The distance to the Pleiades

Main sequence fitting in the near infrared

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK e-mail: [smp,ms]@astro.livjm.ac.uk
² Instituut voor Sterrenkunde, PACS-ICC, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: groen@ster.kuleuven.ac.be

Received: 20 July 2004
Accepted: 6 September 2004

Abstract

Hipparcos parallax measurements of stars in the Pleiades notoriously result in a cluster distance of 118 pc, which is approximately 10% shorter than the “classical” result obtained from earlier main sequence (MS) fitting studies. In an earlier paper we developed a purely empirical MS-fitting method in an attempt to address this problem. This work produced conflicting results for the Pleiades between the and colour indices, indicating that the cluster's photometric metallicity is substantially lower than its generally accepted spectroscopic metallicity of . We were able to reconcile the discrepancy by assuming , the appropriate metallicity indicated from colour–colour plots, and the distance moduli obtained from the two colour indices were in agreement with the Hipparcos result, within the 1σ errors.
With the release of the 2MASS All Sky Catalogue, we now apply our MS-fitting method to the Pleiades using the infrared colours in addition to the optical bands, in order to test the plausibility of our earlier result.
Using the full field dwarf sample and fitting in the and colour planes, we find that assuming a substantially subsolar metallicity does not produce distances in agreement with the and results. However the infrared plus distances are in mutual agreement when adopting the spectroscopic metallicity. By considering only the field dwarfs with , i.e. brighter than the magnitude where the Pleiades colours start to be anomalous (Stauffer et al. 2003, AJ, 126, 833), the infrared and optical colour indices all yield consistent distances using the spectroscopic [Fe/H]. The concordant distances thus obtained from the , , and planes yield a mean of pc, in excellent agreement with both the pre-Hipparcos MS-fitting results, and the most recent determinations from other methods.
We conclude that there are two distinct, and unrelated, issues affecting the Pleiades: 1) the Hipparcos parallax is in error by ~10%, as previously claimed; 2) the colours of the lower MS are anomalous, and we caution against using the index for MS-fitting to the Pleiades and similarly young open clusters.