A nearly pristine star from the Large Magellanic Cloud

Abstract

The first stars formed out of pristine gas, causing them to be so massive that none are expected to have survived until today. If their direct descendants were sufficiently low-mass stars, such stars could exist today and would be recognizable by having the lowest metallicities (abundance of elements heavier than helium). We present the independent identification and detailed chemical analysis of the star SDSS J0715-7334, finding ultra-low elemental abundances of both iron and carbon ([Fe/H] = -4.3, [C/Fe] < -0.2) and total metallicity Z < 7.8 x 10^{-7} (log Z/Zsun < -4.3). The star's orbit indicates that it originates from the halo of the Large Magellanic Cloud. Its heavy element abundance pattern can be explained by a primordial supernova with an initial mass of 30 solar masses. This star is over ten times more chemically pristine than the most extreme high-redshift galaxies currently found by the James Webb Space Telescope. It is sufficiently metal-poor that current models of low-mass star formation require dust cooling to explain its existence.

Figures (9)

• Figure 1: J0715$-$7334 MIKE Spectrum.(a) Ca H and H$\varepsilon$ compared to CD$-$38 245 that has $T_\mathrm{eff}\xspace=4889$ K and $\mathrm{\,[Fe/H]}\xspace=-3.9$Roederer2014Mittal2025 (blue line). J0715$-$7334 (black line) is narrower in both, indicating it is both cooler and more metal-poor. Interstellar medium (ISM) lines are also marked. (b) Mg b region compared to CD$-$38 245. (c) CH G band region. Normal solid and thin dashed red lines indicate synthetic 3D LTE spectra corresponding to a $3\sigma$ and $5\sigma$ upper limit, respectively. Grey regions are masked as the 3D model only includes CH lines. The thick solid lines offset at $+0.9$ indicate the smoothed residuals for the $3\sigma$ upper limit (red) and the smoothed data assuming no carbon (black). The statistical significance of the upper limit is calculated using a profile likelihood and is related to the integrated area between these two lines (see Methods and \ref{['fig:chsynth']}).
• Figure 2: Carbon and iron abundances of ultra-metal-poor stars. J0715$-$7334 is shown as a large red star. Black points show a literature sampleAbohalima2018Sestito2019. Colored points highlight eight other notable stars, with 1D LTE abundances shown as small open symbols and a combination of 1D NLTE, 3D LTE, and 3D NLTE analyses shown as large solid colored symbols (see Methods). The dashed blue contours indicate an approximate total metallicity assuming $\hbox{[Mg/Fe]}=+0.4$ (see Methods). A horizontal dark red line indicates the critical $D_{\rm trans}$ thresholdBromm2003Frebel2007 for atomic fine structure line cooling assuming $\hbox{[C/O]} = -0.6$Amarsi2019, and the two shaded regions show $-1 < \hbox{[C/O]} < 0$ and an extra 0.2 dex theoretical uncertaintyBromm2003.
• Figure 3: Kinematic properties.Top: Past orbit of J0715$-$7334 over $4$ Gyr in Galactocentric coordinates, integrated in a potential that includes the gravitational influence of the LMC (solid lines). Circular markers are placed every $1$ Gyr. For comparison, the past orbit of the LMC itself is shown, along with orbits of the stars J1029$+$1729 (confined to the disk) and LMC-119 (closely bound to the LMC). The dashed red line shows the unbound orbit of J0715$-$7334 in a MW-only potential. Bottom: The past orbit of J0715$-$7334 and the LMC in Galactic coordinates on-sky, overlaid on the distribution of all stars observed by Gaiagai21.
• Figure 4: Population III Supernova Progenitor Constraints.Top: Chemical abundance pattern of J0715$-$7334. Filled red stars show measured abundances with $1\sigma$ uncertainties; open red stars with arrows are upper limits (treated as hard cutoffs). Sc is treated as an upper limit due to model uncertainties; Cr and Zn are excludedHeger2010. The blue line and caption shows the best-fit model; black lines show other models within $95\%$ confidence, with opacity indicating fit quality. Bottom left: Progenitor mass distribution of models weighted using total absolute error (see Methods). Bottom right: Explosion energy vs. progenitor mass for the same models; point size scales with fit quality. Best-fit model shown in blue.
• Figure Extended Data Figure 1: Extended Data Figure 1. Profile likelihood for CH upper limit. Left: the two spectral orders being fit with a 7th degree polynomial, the data is fit well. A red model is plotted indicating the $3\sigma$ upper limit, masked regions shown in grey. Center top: Stitched and normalized spectrum (black line, with $1\sigma$ pixel uncertainties shown as dashed black lines) compared to the best-fit (effectively no-carbon) spectrum (blue line) and the 3$\sigma$ upper limit (red line). The data and blue lines are normalized to 1, while the red $3\sigma$ model is normalized to the dashed red $3\sigma$ continuum line. The dashed red line is not exactly at 1, because the continuum is redetermined at every value of A(C), resulting in a more conservative upper limit when compared to a fixed continuum by about 0.2 dex. We apply an extra +0.2 dex correction to the final results, as the 3D model's stellar parameters are not identical to our adopted parameters (see text). Center bottom: error-normalized residual for the best fit model (blue) and the $3\sigma$ upper limit (red). The per-pixel value is shown as a thin line, while the thick line is smoothed over 2 pixels. The red line is above the blue line where the CH features are. Note this is an approximation for visualization: the calculation is done on each order independently, not on the stitched spectrum. Right:$\chi^2$ as a function of A(C). The blue point marks our minimum $\chi^2$ value. The $3\sigma$ upper limit, corresponding to 99.9% confidence or $\Delta \chi^2 = 10.273$ for 1 degree of freedom, is marked as a red point.