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/Magellan Inamori Kyocera Echelle (MIKE) high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity ( $\sigma ([\mathrm{Fe}\,{\rm\small{II}}/{\rm{H}}])={0.04}_{-0.03}^{+0.11}$ ) 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 ~1 dex, while [Ba II/Fe] shows nearly no intrinsic spread ( $\sigma ([\mathrm{Ba}\,{\rm\small{II}}/{\rm{H}}])={0.03}_{-0.02}^{+0.10}$ ). This abundance signature is best interpreted as slow-neutron-capture element production from a massive fast-rotating metal-poor star (15-20 M_⊙, v_ini/v_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(Li) = 3.1 ± 0.1). At [Fe/H ] = -2.93; it is among the most metal-poor lithium-rich giants known. ^* This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.