243 research outputs found
Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo
We find that the relative contribution of satellite galaxies accreted at high
redshift to the stellar population of the Milky Way's smooth halo increases
with distance, becoming observable relative to the classical smooth halo about
15 kpc from the Galactic center. In particular, we determine
line-of-sight-averaged [Fe/H] and [alpha/Fe] in the metal-poor main-sequence
turnoff (MPMSTO) population along every Sloan Extension for Galactic
Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting
our sample to those lines of sight along which we do not detect elements of
cold halo substructure (ECHOS), we compile the largest spectroscopic sample of
stars in the smooth component of the halo ever observed in situ beyond 10 kpc.
We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population
in the distant half of our sample beyond about 15 kpc from the Galactic center.
Inside of 15 kpc however, we find no significant spatial autocorrelation in
[Fe/H]. At the same time, we perform SEGUE-like observations of N-body
simulations of Milky Way analog formation. While we find that halos formed
entirely by accreted satellite galaxies provide a poor match to our
observations of the halo within 15 kpc of the Galactic center, we do observe
spatial autocorrelation in [Fe/H] in the simulations at larger distances. This
observation is an example of statistical chemical tagging and indicates that
spatial autocorrelation in metallicity is a generic feature of stellar halos
formed from accreted satellite galaxies.Comment: 27 pages, 8 figures, and 7 tables in emulateapj format; accepted for
publication in ApJ. Full tables can be extracted from LaTeX sourc
Metallicity Gradients in the Milky Way Disk as Observed by the SEGUE Survey
The observed radial and vertical metallicity distribution of old stars in the
Milky Way disk provides a powerful constraint on the chemical enrichment and
dynamical history of the disk. We present the radial metallicity gradient,
\Delta[Fe/H]/\Delta R, as a function of height above the plane, |Z|, using 7010
main sequence turnoff stars observed by the Sloan Extension for Galactic
Understanding and Exploration (SEGUE) survey. The sample consists of mostly old
thin and thick disk stars, with a minimal contribution from the stellar halo,
in the region 6 < R < 16 kpc, 0.15 < |Z| < 1.5 kpc. The data reveal that the
radial metallicity gradient becomes flat at heights |Z| > 1 kpc. The median
metallicity at large |Z| is consistent with the metallicities seen in outer
disk open clusters, which exhibit a flat radial gradient at [Fe/H] ~ -0.5. We
note that the outer disk clusters are also located at large |Z|; because the
flat gradient extends to small R for our sample, there is some ambiguity in
whether the observed trends for clusters are due to a change in R or |Z|. We
therefore stress the importance of considering both the radial and vertical
directions when measuring spatial abundance trends in the disk. The flattening
of the gradient at high |Z| also has implications on thick disk formation
scenarios, which predict different metallicity patterns in the thick disk. A
flat gradient, such as we observe, is predicted by a turbulent disk at high
redshift, but may also be consistent with radial migration, as long as mixing
is strong. We test our analysis methods using a mock catalog based on the model
of Sch\"onrich & Binney, and we estimate our distance errors to be ~25%. We
also show that we can properly correct for selection biases by assigning
weights to our targets.Comment: Submitted to ApJ; 22 pages, 14 figures in emulateapj format; Full
resolution version available at
http://www.ucolick.org/~jyc/gradient/cheng_apj_fullres.pd
Advances in instrumentation at the W. M. Keck Observatory
In this paper we describe both recently completed instrumentation projects and our current development efforts in the context of the Observatory's science driven strategic plan which seeks to address key questions in observational astronomy for extra-galactic, Galactic, and planetary science with both seeing limited capabilities and high angular resolution adaptive optics capabilities. This paper will review recently completed projects as well as new instruments in development including MOSFIRE, a near IR multi-object spectrograph nearing completion, a new seeing limited integral field spectrograph for the visible wavelength range called the Keck Cosmic Web Imager, and the Keck Next Generation Adaptive Optics facility and its first light science instrument DAVINCI
Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars
We describe a method for the determination of stellar [C/Fe] abundance ratios
using low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. By
means of a star-by-star comparison with a set of SDSS/SEGUE spectra with
available estimates of [C/Fe] based on published high-resolution analyses, we
demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 to
a precision better than 0.35 dex. Using the measured carbon-to-iron abundance
ratios obtained by this technique, we derive the frequency of carbon-enhanced
stars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars
and other samples from the literature. We find that the differential frequency
slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a
sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to
[Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP stars
varies across different luminosity classes. The giant sample exhibits a
cumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and
33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lower
cumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf population
displays a large change in the cumulative frequency for CEMP stars below [Fe/H]
= -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. When
we impose a restriction with respect to distance from the Galactic mid-plane
(|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low
metallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution of
C-rich material in stars that have undergone mixing with CNO-processed material
from their interiors. The frequency of CEMP stars near the main-sequence
turnoff, which are not expected to have experienced mixing, increases for
[Fe/H] < -3.0. [abridged]Comment: 19 pages, 10 figures, 6 tables, accepted for publication in AJ on
August 20, 201
Swimming with ShARCS: Comparison of On-sky Sensitivity With Model Predictions for ShaneAO on the Lick Observatory 3-meter Telescope
The Lick Observatory's Shane 3-meter telescope has been upgraded with a new
infrared instrument (ShARCS - Shane Adaptive optics infraRed Camera and
Spectrograph) and dual-deformable mirror adaptive optics (AO) system (ShaneAO).
We present first-light measurements of imaging sensitivity in the Ks band. We
compare measured results to predicted signal-to-noise ratio and magnitude
limits from modeling the emissivity and throughput of ShaneAO and ShARCS. The
model was validated by comparing its results to the Keck telescope adaptive
optics system model and then by estimating the sky background and limiting
magnitudes for IRCAL, the previous infra-red detector on the Shane telescope,
and comparing to measured, published results. We predict that the ShaneAO
system will measure lower sky backgrounds and achieve 20\% higher throughput
across the bands despite having more optical surfaces than the current
system. It will enable imaging of fainter objects (by 1-2 magnitudes) and will
be faster to reach a fiducial signal-to-noise ratio by a factor of 10-13. We
highlight the improvements in performance over the previous AO system and its
camera, IRCAL.Comment: 13 pages, 5 figures, SPIE Astronomical Telescopes + Instrumentation,
Montreal 201
Extremely Metal-Poor Stars: The Local High Redshift Universe
Extremely metal-poor (EMP) stars can only have formed early in the history of the Galaxy, and
represent the local equivalent of the high redshift universe. With them, we can study the early
supernovae, the early chemical evolution of the Galaxy, and the history of star formation in the
Milky Way. By analogy we can learn about those epochs of galaxy formation in the distant past
that are currently at such high redshifts that they are beyond the reach of even the largest existing
telescopes, a technique some call “near-field cosmology”.
While H, He, and some Li came out of the Big Bang, all other elements were formed in stars,
and were dispersed by supernovae and stellar winds into the gas from which subsequent stellar
generations formed. The ejecta from supernovae played the most important role in the early
Universe. SN models have many parameters, including the history of the progenitor star (initial
mass, mass loss history, internal nucleosynthesis history prior to the explosion, etc), the details of
the explosion (energy, ejected mass, mixing) etc. There are vigorous groups pursuing the details of
these models both theoretically and computationally in the US and abroad. But there are so many
free or poorly known parameters that these efforts are best guided by observations of metal-poor
stars
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