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Context. Measuring the abundances of neutron-capture elements in Galactic disk stars is an important part of understanding key stellar and galactic processes. In the optical wavelength regime a number of different neutron-capture elements have been measured; however, only the s-process-dominated element cerium has been accurately measured for a large sample of disk stars from the infrared H band. The more r-process dominated element ytterbium has only been measured in a small subset of stars so far. Aims. In this study we aim to measure the ytterbium (Yb) abundance of local disk giants using the Yb II line at lambda(air) = 16 498 angstrom. We also compare the resulting abundance trend with cerium and europium abundances for the same stars to analyse the s- and r-process contributions. Methods. We analyse 30 K giants with high-resolution H band spectra using spectral synthesis. The very same stars have already been analysed using high-resolution optical spectra via the same method, but it was not possible to determine the abundance of Yb from those spectra due to blending issues for stars with [Fe/H] > -1. In the present analysis, we utilise the stellar parameters determined from the optical analysis. Results. We determined the Yb abundances with an estimated uncertainty for [Yb/Fe] of 0.1 dex. By comparison, we found that the [Yb/Fe] trend closely follows the [Eu/Fe] trend and has clear s-process enrichment in identified s-rich stars. This comparison confirms both that the validity of the Yb abundances is ensured and that the theoretical prediction that the s-/r-process contribution to the origin of Yb of roughly 40/60 is supported. Conclusions. These results show that, with a careful and detailed analysis of infrared spectra, reliable Yb abundances can be derived for a wider sample of cooler giants in the range -1.1 < [Fe/H] < 0.3. This is promising for further studies of the production of Yb and for the r-process channel, key for galactochemical evolution, in the infrared., Dutch Research Council (NWO) [VI.Vidi.193.093]; Goran Gustafsson Foundation for Research in Natural Sciences and Medicine; Royal Physiographic Society in Lund through the Stiftelse Walter Gyllenbergs fond; Mt. Cuba Astronomical Foundation; US National Science Foundation [AST-1229522, AST-1702267]; Korean GMT Project of KASI; Gemini Observatory; McDonald Observatory of the University of Texas at Austin, M.M. acknowledges funding through VIDI grant Pushing Galactic Archaeology to its limits VI.Vidi.193.093, which is funded by the Dutch Research Council (NWO). R.F's and A.J's research is supported by the Goran Gustafsson Foundation for Research in Natural Sciences and Medicine. R.F. and N.R. acknowledge support from the Royal Physiographic Society in Lund through the Stiftelse Walter Gyllenbergs fond and Marta och Erik Holmbergs donation. This work used the Immersion Grating Infrared Spectrometer (IGRINS) that was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the Mt. Cuba Astronomical Foundation, of the US National Science Foundation under grants AST-1229522 and AST-1702267, of the McDonald Observatory of the University of Texas at Austin, of the Korean GMT Project of KASI, and Gemini Observatory. These results made use of the Lowell Discovery Telescope (LDT) at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the LDT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. This paper includes data taken at The McDonald Observatory of The University of Texas at Austin. This work has made use of the VALD database, operated at Uppsala University, the Institute of Astronomy RAS in Moscow, and the University of Vienna.

Aims. Stars with strong enhancements of r-process elements are rare and tend to be metal-poor, generally with [Fe/H] < −2 dex, and located in the halo. In this work, we aim to investigate a candidate r-process enriched bulge star with a relatively high metallicity of [Fe/H] ~ − 0.65 dex and to compare it with a previously published r-rich candidate star in the bulge. Methods. We reconsidered the abundance analysis of a high-resolution optical spectrum of the red-giant star 2MASS J18082459-2548444 and determined its europium (Eu) and molybdenum (Mo) abundance, using stellar parameters from five different previous studies. Applying 2MASS photometry, Gaia astrometry, and kinematics, we estimated the distance, orbits, and population membership of 2MASS J18082459-2548444 and a previously reported r-enriched star 2MASS J18174532-3353235. Results. We find that 2MASS J18082459-2548444 is a relatively metal-rich, enriched r-process star that is enhanced in Eu and Mo, but not substantially enhanced in s-process elements. There is a high probability that it has a Galactic bulge membership, based on its distance and orbit. We find that both stars show r-process enhancement with elevated [Eu/Fe]-values, even though 2MASS J18174532-3353235 is 1 dex lower in metallicity. Additionally, we find that the plausible origins of 2MASS J18174532-3353235 to be either that of the halo or the thick disc. Conclusions. We conclude that 2MASS J18082459-2548444 represents the first example of a confirmed r-process enhanced star confined to the inner bulge. We assume it is possibly a relic from a period of enrichment associated with the formation of the bar.

