HE 0144–4657: A Carbon-enhanced Ultra-metal-poor Star (Fe/H ∼ −4.1) from the Helmi Stream Disrupted Dwarf Galaxy*

Abstract We present the discovery of HE 0144−4657, an ultra-metal-poor, CNO-enhanced star dynamically associated with the Helmi Stream disrupted dwarf-galaxy remnant. This star was first identified as a carbon-enhanced, metal-poor star candidate from the Hamburg/ESO objective-prism survey, then followed up with medium- and high-resolution spectroscopy. At Fe/H = −4.11, HE 0144−4657 is the lowest-metallicity star found in a stellar stream to date. Its chemistry is consistent with field halo stars in the same metallicity regime, and the light-element (atomic number Z ≤ 30) chemical abundance pattern suggests that HE 0144−4657 is a bona fide second-generation star with a possible Population III progenitor in the 50 M ⊙ mass range with low explosion energy. One possible scenario for the origin of HE 0144−4657 is that it was formed in an ultrafaint dwarf galaxy accreted by the Helmi Stream progenitor system before merging with the Milky Way. This discovery provides further evidence for the extragalactic origin of carbon-enhanced ultra-metal-poor stars in the Milky Way and for the specific environments conducive to their formation.