[Paper Review] The Geneva-Copenhagen Survey of the Solar neighbourhood II. New uvby calibrations and rediscussion of stellar ages, the G dwarf problem, age-metallicity diagram, and heating mechanisms of the disk

Johan Holmberg, B. Nordström+1 more|ArXiv.org|Jul 13, 2007

Stellar, planetary, and galactic studiesPhysics and Astronomy47 references245 citations

TL;DR

This paper re-evaluates the Geneva-Copenhagen Survey of the Solar neighbourhood using improved uvbyβ photometric calibrations for effective temperature, metallicity, distance, and age in F and G stars. It confirms that isochrone ages are now reliably determined, finds a flat age-metallicity relation with large intrinsic scatter (σ_intrinsic = 0.20 dex), and demonstrates that thin disk heating persists throughout its lifetime, with kinematic criteria for disk separation requiring use of oldest thin-disk stars to avoid contamination.

ABSTRACT

Ages, metallicities, space velocities, and Galactic orbits of stars in the Solar neighbourhood are fundamental observational constraints on models of galactic disk evolution. We aim to consolidate the calibrations of uvby photometry into Te, [Fe/H], distance, and age for F and G stars and rediscuss the results of the Geneva-Copenhagen Survey (Nordstrom et al. 2004; GCS) in terms of the evolution of the disk. We substantially improve the Te and [Fe/H] calibrations for early F stars, where spectroscopic temperatures have large systematic errors. Our recomputed ages are in excellent agreement with the independent determinations by Takeda et al. (2007), indicating that isochrone ages can now be reliably determined. The revised G-dwarf metallicity distribution remains incompatible with closed-box models, and the age-metallicity relation for the thin disk remains almost flat, with large and real scatter at all ages (sigma intrinsic = 0.20 dex). Dynamical heating of the thin disk continues throughout its life; specific in-plane dynamical effects dominate the evolution of the U and V velocities, while the W velocities remain random at all ages. When assigning thick and thin-disk membership for stars from kinematic criteria, parameters for the oldest stars should be used to characterise the thin disk.

Motivation & Objective

• To improve the accuracy of uvbyβ photometric calibrations for effective temperature, metallicity, distance, and age in F and G stars.
• To re-evaluate stellar ages, the G dwarf problem, and the age-metallicity relation using updated data and simulations.
• To assess systematic errors and selection biases in the magnitude-limited GCS sample, particularly regarding kinematic and chemical properties.
• To validate the reliability of isochrone-based age determinations through comparison with independent spectroscopic age estimates.
• To clarify the role of dynamical heating in shaping the kinematics of the Galactic disk, especially for U, V, and W velocity components.

Proposed method

• Utilized high-resolution spectroscopy, Hipparcos parallaxes, V-K photometry, and angular diameters to refine uvbyβ photometric calibrations.
• Performed extensive numerical simulations on synthetic stellar catalogues with known intrinsic properties to test the robustness of age and kinematic derivations.
• Recomputed stellar ages using Bayesian techniques with theoretical isochrones, comparing results with independent age determinations from Takeda et al. (2007).
• Reassessed the Hyades cluster as a calibration standard, identifying offsets in photometry and non-standard helium abundance that invalidate its use for field star calibration.
• Analyzed selection effects from the apparent-magnitude limited sample and blue color cutoff, simulating their impact on observed age-metallicity and age-velocity relations.
• Evaluated kinematic criteria for thin- and thick-disk separation, emphasizing the need to use oldest thin-disk stars to avoid contamination from younger populations.

Experimental results

Research questions

• RQ1 How do improved uvbyβ photometric calibrations affect the determination of effective temperature, metallicity, distance, and age for F and G stars?
• RQ2 To what extent does the revised age-metallicity relation for the thin disk deviate from closed-box model predictions, and what is the intrinsic scatter?
• RQ3 Can isochrone-based ages be reliably determined, and how do they compare with independent spectroscopic age estimates?
• RQ4 What is the role of dynamical heating in shaping the kinematics of the thin disk, and how does it vary with time?
• RQ5 How do selection effects in the magnitude-limited GCS sample influence the observed age-velocity and age-metallicity relations?

Key findings

• The revised effective temperature and metallicity calibrations for early F stars significantly reduce systematic errors, especially where spectroscopic temperatures were previously unreliable.
• The intrinsic scatter in metallicity at any given age is large and real, with σ_intrinsic = 0.20 dex, indicating significant local variations in chemical enrichment history.
• The age-metallicity relation for the thin disk remains essentially flat, with no significant rise in mean metallicity over time, contradicting closed-box model predictions.
• Isochrone ages are now robustly determined, with strong agreement (within ~10%) with independent age estimates from Takeda et al. (2007), validating the method.
• Dynamical heating of the thin disk continues throughout its lifetime, with in-plane velocity components (U and V) showing time-dependent evolution, while W velocities remain random at all ages.
• Kinematic criteria for separating thin and thick disks must be based on the oldest thin-disk stars, as younger samples risk contamination from kinematic substructures like the Hercules stream.