Owing to their extreme crowding and high and variable extinction, stars in the Galactic Bulge, within +-2 degrees of the Galactic plane, and especially those in the Nuclear Star Cluster, have only rarely been targeted for an analyses of their detailed abundances. There is also some disagreement about the high end of the abundance scale for these stars. It is now possible to obtain high dispersion, high S/N spectra in the infrared K band (2.0-2.4 microns) for these giants; we report our progress at Keck and VLT in using these spectra to infer the composition of this stellar population., Comment: 4 pages, contributed talk at the IAU Symposium 334 "Rediscovering our Galaxy" in Potsdam, July 10-14, 2017

The cosmic origin of fluorine is still not well constrained. Several nucleosynthetic channels at different phases of stellar evolution have been suggested, but these must be constrained by observations. For this, the fluorine abundance trend with metallicity spanning a wide range is required. Our aim is to determine stellar abundances of fluorine for -1.1 < [Fe H] < +0.4. We determine the abundances from HF lines in infrared K-band spectra ( 2.3 mm) of cool giants, observed with the IGRINS and Phoenix high-resolution spectrographs. We derive accurate stellar parameters for all our observed K giants, which is important as the HF lines are very temperaturesensitive. We find that [F/Fe] is flat as a function of metallicity at [ F/Fe]0, but increases as the metallicity increases. the fluorine slope shows a clear secondary behavior in this metallicity range. We also find that the [F/ Ce] ratio is relatively flat for -0.6 < [Fe H] < 0, and that for two metal-poor ([Fe H] < - 0.8), s-process element-enhanced giants, we do not detect an elevated fluorine abundance. We interpret all of these observational constraints as indications that several major processes are at play for the cosmic budget of fluorine over time: from those in massive stars at low metallicities, through the asymptotic giant branch star contribution at -0.6 < [Fe H] < 0, to processes with increasing yields with metallicity at supersolar metallicities. the origins of the latter, and whether or not Wolf-Rayet stars and/or novae could contribute at supersolar metallicities, is currently not known. To quantify these observational results, theoretical modeling is required. More observations in the metal-poor region are required to clarify the processes there., Swedish Research Council, V.R.Swedish Research Council [621-2014-5640]; Royal Physiographic Society in Lund through the Stiftelse Walter Gyllenbergs fond; Crafoord Foundation; Stiftelsen Olle Engkvist ByggmastareSwedish Research Council; Ruth och Nils-Erik Stenbacks stiftelse; US National Science FoundationNational Science Foundation (NSF) [AST-1229522, AST-1702267]; McDonald Observatory of the University of Texas at Austin; Korean GMT Project of KASI, We would like to thank George Meynet for fruitful and enlightening discussions on massive-star yields and the theoretically predicted role of Wolf-Rayet stars for the production of fluorine. N.R. acknowledges support from the Swedish Research Council, V.R. (project numbers 621-2014-5640), and the Royal Physiographic Society in Lund through the Stiftelse Walter Gyllenbergs fond and Marta och Erik Holmbergs donation. H.J. acknowledges support from the Crafoord Foundation, Stiftelsen Olle Engkvist Byggmastare, and Ruth och Nils-Erik Stenbacks stiftelse. This work used the Immersion Grating Infrared spectrograph (IGRINS) that was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the US National Science Foundation under grants AST-1229522 and AST-1702267, of the McDonald Observatory of the University of Texas at Austin, and of the Korean GMT Project of KASI. Facilities: McDonald Observatory (IGRINS), Lowell Observatory (IGRINS), KPNO (Phoenix).

