---
title: 'GDR3_526285: Ultra Metal-Poor Red Giant'
url: https://www.emergentmind.com/topics/gdr3_526285
type: topic
---

# GDR3_526285: Ultra Metal-Poor Red Giant

GDR3_526285, identified as Gaia DR3 Source ID 5262850721755411072, is an ultra metal-poor red giant-branch star in the Milky Way’s outer halo with a measured iron abundance of $\rm[Fe/H] = -4.82 \pm 0.25$. It was discovered through Gaia BP/RP (XP) spectro-photometric pre-selection and subsequently confirmed by multi-band photometry and high-resolution spectroscopy under local thermodynamic equilibrium. With $\rm[Fe/H] < -4$, it belongs to the ultra metal-poor regime, and its inferred total metal-mass fraction, $Z_{\rm GDR3\_526285}\lesssim1.0\times10^{-6}$, places it among the stars with the lowest known metal content. The object is therefore significant both as a surviving probe of early chemical enrichment and as a validation case for all-sky Gaia XP searches for the most iron-poor stars [2508.00067].

## 1. Classification and defining properties

GDR3_526285 is a bona-fide ultra metal-poor red giant-branch star. The discovery analysis classifies it as an RGB star with $T_{\rm eff}=4596\pm65\,{\rm K}$ and $\log g=0.88\pm0.15$, located in the Milky Way’s outer halo at a heliocentric distance of $d_{\rm helio}=24.1^{+4.9}_{-4.1}\,{\rm kpc}$. Its line-of-sight velocity is $v_{\rm los}=+428.7\pm2.0\,{\rm km\,s^{-1}}$, and its photospheric iron abundance is derived from 30 Fe I lines as $\rm[Fe/H]=-4.82\pm0.25$ [2508.00067].

The star is notable not only for its low iron abundance but also for its extremely small total metal content. Assuming $\rm[O,N/Fe]\simeq+0.6$ and summing all measured species, the total metal-mass fraction is estimated as $Z_{\rm GDR3\_526285}\lesssim1.0\times10^{-6}$. In the terminology used in the discovery paper, “metal” refers to elements with atomic number $>2$. This places GDR3_526285 among the chemically most primitive stellar survivors presently known [2508.00067].

| Quantity | Value |
|---|---|
| Gaia DR3 Source ID | 5262850721755411072 |
| Classification | UMP RGB star |
| $\rm[Fe/H]$ | $-4.82\pm0.25$ |
| $T_{\rm eff}$ | $4596\pm65\,{\rm K}$ |
| $\log g$ | $0.88\pm0.15$ |
| $d_{\rm helio}$ | $24.1^{+4.9}_{-4.1}\,{\rm kpc}$ |
| $v_{\rm los}$ | $+428.7\pm2.0\,{\rm km\,s^{-1}}$ |
| $Z_{\rm GDR3\_526285}$ | $\lesssim1.0\times10^{-6}$ |

## 2. Discovery through Gaia XP pre-selection

The source was first flagged in the Gaia DR3 BP/RP (XP) spectro-photometric catalog using the machine-learning catalog of Yao et al. 2023, which trained an XGBoost classifier on low-resolution Gaia XP spectra with $R\sim50$ to isolate likely very metal-poor stars with $\rm[Fe/H]<-2$. The candidate list was then refined by training an XGBoost regression on approximately 1200 literature stars with high-resolution [Fe/H] measurements drawn from SAGA and JINAbase, restricting attention to “golden” RGB candidates satisfying $\log g<3$ and $T_{\rm eff}<5200\,{\rm K}$ [2508.00067].

Within that search space, GDR3_526285 stood out because of its red color, Gaia BP–RP $=1.44$, its XP-predicted metallicity of approximately $\rm[Fe/H]\simeq-3.5$, and its brightness, with BP $\approx15.6$. These properties made it an optimal ultra metal-poor RGB target for high-resolution spectroscopic follow-up. The discovery thus served as a direct demonstration that Gaia XP spectroscopy, coupled to supervised machine-learning pre-selection, can recover exceptionally iron-poor halo giants from an all-sky catalog [2508.00067].

## 3. Spectroscopic characterization and abundance analysis

The high-resolution follow-up used Magellan/MIKE, with two 20 min exposures at resolving power $R\sim35{,}000$ on the blue arm and $R\sim28{,}000$ on the red arm. The resulting signal-to-noise ratio was approximately $20\,{\rm pix}^{-1}$ at 4000 Å and $64\,{\rm pix}^{-1}$ at 6500 Å. Atmospheric parameters were obtained through a hybrid photometric and evolutionary approach: $T_{\rm eff}$ from Gaia $G$, BP, RP and 2MASS $K$ photometry using the color–$T_{\rm eff}$ calibrations of Mucciarelli et al. (2021) with $E(B-V)=0.162$, and $\log g$ from interpolation on a 12 Gyr, $\rm[Fe/H]=-3.5$, $\rm[\alpha/Fe]=+0.4$ Y$^2$ isochrone. Monte Carlo sampling yielded $T_{\rm eff}=4596\pm65\,{\rm K}$, while the isochrone interpolation gave $\log g=0.88\pm0.15$ [2508.00067].

