HE0144-4657: A Carbon-Enhanced Ultra Metal-Poor Star ([Fe/H] ~ -4.1) from the Helmi Stream Disrupted Dwarf Galaxy

TL;DR

HE0144-4657 is a carbon-enhanced ultra metal-poor star with $[\mathrm{Fe}/\mathrm{H}] = -4.11$ that is dynamically associated with the Helmi Stream. The study combines Hamburg/ESO candidate selection, medium- and high-resolution spectroscopy, and a chemo-dynamical analysis to determine abundances for 18 elements and to characterize its orbit in a Milky Way potential. The light-element pattern and abundance fits strongly favor a mono-enriched second-generation origin formed from gas polluted by a single low-energy Population III supernova of approximately $50\ M_\odot$ and $E_{ m SN} \le 1.8\times10^{51}$ erg. This work provides compelling evidence that some UMP CEMP stars originate in disrupted dwarf galaxy environments and highlights the role of UFD-like birthplaces in shaping the early chemical evolution of the Milky Way.

Abstract

We present the discovery of HE0144-4657, 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, HE0144-4657 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 HE0144-4657 is a bona-fide second-generation star with a possible Population III progenitor in the 50Msun mass range with low explosion energy. One possible scenario for the origin of HE0144-4657 is that it was formed in an ultra-faint 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.

Figures (3)

• Figure 1: HE 0144$-$4657, as observed from HES (top left), NTT/EFOSC-2 (second row from the top left), and Magellan/MIKE (bottom two rows). The different color lines reference the same regions in each spectrum, highlighting the difference in resolution. Absorption features and molecular bands of interest are labeled throughout. Top right panel: $T_\mathrm{eff}$ vs. $\log\,g$ for HE 0144$-$4657 (blue circle), together with data from barklem2005 (gray squares, with green highlights for RHB stars). Solid lines show YY Isochrones from demarque2004 with 12 Gyr, 0.8$M_\odot$, [$\alpha$/Fe]=$+0.4$, and $[\mathrm{Fe}/\mathrm{H}]$=$-2.0,-2.5,-3.0$. The horizontal-branch tracks were taken from dotter2008. The inset shows $\xi$ (km/s) vs. $\log\,g$ for the same objects.
• Figure 2: First and second rows: Abundance determination via spectral synthesis for nitrogen and carbon. The filled gray squares connected by the black line represent the MIKE spectrum, the teal line is the best fit, and the shaded regions represent $\pm0.15$ and $\pm0.30$ dex from the best-fit abundance. Also shown is a synthetic spectrum without carbon and nitrogen (gray line). Third row from the top: Selected LTE [X/Fe] abundance ratios as a function of $[\mathrm{Fe}/\mathrm{H}]$ for HE 0144$-$4657 (blue circle), compared with the JINAbasejinabase and SAGAsaga2008 literature compilations (density map). Additional references include limberg2025placco2025. Bottom panel: Light-element abundance pattern for HE 0144$-$4657 (LTE and NLTE - blue points), compared with Population III supernova yields (solid lines, residuals given as vertical colored bars). See text for details.
• Figure 3: Top panels: fiducial orbit for HE 0144$-$4657 (blue circle) in Cartesian Galactocentric coordinates within the mcmillan2017 model potential. Top left: $(X,Z)$; MW edge-on view. Top right: $(X,Y)$; face-on. Blue arrows exhibit the present-day velocity vector of HE 0144$-$4657. Orange and green lines are forward and backward trajectories, respectively. Thicker lines show 200 Myr integration, while thinner transparent lines represent 2 Gyr total. Black dashed lines are the orbit of the Sun with the same assumptions (Section \ref{['kindyn']}) and the black symbol marks its current Galactic location. The gray regions illustrate the MW's thin disk with 18 kpc radius and 1 kpc maximum height. The thicker portion in the top left panel is the thick disk (14 kpc radius and 3 kpc height). Bottom panels: dynamical parameter spaces. Bottom left: $(L_z,E)$. Bottom center: projected action space (or action 'diamond'). The horizontal axis displays $L_z/J_{\rm total} \in [-1,+1]$, where $J_{\rm total} = J_R + |L_z| + J_z$ (see text). Vertical axis show $(J_z - J_R)/J_{\rm total} \in [-1,+1]$. Bottom right: $(L_z,L_\perp)$, where $L_\perp = \sqrt{L_x^2 + L_y^2}$. The white dashed rectangles delineate the Helmi Stream kinematic locus. In all bottom panels, green, yellow, and pink dots are Helmi Stream members from the SEGUE survey myeong2018shards, a very metal-poor sample (${\rm[Fe/H]}\xspace\leq-1.8$) including HES data limberg2021b, and Gaia DR3 RVS instrument dodd2023subs. We also highlight the dynamical volume occupied by the most relevant halo substructures, namely Gaia-Sausage/Enceladus belokurov2018helmi2018 and Sagittarius stream ibata1994majewski2003sgr. The colored background 2D histograms consist of the SEGUE/StarHorse catalog limberg2023queiroz2023sh.