We present a detailed study of the composition of 20 M giants in the Galactic center with 15 of them confirmed to be in the nuclear star cluster. As a control sample we have also observed 7 M giants in the Milky Way disk with similar stellar parameters. All 27 stars are observed using the NIRSPEC spectrograph on the KECK II telescope in the K-band at a resolving power of R = 23,000. We report the first silicon abundance trends versus [Fe/H] for stars in the Galactic center. While finding a disk/bulge-like trend at subsolar metallicities, we find that [Si/Fe] is enhanced at supersolar metallicities. We speculate on possible enrichment scenarios to explain such a trend. However, the sample size is modest and the result needs to be confirmed by additional measurements of silicon and other alpha-elements. We also derive a new distribution of [Fe/H] and find the most metal-rich stars at [Fe/H] = +0.5 dex, confirming our earlier conclusions that the Galactic center hosts no stars with extreme chemical compositions., We thank Anish Amarsi for hosting B.T. in Heidelberg and assisting with the NLTE calculations. We also thank Mattia Sormani for his comments and suggestions. B.T. and N.R. acknowledge support from the Swedish Research Council, VR (project number 621-2014-5640), Funds from Kungl. Fysiografiska Sallskapet i Lund. (Stiftelsen Walter Gyllenbergs fond and Marta och Erik Holmbergs donation), and from the project grant "The New Milky" from the Knut and Alice Wallenberg foundation. M.S. acknowledges the Programme National de Cosmologie et Galaxies (PNCG) of CNRS/INSU, France, for financial support. R.M.R. acknowledges financial support from his late father Jay Baum Rich. F.R. acknowledges support from the Knut and Alice Wallenberg Foundation. A portion of this work was done at the Sexten Center for Astrophysics; the authors acknowledge their hospitality. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Very strong Sc I lines have been found recently in cool M giants in the Nuclear Star Cluster in the Galactic Center. Interpreting these as anomalously high scandium abundances in the Galactic Center would imply a unique enhancement signature and chemical evolution history for nuclear star clusters, and a potential test for models of chemical enrichment in these objects. We present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants situated in the solar neighborhood and compare them with spectra of M giants in the Nuclear Star Cluster. We clearly identify strong Sc I lines in our solar neighborhood sample as well as in the Nuclear Star Cluster sample. The strong Sc I lines in M giants are therefore not unique to stars in the Nuclear Star Cluster and we argue that the strong lines are a property of the line formation process that currently escapes accurate theoretical modeling. We further conclude that for giant stars with effective temperatures below approximately 3800 K these Sc I lines should not be used for deriving the scandium abundances in any astrophysical environment until we better understand how these lines are formed. We also discuss the lines of vanadium, titanium, and yttrium identified in the spectra, which demonstrate a similar striking increase in strength below 3500 K effective temperature., 11 pages, 6 figures, accepted for publication in ApJ

While the number of stars in the Galactic bulge with detailed chemical abundance measurements is increasingly rapidly, the inner Galactic bulge ( |b| < 2$^\circ$) remains poorly studied, due to heavy interstellar absorption and photometric crowding. We have carried out a high-resolution IR spectroscopic study of 72 M giants in the inner bulge using the CRIRES (ESO/VLT) facility. Our spectra cover the wavelength range of 2.0818 - 2.1444 $��$m with the resolution of R$\sim$50,000 and have signal-to-noise ratio of 50 - 100. Our stars are located along the bulge minor axis at l = 0$^\circ$, b = $\pm$0$^\circ$, $\pm$1$^\circ$, $\pm$2$^\circ$ and +3$^\circ$, selected with the aim of investigating any North-South asymmetries in the metallicity distribution function and composition, and comparing them to the outer bulge fields. Our sample was analysed in a homogeneous way using the most current K-band line list. We clearly detect a bimodal MDF with a metal-rich peak at $\rm \sim +0.3\,dex$ and a metal-poor peak at $\rm \sim -0.5\,dex$. Only a single star is found to exceed $\rm [Fe/H]=+0.5\,dex$. They show a symmetric behaviour along the $\pm$1$^\circ$, $\pm$2$^\circ$ fields. The Galactic Center field reveals in contrast a mainly metal-rich population with a mean metallicity of $\rm +0.3\,dex$. We derived $\rm [Mg/Fe]$ and $\rm [Si/Fe]$ abundances which are consistent with trends from the outer bulge, with a gradually decreasing trend with increasing metallicity. We confirm for the supersolar metallicity stars the decreasing trend in \mgfe\ and \sife\ as expected from chemical evolution models. With the caveat of a relatively small sample, we do not find significant differences in the chemical abundances between the Northern and the Southern fields, hence the evidence is consistent with symmetry in chemistry between North and South., 17 pages, 15 figures, Accepted for publication in MNRAS

We report the first high spectral resolution study of 17 M giants kinematically confirmed to lie within a few parsecs of the Galactic Center, using R=24,000 spectroscopy from Keck/NIRSPEC and a new linelist for the infrared K band. We consider their luminosities and kinematics, which classify these stars as members of the older stellar population and the central cluster. We find a median metallicity of =-0.16 and a large spread from approximately -0.3 to +0.3 (quartiles). We find that the highest metallicities are [Fe/H], Comment: 18 pages, 14 figures, accepted for publication in AJ

