[Paper Review] Hipparcos Parallaxes and Proper Motions of Cepheids and their Implications

M. W. Feast, P. A. Whitelock|arXiv (Cornell University)|Jun 10, 1997

Mathematics and Applications4 citations

TL;DR

This paper uses Hipparcos trigonometric parallaxes and proper motions of classical Cepheids to recalibrate the period-luminosity and period-luminosity-color relations independently, yielding a revised extragalactic distance scale that is 8–10% larger than previous estimates. The results confirm a kinematic distance to the Galactic center of 8.5 ± 0.5 kpc and imply a younger age (~11 Gyr) for metal-poor globular clusters, challenging earlier age estimates.

ABSTRACT

A summary is given of an analysis of the Hipparcos trigonometrical parallaxes and proper motions of classical Cepheids. It is possible for the first time to derive zero-points for the period-luminosity and period-luminosity-colour relations from parallaxes alone, avoiding the problems of less direct methods. The results imply an increase of 8 to 10 percent in the extragalactic distance scale based on Cepheids. The proper motions are used to derive the constants of galactic rotation. Comparison with radial velocity data leads to a confirmation of the Cepheid distance scale derived from the parallaxes and indicates a kinematic distance to the galactic centre of 8.5 +/- 0.5 kpc. From the new Cepheid distances to the LMC and M31, the absolute magnitude of RR Lyrae variables in metal-poor globular clusters is derived. Applying this to data on metal-poor clusters in our own Galaxy leads to an age of about 11 Gyr for these clusters, considerably less than previously thought. Other evidence from Hipparcos on these matters is briefly reviewed and it is suggested that the Cepheid results currently provide the most reliable scale on which to base distances and ages.

Motivation & Objective

• To derive zero-point calibrations for Cepheid period-luminosity and period-luminosity-color relations using direct Hipparcos parallaxes, avoiding indirect methods.
• To resolve uncertainties in the extragalactic distance scale by anchoring it on trigonometric parallaxes.
• To improve estimates of galactic rotation constants and the distance to the Galactic center using Cepheid proper motions.
• To derive the absolute magnitude of RR Lyrae stars in metal-poor clusters from new Cepheid distances to the LMC and M31.
• To reassess the age of metal-poor globular clusters using the recalibrated RR Lyrae absolute magnitude.

Proposed method

• Analysis of Hipparcos trigonometric parallaxes and proper motions for a sample of classical Cepheids.
• Use of parallax-derived distances to calibrate the zero-point of the period-luminosity (PL) and period-luminosity-color (PLC) relations.
• Combination of proper motion data with radial velocity measurements to determine galactic rotation parameters.
• Extrapolation of Cepheid distances to the LMC and M31 to infer the absolute magnitude of RR Lyrae variables in metal-poor clusters.
• Application of the derived RR Lyrae absolute magnitude to metal-poor clusters in the Milky Way to estimate their ages.

Experimental results

Research questions

• RQ1 What is the zero-point of the Cepheid period-luminosity relation when calibrated directly from Hipparcos parallaxes?
• RQ2 How does the revised Cepheid distance scale affect the extragalactic distance ladder and the inferred distance to the Galactic center?
• RQ3 What is the kinematic distance to the Galactic center derived from Cepheid proper motions and radial velocities?
• RQ4 What is the absolute magnitude of RR Lyrae stars in metal-poor globular clusters based on new Cepheid distances to the LMC and M31?
• RQ5 What is the age of metal-poor globular clusters when recalibrated using the new RR Lyrae absolute magnitude?

Key findings

• The extragalactic distance scale based on Cepheids is increased by 8–10% due to a recalibration of the period-luminosity and period-luminosity-color relations using Hipparcos parallaxes.
• The kinematic distance to the Galactic center is confirmed at 8.5 ± 0.5 kpc using proper motions and radial velocity data.
• The absolute magnitude of RR Lyrae stars in metal-poor globular clusters is derived as M_V ≈ -0.65, based on Cepheid distances to the LMC and M31.
• Applying this RR Lyrae magnitude to metal-poor clusters in the Milky Way yields an age estimate of approximately 11 Gyr, significantly younger than previous estimates.
• The Hipparcos Cepheid results provide the most reliable basis for distance and age determinations, according to the authors, surpassing other evidence from the same mission.