A Fossil Star Without Planets? A High-Precision Look at BD+44°493
BD+44°493 is one of the brightest and most chemically pristine stars known, making it a rare “fossil” from the early Universe. Placco and collaborators focus on this extremely metal-poor, carbon-enhanced star because it preserves the chemical signatures of some of the first explosions in cosmic history. These types of stars are difficult to study, many of their absorption lines are faint, but they allow astronomers to infer the properties of Population III stars and the gas clouds they enriched. The authors also highlight a larger scientific question: can planets form around such metal-poor stars, when traditional planet-formation theories predict they should be scarce?
Observations and Data Processing with NEID
To investigate this question, the team collected fourteen nights of high-resolution spectra with the NEID spectrograph between October 2023 and January 2024. These data, taken at R ~ 110,000 and extremely high signal-to-noise, represent some of the best spectra ever acquired for an EMP star. After removing cosmic rays, aligning the spectra, and combining the 14 exposures, the authors produced a clean, precise dataset. They also incorporated archival Subaru/HDS and Hubble (STIS and COS) observations to re-derive additional elemental abundances using consistently updated stellar parameters.
Determining Atmospheric Parameters and Chemical Abundances
With the combined datasets, Placco et al. determined updated atmospheric parameters for BD+44°493, finding an effective temperature of 5351 K, surface gravity of log g = 3.12, and metallicity of [Fe/H] = −3.96. They measured or constrained 17 elements using NEID, and recalculated 11 more using Subaru and HST spectra. These measurements included lithium, carbon, nitrogen, oxygen, α-elements such as magnesium and calcium, and heavy elements like strontium and barium. The resulting chemical pattern strongly suggests that BD+44°493 formed from gas enriched by a single 20.5-solar-mass Population III supernova. Its high carbon, low neutron-capture elements, and very low europium upper limit align with expectations for a second-generation star formed shortly after the first stars died.
Galactic Motion and Age Determination
Using Gaia DR3 astrometry combined with precise radial velocities, the team calculated the orbit of BD+44°493 through the Milky Way. The star follows an eccentric, disk-like orbit and does not appear linked to any major merger events in the Galaxy’s early history. By combining stellar evolution models with this dynamical information, the authors estimate an age between 12.1 and 13.2 billion years, consistent with a very early formation, shortly after the birth of the first stars.
Radial Velocities and the Search for Planets
The NEID radial velocities reach a median absolute deviation of only 16 m/s, the most precise RVs ever obtained for an EMP star. With this precision, the authors searched for periodic signals indicative of orbiting planets. None were found. Instead, they place strict limits on possible companions: BD+44°493 cannot host planets more massive than about 2 Jupiter masses with orbital periods shorter than 100 days. These are the tightest constraints on planets around any star with [Fe/H] ≈ −4, implying that either planets are rare around such metal-poor stars or that only very low-mass planets, below the detection limits, could exist.
Conclusions and Implications for Early-Universe Planet Formation
Through this detailed chemo-dynamical study, the authors show that BD+44°493 remains one of the most informative “stellar fossils” known. It preserves signs of a single Population III progenitor and follows an orbit consistent with an ancient disk star. The NEID data also establish foundational limits on planet formation in the extremely metal-poor regime, linking stellar archaeology with modern exoplanet science. Even without detectable planets, BD+44°493 provides a unique laboratory for exploring what the earliest stars, and possibly the earliest planetary systems, were like.
Source: Placco
