[Paper Review] Distances and Absolute Ages of Galactic Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs

R. Gratton, F. Fusi Pecci+4 more|arXiv (Cornell University)|Jul 9, 1997

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper uses high-precision Hipparcos parallaxes of 30 local subdwarfs and their metal abundances to recalibrate distances and absolute ages of nine Galactic globular clusters. It finds a longer distance scale (0.2 mag) and younger ages (~2.8 Gyr) than previous estimates, resolving tensions with standard cosmological models by yielding a globular cluster age of 11.8 (+2.1/-2.5) Gyr for old clusters.

ABSTRACT

High precision trigonometric parallaxes from the HIPPARCOS satellite and accurate metal abundances ([Fe/H], [O/Fe], and [alpha/Fe]) from high resolution spectroscopy for about 30 local subdwarfs have been used to derive distances and ages for a carefully selected sample of nine globular clusters. We find that HIPPARCOS parallaxes are smaller than the corresponding ground-based measurements leading, to a longer distance scale (~ 0.2 mag) and to ages ~ 2.8 Gyr younger. The relation between the zero age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine programme clusters is : Mv(ZAHB) = (0.22 +/- 0.09)([Fe/H]+1.5) + (0.48 +/- 0.04) The corresponding Large Magellanic Cloud distance modulus is (m-M)o=18.61 +/- 0.07. The age of the bona fide old globular clusters (Oosterhoff II and Blue Horizontal Branch) based on the absolute magnitude of the turn-off is: Age = 11.8 (+2.1/-2.5) Gyr The present age of globular clusters does no longer conflict with standard inflationary models for the Universe.

Motivation & Objective

• To improve the accuracy of distance and age estimates for Galactic globular clusters using independent distance indicators.
• To resolve discrepancies between globular cluster age estimates and standard cosmological models.
• To calibrate the zero-age horizontal branch (ZAHB) magnitude-metallicity relation using local subdwarfs as distance anchors.
• To assess the impact of Hipparcos parallax measurements on the cosmic distance scale and cluster age determination.
• To determine the distance modulus of the Large Magellantic Cloud using the calibrated cluster distances.

Proposed method

• High-precision trigonometric parallaxes from the Hipparcos satellite were used for 30 nearby subdwarfs.
• High-resolution spectroscopy provided accurate metal abundances ([Fe/H], [O/Fe], [alpha/Fe]) for the subdwarfs.
• The absolute magnitude of the ZAHB was calibrated using the subdwarf parallaxes and metallicities.
• The distance modulus of the Large Magellantic Cloud was derived from the calibrated cluster distances.
• Age estimates were computed from the absolute magnitude of the main-sequence turnoff, using isochrone fitting.
• Systematic differences between Hipparcos and ground-based parallaxes were quantified and corrected.

Experimental results

Research questions

• RQ1 What is the impact of Hipparcos parallaxes on the derived distances to Galactic globular clusters?
• RQ2 How does the ZAHB absolute magnitude depend on metallicity, and what is its calibrated relation?
• RQ3 What is the revised age of old globular clusters when based on Hipparcos-calibrated distances?
• RQ4 Does the recalibrated age of globular clusters remain consistent with standard inflationary cosmological models?
• RQ5 What is the distance modulus of the Large Magellantic Cloud based on the new calibration?

Key findings

• The Hipparcos parallaxes are systematically smaller than ground-based measurements, leading to a longer distance scale of approximately 0.2 mag.
• The ZAHB absolute magnitude-metallicity relation is Mv(ZAHB) = (0.22 ± 0.09)([Fe/H] + 1.5) + (0.48 ± 0.04).
• The distance modulus of the Large Magellantic Cloud is (m-M)o = 18.61 ± 0.07.
• The age of bona fide old globular clusters (Oosterhoff II and Blue Horizontal Branch) is 11.8 (+2.1/-2.5) Gyr, with statistical uncertainties.
• The revised age no longer conflicts with standard inflationary models of the Universe.
• The systematic shift in parallaxes implies a ~2.8 Gyr younger age estimate compared to previous ground-based calibrations.