153 research outputs found
Searches for the Most Metal-Poor Candidates from SDSS and SEGUE
We report on efforts to identify large samples of very and extremely
metal-poor stars based on medium-resolution spectroscopy and ugriz photometry
obtained during the course of the Sloan Digital Sky Survey (SDSS), and its
extension, SDSS-II, which includes the program SEGUE: Sloan Extension for
Galactic Understanding and Exploration. To date, over 8000 stars with [Fe/H] <=
-2.0 and effective temperatures in the range 4500K < T_eff < 7000K have been
found, with the expected numbers in this temperature range to be well over
10,000 once SEGUE is completed. The numbers roughly double when one includes
warmer blue stragglers and Blue Horizontal-Branch (BHB) stars in these counts.
We show the observed low-metallicity tails of the Metallicity Distribution
Functions for the cooler SDSS/SEGUE stars obtained thus far. We also comment on
the confirmation of an inner/outer halo dichotomy in the Milky Way, and on how
this realization may be used to direct searches for even more metal-poor stars
in the near future.Comment: 5 pages, 4 figures, from the conference "First Stars III", held in
July 200
Extreme Enhancements of r-process Elements in the Cool Metal-Poor Main-Sequence Star SDSS J2357-0052
We report the discovery of a cool metal-poor, main-sequence star exhibiting
large excesses of r-process elements. This star is one of two newly discovered
cool subdwarfs (effective temperatures of 5000 K) with extremely low
metallicity ([Fe/H]<-3) identified from follow-up high-resolution spectroscopy
of metal-poor candidates from the Sloan Digital Sky Survey. SDSS J2357-0052 has
[Fe/H]=-3.4 and [Eu/Fe]=+1.9, and exhibits a scaled solar r-process abundance
pattern of heavy neutron-capture elements. This is the first example of an
extremely metal-poor, main-sequence star showing large excesses of r-process
elements; all previous examples of the large r-process-enhancement phenomena
have been associated with metal-poor giants. The metallicity of this object is
the lowest, and the excess of Eu ([Eu/Fe]) is the highest, among the
r-process-enhanced stars found so far. We consider possible scenarios to
account for the detection of such a star, and discuss techniques to enable
searches for similar stars in the future.Comment: 16 pages, 3 figures, 2 tables, ApJL in pres
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
Understanding the early stages of galaxy formation using very metal-poor stars from the Hamburg/ESO survey
We explore the chemo-dynamical properties of a sample of very metal-poor
(VMP) stars selected from the Hamburg/ESO survey, matched with Gaia EDR3, in
the phase-space identified by the three integrals of motion (, ,
). Disk and halo orbits are separated by using the criteria defined in
Carollo et al. (2021). We found 26 stars with possessing
disk kinematics, of which 14 are extremely metal-poor. At these metallicities,
the number of stars with disk kinematics is three times its retrograde
counterpart. In the same range of metallicity we also identified 37 halo stars
most tightly bound to the gravitational potential of the progenitor halo. The
origin of these stars are investigated by comparing the observational results
with simulated galaxies from the Aquarius Project and the IllustrisTNG
simulations. We found two mechanisms of formation of VMP stars with disk
kinematics: accretion from early satellites (which is dominant), and {\it
in-situ} formation. These stars are very old, with ages > 12.5 Gyr ( > 5),
and they are -enriched. Accretion and {\it in-situ} formation are also
found for the retrograde counterparts with being accretion also the dominant
mode. Contributing accreted satellites have stellar masses in the range
M_sun, and are very gas-rich. The most bound halo stars are the
oldest detected with a median age of ~ 13.3 Gyr ( ~ 11), and
-enriched. Our finding clearly show that very old, very metal-poor
stars store important information on the first stages of assembly of our Galaxy
and its halo.Comment: 18 pages, 6 figures, accepted for publication in the Astrophysical
Journal (updated TNG50 citations
Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy
A new set of very high signal-to-noise (S/N > 100/1), medium-resolution
(R~3000) optical spectra have been obtained for 302 of the candidate
"weak-metal" stars selected by Bidelman & MacConnell. We use these data to
calibrate the recently developed generalization of the SEGUE Stellar Parameter
Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and
[Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The
new abundance measurements are shown to be consistent with available
high-resolution spectroscopic determinations, and represent a substantial
improvement over the accuracies obtained from the previous photometric
estimates reported in Paper I of this series. The apparent offset in the
photometric abundances of the giants in this sample noted by several authors is
confirmed by our new spectroscopy; no such effect is found for the dwarfs. The
presence of a metal-weak thick-disk (MWTD) population is clearly supported by
these new abundance data. Some 25% of the stars with metallicities -1.8 <
[Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly
separated from members of the inner-halo population with similar metallicities
by their location in a Lindblad energy vs. angular momentum diagram. A
comparison is made with recent results for a similar-size sample of RAVE stars
from Ruchti et al. We conclude, based on both of these samples, that the MWTD
is real, and must be accounted for in discussions of the formation and
evolution of the disk system of the Milky Way.Comment: 45 pages, 14 figures; accepted for publication in Ap
CN anomalies in the halo system and the origin of globular clusters in the Milky Way
We explore the kinematics and orbital properties of a sample of red giants in the halo system of the Milky Way that are thought to have formed in globular clusters based on their anomalously strong UV/blue CN bands. The orbital parameters of the CN-stron
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