270 research outputs found
Stellar loci I. Metallicity dependence and intrinsic widths
Stellar loci are widely used for selection of interesting outliers, reddening
determinations, and calibrations. However, hitherto the dependence of stellar
loci on metallicity has not been fully explored and their intrinsic widths are
unclear. In this paper, by combining the spectroscopic and re-calibrated
imaging data of the SDSS Stripe 82, we have built a large, clean sample of
dwarf stars with accurate colors and well determined metallicities to
investigate the metallicity dependence and intrinsic widths of the SDSS stellar
loci. Typically, one dex decrease in metallicity causes 0.20 and 0.02 mag
decrease in colors u-g and g-r, and 0.02 and 0.02 mag increase in colors r-i
and i-z, respectively. The variations are larger for metal-rich stars than for
metal-poor ones, and for F/G/K stars than for A/M ones. Using the sample, we
have performed two dimensional polynomial fitting to the u-g, g-r, r-i, and i-z
colors as a function of color g-i and metallicity [Fe/H]. The residuals, at the
level of 0.029, 0.008, 0.008 and 0.011 mag for the u-g, g-r, r-i, and i-z
colors, respectively can be fully accounted for by the photometric errors and
metallicity uncertainties, suggesting that the intrinsic widths of the loci are
at maximum a few mmag. The residual distributions are asymmetric, revealing
that a significant fraction of stars are binaries. In a companion paper, we
will present an unbiased estimate of the binary fraction for field stars. Other
potential applications of the metallicity dependent stellar loci are briefly
discussed.Comment: 6 pages, 4 figures, 1 table, ApJ in pres
Stellar loci III: Photometric metallicities for half million FGK stars of Stripe 82
We develop a method to estimate photometric metallicities by simultaneously
fitting the dereddened colors u-g, g-r, r-i and i-z from the SDSS with those
predicted by the metallicity-dependent stellar loci. The method is tested with
a spectroscopic sample of main-sequence stars in Stripe 82 selected from the
SDSS DR9 and three open clusters. With 1 per cent photometry, the method is
capable of delivering photometric metallicities precise to about 0.05, 0.12,
and 0.18 dex at metallicities of 0.0, -1.0, and -2.0, respectively, comparable
to the precision achievable with low-resolution spectroscopy at a
signal-to-noise ratio of 10. We apply this method to the re-calibrated Stripe
82 catalog and derive metallicities for about 0.5 million stars of colors 0.3 <
g-i < 1.6 mag and distances between 0.3 -- 18 kpc. Potential systematics in the
metallicities thus derived, due to the contamination of giants and binaries,
are investigated. Photometric distances are also calculated. About 91, 72, and
53 per cent of the sample stars are brighter than r = 20.5, 19.5, and 18.5 mag,
respectively. The median metallicity errors are around 0.19, 0.16, 0.11, and
0.085 dex for the whole sample, and for stars brighter than r = 20.5, 19.5, and
18.5 mag, respectively. The median distance errors are 8.8, 8.4, 7.7, and 7.3
per cent for the aforementioned four groups of stars, respectively. The data
are publicly available. Potential applications of the data in studies of the
distribution, (sub)structure, and chemistry of the Galactic stellar
populations, are briefly discussed. The results will be presented in future
papers.Comment: 10 pages, 10 figures, ApJ accepte
Stellar color regression: a spectroscopy based method for color calibration to a few mmag accuracy and the recalibration of Stripe 82
In this paper, we propose a spectroscopy based Stellar Color Regression (SCR)
method to perform accurate color calibration for modern imaging surveys, taking
advantage of millions of stellar spectra now available. The method is
straightforward, insensitive to systematic errors in the spectroscopically
determined stellar atmospheric parameters, applicable to regions that are
effectively covered by spectroscopic surveys, and capable of delivering an
accuracy of a few millimagnitudes for color calibration. As an illustration, we
have applied the method to the SDSS Stripe 82 data (Ivezic et al; I07
hereafter). With a total number of 23,759 spectroscopically targeted stars, we
have mapped out the small but strongly correlated color zero point errors
present in the photometric catalog of Stripe 82, and improve the color
calibration by a factor of 2 -- 3. Our study also reveals some small but
significant magnitude dependence errors in z-band for some CCDs. Such errors
are likely to be present in all the SDSS photometric data. Our results are
compared with those from a completely independent test based on the intrinsic
colors of red galaxies presented by I07. The comparison as well as other tests
shows that the SCR method has achieved a color calibration internally
consistent at a level of about 5 mmag in u-g, 3 mmag in g-r, and 2 mmag in r-i
and i-z, respectively. Given the power of the SCR method, we discuss briefly
the potential benefits by applying the method to existing, on-going, and
up-coming imaging surveys.Comment: 17 pages, 14 figures, 3 tables, ApJ in pres
Three-dimensional structure of the milky way dust: modeling of LAMOST data
We present a three-dimensional modeling of the Milky Way dust distribution by
fitting the value-added star catalog of LAMOST spectral survey. The global dust
distribution can be described by an exponential disk with scale-length of 3,192
pc and scale height of 103 pc. In this modeling, the Sun is located above the
dust disk with a vertical distance of 23 pc. Besides the global smooth
structure, two substructures around the solar position are also identified. The
one located at and is
consistent with the Gould Belt model of \citet{Gontcharov2009}, and the other
one located at and is
associated with the Camelopardalis molecular clouds.Comment: 15 pages, 6 figure, accepted by Ap
Determination of the Local Standard of Rest using the LSS-GAC DR1
We re-estimate the peculiar velocity of the Sun with respect to the local
standard of rest using a sample of local stars within 600 pc of the Sun,
selected from the LAMOST Spectroscopic Survey of the Galactic Anti-centre
(LSS-GAC). The sample consists of 94332 FGK main-sequence stars with
well-determined radial velocities and atmospheric parameters. To derive the
LSR, two independent analyses are applied to the data. Firstly, we determine
the solar motion by comparing the observed velocity distribution to that
generated with the analytic formulism of Schonrich & Binney that has been
demonstrated to show excellent agreement with rigorous torus-based dynamics
modelling by Binney & McMillan. Secondly, we propose that cold populations of
thin disc stars, selected by applying an orbital eccentricity cut, can be
directly used to determine the LSR without the need of asymmetric drift
corrections. Both approaches yield consistent results of solar motion in the
direction of Galactic rotation, V_sun, that are much higher than the standard
value adopted hitherto, derived from Stromgren's equation. The newly deduced
values of V_sun are 1-2 km/s smaller than the more recent estimates derived
from the Geneva-Copenhagen Survey sample of stars in the solar neighbourhood
(within 100 pc). We attribute the small difference to the presence of several
well-known moving groups in the GCS sample that, fortunately, hardly affect the
LSS-GAC sample. The newly derived radial and vertical components of the solar
motion agree well with the previous studies. In addition, for all components of
the solar motion, the values yielded by stars of different spectral types in
the LSS-GAC sample are consistent with each other, suggesting that the local
disk is well relaxed and that the LSR reported in the current work is robust.