The microturbulent velocity was set from the quadratic $\xi$–$\log g$ relation of Barklem (2005), as refit by Ji et al. 2023, giving $\xi=2.27\pm0.10\,{\rm km\,s^{-1}}$. Elemental abundances were derived in LTE with MOOG 2017 plus scattering via smhr, using 1D Castelli & Kurucz atmospheres at $\rm[M/H]=-4$. The iron abundance, based on 30 Fe I lines, is $\rm[Fe/H]=-4.82\pm0.25$. This very low value is the principal basis for the star’s classification as ultra metal-poor and for its importance in empirical studies of the low-metallicity tail of Galactic stellar populations [2508.00067].

## 4. Carbon constraint and implications for star-forming gas cooling

The CH G-band region near 4300 Å shows no detectable carbon. A $\chi^2$ minimization over four 2.5 Å windows yields an upper limit of $\log\epsilon({\rm C})<4.11$, corresponding to $\rm[C/H]<-4.32$ and therefore $\rm[C/Fe]<+0.50$. After accounting for evolutionary depletion on the RGB using the Placco et al. (2014) correction $\Delta[\mathrm{C/Fe}]=+0.68$, the corrected upper limit becomes $\rm[C/Fe]_{\rm cor}<+1.18$ [2508.00067].

This upper limit is astrophysically consequential because it constrains the cooling channel of the natal gas. With $\rm[C/H]_{\rm cor}<-3.64$ and adopting $\rm[O/Fe]\simeq+0.6$, the Frebel & Bromm (2007) transition discriminant,
$$
D_{\rm trans}=\log_{10}\bigl(10^{[\mathrm{C/H}]}+0.3\times10^{[\mathrm{O/H}]}\bigr),
$$
satisfies $D_{\rm trans}\lesssim-3.6$, which is at or below the critical threshold of approximately $-3.5\pm0.2$. The discovery paper interprets this as evidence that fine-structure line cooling alone would fail in the progenitor gas, making dust-grain cooling the most plausible mechanism by which the gas fragmented to form GDR3_526285 [2508.00067].

A common over-interpretation would be to treat the carbon result as a positive measurement of enhancement. The published result is instead an upper limit: the analysis establishes no detectable carbon and constrains the corrected ratio only to $\rm[C/Fe]_{\rm cor}<+1.18$ [2508.00067].

## 5. Outer-halo dynamics and possible Magellanic origin

The star’s Galactic context is defined not only by its chemistry but also by its dynamics. Using the spectro-photometric $(T_{\rm eff},\log g)$ solution together with the Gaia $G$ magnitude, the heliocentric distance is estimated as $24.1^{+4.9}_{-4.1}\,{\rm kpc}$. Orbital quantities computed with Astropy and the McMillan (2017) potential give a high orbital energy, $E_{\rm tot}\gtrsim-0.5\times10^5\,{\rm km}^2\,{\rm s}^{-2}$, and a large $L_X\sim-9500\,{\rm kpc}\,{\rm km\,s^{-1}}$ [2508.00067].

Monte Carlo backward integrations in a time-varying Milky Way plus Large Magellanic Cloud potential following Vasiliev & Bovy (2021) show that 53% of realizations become bound to the LMC more than 2 Gyr ago, with pericenter $<60\,{\rm kpc}$. The kinematics therefore suggest two non-exclusive scenarios: either GDR3_526285 was dynamically perturbed by the infall of the Magellanic system, or it was formerly a member of the Magellanic Clouds and was later stripped by the Milky Way. The current data support a Magellanic connection but do not uniquely fix a single origin channel [2508.00067].

## 6. Methodological significance and catalog context

The discovery has methodological importance beyond the properties of a single star. The results explicitly showcase the potential of an all-sky search for low-metallicity targets with Gaia XP and confirm the described methodology as a useful “treasure map” for finding additional ultra metal-poor stars. In that sense, GDR3_526285 is both an astrophysical object of interest and a proof of concept for Gaia-based, machine-learning-assisted identification of rare chemically primitive giants [2508.00067].

A related point of catalog interpretation is that the source identifier also appears in a worked example in the Gaia Andromeda Photometric Survey literature. That example describes how to retrieve, clean, renormalize, and analyze Gaia DR3 epoch photometry for a single source and explicitly states that the quoted numbers are illustrative of the procedure rather than direct published measurements of the star. A plausible implication is that source-identifier searches should distinguish between the discovery-based physical characterization of GDR3_526285 and later methodological examples that reuse the identifier for demonstration [2206.05591].

In the present literature, the core scientific significance of GDR3_526285 lies in the conjunction of three properties: an iron abundance of $\rm[Fe/H]=-4.82$, a carbon upper limit low enough to place it near or below the fine-structure cooling threshold, and outer-halo kinematics consistent with a possible Magellanic connection. Together these make it a valuable empirical datum for studies of early chemical enrichment, low-metallicity star formation, and the assembly history of the Galactic halo [2508.00067].

Source: https://www.emergentmind.com/topics/gdr3_526285