Melike Afşar

CHEMICAL COMPOSITIONS OF THIN-DISK, HIGH-METALLICITY RED HORIZONTAL-BRANCH FIELD STARS

We present a detailed abundance analysis and atmospheric parameters of 76 stars from a survey to identify field Galactic red horizontal-branch (RHB) stars. High-resolution echelle spectra (R 60,000, S/N ? 100) were obtained with the 2.7?m Harlan J. Smith Telescope at McDonald Observatory. The target stars were selected only by color and parallax information. Overall metallicities and relative abundances of proton-capture elements (C, N, O, Li), ?-elements (Ca and Si), and neutron-capture elements (Eu and La) were determined by either equivalent width or synthetic spectrum analyses. We used CN features at the ??7995-8040 region in order to determine the 12C/13C ratios of our targets. Investigation of the evolutionary stages, using spectroscopic T eff and log g values along with derived 12C/13C ratios, revealed the presence of 18 probable RHB stars in our sample. We also derived kinematics of the stars with available distance information. Taking into account both the kinematics and probable evolutionary stages, we conclude that our sample contains 5 thick-disk and 13 thin-disk RHB stars. Up until now, RHB stars have been considered as members of the thick disk, and were expected to have large space velocities and sub-solar metallicities. However, our sample is dominated by low-velocity solar-metallicity RHB stars; their existence cannot be easily explained with standard stellar evolution.

THE CHEMICAL COMPOSITIONS OF VERY METAL-POOR STARS HD 122563 AND HD 140283: A VIEW FROM THE INFRARED

From high resolution (R = 45,000), high signal-to-noise (S/N > 400) spectra gathered with the Immersion Grating Infrared Spectrograph (IGRINS) in the H and K photometric bands, we have derived elemental abundances of two bright, well-known metal-poor halo stars: the red giant HD 122563 and the subgiant HD 140283. Since these stars have metallicities approaching [Fe/H] = -3, their absorption features are generally very weak. Neutral-species lines of Mg, Si, S and Ca are detectable, as well as those of the light odd-Z elements Na and Al. The derived IR-based abundances agree with those obtained from optical-wavelength spectra. For Mg and Si the abundances from the infrared transitions are improvements to those derived from shorter wavelength data. Many useful OH and CO lines can be detected in the IGRINS HD 122563 spectrum, from which derived O and C abundances are consistent to those obtained from the traditional [O I] and CH features. IGRINS high resolutions H- and K-band spectroscopy offers promising ways to determine more reliable abundances for additional metal-poor stars whose optical features are either not detectable, or too weak, or are based on lines with analytical difficulties.

300 nights of science with IGRINS at McDonald Observatory

The Immersion Grating Infrared Spectrometer (IGRINS) is a revolutionary instrument that exploits broad spectral coverage at high-resolution in the near-infrared. IGRINS employs a silicon immersion grating as the primary disperser, and volume-phase holographic gratings cross-disperse the H and K bands onto Teledyne Hawaii-2RG arrays. The use of an immersion grating facilitates a compact cryostat while providing simultaneous wavelength coverage from 1.45 - 2.5 μm. There are no cryogenic mechanisms in IGRINS and its high-throughput design maximizes sensitivity. IGRINS on the 2.7 meter Harlan J. Smith Telescope at McDonald Observatory is nearly as sensitive as CRIRES at the 8 meter Very Large Telescope. However, IGRINS at R≈45,000 has more than 30 times the spectral grasp of CRIRES* in a single exposure. Here we summarize the performance of IGRINS from the first 300 nights of science since commissioning in summer 2014. IGRINS observers have targeted solar system objects like Pluto and Ceres, comets, nearby young stars, star forming regions like Taurus and Ophiuchus, the interstellar medium, photo dissociation regions, the Galactic Center, planetary nebulae, galaxy cores and super novae. The rich near-infrared spectra of these objects motivate unique science cases, and provide information on instrument performance. There are more than ten submitted IGRINS papers and dozens more in preparation. With IGRINS on a 2.7m telescope we realize signal-to-noise ratios greater than 100 for K=10.3 magnitude sources in one hour of exposure time. Although IGRINS is Cassegrain mounted, instrument flexure is sub-pixel thanks to the compact design. Detector characteristics and stability have been tested regularly, allowing us to adjust the instrument operation and improve science quality. A wide variety of science programs motivate new tools for analyzing high-resolution spectra including multiplexed spectral extraction, atmospheric model fitting, rotation and radial velocity, unique line identification, and circumstellar disk modeling. Here we discuss details of instrument performance, summarize early science results, and show the characteristics of IGRINS as a versatile near-infrared spectrograph and forerunner of future silicon immersion grating spectrographs like iSHELL2 and GMTNIRS.3

The chemical compositions and evolutionary status of red giants in the open cluster NGC 6940

We present the high resolution (R

Aliphatic hydrocarbon content of interstellar dust

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Chemical Compositions of Evolved Stars from Near-infrared IGRINS High-resolution Spectra. I. Abundances in Three Red Horizontal Branch Stars

60000), high signal-to-noise (S/N

A Spectroscopic Survey of Field Red Horizontal-branch Stars

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BINARY CENTRAL STARS OF PLANETARY NEBULAE DISCOVERED THROUGH PHOTOMETRIC VARIABILITY. II. MODELING THE CENTRAL STARS OF NGC 6026 AND NGC 6337

120) spectroscopic analysis of 12 red giant members of the Galactic open cluster NGC 6940. We applied Yonsei-Yale isochrones to the color-magnitude diagram, which suggested an age of 1.1 Gyr for the cluster with a turn-off mass of 2 M

A Young Stellar Cluster Surrounding the Peculiar Eruptive Variable V838 Monocerotis

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Line strengths of rovibrational and rotational transitions in the X2Π ground state of OH

. Atmospheric parameters (