The open cluster distance scale

A new empirical approach

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK e-mail: ms@astro.livjm.ac.uk
² South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa e-mail: dmk@saao.ac.za

Corresponding author: S. M. Percival, smp@astro.livjm.ac.uk

Received: 21 November 2002
Accepted: 9 January 2003

Abstract

We present new photometry for a sample of 54 local G and K stars with accurate Hipparcos parallaxes in the metallicity range . We use this sample to develop a completely model-independent main sequence (MS) fitting method which we apply to 4 open clusters – the Hyades, Praesepe, the Pleiades and NGC 2516 – which all have direct Hipparcos parallax distance determinations. Comparison of our MS-fitting results with distances derived from Hipparcos parallaxes enables us to explore whether the discrepancy between the Hipparcos distance scale and other MS-fitting methods found for some clusters is a consequence of model assumptions. We find good agreement between our results and the Hipparcos ones for the Hyades and Praesepe. For the Pleiades and NGC 2516, when adopting the solar abundance determined from spectroscopy, we find significant disagreement at a level similar to that found by other MS-fitting studies. However, the colour-colour relationship for both these clusters suggests that their metallicity is significantly subsolar. Since the MS-fitting method relies on matching the cluster colours to a template MS, we argue that, when applying this method, the appropriate metallicity to adopt is the photometric subsolar one, not the solar abundance indicated by spectroscopy. Adopting photometric metallicities for all 4 clusters, we find complete agreement with the Hipparcos results and hence we conclude that the mismatch between the spectroscopic and photometric abundances for the Pleiades and NGC 2516 is responsible for the discrepancies in distance estimates found by previous studies. The origin of this mismatch in abundance scales remains an unsolved problem and some possible causes are discussed.