Signature of a massive rotating metal-poor star imprinted in the Phoenix stellar stream

Abstract

The Phoenix stellar stream has a low intrinsic dispersion in velocity and metallicity that implies the progenitor was probably a low mass globular cluster. In this work we use Magellan/MIKE high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity ($\ddot{I}ƒ([\mathrm{Fe}II/H])=0.04_{-0.03}^{+0.11}$) and a large peak-to-peak range in [Na/Fe] and [Al/Fe] abundance ratios, consistent with the light element abundance patterns seen in the most metal-poor globular clusters. However, unlike any other globular cluster, we also find an intrinsic spread in [Sr II/Fe] spanning $\sim$1 dex, while [Ba II/Fe] shows nearly no intrinsic spread ($\ddot{I}ƒ([\mathrm{Ba}II/H])={0.03}_{-0.02}^{+0.10}$). This abundance signature is best interpreted as slow neutron capture element production from a massive fast-rotating metal-poor star ($15-20{\mathrm{M}}_{\odot }$, $v_{\mathrm{ini}}/v_{\mathrm{crit}}=0.4$, $[Fe/H]=-3.8$). The low inferred cluster mass suggests the system would have been unable to retain supernovae ejecta, implying that any massive fast-rotating metal-poor star that enriched the interstellar medium must have formed and evolved before the globular cluster formed. Neutron capture element production from asymptotic giant branch stars or magneto-rotational instabilities in core-collapse supernovae provide poor fits to the observations. We also report one Phoenix stream star to be a lithium-rich giant ($A(\mathrm{Li})=3.1\pm 0.1$). At $[Fe/H]=-2.93$ it is among the most metal-poor lithium-rich giants known.

Accepted to ApJ 2021-07-09