NLTE effects on Fe I/II in the atmospheres of FGK stars and application to abundance analysis of their spectra

We describe the first results from our project aimed at large-scale calculations of NLTE abundance corrections for important astrophysical atoms and ions. In this paper, the focus is on Fe which is a proxy of stellar metallicity and is commonly used to derive effective temperature and gravity. We present a small grid of NLTE abundance corrections for Fe I lines and discuss how NLTE effects influence determination of effective temperature, surface gravity, and metallicity for late-type stars.Comment: 6 pages, to be published in IOP The Journal of Physics: Conference Series, proceedings of the Workshop: 'Stellar Atmospheres in the Gaia Era: Quantitative Spectroscopy and Comparative Spectrum Modelling', Brussels, June 201

Discovery and Characterization of 3000+ Main-Sequence Binaries from APOGEE Spectra

We develop a data-driven spectral model for identifying and characterizing spatially unresolved multiple-star systems and apply it to APOGEE DR13 spectra of main-sequence stars. Binaries and triples are identified as targets whose spectra can be significantly better fit by a superposition of two or three model spectra, drawn from the same isochrone, than any single-star model. From an initial sample of $\sim$20,000 main-sequence targets, we identify $\sim$2,500 binaries in which both the primary and secondary star contribute detectably to the spectrum, simultaneously fitting for the velocities and stellar parameters of both components. We additionally identify and fit $\sim$200 triple systems, as well as $\sim$700 velocity-variable systems in which the secondary does not contribute detectably to the spectrum. Our model simplifies the process of simultaneously fitting single- or multi-epoch spectra with composite models and does not depend on a velocity offset between the two components of a binary, making it sensitive to traditionally undetectable systems with periods of hundreds or thousands of years. In agreement with conventional expectations, almost all the spectrally-identified binaries with measured parallaxes fall above the main sequence in the color-magnitude diagram. We find excellent agreement between spectrally and dynamically inferred mass ratios for the $\sim$600 binaries in which a dynamical mass ratio can be measured from multi-epoch radial velocities. We obtain full orbital solutions for 64 systems, including 14 close binaries within hierarchical triples. We make available catalogs of stellar parameters, abundances, mass ratios, and orbital parameters.Comment: Accepted to MNRAS with minor revisions since v1. 19 pages, 12 figures, plus Appendice

Parent Stars of Extrasolar Planets. XI. Trends with Condensation Temperature Revisited

We report the results of abundance analyses of new samples of stars with planets and stars without detected planets. We employ these data to compare abundance-condensation temperature trends in both samples. We find that stars with planets have more negative trends. In addition, the more metal-rich stars with planets display the most negative trends. These results confirm and extend the findings of Ramirez et al. (2009) and Melendez et al. (2009), who restricted their studies to solar analogs. We also show that the differences between the solar photospheric and CI meteoritic abundances correlate with condensation temperature.Comment: 7 pages, 11 figures; to be published in MNRA

Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles

Measurements of quantum oscillations in the cuprate superconductors afford a new opportunity to assess the extent to which the electronic properties of these materials yield to a description rooted in Fermi liquid theory. However, such an analysis is hampered by the small number of oscillatory periods observed. Here we employ a genetic algorithm to globally model the field, angular, and temperature dependence of the quantum oscillations observed in the resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key feature of the data is the first identification of the effect of Zeeman splitting, which separates spin-up and spin-down contributions, indicating that the quasiparticles in the cuprates behave as nearly free spins, constraining the source of the Fermi surface reconstruction to something other than a conventional spin density wave with moments parallel to the CuO2 planes.Comment: 8 pages, 4 figure

Analysis of stellar spectra with 3D and NLTE models

Models of radiation transport in stellar atmospheres are the hinge of modern astrophysics. Our knowledge of stars, stellar populations, and galaxies is only as good as the theoretical models, which are used for the interpretation of their observed spectra, photometric magnitudes, and spectral energy distributions. I describe recent advances in the field of stellar atmosphere modelling for late-type stars. Various aspects of radiation transport with 1D hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E. Niemczura, B. Smalley, W. Pyc

Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba

A previous study (Nissen & Schuster 2010) of 94 dwarf stars with -1.6 < [Fe/H] < -0.4 has revealed the existence of two distinct halo populations with a systematic difference in [alpha/Fe] at a given metallicity. In continuation of that work, abundances of Mn, Cu, Zn, Y, and Ba are determined for the same sample of stars. Equivalent widths of atomic lines are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive precise abundance ratios from an LTE analysis based on MARCS model atmospheres. Systematic differences between the `high-alpha' and `low-alpha' halo populations are found for [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significant difference in the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] shows a large scatter that is closely correlated with a corresponding scatter in [Na/Fe] and [Ni/Fe]. The metallicity trends of [Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from existing nucleosynthesis calculations if the high-alpha stars formed in regions with such a high star formation rate that only massive stars and Type II supernovae contributed to the chemical enrichment. The low-alpha stars, on the other hand, most likely originate from systems with a slower chemical evolution, characterized by additional enrichment from Type Ia supernovae and low-mass AGB stars.Comment: Accepted for publication in A&

Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy

Our Galaxy is thought to have undergone an active evolutionary history dominated by star formation, the accretion of cold gas, and, in particular, mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of these interactions in the form of stellar streams, substructures, and chemically distinct stellar components. The impact of dwarf galaxy mergers on the content and morphology of the Galactic disk is still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups, which may have extragalactic origin. However, there is mounting evidence that stellar overdensities at the outer disk/halo interface could have been caused by the interaction of a dwarf galaxy with the disk. Here we report detailed spectroscopic analysis of 14 stars drawn from two stellar overdensities, each lying about 5 kiloparsecs above and below the Galactic plane - locations suggestive of association with the stellar halo. However, we find that the chemical compositions of these stars are almost identical, both within and between these groups, and closely match the abundance patterns of the Milky Way disk stars. This study hence provides compelling evidence that these stars originate from the disk and the overdensities they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies.Comment: accepted for publication in Natur

Summary of IAU GA SpS 5 II: Stellar and Wind Parameters

The development of infrared observational facilities has revealed a number of massive stars in obscured environments throughout the Milky Way and beyond. The determination of their stellar and wind properties from infrared diagnostics is thus required to take full advantage of the wealth of observations available in the near and mid infrared. However, the task is challenging. This session addressed some of the problems encountered and showed the limitations and successes of infrared studies of massive stars

Dynamical Mean Field Theory of Double Perovskite Ferrimagnets

The dynamical mean field method is used to analyze the magnetic transition temperature and optical conductivity of a model for the ferrimagnetic double perovskites such as $Sr_2FeMoO_6$. The calculated transition temperatures and optical conductivities are found to depend sensitively on the band structure. For parameters consistent with local spin density approximation band calculations, the computed transition temperatures are lower than observed, and in particular decrease dramatically as band filling is increased, in contradiction to experiment. Band parameters which would increase the transition temperature are identified.Comment: Supercedes cond-mat/000628 (PRB64 024424/1-4 (2001

First metallicity determination from Near-Infrared spectra for five obscured Cepheids discovered in the inner Disk

We report the discovery of five new classical Cepheids located in the inner Galactic Disk at longitude l ≃ −40° in our IRSF/SIRIUS Near-Infrared (NIR) variability survey. The new Cepheids are unique in probing the kinematics and metallicity of young stars at the transition between the inner Disk and the minor axis of the central Bar, where they are expected to be less affected by its dynamical influence. This is also the first time that metallicity of Cepheids is estimated on the basis of medium-resolution (R ∼ 3, 000) NIR spectra, and we validated our results with data in the literature, finding a minimal dependence on the adopted spectroscopic diagnostics. This result is very promising for using Cepheids as stellar proxy of the present-time chemical content of the obscured regions in the Disk. We found that the three Cepheids within 8–10 kpc from us have metallicities consistent with the mean radial metallicity gradient, and kinematics consistent with the Galactic rotation curve. Instead, the closest (∼4 kpc)/farthest (∼12 kpc) Cepheids have significant negative/positive residuals, both in velocity and in iron content. We discuss the possibility that such residuals are related to large-scale dynamical instabilities, induced by the bar/spiral-arm pattern, but the current sample is too limited to reach firm conclusion