this region, $\sim$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 $3.38_{-0.02}^{+0.03}{M}_{\odot }$ 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 $\sim$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 AGB stars with initial masses as low as $3.4M_{\odot }$.

18 pages, 5 figures, 4 tables