Three Ancient Halo Subgiants: Precise Parallaxes, Compositions, Ages, and Implications for Globular Clusters

Abstract: The most accurate ages for the oldest stars are those obtained for nearby halo subgiants, because they depend almost entirely on just the measured parallaxes and absolute oxygen abundances. In this study, we have used the Fine Guidance Sensors on the Hubble Space Telescope to determine trigonometric parallaxes, with precisions of 2.1% or better, for the Population II subgiants HD 84937, HD 132475, and HD 140283. High quality spectra have been used to derive their surface abundances of O, Fe, Mg, Si, and Ca, which are assumed to be 0.1-0.15 dex less than their initial abundances due to the effects of diffusion. Comparisons of isochrones with the three subgiants on the $(\log\,T_{\rm eff}, M_V)$-diagram yielded ages of $12.08 \pm 0.14, 12.56 \pm 0.46$, and $14.27 \pm 0.38$ Gyr for HD 84937, HD 132475, and HD 140283, in turn, where each error bar includes only the parallax uncertainty. The total uncertainty is estimated to be $\sim\pm 0.8$ Gyr (larger in the case of the near-turnoff star HD 84937). Although the age of HD 140283 is greater than the age of the universe as inferred from the cosmic microwave background by $\sim$ 0.4-0.5 Gyr, this discrepancy is at a level of $< 1\,\sigma$. Nevertheless, the first Population II stars apparently formed very soon after the Big Bang. (Stellar models that neglect diffusive processes seem to be ruled out as they would predict that HD 140283 is $\sim 1.5$ Gyr older than the universe.) The halo field subgiants appear to be older than globular clusters of similar metallicities: if distances close to those implied by the RR Lyrae standard candle are assumed, M92 and M5 are younger than HD 140283 and HD 132475 by $\sim 1.5$ and $\sim 1.0$ Gyr, respectively.

• The paper analyzes three ancient field halo subgiants (HD 84937, HD 132475, HD 140283) using precise astrometry and spectroscopy to determine their ages and compositions.
• Using HST parallaxes and high-resolution spectroscopy, the study refined distance and metallicity, allowing for accurate age estimations via isochrone fitting.
• The findings indicate ages up to 14.27 Gyr for HD 140283 and suggest age discrepancies between field subgiants and globular cluster stars of similar metallicity, impacting stellar population models.

Overview of "Three Ancient Halo Subgiants: Precise Parallaxes, Compositions, Ages, and Implications for Globular Clusters"

This paper presents a comprehensive analysis of three field halo subgiants - HD 84937, HD 132475, and HD 140283 - using precise parallax measurements and chemical abundance analyses to infer their ages. These subgiants are among the oldest stars in the Galactic halo and provide key insights into the early stages of star formation following the Big Bang. High-precision data from the Fine Guidance Sensors on the Hubble Space Telescope (HST) were employed, alongside new spectroscopic analyses, to refine the distance and metallicity determinations, thereby allowing for more accurate age assessments.

Methodology

The study utilized trigonometric parallaxes from the HST to reduce the uncertainties in distance measurements substantially compared to those available from Hipparcos data. By correcting for instrumental and observational biases, the authors were able to achieve parallax precisions of better than 2.1%. With this robust astrometric baseline, they coupled high-resolution spectroscopy to deduce surface abundances for oxygen, iron, and other key elements, while considering corrections for diffusive processes known to affect surface abundances over the stellar lifetime.

Results

The isochrone fitting technique, applied to the derived astronomical and spectroscopic parameters, suggested ages of approximately 12.09 Gyr for HD 84937, 12.56 Gyr for HD 132475, and a notably older 14.27 Gyr for HD 140283. The age determination for HD 140283 is of particular interest as it slightly exceeds the inferred age of the universe from cosmic microwave background measurements, albeit within a statistical error margin of less than 1σ. This star, therefore, represents one of the oldest objects in the Milky Way and hints at the near-simultaneous formation of Population II stars and the early universe's creation. The paper discusses how adopting models allowing for helium diffusion reconciles the ages with cosmological constraints, effectively ruling out models that do not incorporate such processes.

Implications for Globular Clusters

The study extends its implications to globular clusters (GCs), specifically M 92 and M 5, showing an apparent age discrepancy where these subgiants appear older than stars in GCs of similar metallicity. This suggests that field stars and GC stars may have different formation histories or initial conditions, which might cause variations in ages deduced from nuclear chronology and the chemistry of stellar atmospheres. The authors highlight that the distance modulus derived from field subgiants does not align well with those derived from other standard candles like RR Lyrae stars or GC main sequence fitting, posing new questions on homogeneous stellar population assumptions in these GCs.

Discussion and Future Directions

The analysis underscores the critical role of precise parallax and consistent spectroscopic constraints in stellar age determination, an area that will further benefit from upcoming Gaia mission data. Theoretical models of stellar evolution, particularly concerning diffusion processes and their interaction with convection zones, remain areas for improved characterization to better match empirical results. Additionally, understanding the precise age and chemical composition of primordial stars could influence models of early galactic formation and evolution.

In summary, this research exemplifies how combining astrometric precision with refined spectroscopic techniques can yield significant insights into stellar evolution and cosmic chronology. The findings invite further investigation into the potential variances in age and composition between field stars and globular cluster companions, offering the potential to enrich our understanding of galactic and cosmic history.