This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified sakaricm@u.washington.edu 2 Sakari et al.as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed-up at high spectral resolution (R ∼ 31, 500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe I lines, taking into account <3D> non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H] < −1.5, 105 have [Fe/H] < −2.0, and 4 have [Fe/H] < −3.0. Nine new carbonenhanced metal-poor stars have been discovered, 3 of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ [Eu/Fe] ≤ + 1.0 and [Ba/Eu] < 0) and 4 new r-II stars (with [Eu/Fe] > +1.0). Nineteen stars are found to exhibit a "limited-r" signature ([Sr/Ba] > +0.5, [Ba/Eu] < 0). For the r-II stars, the second-and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, α, and Fe-peak elements match those of typical Milky Way halo stars, except for one r-I star which has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UV W space velocities for these stars which are consistent with membership in the Milky Way halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.
A high-resolution spectroscopic analysis is presented for a new highly r-process-enhanced ([Eu/Fe]=1.27, [Ba/Eu]=−0.65), very metal-poor ([Fe/H]=−2.09), retrograde halo star, RAVE J153830.9-180424, discovered as part of the R-Process Alliance survey. At V = 10.86, this is the brightest and most metal-rich r-II star known in the Milky Way halo. Its brightness enables high-S/N detections of a wide variety of chemical species that are mostly created by the r-process, including some infrequently detected lines from elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and U. This is the most complete r-process census in a very metal-poor r-II star. J1538-1804 shows no signs of s-process contamination, based on its low [Ba/Eu] and [Pb/Fe]. As with many other r-process-enhanced stars, J1538-1804ʼs r-process pattern matches that of the Sun for elements between the first, second, and third peaks, and does not exhibit an actinide boost. Cosmo-chronometric age-dating reveals the r-process material to be quite old. This robust main r-process pattern is a necessary constraint for r-process formation scenarios (of particular interest in light of the recent neutron star merger, GW170817), and has important consequences for the origins of r-II stars. Additional r-I and r-II stars will be reported by the R-Process Alliance in the near future.
Corresponding author: Charli M. Sakari sakaricm@u.washington.edu 2 Sakari et al.A new moderately r-process-enhanced metal-poor star, RAVE J093730.5−062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937−0626 is likely a horizontal branch star. At [Fe/H] = −1.86, J0937−0626 is found to have subsolar [X/Fe] ratios for nearly every light, α, and Fe-peak element. The low [α/Fe] ratios can be explained by an ∼ 0.6 dex excess of Fe; J0937−0626 is therefore similar to the subclass of "iron-enhanced" metal-poor stars. A comparison with Milky Way field stars at [Fe/H] = −2.5 suggests that J0937−0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937−0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937−0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937−0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment.
We analyzed 62 high-resolution spectra of 30 Galactic Field RR Lyrae-type stars with the aim of deriving their atmospheric parameters (T eff , g log , V t ), metallicity ([Fe/H]), radial velocities, and NLTE abundances of oxygen and sodium. We found that there is no clear anti-correlation between [O/Fe] and [Na/Fe] as is seen in globular clusters. On this basis, we conclude that the majority of field RR Lyrae-type stars should hardly be considered to be remnants of the dissolution of globular clusters.
Cepheid variable stars calibrate the cosmic distance ladder and exhibit large-amplitude pulsations that provide interesting hydrodynamical insights into stellar atmospheres. Here, we investigate velocities of different spectral line species (ionization levels, elements) to provide new detail into the complex velocity structure of the particularly large-amplitude Cepheid X Cygni, P puls =16.3 d. This is done against the backdrop of studies aimed at understanding the projection factor for Baade-Wesselink-type distance methods, as well as the timeseries velocimetry being gathered by the Gaia satellite. We analyze and compare velocities measured by cross-correlation using thousands of optical metallic lines, as well as individual line velocities of hydrogen (Balmer and Paschen lines), oxygen, ionized silicon, potassium, sodium, and ionized calcium. We compare individual line velocity curves to a reference velocity curve based on cross-correlation and discuss observed differences as a function of pulsation phase. This purely empirical approach yields detailed insights into the dependence of certain lines on NLTE effects, shocks, and phase lags in velocity space, which can be used as strong constraints for hydrodynamical modeling. Of particular importance is the velocity curve of the Ca II IR triplet, which dominates the Gaia radial velocities of F, G, and K type variable stars.
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