Nitrogen Abundances and the Distance Moduli of the Pleiades and Hyades

Abstract

Recent reanalyses of Hipparcos parallax data confirm a previously noted discrepancy with the Pleiades distance modulus estimated from main-sequence fitting in the color-magnitude diagram. One proposed explanation of this distance modulus discrepancy is a Pleiades He abundance that is significantly larger than the Hyades value. We suggest that, based on our theoretical and observational understanding of Galactic chemical evolution, nitrogen abundances may serve as a proxy for helium abundances of disk stars. Utilizing high-resolution near-UV Keck/HIRES spectroscopy, we determine N abundances in the Pleiades and Hyades dwarfs from NH features in the λ3330 region. While our Hyades N abundances show a modest ∼0.2 dex trend over a 800 K T eff range, we find the Pleiades N abundance (by number) is 0.13 ± 0.05 dex lower than in the Hyades for stars in a smaller overlapping T eff range around 6000 K; possible systematic errors in the lower Pleiades N abundance result are estimated to be at the ≤ 0.10 dex level. Our results indicate [N/Fe] ∼ 0 for both the Pleiades and Hyades, consistent with the ratios exhibited by local Galactic disk field stars in other studies. If N production is a reliable tracer of He production in the disk, then our results suggest the Pleiades He abundance is smaller than that in the Hyades. This finding is supported by the relative Pleiades–Hyades C, O, and Fe abundances interpreted in the current context of Galactic chemical evolution and is resistant to the effects on our derived N abundances of a He abundance difference like that needed to explain the Pleiades distance modulus discrepancy. A physical explanation of the Pleiades distance modulus discrepancy does not appear to be related to He abundance.