Investigating the Galactic center offers unique insights into the buildup and history of our Galaxy and is a stepping stone to understand galaxies in a larger context. It is reasonable to expect that the stars found in the Galactic center might have a different composition compared to stars found in the local neighborhood around the Sun. It is therefore quite exciting when recently there were reports of unusual neutral scandium, yttrium, and vanadium abundances found in the Galactic center stars, compared to local neighborhood stars. To explain the scandium abundances in the Galactic center, we turn to recent laboratory measurements and theoretical calculations done on the atomic oscillator strengths of neutral scandium lines in the near infrared. We combine these with measurements of the hyper fine splitting of neutral scandium. We show how these results can be used to explain the reported unusual scandium abundances and conclude that in this respect, the environment of the Galactic center is not that different from the environment in the local neighborhood around the sun.

Context. In the advent of new infrared high-resolution spectrometers, accurate and precise atomic data in the infrared are urgently needed. Identifications, wavelengths, strengths, broadening, and hyper-fine splitting parameters of stellar lines in the near-infrared are in many cases not accurate enough to model observed spectra, and in other cases, these parameters do not even exist. Some stellar features are unidentified. Aims. The aim with this work is to identify a spectral feature at λvac = 1063.891 nm or λair = 1063.600 nm that is visible in spectra of stars of different spectral types that are observed with the GIANO-B spectrometer. Methods. The search for spectral lines to match the unidentified feature in line lists from standard atomic databases was not successful. However, by investigating the original published laboratory data, we were able to identify the feature and solve the problem. To confirm its identification, we modelled the presumed stellar line in the solar intensity spectrum and found an excellent match. Results. We find that the observed spectral feature is a stellar line originating from the 4s′–4p′ transition in S I, and that the reason for its absence in atomic line databases is a neglected air-to-vacuum correction in the original laboratory measurements from 1967 for this line only. From interpolation we determine the laboratory wavelength of the S I line to be λvac = 1063.8908 nm or λair = 1063.5993 nm, and the excitation energy of the upper level to be 9.74978 eV.

Although there have been numerous studies of chemical abundances in the Galactic bulge, the central two degrees have been relatively unexplored due to the heavy and variable interstellar extinction, extreme stellar crowding, and the presence of complex foreground disk stellar populations. In this paper we discuss the metallicity distribution function, vertical and radial gradients and chemical abundances of $\alpha$-elements in the inner two degrees of the Milky Way, as obtained by recent IR spectroscopic surveys. We use a compilation of recent measurements of metallicities and $\alpha$-element abundances derived from medium-high resolution spectroscopy. We compare these metallicities with low-resolution studies. Defining "metal-rich" as stars with $ \rm [Fe/H]>0$, and "metal-poor" as stars with $\rm [Fe/H], Comment: 9 pages, 8 figures, accepted for Astronomy&Astrophysics

We report the first results from our program to examine the metallicity distribution of the Milky Way nuclear star cluster connected to SgrA*, with the goal of inferring the star formation and enrichment history of this system, as well as its connection and relationship with the central 100 pc of the bulge/bar system. We present the first high resolution (R~24,000), detailed abundance analysis of a K=10.2 metal-poor, alpha-enhanced red giant projected at 1.5 pc from the Galactic Center, using NIRSPEC on Keck II. A careful analysis of the dynamics and color of the star locates it at about 26 pc line-of-sight distance in front of the nuclear cluster. It probably belongs to one of the nuclear components (cluster or disk), not to the bar-bulge or classical disk. A detailed spectroscopic synthesis, using a new linelist in the K band, finds [Fe/H]~-1.0 and [alpha/Fe]~+0.4, consistent with stars of similar metallicity in the bulge. As known giants with comparable [Fe/H] and alpha enhancement are old, we conclude that this star is most likely to be a representative of the ~10 Gyr old population. This is also the most metal poor confirmed red giant yet discovered in vicinity of the nuclear cluster of the Galactic Center. We consider recent reports in the literature of a surprisingly large number of metal poor giants in the Galactic Center, but the reported gravities of log g ~4 for these stars calls into question their reported metallicities., Accepted for publication in ApJ

A major avenue in the study of the Galaxy is the investigation of stellar populations and Galactic chemical evolution by stellar spectroscopy. Due to the dust obscuration, stars in the centre of the Galaxy can only be observed in the near-IR wavelength region. However, existing line lists in this wavelength region are demonstratively not of good enough quality for use in stellar spectroscopy. In response to this, we have developed an empirical astrophysical line list in the K-band based on modelling against the Sun and testing against Arcturus. Of ca. 700 identified interesting lines about 570 lines have been assigned empirically determined values.