Chemical Abundances of the Typhon Stellar Stream

TL;DR

This study provides the first high-resolution chemical abundance analysis of stars in the Typhon stellar stream, revealing its likely dwarf galaxy origin and complex formation history through detailed chemical and kinematic data.

Contribution

It offers the first detailed chemical characterization of Typhon, a distant stellar stream, and discusses its possible origins and implications for the Milky Way's halo assembly.

Findings

Typhon's chemical abundances resemble a dwarf galaxy more than a globular cluster.

Typhon stars show enhanced alpha-element abundances and increasing r-process elements with metallicity.

Typhon's properties suggest a complex formation history involving dynamical interactions or group preprocessing.

Abstract

We present the first high-resolution chemical abundances of seven stars in the recently discovered high-energy stream Typhon. Typhon stars have apocenters >100 kpc, making this the first detailed chemical picture of the Milky Way's very distant stellar halo. Though the sample size is limited, we find that Typhon's chemical abundances are more like a dwarf galaxy than a globular cluster, showing a metallicity dispersion and no presence of multiple stellar populations. Typhon stars display enhanced $\alpha$-element abundances and increasing r-process abundances with increasing metallicity. The high-$\alpha$ abundances suggest a short star formation duration for Typhon, but this is at odds with expectations for the distant Milky Way halo and the presence of delayed r-process enrichment. If the progenitor of Typhon is indeed a new dwarf galaxy, possible scenarios explaining this apparent contradiction include a dynamical interaction that increases Typhon's orbital energy, a burst of enhanced late-time star formation that raises [$\alpha$/Fe], and/or group preprocessing by another dwarf galaxy before infall into the Milky Way. Alternatively, Typhon could be the high-energy tail of a more massive disrupted dwarf galaxy that lost energy through dynamical friction. We cannot clearly identify a known low-energy progenitor of Typhon in the Milky Way, but 70% of high-apocenter stars in cosmological simulations are from high-energy tails of large dwarf galaxies. Typhon's surprising combination of kinematics and chemistry thus underscores the need to fully characterize the dynamical history and detailed abundances of known substructures before identifying the origin of new substructures.