We present the first investigation of Th abundances in Solar twins and analogues to understand the possible range of this radioactive element and its effect on rocky planet interior dynamics and potential habitability. The abundances of the radioacti
ve elements Th and U are key components of a planets energy budget, making up 30% to 50% of the Earths (Korenaga 2008; All`egre et al. 2001; Schubert et al. 1980; Lyubetskaya & Korenaga 2007; The KamLAND Collaboration 2011; Huang et al. 2013). Radiogenic heat drives interior mantle convection and surface plate tectonics, which sustains a deep carbon and water cycle and thereby aides in creating Earths habitable surface. Unlike other heat sources that are dependent on the planets specific formation history, the radiogenic heat budget is directly related to the mantle concentration of these nuclides. As a refractory element, the stellar abundance of Th is faithfully reflected in the terrestrial planets concentration. We find that log eps Th varies from 59% to 251% that of Solar, suggesting extrasolar planetary systems may possess a greater energy budget with which to support surface to interior dynamics and thus increase their likelihood to be habitable compared to our Solar System.
The SDSS-III/APOGEE survey operated from 2011-2014 using the APOGEE spectrograph, which collects high-resolution (R~22,500), near-IR (1.51-1.70 microns) spectra with a multiplexing (300 fiber-fed objects) capability. We describe the survey data produ
cts that are publicly available, which include catalogs with radial velocity, stellar parameters, and 15 elemental abundances for over 150,000 stars, as well as the more than 500,000 spectra from which these quantities are derived. Calibration relations for the stellar parameters (Teff, log g, [M/H], [alpha/M]) and abundances (C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Mn, Fe, Ni) are presented and discussed. The internal scatter of the abundances within clusters indicates that abundance precision is generally between 0.05 and 0.09 dex across a broad temperature range; within more limited ranges and at high S/N, it is smaller for some elemental abundances. We assess the accuracy of the abundances using comparison of mean cluster metallicities with literature values, APOGEE observations of the solar spectrum and of Arcturus, comparison of individual star abundances with other measurements, and consideration of the locus of derived parameters and abundances of the entire sample, and find that it is challenging to determine the absolute abundance scale; external accuracy may be good to 0.1-0.2 dex. Uncertainties may be larger at cooler temperatures (Teff<4000K). Access to the public data release and data products is described, and some guidance for using the data products is provided.
The open cluster NGC 6791 is among the oldest, most massive and metal-rich open clusters in the Galaxy. High-resolution $H$-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) of 11 red giants in NGC 6791 are analyze
d for their chemical abundances of iron, oxygen, and sodium. The abundances of these three elements are found to be homogeneous (with abundance dispersions at the level of $sim$ 0.05 - 0.07 dex) in these cluster red giants, which span much of the red-giant branch (T$_{rm eff}$ $sim$ 3500K - 4600K), and include two red-clump giants. From the infrared spectra, this cluster is confirmed to be among the most metal-rich clusters in the Galaxy ($<$[Fe/H]$>$ = 0.34 $pm$ 0.06), and is found to have a roughly solar value of [O/Fe] and slightly enhanced [Na/Fe]. Non-LTE calculations for the studied Na I lines in the APOGEE spectral region ($lambda$16373.86AA and $lambda$16388.85AA) indicate only small departures from LTE ($leq$ 0.04 dex) for the parameter range and metallicity of the studied stars. The previously reported double population of cluster members with different Na abundances is not found among the studied sample.
I report on the APOKASC catalog, a joint effort between the Kepler Asteroseismic Science Consortium and the SDSS-III APOGEE spectroscopic survey. It will contain both seismic and spectroscopic values for stars observed by both surveys. I discuss the
derivation of spectroscopic parameters and their uncertainties from the H-band spectra delivered by the APOGEE spectrograph, illustrating the sensitivity of stellar spectra to some parameters, such as Teff, and lack of sensitivity to others, such as logg.
Using G dwarfs from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey, we have determined a vertical metallicity gradient over a large volume of the Milky Ways disk, and examined how this gradient varies for different [a/F
e] subsamples. This sample contains over 40,000 stars with low-resolution spectroscopy over 144 lines of sight. We employ the SEGUE Stellar Parameter Pipeline (SSPP) to obtain estimates of effective temperature, surface gravity, [Fe/H], and [a/Fe] for each star and extract multiple volume-complete subsamples of approximately 1000 stars each. Based on the surveys consistent target-selection algorithm, we adjust each subsample to determine an unbiased picture of the disk in [Fe/H] and [a/Fe]; consequently, each individual star represents the properties of many. The SEGUE sample allows us to constrain the vertical metallicity gradient for a large number of stars over a significant volume of the disk, between ~0.3 and 1.6 kpc from the Galactic plane, and examine the in situ structure, in contrast to previous analyses which are more limited in scope. This work does not pre-suppose a disk structure, whether composed of a single complex population or a distinct thin and thick disk component. The metallicity gradient is -0.243 +0.039 -0.053 dex/kpc for the sample as a whole, which we compare to various literature results. Each [a/Fe] subsample dominates at a different range of heights above the plane of the Galaxy, which is exhibited in the gradient found in the sample as a whole. Stars over a limited range in [a/Fe] show little change in median [Fe/H] with height. If we associate [a/Fe] with age, our consistent vertical metallicity gradients with [a/Fe] suggest that stars formed in different epochs exhibit comparable vertical structure, implying similar star-formation processes and evolution.
