- ID:
- ivo://CDS.VizieR/J/AJ/144/95
- Title:
- Abundance in stars of the outer galactic disk. IV.
- Short Name:
- J/AJ/144/95
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present radial velocities and chemical abundances for nine stars in the old, distant open clusters Be18, Be21, Be22, Be32, and PWM4. For Be18 and PWM4, these are the first chemical abundance measurements. Combining our data with literature results produces a compilation of some 68 chemical abundance measurements in 49 unique clusters. For this combined sample, we study the chemical abundances of open clusters as a function of distance, age, and metallicity. We confirm that the metallicity gradient in the outer disk is flatter than the gradient in the vicinity of the solar neighborhood. We also confirm that the open clusters in the outer disk are metal-poor with enhancements in the ratios [{alpha}/Fe] and perhaps [Eu/Fe]. All elements show negligible or small trends between [X/Fe] and distance (<0.02dex/kpc), but for some elements, there is a hint that the local (R_GC_<13kpc) and distant (R_GC_>13kpc) samples may have different trends with distance. There is no evidence for significant abundance trends versus age (<0.04dex/Gyr). We measure the linear relation between [X/Fe] and metallicity, [Fe/H], and find that the scatter about the mean trend is comparable to the measurement uncertainties. Comparison with solar neighborhood field giants shows that the open clusters share similar abundance ratios [X/Fe] at a given metallicity. While the flattening of the metallicity gradient and enhanced [{alpha}/Fe] ratios in the outer disk suggest a chemical enrichment history different from that of the solar neighborhood, we echo the sentiments expressed by Friel et al. that definitive conclusions await homogeneous analyses of larger samples of stars in larger numbers of clusters. Arguably, our understanding of the evolution of the outer disk from open clusters is currently limited by systematic abundance differences between various studies.
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- ID:
- ivo://CDS.VizieR/J/ApJS/191/352
- Title:
- Abundances in stars of Milky Way dwarf satellites
- Short Name:
- J/ApJS/191/352
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalog of Fe, Mg, Si, Ca, and Ti abundances for 2961 stars in eight dwarf satellite galaxies of the Milky Way (MW): Sculptor, Fornax, Leo I, Sextans, Leo II, Canes Venatici I, Ursa Minor, and Draco. For the purposes of validating our measurements, we also observed 445 red giants in MW globular clusters and 21 field red giants in the MW halo. The measurements are based on Keck/DEIMOS medium-resolution spectroscopy (MRS) combined with spectral synthesis. We estimate uncertainties in [Fe/H] by quantifying the dispersion of [Fe/H] measurements in a sample of stars in monometallic globular clusters (GCs). We estimate uncertainties in Mg, Si, Ca, and Ti abundances by comparing to high-resolution spectroscopic abundances of the same stars. For this purpose, a sample of 132 stars with published high-resolution spectroscopy in GCs, the MW halo field, and dwarf galaxies has been observed with MRS. The standard deviations of the differences in [Fe/H] and <[{alpha}/Fe]> (the average of [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe]) between the two samples is 0.15 and 0.16, respectively.
- ID:
- ivo://CDS.VizieR/J/MNRAS/492/1164
- Title:
- Abundances of Gaia DR2 wide binaries
- Short Name:
- J/MNRAS/492/1164
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- One of the high-level goals of Galactic archaeology is chemical tagging of stars across the Milky Way to piece together its assembly history. For this to work, stars born together must be uniquely chemically homogeneous. Wide binary systems are an important laboratory to test this underlying assumption. Here we present the detailed chemical abundance patterns of 50 stars across 25 wide binary systems comprised of main-sequence stars of similar spectral type identified in Gaia DR2 with the aim of quantifying their level of chemical homogeneity. Using high-resolution spectra obtained with McDonald Observatory, we derive stellar atmospheric parameters and precise detailed chemical abundances for light/odd-Z (Li, C, Na, Al, Sc, V, Cu), alpha (Mg, Si, Ca), Fe-peak (Ti, Cr, Mn, Fe, Co, Ni, Zn), and neutron capture (Sr, Y, Zr, Ba, La, Nd, Eu) elements. Results indicate that 80% (20 pairs) of the systems are homogeneous in [Fe/H] at levels below 0.02dex. These systems are also chemically homogeneous in all elemental abundances studied, with offsets and dispersions consistent with measurement uncertainties. We also find that wide binary systems are far more chemically homogeneous than random pairings of field stars of similar spectral type. These results indicate that wide binary systems tend to be chemically homogeneous but in some cases they can differ in their detailed elemental abundances at a level of [X/H]~0.10dex, overall implying chemical tagging in broad strokes can work.
