[Paper Review] New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey

L. Casagrande, Ralph Schönrich+6 more|Scientific Electronic Library Online (São Paulo Research Foundation, Latin American and Caribbean Center on Health Sciences Information, Conselho Nacional de Desenvolvimento Científico e Tecnológico)|Mar 23, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy127 references452 citations

TL;DR

This study reanalyzes the Geneva-Copenhagen Survey using improved stellar effective temperatures from the infrared flux method and recalibrates metallicities against high-resolution spectroscopy, resulting in a 100 K hotter temperature scale and a 0.1 dex higher metallicity. The revised data show the Sun is now statistically typical in metallicity and enable the first photometric proxy for [α/Fe], revealing a thick disc with old, mildly alpha-enhanced stars extending to super-solar metallicities.

ABSTRACT

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Stromgren photometry we are able to derive for the first time a proxy for alpha elements, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context.

Motivation & Objective

• To improve the accuracy of stellar effective temperatures and metallicities in the Geneva-Copenhagen Survey using the infrared flux method.
• To recalibrate the metallicity scale using high-resolution spectroscopy and independent benchmarks like open clusters and a moving group.
• To enable a kinematically unbiased, age-resolved analysis of the solar neighbourhood with consistent astrophysical parameters.
• To derive a photometric proxy for [α/Fe] from Strömgren photometry to probe chemical substructures in the Galactic disc.
• To test the radial migration hypothesis by analyzing correlations between kinematics, metallicity, and age in the disc population.

Proposed method

• Apply the infrared flux method (IRFM) to derive more accurate effective temperatures, reducing systematic uncertainties to ~20 K.
• Use the new $T_{\mathrm{eff}}$ scale to recalibrate metallicities against high-resolution spectroscopic data from ~1500 stars.
• Cross-validate the metallicity scale using four open clusters and a moving group, achieving an intrinsic scatter below 0.10 dex in [Fe/H].
• Derive a photometric proxy for [α/Fe] using Strömgren $uvby$ photometry, calibrated against spectroscopic data.
• Perform Bayesian isochrone fitting using the improved parameters to derive consistent ages, masses, and distances.
• Combine kinematics, metallicities, and ages to investigate chemo-dynamical trends and test radial migration models.

Experimental results

Research questions

• RQ1 How do improved effective temperatures from the infrared flux method affect the metallicity scale and stellar age estimates in the Geneva-Copenhagen Survey?
• RQ2 To what extent does the revised metallicity scale resolve discrepancies in the age–metallicity relation and the metallicity distribution function?
• RQ3 Can Strömgren photometry provide a reliable proxy for [α/Fe] abundances in F and G dwarfs, enabling a photometric dissection of the thin and thick discs?
• RQ4 What evidence does the data provide for radial migration in the Galactic disc, based on correlations between kinematics, age, and metallicity?
• RQ5 Is the Sun still an atypical star in metallicity, or does the revised scale place it within the typical range of solar neighbourhood stars?

Key findings

• The new effective temperature scale is on average 100 K hotter than previous estimates, primarily due to improved infrared flux method calibration.
• The revised metallicity scale increases the mean metallicity of the sample by 0.1 dex, shifting the peak of the metallicity distribution function to [M/H] ≈ −0.01 dex.
• The Sun is now found to be statistically typical in metallicity, resolving prior assumptions of solar metallicity excess.
• The first photometric proxy for [α/Fe] is successfully derived from Strömgren photometry, enabling tentative chemical decomposition of the thin and thick discs.
• Evidence is found for a thick disc composed of old, mildly alpha-enhanced stars extending to super-solar metallicities, consistent with spectroscopic studies.
• The data show a broadening of the metallicity distribution at older ages and distinct patterns in the rotation velocity–abundance plane, supporting the radial migration scenario in disc evolution.