1,935 research outputs found
Possible Evidence for Metal Accretion onto the Surfaces of Metal-Poor Main-Sequence Stars
The entire evolution of the Milky Way, including its mass-assembly and
star-formation history, is imprinted onto the chemo-dynamical distribution
function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase
space spanned by position, velocity, and elemental abundance ratios. In
particular, the chemo-dynamical distribution functions for low-mass stars
(e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the
Milky Way's formation, since their main-sequence lifetimes approach or exceed
the age of the universe. A basic tenet of essentially all previous analyses is
that the stellar metallicity, usually parametrized as [Fe/H], is conserved over
time for main-sequence stars (at least those that have not been polluted due to
mass transfer from binary companions). If this holds true, any correlations
between metallicity and kinematics for long-lived main-sequence stars of
different masses, effective temperatures, or spectral types must strictly be
the same, since they reflect the same mass-assembly and star-formation
histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars
on the main sequence, taken from Data Release 8 of the Sloan Digital Sky
Survey, we find that the median metallicity of G-type dwarfs is systematically
higher (by about 0.2 dex) than that of K-type dwarfs having the same median
rotational velocity about the Galactic center. If it can be confirmed, this
finding may invalidate the long-accepted assumption that the atmospheric
metallicities of long-lived stars are conserved over time.Comment: 12 pages, 7 figures, ApJ accepted, comments welcom
Very Metal-Poor Outer-Halo Stars with Round Orbits
The orbital motions of halo stars in the Milky Way reflect the orbital
motions of the progenitor systems in which they formed, making it possible to
trace the mass-assembly history of the Galaxy. Direct measurement of
three-dimensional velocities, based on accurate proper motions and
line-of-sight velocities, has revealed that the majority of halo stars in the
inner-halo region move on eccentric orbits. However, our understanding of the
motions of distant, in-situ halo-star samples is still limited, due to the lack
of accurate proper motions for these stars. Here we explore a model-independent
analysis of the line-of-sight velocities and spatial distribution of a recent
sample of 1865 carefully selected halo blue horizontal-branch (BHB) stars
within 30 kpc of the Galactic center. We find that the mean rotational velocity
of the very metal-poor ([Fe/H] < -2.0) BHB stars significantly lags behind that
of the relatively more metal-rich ([Fe/H] > -2.0) BHB stars. We also find that
the relatively more metal-rich BHB stars are dominated by stars with eccentric
orbits, as previously observed for other stellar samples in the inner-halo
region. By contrast, the very metal-poor BHB stars are dominated by stars on
rounder, lower-eccentricity orbits. Our results indicate that the motion of the
progenitor systems of the Milky Way that contributed to the stellar populations
found within 30 kpc correlates directly with their metal abundance, which may
be related to their physical properties such as gas fractions. These results
are consistent with the existence of an inner/outer halo structure for the halo
system, as advocated by Carollo et al. (2010).Comment: 5 pages, 3 figures, ApJ Letter accepted, comments welcom
Automated Determination of [Fe/H] and [C/Fe] from Low-Resolution Spectroscopy
We develop an automated spectral synthesis technique for the estimation of
metallicities ([Fe/H]) and carbon abundances ([C/Fe]) for metal-poor stars,
including carbon-enhanced metal-poor stars, for which other methods may prove
insufficient. This technique, autoMOOG, is designed to operate on relatively
strong features visible in even low- to medium-resolution spectra, yielding
results comparable to much more telescope-intensive high-resolution studies. We
validate this method by comparison with 913 stars which have existing
high-resolution and low- to medium-resolution to medium-resolution spectra, and
that cover a wide range of stellar parameters. We find that at low
metallicities ([Fe/H] < -2.0), we successfully recover both the metallicity and
carbon abundance, where possible, with an accuracy of ~ 0.20 dex. At higher
metallicities, due to issues of continuum placement in spectral normalization
done prior to the running of autoMOOG, a general underestimate of the overall
metallicity of a star is seen, although the carbon abundance is still
successfully recovered. As a result, this method is only recommended for use on
samples of stars of known sufficiently low metallicity. For these
low-metallicity stars, however, autoMOOG performs much more consistently and
quickly than similar, existing techniques, which should allow for analyses of
large samples of metal-poor stars in the near future. Steps to improve and
correct the continuum placement difficulties are being pursued.Comment: 8 pages, 7 figures; accepted for publication in A
The stellar content of the Hamburg/ESO survey VI. The metallicity distribution of main-sequence turnoff stars in the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactic halo
based on metal-poor main-sequence turnoff-stars (MSTO) which were selected from
the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up
moderateresolution observations (R ~ 2000) of some 682 stars (among which 617
were accepted program stars) were carried out with the 2.3m telescope at the
Siding Spring Observatory (SSO). Corrections for the survey volume covered by
the sample stars were quantitatively estimated and applied to the observed MDF.
The corrections are quite small, when compared with those for a previously
studied sample of metal-poor giants. The corrected observational MDF of the
turnoff sample was then compared with that of the giants, as well as with a
number of theoretical predictions of Galactic chemical evolution, including the
mass-loss modified Simple Model. Although the survey-volume corrected MDFs of
the metal-poor turnoff and the halo giants notably differ in the region of
[Fe/H] > -2.0, below [Fe/H] ~ -2.0, (the region we scientifically focus on
most) both MDFs show a sharp drop at [Fe/H] ~ -3.6 and present rather similar
distributions in the low-metallicity tail. Theoretical models can fit some
parts of the observed MDF, but none is found to simultaneously reproduce the
peak as well as the features in the metal-poor region with [Fe/H] between -2.0
to -3.6. Among the tested models only the GAMETE model, when normalized to the
tail of the observed MDF below [Fe/H] ~ -3.0, and with Z_{cr} =
10^{-3.4}Z_{\odot}, is able to predict the sharp drop at [Fe/H] ~ -3.6.Comment: 10 pages, 11 figures, accepted for publication in A&
Carbon-Enhanced Metal-Poor Stars in the Inner and Outer Halo Components of the Milky Way
(Abridged) Carbon-enhanced metal-poor (CEMP) stars in the halo components of
the Milky Way are explored, based on accurate determinations of the
carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over
30000 calibration stars from the Sloan Digital Sky Survey (SDSS). Using our
present criterion that low-metallicity stars exhibiting [C/Fe] ratios
("carbonicity") in excess of [C/Fe] are considered CEMP stars, the
global frequency of CEMP stars in the halo system for \feh\ is 8%; for
\feh\ it is 12%; for \feh\ it is 20%. We also confirm a
significant increase in the level of carbon enrichment with declining
metallicity, growing from $\sim +1.0$ at \feh\ $= -1.5$ to
at \feh\ . The nature of the carbonicity
distribution function (CarDF) changes dramatically with increasing distance
above the Galactic plane, Z. For Z kpc, relatively few CEMP
stars are identified. For distances Z kpc, the CarDF exhibits a
strong tail towards high values, up to [C/Fe] +3.0. We also find a clear
increase in the CEMP frequency with Z. For stars with [Fe/H] 1.5, the frequency grows from 5% at Z kpc to 10% at Z
kpc. For stars with [Fe/H] 2.0, the frequency grows from 8% at
Z kpc to 25% at Z kpc. For stars with
[Fe/H] $\sim +1.0$ for 0 kpc $<$
$|$Z$|$ $<$ 10 kpc, with little dependence on $|$Z$|$; for [Fe/H] $< -$2.0,
, again roughly independent of Z.Comment: Accepted for publication in the Astrophysical Journal, 32 pages, 15
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