- ID:
- ivo://CDS.VizieR/J/ApJ/856/142
- Title:
- Abundances of metal-poor stars in Sculptor
- Short Name:
- J/ApJ/856/142
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The study of the chemical abundances of metal-poor stars in dwarf galaxies provides a venue to constrain paradigms of chemical enrichment and galaxy formation. Here we present metallicity and carbon abundance measurements of 100 stars in Sculptor from medium-resolution (R~2000) spectra taken with the Magellan/Michigan Fiber System mounted on the Magellan-Clay 6.5m telescope at Las Campanas Observatory. We identify 24 extremely metal-poor star candidates ([Fe/H]{<}-3.0) and 21 carbon-enhanced metal-poor (CEMP) star candidates. Eight carbon-enhanced stars are classified with at least 2{sigma} confidence, and five are confirmed as such with follow-up R~6000 observations using the Magellan Echellette Spectrograph on the Magellan-Baade 6.5m telescope. We measure a CEMP fraction of 36% for stars below [Fe/H]=-3.0, indicating that the prevalence of carbon-enhanced stars in Sculptor is similar to that of the halo (~43%) after excluding likely CEMP-s and CEMP-r/s stars from our sample. However, we do not detect that any CEMP stars are strongly enhanced in carbon ([C/Fe]>1.0). The existence of a large number of CEMP stars both in the halo and in Sculptor suggests that some halo CEMP stars may have originated from accreted early analogs of dwarf galaxies.
- ID:
- ivo://CDS.VizieR/J/AJ/157/154
- Title:
- Abundances of RGs in Galactic globular clusters
- Short Name:
- J/AJ/157/154
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Among stars in Galactic globular clusters the carbon abundance tends to decrease with increasing luminosity on the upper red giant branch, particularly within the lowest metallicity clusters. While such a phenomena is not predicted by canonical models of stellar interiors and evolution, it is widely held to be the result of some extra mixing operating during red giant branch ascent which transports material exposed to the CN(O)-cycle across the radiative zone in the stellar interior and into the base of the convective envelope, whereupon it is brought rapidly to the stellar surface. Here we present measurements of [C/Fe] abundances among 67 red giants in 19 globular clusters within the Milky Way. Building on the work of Martell et al. (2008AJ....136.2522M), we have concentrated on giants with absolute magnitudes of M_V_~-1.5 within clusters encompassing a range of metallicity (-2.4<[Fe/H]<-0.3). The Kitt Peak National Observatory (KPNO) 4 m and Southern Astrophysical Research (SOAR) 4.1 m telescopes were used to obtain spectra covering the {lambda}4300 CH and {lambda}3883 CN bands. The CH absorption features in these spectra have been analyzed via synthetic spectra in order to obtain [C/Fe] abundances. These abundances and the luminosities of the observed stars were used to infer the rate at which C abundances change with time during upper red giant branch evolution (i.e., the mixing efficiency). By establishing rates over a range of metallicity, the dependence of deep mixing on metallicity is explored. We find that the inferred carbon depletion rate decreases as a function of metallicity, although our results are dependent on the initial [C/Fe] composition assumed for each star.
- ID:
- ivo://CDS.VizieR/J/ApJ/836/77
- Title:
- A library of high-S/N optical spectra of FGKM stars
- Short Name:
- J/ApJ/836/77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Classification of stars, by comparing their optical spectra to a few dozen spectral standards, has been a workhorse of observational astronomy for more than a century. Here, we extend this technique by compiling a library of optical spectra of 404 touchstone stars observed with Keck/HIRES by the California Planet Search. The spectra have high resolution (R~60000), high signal-to-noise ratio (S/N~150/pixel), and are registered onto a common wavelength scale. The library stars have properties derived from interferometry, asteroseismology, LTE spectral synthesis, and spectrophotometry. To address a lack of well-characterized late-K dwarfs in the literature, we measure stellar radii and temperatures for 23 nearby K dwarfs, using modeling of the spectral energy distribution and Gaia parallaxes. This library represents a uniform data set spanning the spectral types ~M5-F1 (T_eff_~3000-7000K, R_*_~0.1-16R_{Sun}_). We also present "Empirical SpecMatch" (SpecMatch-Emp), a tool for parameterizing unknown spectra by comparing them against our spectral library. For FGKM stars, SpecMatch-Emp achieves accuracies of 100K in effective temperature (T_eff_), 15% in stellar radius (R_*_), and 0.09dex in metallicity ([Fe/H]). Because the code relies on empirical spectra it performs particularly well for stars ~K4 and later, which are challenging to model with existing spectral synthesizers, reaching accuracies of 70K in T_eff_, 10% in R_*_, and 0.12dex in [Fe/H]. We also validate the performance of SpecMatch-Emp, finding it to be robust at lower spectral resolution and S/N, enabling the characterization of faint late-type stars. Both the library and stellar characterization code are publicly available.
