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Register a new userChemical abundances of solar neighborhood RR Lyrae stars
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We have analysed a sample of 18 RR Lyrae stars (17 fundamental-mode - RRab - and one first overtone - RRc) and three Population II Cepheids (two BL Her stars and one W Vir star), for which high-resolution (R $ge$30000), high signal-to-noise (S/N$ge$30) spectra were obtained with either SARG at the Telescopio Nazionale Galileo (La Palma, Spain) or UVES at the ESO Very Large Telescope (Paranal, Chile). Archival data were also analyzed for a few stars, sampling $gtrsim$3 phases for each star. We obtained atmospheric parameters (T$_{rm{eff}}$, log$g$, v$_{rm{t}}$, and [M/H]) and abundances of several iron-peak and $alpha$-elements (Fe, Cr, Ni, Mg, Ca, Si, and Ti) for different pulsational phases, obtaining $<$[$alpha$/Fe]$>$=+0.31$pm$0.19 dex over the entire sample covering -2.2$<$[Fe/H]$<$-1.1 dex. We find that silicon is indeed extremely sensitive to the phase, as reported by previous authors, and cannot be reliably determined. Apart from this, metallicities and abundance ratios are consistently determined, regardless of the phase, within 0.10-0.15 dex, although caution should be used in the range $0lesssimphilesssim0.15$. Our results agree with literature determinations for both variable and non-variable field stars, obtained with very different methods, including low and high-resolution spectroscopy. W Vir and BL Her stars, at least in the sampled phases, appear indistinguishable from RRab from the spectroscopic analysis point of view. Our large sample, covering all pulsation phases, confirms that chemical abundances can be obtained for RR Lyrae with the classical EW-based technique and static model atmospheres, even rather close to the shock phases.
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We present a detailed abundance study of 11 RR Lyrae ab-type variables: AS Vir, BS Aps, CD Vel, DT Hya, RV Oct, TY Gru, UV Oct, V1645 Sgr, WY Ant, XZ Aps, and Z Mic.High resolution and high S/N echelle spectra of these variables were obtained with 2.5 m du Pont telescope at the Las Campanas Observatory. We obtained more than 2300 spectra, roughly 200 spectra per star, distributed more or less uniformly throughout the pulsational cycles. A new method has been developed to obtain initial effective temperature of our sample stars at a specific pulsational phase. We find that the abundance ratios are generally consistent with those of similar metallicity field stars in different evolutionary states and throughout the pulsational cycles for RR Lyrae stars. TY Gru remains the only n-capture enriched star among the RRab in our sample. A new relation is found between microturbulence and effective temperature among stars of the HB population. In addition, the variation of microturbulence as a function of phase is empirically shown to be similar to the theoretical variation. Finally, we conclude that the derived teffand log g values of our sample stars follow the general trend of a single mass evolutionary track.
Low-mass, variable, high-velocity stars are interesting study cases for many aspects of Galactic structure and evolution. Until recently, the only known high- or hyper-velocity stars were young stars thought to originate from the Galactic centre. Wide-area surveys like APOGEE and BRAVA have found several low-mass stars in the bulge with Galactic rest-frame velocities larger than 350 km/s. In this study we present the first abundance analysis of a low-mass, RR Lyrae star, located close to the Galactic bulge, with a space motion of ~ -400 km/s. Using medium-resolution spectra, we derive abundances (including upper limits) of 11 elements. These allow us to chemically tag the star and discuss its origin, although our derived abundances and metallicity, at [Fe/H] =-0.9 dex, do not point toward one unambiguous answer. Based on the chemical tagging, we cannot exclude that it originated in the bulge. However, combining its retrograde orbit and the derived abundances suggests that the star was accelerated from the outskirts of the inner (or even outer) halo during many-body interactions. Other possible origins include the bulge itself, or the star could be stripped from a star cluster or the Sagittarius dwarf galaxy when it merged with the Milky Way.
The serendipitous discovery by Preston and colleagues of the neutron-capture-enhanced RR Lyrae variable star TY Gru (a.k.a. CS 22881-071 in the HK survey of very metal-poor halo stars) has resulted in a growing set of initiatives on the chemical compositions of RR Lyrae stars and their application to broader topics in Galactic halo structure. Here we summarize the main aspects of our work on TY Gru, including a new discussion of our search for possible orbital motion of this star around a putative unseen companion. Then we describe a few of the results of a newly-completed intensive spectroscopic investigation of 10 additional field RR Lyr stars. We finish by outlining current projects that seek to contrast the atmospheres and chemical compositions of RRc stars with those of the RRab stars, and that employ a much larger RRab sample in a chemo-dynamical study of Galactic halo RR Lyr.
We present a kinematical study of 314 RR~Lyrae stars in the solar neighbourhood using the publicly available photometric, spectroscopic, and {it Gaia} DR2 astrometric data to explore their distribution in the Milky Way. We report an overdensity of 22 RR~Lyrae stars in the solar neighbourhood at a pericenter distance of between 5--9,kpc from the Galactic center. Their orbital parameters and their chemistry indicate that these 22 variables share the kinematics and the [Fe/H] values of the Galactic disc, with an average metallicity and tangential velocity of [Fe/H]=$-0.60$,dex and $v_{theta} = 241$,km,s$^{-1}$, respectively. From the distribution of the Galactocentric spherical velocity components, we find that these 22 disc-like RR~Lyrae variables are not consistent with the {it Gaia} Sausage ({it Gaia}-Enceladus), unlike almost half of the local RR~Lyrae stars. Chemical information from the literature shows that the majority of the selected pericenter peak RR~Lyrae variables are $alpha$-poor, a property shared by typically much younger stars in the thin disc. Using the available photometry we rule out a possible misclassification with the known classical and anomalous Cepheids. The similar kinematic, chemical, and pulsation properties of these disc RR~Lyrae stars suggest they share a common origin. In contrast, we find the RR~Lyrae stars associated with the {it Gaia}-Enceladus based on their kinematics and chemical composition show a considerable metallicity spread in the old population ($sim$~1,dex).
We present a detailed spectroscopic analysis of RR Lyrae (RRL) variables in the globular cluster NGC 5139 (omega Cen). We collected optical (4580-5330 A), high resolution (R = 34,000), high signal-to-noise ratio (200) spectra for 113 RRLs with the multi-fiber spectrograph M2FS at the Magellan/Clay Telescope at Las Campanas Observatory. We also analysed high resolution (R = 26,000) spectra for 122 RRLs collected with FLAMES/GIRAFFE at the VLT, available in the ESO archive. The current sample doubles the literature abundances of cluster and field RRLs in the Milky Way based on high resolution spectra. Equivalent width measurements were used to estimate atmospheric parameters, iron, and abundance ratios for alpha (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn), and s-process (Y) elements. We confirm that omega Cen is a complex cluster, characterised by a large spread in the iron content: -2.58 < [Fe/H] < -0.85. We estimated the average cluster abundance as [Fe/H] = -1.80 +- 0.03, with sigma = 0.33 dex. Our findings also suggest that two different RRL populations coexist in the cluster. The former is more metal-poor ([Fe/H] < -1.5), with almost solar abundance of Y. The latter is less numerous, more metal-rich, and yttrium enhanced ([Y/Fe] > 0.4). This peculiar bimodal enrichment only shows up in the s-process element, and it is not observed among lighter elements, whose [X/Fe] ratios are typical for Galactic globular clusters.
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