On the use of field RR lyrae as galactic probes. II. A new Δs calibration to estimate their metallicity

J. Crestani, M. Fabrizio, V. F. Braga, C. Sneden, G. Preston, I. Ferraro, G. Iannicola, G. Bono, A. Alves-Brito, M. Nonino, V. D'Orazi, L. Inno, M. Monelli, J. Storm, G. Altavilla, B. Chaboyer, M. Dall'Ora, G. Fiorentino, C. Gilligan, E. K. GrebelH. Lala, B. Lemasle, M. Marengo, S. Marinoni, P. M. Marrese, C. E. Martinez-Vazquez, N. Matsunaga, J. P. Mullen, J. Neeley, Z. Prudil, R. Da Silva, P. B. Stetson, F. Thévenin, E. Valenti, A. Walker, M. Zoccali

Research output: Contribution to journalArticlepeer-review

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 ΔS method. This diagnostic, based on the equivalent widths of Ca II K and three Balmer (Hδ, 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.

Original languageEnglish
Article numberabd183
JournalAstrophysical Journal
Volume908
Issue number1
DOIs
Publication statusPublished - 10 Feb 2021
Externally publishedYes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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