Fundamental stellar properties, such as mass, radius, and age, can be inferred using asteroseismology. Cool stars with convective envelopes have turbulent motions that can stochastically drive and damp pulsations. The properties of the oscillation fr
equency power spectrum can be tied to mass and radius through solar-scaled asteroseismic relations. Stellar properties derived using these scaling relations need verification over a range of metallicities. Because the age and mass of halo stars are well-constrained by astrophysical priors, they provide an independent, empirical check on asteroseismic mass estimates in the low-metallicity regime. We identify nine metal-poor red giants (including six stars that are kinematically associated with the halo) from a sample observed by both the Kepler space telescope and the Sloan Digital Sky Survey-III APOGEE spectroscopic survey. We compare masses inferred using asteroseismology to those expected for halo and thick-disk stars. Although our sample is small, standard scaling relations, combined with asteroseismic parameters from the APOKASC Catalog, produce masses that are systematically higher (<{Delta}M>=0.17+/-0.05 Msun) than astrophysical expectations. The magnitude of the mass discrepancy is reduced by known theoretical corrections to the measured large frequency separation scaling relationship. Using alternative methods for measuring asteroseismic parameters induces systematic shifts at the 0.04 Msun level. We also compare published asteroseismic analyses with scaling relationship masses to examine the impact of using the frequency of maximum power as a constraint. Upcoming APOKASC observations will provide a larger sample of ~100 metal-poor stars, important for detailed asteroseismic characterization of Galactic stellar populations.
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 ev
olution 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 ~100,000 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.
We present the metallicity distribution function (MDF) for 24,270 G and 16,847 K dwarfs at distances from 0.2 to 2.3 kpc from the Galactic plane, based on spectroscopy from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey
. This stellar sample is significantly larger in both number and volume than previous spectroscopic analyses, which were limited to the solar vicinity, making it ideal for comparison with local volume-limited samples and Galactic models. For the first time, we have corrected the MDF for the various observational biases introduced by the SEGUE target selection strategy. The SEGUE sample is particularly notable for K dwarfs, which are too faint to examine spectroscopically far from the solar neighborhood. The MDF of both spectral types becomes more metal-poor with increasing |Z|, which reflects the transition from a sample with small [alpha/Fe] values at small heights to one with enhanced [alpha/Fe] above 1 kpc. Comparison of our SEGUE distributions to those of two different Milky Way models reveals that both are more metal-rich than our observed distributions at all heights above the plane. Our unbiased observations of G and K dwarfs provide valuable constraints over the |Z|-height range of the Milky Way disk for chemical and dynamical Galaxy evolution models, previously only calibrated to the solar neighborhood, with particular utility for thin- and thick-disk formation models.
We present the N, O, F and Na abundance and 12C/13C isotopic ratio measurements or upper limits for a sample of 10 C-rich, metal-poor giant stars, eight enhanced in s-process (CEMP-s) elements and two poor in n-capture elements (CEMP-no). The abundan
ces are derived from IR, K-band, high-resolution CRIRES@VLT spectra obtained. The metallicity of our sample ranges from [Fe/H]=-3.4 to -1.3. F abundance could be measured only in two CEMP-s stars. With [F/Fe]=0.64, one is mildly F-overabundant, while the other is F-rich, at [F/Fe]=1.44. For the remaining eight objects, including both CEMP-no in our sample, only upper limits on F abundance could be placed. Our measurements and upper limits show that there is a spread in [F/C+N] ratio in CEMP-s stars as predicted by theory. Predictions from nucleosynthetic models for low-mass, low-metallicity Asymptotic Giant Branch stars, account for the derived F abundances, while the upper limits on F content derived for most of the stars are lower than the predicted values. The measured Na content is accounted for by AGB models in the 1.25 to 1.7 Msun range, confirming that the stars responsible for the peculiar abundance pattern observed in CEMP-s stars are low-mass, low-metallicity AGB stars, in agreement with the most accepted astrophysical scenario. We conclude that the mechanism of F production in current state-of-the-art low-metallicity low-mass AGB models needs further scrutiny and that F measurements in a larger number of metal-poor stars are needed to better constraint the models.
Using numerical modeling and a grid of synthetic spectra, we examine the effects that unresolved binaries have on the determination of various stellar atmospheric parameters for SEGUE targets measured using the SEGUE Stellar Parameter Pipeline (SSPP)
. To model undetected binaries that may be in the SEGUE sample, we use a variety of mass distributions for the primary and secondary stars in conjunction with empirically determined relationships for orbital parameters to determine the fraction of G-K dwarf stars, as defined by SDSS color cuts, that will be blended with a secondary companion. We focus on the G-K dwarf sample in SEGUE as it records the history of chemical enrichment in our galaxy. To determine the effect of the secondary on the spectroscopic parameters, we synthesize a grid of model spectra from 3275 to 7850 K (~0.1 to 1.0 msun) and [Fe/H]=-0.5 to -2.5 from MARCS model atmospheres using TurboSpectrum. We analyze both infinite signal-to-noise ratio (S/N) models and degrad
