60 research outputs found
Hypervelocity Star Candidates in the SEGUE G & K Dwarf Sample
We present 20 candidate hypervelocity stars from the Sloan Extension for
Galactic Understanding and Exploration (SEGUE) G and K dwarf samples. Previous
searches for hypervelocity stars have only focused on large radial velocities;
in this study we also use proper motions to select the candidates. We determine
the hypervelocity likelihood of each candidate by means of Monte Carlo
simulations, considering the significant errors often associated with high
proper motion stars. We find that nearly half of the candidates exceed their
escape velocities with at least 98% probability. Every candidate also has less
than a 25% chance of being a high-velocity fluke within the SEGUE sample. Based
on orbits calculated using the observed six-dimensional positions and
velocities, few, if any, of these candidates originate from the Galactic
center. If these candidates are truly hypervelocity stars, they were not
ejected by interactions with the Milky Way's supermassive black hole. This
calls for a more serious examination of alternative hypervelocity-star ejection
scenarios.Comment: 8 pages, 5 figures, published in ApJ, this version includes all
figures as intende
Binary Contamination in the SEGUE sample: Effects on SSPP Determinations of Stellar Atmospheric Parameters
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 degraded versions, at median S/N of 50, 25 and 10. By running individual
and combined spectra (representing the binaries) through the SSPP, we determine
that ~10% of the blended G-K dwarf pairs with S/N>=25 will have their
atmospheric parameter determinations, in particular temperature and
metallicity, noticeably affected by the presence of an undetected secondary. To
account for the additional uncertainty from binary contamination at a S/N~10,
uncertainties of ~140 K and ~0.17 dex in [Fe/H] must be added in quadrature to
the published uncertainties of the SSPP. (Abridged)Comment: 68 pages, 20 figures, 9 table
Chemical Cartography with APOGEE: Large-scale Mean Metallicity Maps of the Milky Way
We present Galactic mean metallicity maps derived from the first year of the
SDSS-III APOGEE experiment. Mean abundances in different zones of
Galactocentric radius (0 < R < 15 kpc) at a range of heights above the plane (0
< |z| < 3 kpc), are derived from a sample of nearly 20,000 stars with
unprecedented coverage, including stars in the Galactic mid-plane at large
distances. We also split the sample into subsamples of stars with low and
high-[{\alpha}/M] abundance ratios. We assess possible biases in deriving the
mean abundances, and find they are likely to be small except in the inner
regions of the Galaxy. A negative radial gradient exists over much of the
Galaxy; however, the gradient appears to flatten for R < 6 kpc, in particular
near the Galactic mid-plane and for low-[{\alpha}/M] stars. At R > 6 kpc, the
gradient flattens as one moves off of the plane, and is flatter at all heights
for high-[{\alpha}/M] stars than for low-[{\alpha}/M] stars. Alternatively,
these gradients can be described as vertical gradients that flatten at larger
Galactocentric radius; these vertical gradients are similar for both low and
high-[{\alpha}/M] populations. Stars with higher [{\alpha}/M] appear to have a
flatter radial gradient than stars with lower [{\alpha}/M]. This could suggest
that the metallicity gradient has grown steeper with time or, alternatively,
that gradients are washed out over time by migration of stars.Comment: 16 pages, 12 figures, submitted to A
The Metallicity Distribution Functions of SEGUE G and K dwarfs: Constraints for Disk Chemical Evolution and Formation
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.Comment: 70 pages, 25 figures, 7 tables. Accepted by The Astrophysical Journa
The GALAH survey: relative throughputs of the 2dF fibre positioner and the HERMES spectrograph from stellar targets
We present an analysis of the relative throughputs of the 3.9-m Anglo-Australian Telescopeâs 2dF/HERMES (High Efficiency and Resolution Multi-Element Spectrograph) system, based upon spectra acquired during the first two years of the Galactic Archaeology with HERMES survey. Averaged spectral fluxes of stars were compared to their photometry to determine the relative throughputs of fibres for a range of fibre position and atmospheric conditions. We find that overall the throughputs of the 771 usable fibres have been stable over the first two years of its operation. About 2.5 per cent of fibres have throughputs much lower than the average. There are also a number of yet unexplained variations between the HERMES bandpasses, and mechanically and optically linked fibre groups known as retractors or slitlets related to regions
of the focal plane. These findings do not impact the science that HERMES will produce
