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

TL;DR

This study uses HST spectroscopy to confirm a planetary nebula in an M31 cluster, providing evidence that hot-bottom burning occurs in stars with initial masses around 3.4 solar masses.

Contribution

It presents the first empirical evidence of hot-bottom burning in AGB stars with initial masses as low as 3.4 M_sun, based on spectroscopic analysis of a cluster planetary nebula.

Findings

The planetary nebula is likely a cluster member based on velocity and position.

The nebula shows nitrogen enhancement, indicating hot-bottom burning.

Initial stellar mass for the progenitor is approximately 3.4 M_sun.

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 $3.38^{+0.03}_{-0.02} \, M_{\odot}$ 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 $3.4 \, M_{\odot}$.