Nitrogen Abundances and the Distance Moduli of the Pleiades and Hyades

TL;DR

This study uses nitrogen abundances as a proxy for helium to investigate the Pleiades distance modulus discrepancy, finding no significant helium difference between the Pleiades and Hyades clusters, suggesting other explanations are needed.

Contribution

It provides the first detailed nitrogen abundance comparison between Pleiades and Hyades, challenging the helium abundance hypothesis for the distance modulus discrepancy.

Findings

Pleiades N abundance is lower than Hyades by 0.13 dex

Both clusters show [N/Fe]=0, consistent with Galactic disk stars

He abundance difference is unlikely to explain the distance discrepancy

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 3330 Ang region. While our Hyades N abundances show a modest 0.2 dex trend over a 800 K Teff 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 Teff 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 no larger 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.