Our final recommended LSR is, (U,V,W)_sun = (7.01+/-0.20, 10.13+/-0.12,
4.95+/-0.09) km/s.Comment: MNRAS accepted, 13 pages, 11 figures, 7 table
White dwarf-main sequence binaries from LAMOST: the DR1 catalogue
Context. White dwarf-main sequence (WDMS) binaries are used to study several
different important open problems in modern astrophysics.
Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of
WDMS binaries currently known. However, this sample is seriously affected by
selection effects and the population of systems containing cool white dwarfs
and early-type companions is under-represented.Here we search for WDMS binaries
within the spectroscopic data release 1 of the LAMOST (Large sky Area
Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow
different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn
from a different parent population and thus help in overcoming the selection
effects incorporated by SDSS on the current observed population.
Methods. We develop a fast and efficient routine based on the wavelet
transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M
dwarf companion, and apply a decomposition/fitting routine to their LAMOST
spectra to estimate their distances and measure their stellar parameters,
namely the white dwarf effective temperatures, surface gravities and masses,
and the secondary star spectral types.
Results. We identify 121 LAMOST WDMS binaries, 80 of which are new
discoveries, and estimate the sample to be \sim90 per cent complete. The LAMOST
and SDSS WDMS binaries are found to be statistically different. However, this
result is not due to the different target selection criteria of both surveys,
but likely a simple consequence of the different observing conditions. Thus,
the LAMOST population is found at considerably shorter distances (\sim50-450
pc) and is dominated by systems containing early-type companions and hot white
dwarfs. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A&
Discovery of two new hypervelocity stars from the LAMOST spectroscopic surveys
We report the discovery of two new unbound hypervelocity stars (HVSs) from
the LAMOST spectroscopic surveys. They are respectively a B2V type star of ~ 7
M with a Galactic rest-frame radial velocity of 502 km/s at a
Galactocentric radius of ~ 21 kpc and a B7V type star of ~ 4 M
with a Galactic rest-frame radial velocity of 408 km/s at a Galactocentric
radius of ~ 30 kpc. The origins of the two HVSs are not clear given their
currently poorly measured proper motions. However, the future data releases of
Gaia should provide proper motion measurements accurate enough to solve this
problem. The ongoing LAMOST spectroscopic surveys are expected to yield more
HVSs to form a statistical sample, providing vital constraint on understanding
the nature of HVSs and their ejection mechanisms.Comment: 5 pages, 3 figures, 1 table, accepted for publication in ApJ
Contribution of increasing CO2 and climate change to the carbon cycle in China\u27s ecosystems
Atmospheric CO2 and China\u27s climate have changed greatly during 1961–2000. The influence of increased CO2 and changing climate on the carbon cycle of the terrestrial ecosystems in China is still unclear. In this article we used a process-based ecosystem model, Biome-BGC, to assess the effects of changing climate and elevated atmospheric CO2 on terrestrial China\u27s carbon cycle during two time periods: (1) the present (1961–2000) and (2) a future with projected climate change under doubled CO2 (2071–2110). The effects of climate change alone were estimated by driving Biome-BGC with a fixed CO2 concentration and changing climate, while the CO2 fertilization effects were calculated as the difference between the results driven by both increasing CO2 and changing climate and those of variable climate alone. Model simulations indicate that during 1961–2000 at the national scale, changes in climate reduced carbon storage in China\u27s ecosystems, but increasing CO2 compensated for these adverse effects of climate change, resulting in an overall increase in the carbon storage of China\u27s ecosystems despite decreases in soil carbon. The interannual variability of the carbon cycle was associated with climate variations. Regional differences in climate change produced differing regional carbon uptake responses. Spatially, reductions in carbon in vegetation and soils and increases in litter carbon were primarily caused by climate change in most parts of east China, while carbon in vegetation, soils, and litter increased for much of west China. Under the future scenario (2071–2110), with a doubling CO2, China will experience higher precipitation and temperature as predicted by the Hadley Centre HadCM3 for the Intergovernmental Panel on Climate Change Fourth Assessment. The concomitant doubling of CO2 will continue to counteract the negative effects of climate change on carbon uptake in the future, leading to an increase in carbon storage relative to current levels. This study highlights the role of CO2 fertilization in the carbon budget of China\u27s ecosystems, although future studies should include other important processes such as land use change, human management (e.g., fertilization and irrigation), environmental pollution, etc
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