C. Allende Prieto

Line formation in solar granulation IV. (O I), O I and OH lines and the photospheric O abundance

The solar photospheric oxygen abundance has been determined from (O I), O I, OH vibration-rotation and OH pure rotation lines by means of a realistic time-dependent, 3D, hydrodynamical model of the solar atmosphere. In the case of the O I lines, 3D non-LTE calculations have been performed, revealing significant departures from LTE as a result of photon loss es in the lines. We derive a solar oxygen abundance of log ǫO = 8.66 ± 0.05. All oxygen diagnostics yield highly consistent abundances, in sharp contrast with the results of classical 1D model atmospheres. This low value is in good agreement with measurements of the local interstellar medium and nearby B stars. This low abundance is also supported by the excellent correspondence between lines of very different line formation sensitivities, and between the observed and predicted line shapes and center-to-limb variations. Together with the corresponding down-ward revisions of the solar carbon, nitrogen and neon abundances, the resulting significant decrease in solar met al mass fraction to Z = 0.0126 can, however, potentially spoil the impressive agreement between predicted and observed sound speed in the solar interior determined from helioseismology.

Line formation in solar granulation VI. [Cl], Cl, CH and C2 lines and the photospheric C abundance

The solar photospheric carbon abundance has been determined from (C ), C , CH vibration-rotation, CH A-X electronic and C2 Swan electronic lines by means of a time-dependent, 3D, hydrodynamical model of the solar atmosphere. Departures from LTE have been considered for the C  lines. These turned out to be of increasing importance for stronger lines and are crucial to remove a trend in LTE abundances with the strengths of the lines. Very gratifying agreement is found among all the atomic and molecular abundance diagnostics in spite of their widely different line formation sensitivities. The mean value of the solar carbon abundance based on the four primary abundance indicators ((C ), C , CH vibration-rotation, C2 Swan) is logC = 8.39 ± 0.05, including our best estimate of possible systematic errors. Consistent results also come from the CH electronic lines, which we have relegated to a supporting role due to their sensitivity to the line broadening. The new 3D based solar C abundance is significantly lower than previously estimated in studies using 1D model atmospheres.

Circumstellar Material in Type Ia Supernovae via Sodium Absorption Features

Most of the progenitors of type Ia supernovae in nearby spiral galaxies may be white dwarf−normal star binary systems. Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus, many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems.

Target selection for the Apache Point Observatory Galactic Evolution Experiment (APOGEE)

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a high-resolution infrared spectroscopic survey spanning all Galactic environments (i.e., bulge, disk, and halo), with the principal goal of constraining dynamical and chemical evolution models of the Milky Way. APOGEE takes advantage of the reduced effects of extinction at infrared wavelengths to observe the inner Galaxy and bulge at an unprecedented level of detail. The surveys broad spatial and wavelength coverage enables users of APOGEE data to address numerous Galactic structure and stellar populations issues. In this paper we describe the APOGEE targeting scheme and document its various target classes to provide the necessary background and reference information to analyze samples of APOGEE data with awareness of the imposed selection criteria and resulting sample properties. APOGEEs primary sample consists of ~105 red giant stars, selected to minimize observational biases in age and metallicity. We present the methodology and considerations that drive the selection of this sample and evaluate the accuracy, efficiency, and caveats of the selection and sampling algorithms. We also describe additional target classes that contribute to the APOGEE sample, including numerous ancillary science programs, and we outline the targeting data that will be included in the public data releases.

Oxygen abundances in nearby stars - Clues to the formation and evolution of the Galactic disk

The abundances of iron and oxygen are homogeneously determined in a sample of 523 nearby (d 5500 K and [Fe/H] −0.3, we find no obvious indication of a sudden decrease (i.e., a “knee”) in the [O/Fe] vs. [Fe/H] pattern of thick-disk stars that would connect the thick and thin disk trends at a high metallicity. We conclude that Type Ia supernovae (SN Ia) did not contribute significantly to the chemical enrichment of the thick disk. In the −0.8 < [Fe/H] < +0.3 range, thin-disk stars show decreasing [O/Fe] ratios from about 0.4 to 0.0 that require a SN Ia contribution. The implications of these results for studies of the formation and evolution of the Galactic disk are discussed.

Detailed analysis of Balmer lines in cool dwarf stars

An analysis of H and H spectra in a sample of 30 cool dwarf and subgiant stars is presented using MARCS model atmospheres based on the most recent calculations of the line opacities. A detailed quantitative comparison of the solar flux spectra with model spectra shows that Balmer line prole shapes, and therefore the temperature structure in the line formation region, are best represented under the mixing length theory by any combination of a low mixing-length parameter and a low convective structure parameter y. A slightly lower eective temperature is obtained for the sun than the accepted value, which we attribute to errors in models and line opacities. The programme stars span temperatures from 4800 to 7100 K and include a small number of population II stars. Eective temperatures have been derived using a quantitative tting method with a detailed error analysis. Our temperatures nd good agreement with those from the Infrared Flux Method (IRFM) near solar metallicity but show dierences at low metallicity where the two available IRFM determinations themselves are in disagreement. Comparison with recent temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who employed a dierent description of the wing absorption due to self-broadening, does not show the large dierences predicted by Barklem et al. (2000b). In fact, perhaps fortuitously, reasonable agreement is found near solar metallicity, while we nd signicantly cooler temperatures for low metallicity stars of around solar temperature.

Orbital parameters of the microquasar LS I +61 303

New optical spectroscopy of the high-mass X-ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X-ray/radio outbursts are triggered 2.5-4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ∼65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i ≃ 113 km s -1 .

APOGEE: The Apache Point Observatory Galactic Evolution Experiment

APOGEE is a large-scale, NIR, high-resolution (R ∼ 20 000) spectroscopic survey of Galactic stars. It is one of the four experiments in SDSS-III. Because APOGEE will observe in the H band, where the extinction is six times smaller than in V, it will be the first survey to pierce through Galactic dust and provide a vast, uniform database of chemical abundances and radial velocities for stars across all Galactic populations (bulge, disk, and halo). The survey will be conducted with a dedicated, 300-fiber, cryogenic, spectrograph that is being built at the University of Virginia, coupled to the ARC 2.5-m telescope at Apache Point Observatory. APOGEE will use a significant fraction of the SDSS-III bright time during a three-year period to observe, at high signal-to-noise ratio (S /N > 100), about 100 000 giant stars selected directly from 2MASS down to a typical flux limit ofH < 13. The main scientific objectives of APOGEE are: (1) measuring unbiased metallicity distributions and abundance patterns for the different Galactic stellar populations, (2) studying the processes of star formation, feedback, and chemical mixing in theMilkyWay, (3) surveying the dynamics of the bulge and disk, placing constraints on the nature and influence of the Galactic bar and spiral arms, and (4) using extensive chemodynamical data, particularly in the inner Galaxy, to unravel its formation and evolution. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk

We study the relationship between age, metallicity, and alpha-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more alpha-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.

Chemodynamics of the Milky Way - I. The first year of APOGEE data

We investigate the chemo-kinematic properties of the Milky Way disc by exploring the first year of data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and compare our results to smaller optical high-resolution samples in the literature, as well as results from lower resolution surveys such as GCS, SEGUE and RAVE. We start by selecting a high-quality sample in terms of chemistry (