- ID:
- ivo://CDS.VizieR/J/AJ/156/94
- Title:
- APOGEE and Gaia DR2 analysis of IC 166
- Short Name:
- J/AJ/156/94
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- IC 166 is an intermediate-age open cluster (OC) (~1 Gyr) that lies in the transition zone of the metallicity gradient in the outer disk. Its location, combined with our very limited knowledge of its salient features, make it an interesting object of study. We present the first high-resolution spectroscopic and precise kinematical analysis of IC 166, which lies in the outer disk with R_GC_~12.7 kpc. High-resolution H-band spectra were analyzed using observations from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment survey. We made use of the Brussels Automatic Stellar Parameter code to provide chemical abundances based on a line-by-line approach for up to eight chemical elements (Mg, Si, Ca, Ti, Al, K, Mn, and Fe). The {alpha}-element (Mg, Si, Ca, and whenever available Ti) abundances, and their trends with Fe abundances have been analyzed for a total of 13 high-likelihood cluster members. No significant abundance scatter was found in any of the chemical species studied. Combining the positional, heliocentric distance, and kinematic information, we derive, for the first time, the probable orbit of IC 166 within a Galactic model including a rotating boxy bar, and found that it is likely that IC 166 formed in the Galactic disk, supporting its nature as an unremarkable Galactic OC with an orbit bound to the Galactic plane.
- ID:
- ivo://CDS.VizieR/J/AJ/155/240
- Title:
- A spectroscopic survey of field RHB stars
- Short Name:
- J/AJ/155/240
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A metallicity, chemical composition, and kinematic survey has been conducted for a sample of 340 candidate field red horizontal-branch (RHB) stars. Spectra with high resolution and high signal-to-noise ratio were gathered with the McDonald Observatory 2.7 m Tull and the Hobby-Eberly Telescope echelle spectrographs, and were used to determine effective temperatures, surface gravities, microturbulent velocities, [Fe/H] metallicities, and abundance ratios [X/Fe] for seven {alpha} and Fe-group species. The derived temperatures and gravities confirm that at least half of the candidates are true RHB stars, with (average) parameters T_eff_~5000 K and log g~2.5. From the {alpha} abundances alone, the thin and thick Galactic populations are apparent in our sample. Space motions for 90% of the program stars were computed from Hipparcos and Gaia parallaxes and proper motions. Correlations between chemical compositions and Galactic kinematics clearly indicate the existence of both thin-disk and thick-disk RHB stars.
- ID:
- ivo://CDS.VizieR/J/AJ/157/153
- Title:
- Atomic lines in spectra of RR Lyrae & RHB stars
- Short Name:
- J/AJ/157/153
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have derived relations between FWHMs and equivalent widths of metallic absorption lines in the spectra of RR Lyrae stars to estimate new upper limits on the axial equatorial rotational velocities of RR Lyrae and metal-poor red horizontal branch (RHB) stars. We have also derived the variations of RR Lyrae macroturbulent velocities during the pulsation cycles. In RRab cycles, the line widths are dominated by phase-dependent convolutions of axial rotation and macroturbulence, which we designate as V_macrot_. The behavior of V_macrot_ is remarkably uniform among the RRab stars, but the behavior of V_macrot_ among RRc stars varies strongly from star to star. The RRab stars exhibit an upper limit on V_macrot_ of 5+/-1 km/s with weak evidence of an anticorrelation with T_eff_. The RRc minima range from 2 to 12 km/s. The abrupt decline in large rotations with decreasing T_eff_ at the blue boundary of the instability strip and the apparently smooth continuous variation among the RRab and RHB stars suggests that HB stars gain/lose surface angular momentum on timescales that are short compared to HB lifetimes. The V_macrot_ values for our metal-poor RHB stars agree well with those derived by Fourier analysis of an independent but less metal-poor sample of Carney et al. (2008AJ....135..892C); they conform qualitatively to the expectations of Tanner et al. (2013ApJ...778..117T). A general conclusion of our investigation is that surface angular momentum as measured by V_rot_sin(i) is not a reliable indicator of total stellar angular momentum anywhere along the HB.
- ID:
- ivo://CDS.VizieR/J/ApJ/869/50
- Title:
- Barium abundances of red giant branch stars
- Short Name:
- J/ApJ/869/50
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- There are many candidate sites of the r-process: core-collapse supernovae (CCSNe; including rare magnetorotational core-collapse supernovae), neutron star mergers (NSMs), and neutron star/black hole mergers. The chemical enrichment of galaxies --specifically dwarf galaxies-- helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference --how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [{alpha}/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than CCSNe in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] versus [Fe/H] seen for [Fe/H]<~-1.6 when compared to the [Mg/Fe] versus [Fe/H] trend. We conclude that NSMs are the most likely source of r-process enrichment in dwarf galaxies at early times.
