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D551AFC4-F44B-4861-9A24-B646C5B0CD87

# Chemical Abundances of the Typhon Stellar Stream

Alexander P. Ji, Rohan P. Naidu, Kaley Brauer, Yuan-Sen Ting, Joshua D. Simon
arXiv:2207.04016

Submitted on 8 July 2022

## Abstract

We present the first high-resolution chemical abundances of seven stars in the recently discovered high-energy dwarf galaxy stream Typhon. Typhon stars have apocenters reaching to $\gtrsim 100$ kpc, making this the first detailed chemical picture of the Milky Way's very distant stellar halo. 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. We find that 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 within the Milky Way 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. An alternate explanation is that Typhon is 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, but cosmological simulations give about 2:1 odds that Typhon is a high-energy tail of a massive galaxy instead of a new disrupted galaxy. 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.

## Preprint

Comment: 12 pages, 4 figures, 2 tables, submitted to MNRAS

Subject: Astrophysics - Astrophysics of Galaxies