2008/04/30 by Evan N. Kirby, Puragra Guhathakurta, Christopher Sneden
Physics and Astronomy · #Astronomy and Astrophysical Research #Scientific Research and Discoveries #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/589627
published as ApJ 2008, 682, 1217 · 16 pages, 17 figures, 7 tables, accepted for publication in ApJ
arxiv created 2008/06/24 · openalex publication_date 2008/07/29 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
We present a technique that applies spectral synthesis to medium-resolution spectroscopy (MRS; R ∼ 6000) in the red (6300 Å < λ < 9100 Å) to measure [Fe/H] and [α/Fe] of individual red giant stars over a wide metallicity range. We apply our technique to 264 red giant stars in seven Galactic globular clusters and demonstrate that it reproduces the metallicities and α-enhancements derived from high-resolution spectroscopy (HRS). The MRS technique excludes the three Ca II triplet lines and instead relies on a plethora of weaker lines. Unlike empirical metallicity estimators, such as the equivalent width of the Ca II triplet, the synthetic method presented here is applicable over an arbitrarily wide metallicity range and is independent of assumptions about the α-enhancement. Estimates of cluster mean [Fe/H] from different HRS studies show typical scatter of ~0.1 dex but can be larger than 0.2 dex for metal-rich clusters. The scatter in HRS abundance estimates among individual stars in a given cluster is also comparable to 0.1 dex. By comparison, the scatter among MRS [Fe/H] estimates of individual stars in a given cluster is ~0.1 dex for most clusters but 0.17 dex for the most metal-rich cluster, M71 (⟨ [ Fe/H ] ⟩ = − 0.8). A star-by-star comparison of HRS versus MRS [α/Fe] estimates indicates that the precision in [ α/Fe ] MRS is 0.05 dex. The errors in [ Fe/H ] MRS and [ α/Fe ] MRS increase beyond 0.25 dex only below signal-to-noise ratios of 20 Å −1 , which is typical for existing MRS of the red giant stars in Leo I, one of the most distant Milky Way satellites (250 kpc).