Chemical Compositions of Field and Globular Cluster RR Lyrae Stars. I. NGC 3201^∗

Abstract

We present a detailed spectroscopic analysis of horizontal branch stars in the globular cluster NGC 3201. We collected optical (4580-5330Å), high resolution (∼34,000), high signal-to-noise ratio (∼200) spectra for eleven RR Lyrae stars and one red horizontal branch star with the multifiber spectrograph M2FS at the 6.5m Magellan telescope at the Las Campanas Observatory. From measured equivalent widths we derived atmospheric parameters and abundance ratios for α (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn) and s-process (Y) elements. We found that NGC 3201 is a homogeneous, monometallic ([Fe/H]=−1.47 ± 0.04), α-enhanced ([α/Fe]=0.37 ± 0.04) cluster. The relative abundances of the iron peak and s-process elements were found to be consistent with solar values. In comparison with other large stellar samples, NGC 3201 RR Lyraes have similar chemical enrichment histories as do those of other old (t≥10 Gyr) Halo components (globular clusters, red giants, blue and red horizontal branch stars, RR Lyraes). We also provided a new average radial velocity estimate for NGC 3201 by using a template velocity curve to overcome the limit of single epoch measurements of variable stars: V rad = 494 ± 2 km s −1 (σ=8 km s −1 ).

Local RR Lyrae stars: native and alien

“…Halo stars divide into two groups by [α/Fe], which also separates them by kinematics and age (Nissen & Schuster 2010;Schuster et al 2012), in the sense that the low α stars are younger and have hotter kinematics. Recent data from the APOGEE project has greatly increased the number of stars with measured abundances and reinforced the importance of the α abundances for interpreting the evolution of the MW (e.g., Hayes et al 2018; Fernández-Alvar et al Magurno et al (2018), which was in the predicted direction, was not due to the heterogeneity of the data sources.…”

“…Many of the designations as type ab or c, <V>, and the interstellar extinctions in the V pass-band (Vext) were taken from Dambis et al (2013). Other sources for these data were the references listed by Magurno et al (2018), Muraveva et al (2018), and the Simbad database.…”

“…A good example of the utility of this procedure is provided by the very metalpoor star X Ari, for which the results from high-resolution spectroscopy disagree (see also Chadid et al 2017). The 6 measurements compiled by Magurno et al (2018) range from -2.19 to -2.74, and have a mean, <[Fe/H]>, of -2.52 and σ = 0.19. After transforming the individual measurements to the L94 scale, the range narrows to -2.34 to -2.58 with <[Fe/H]>= −2.44 and σ = 0.11.…”

“…Another discriminant between thin-and thick-disc stars near the Sun is the ratio of the abundances of the α elements to Fe, which at the same [Fe/H] is significantly higher than solar in thick disk stars and near solar in thin-disc ones (e.g., Haywood et al 2013;Bensby et al 2014). For 96 field RRL, Magurno et al (2018) have recently compiled and re-scaled to the same solar abundance the measurements in the literature of [Mg/Fe], [Ca/Fe], and [Ti/Fe] from high-resolution spectroscopy. Fig.…”

“…Fig. 10 in Magurno et al (2018), shows the dependence of these ratios on [Fe/H] and also illustrates the scatter in the measurements. Eighty-six stars of their sample are in our catalog, and for these stars we averaged where possible the abundance ratios that Magurno et al (2018) list from multiple sources.…”