On the Use of Field RR Lyrae as Galactic Probes. II. A New ΔS Calibration to Estimate Their Metallicity

• Published in: The Astrophysical journal Vol. 908; no. 1; p. 20
• Main Authors: Crestani, J., Fabrizio, M., Braga, V. F., Sneden, C., Preston, G., Ferraro, I., Iannicola, G., Bono, G., Alves-Brito, A., Nonino, M., D’Orazi, V., Inno, L., Monelli, M., Storm, J., Altavilla, G., Chaboyer, B., Dall’Ora, M., Fiorentino, G., Gilligan, C., Grebel, E. K., Lala, H., Lemasle, B., Marengo, M., Marinoni, S., Marrese, P. M., Martínez-Vázquez, C. E., Matsunaga, N., Mullen, J. P., Neeley, J., Prudil, Z., Silva, R. da, Stetson, P. B., Thévenin, F., Valenti, E., Walker, A., Zoccali, M.
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 01-02-2021; American Astronomical Society
• Subjects: Abundance; Astrophysics; Balmer lines; Calibration; Diagnostic systems; Empirical analysis; Halo formation; Iron; Metallicity; Metals; Parameter estimation; Physics; Spectra; Spectroscopy
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abd183

Abstract
We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured ≈6300 high-resolution (HR,
R
∼ 35,000) spectra for 143 RRLs (111 fundamental, RRab; 32 first-overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well-defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the Δ
S
method. This diagnostic, based on the equivalent widths of Ca
ii
K and three Balmer (H
δ
,
γ
,
β
) lines, traces the metallicity of RRLs. For the first time, the new empirical calibration: (i) includes spectra collected over the entire pulsation cycle; (ii) includes RRc variables; (iii) relies on spectroscopic calibrators covering more than three dex in iron abundance; and (iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to a data set of both SEGUE-SDSS and degraded HR spectra totalling 6451 low-resolution (
R
∼ 2000) spectra for 5001 RRLs (3439 RRab, 1562 RRc). This resulted in an iron distribution with a median
η
= −1.55 ± 0.01 and
σ
= 0.51 dex, in good agreement with literature values. We also found that RRc are 0.10 dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.