The distance to the Pleiades - Main sequence fitting in the near infrared

摘要

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 (B-V) and (V-I) colour indices, indicating that the cluster's photometric metallicity is substantially lower than its generally accepted spectroscopic metallicity of Fe/H = -0.03. We were able to reconcile the discrepancy by assuming Fe/H = -0.4, the appropriate metallicity indicated from (B-V)/(V-I) colour-colour plots, and the distance moduli obtained from the two colour indices were in agreement with the Hipparcos result, within the 1sigma 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 V/(V-K) and K/(J-K) colour planes, we find that assuming a substantially subsolar metallicity does not produce distances in agreement with the (B-V) and (V-I) results. However the infrared plus (V-I) distances are in mutual agreement when adopting the spectroscopic metallicity. By considering only the field dwarfs with M-V less than or equal to 6, i.e. brighter than the magnitude where the Pleiades (B-V) 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 V/(B-V), V/(V-I), V/(V-K) and K/(J-K) planes yield a mean of 133.8 +/- 3 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 similar to10, as previously claimed; 2) the (B-V) colours of the lower MS are anomalous, and we caution against using the (B-V) index for MS-fitting to the Pleiades and similarly young open clusters.