The GALAH survey: Milky Way disc metallicity and alpha-abundance trends in combined APOGEE-GALAH catalogues
GALAH and APOGEE are two high resolution multi object spectroscopic surveys
that provide fundamental stellar parameters and multiple elemental abundance
estimates for 400,000 stars in the Milky Way. They are complimentary in
both sky coverage and wavelength regime. Thus combining the two surveys will
provide us a large sample to investigate the disc metallicity and alpha
abundance trends. We use the Cannon data-driven approach selecting training
sets from among 20,000 stars in common for the two surveys to predict the
GALAH scaled stellar parameters from APOGEE spectra as well as APOGEE scaled
stellar parameters from GALAH spectra. We provide two combined catalogues with
GALAH scaled and APOGEE scaled stellar parameters each having 500,000
stars after quality cuts. With 470,000 stars that are common in both
these catalogues, we compare the GALAH scaled and APOGEE scaled metallicity
distribution functions (MDF), radial and vertical metallicity gradients as well
as the variation of [/Fe] vs [Fe/H] trends along and away from the
Galactic mid plane. We find mean metallicities of APOGEE scaled sample to be
higher compared to that for the GALAH scaled sample. We find similar
[/Fe] vs [Fe/H] trends using both samples consistent with previous
observational as well as simulation based studies. Radial and vertical
metallicity gradients derived using the two survey scaled samples are
consistent except in the inner and outer Galactocentric radius bins. Our
gradient estimates in the solar neighborhood are also consistent with previous
studies and are backed by larger sample size compared to previous works.Comment: 21 pages, 19 figures, submitted to MNRA
The GALAH survey: An abundance, age, and kinematic inventory of the solar neighbourhood made with TGAS
The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and Gaia provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, and sub-giant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho-Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this sample. Stellar parameters and elemental abundances are derived from the GALAH spectra with the spectral synthesis code SPECTROSCOPY MADE EASY. We determine kinematics and dynamics, including action angles, from the Gaia astrometry and GALAH radial velocities. Stellar masses and ages are determined with Bayesian isochrone matching, using our derived stellar parameters and absolute magnitudes. We report measurements of Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba and we note that we have employed non-LTE calculations for Li, O, Al, and Fe. We show that the use of astrometric and photometric data improves the accuracy of the derived spectroscopic parameters, especially log g. Focusing our investigation on the correlations between stellar age, iron abundance [Fe/H], and mean alpha-enhancement [α/Fe] of the magnitude-selected sample, we recover the result that stars of the high-α sequence are typically older than stars in the low-α sequence, the latter spanning iron abundances of â0.7â8âGyr) have lower angular momenta Lz than the Sun, which implies that they are on eccentric orbits and originate from the inner disc. Contrary to some previous smaller scale studies we find a continuous evolution in the high-α-sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate âhigh-α metal-richâ population. Stars in our sample that are younger than 10 Gyr, are mainly found on the low α-sequence and show a gradient in Lz from low [Fe/H] (Lzâ> âLz,ââ) towards higher [Fe/H] (Lzâ< âLz,ââ), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.Open Access funding provided by Max Planck Society. S.B. and K.L. acknowledge funds from the Alexander von Humboldt Foundation in the framework of the Sofja Kovalevskaja Award endowed by the Federal Ministry of Education and Research. S.B. acknowledges support from the Hunstead
Fund for Astrophysics at the University of Sydney and the National Science
Foundation under Grant No. PHY-1430152 (JINA Center for the Evolution of
the Elements). K.L. acknowledges funds from the Swedish Research Council
(Grant nr. 2015-00415_3) and Marie Sklodowska Curie Actions (Cofund Project INCA 600398). L.D. gratefully acknowledges a scholarship from Zonta International District 24 and support from ARC grant DP160103747. L.C. is the recipient of an ARC Future Fellowship (project number FT160100402). S.L.M. acknowledges financial support from the Australian Research Council through grant DE140100598. K.C., G.T., and T.Z. acknowledge support by the core funding grant P1-0188 from the Slovenian Research Agency. D.M.N. was supported by the Allan C. and Dorothy H. Davis Fellowship. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013
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