27 research outputs found
The Spatial Distribution of the Galactic First Stars II: SPH Approach
We use cosmological, chemo-dynamical, smoothed particle hydrodynamical
simulations of Milky-Way-analogue galaxies to find the expected present-day
distributions of both metal-free stars that formed from primordial gas and the
oldest star populations. We find that metal-free stars continue to form until
z~4 in halos that are chemically isolated and located far away from the biggest
progenitor of the final system. As a result, if the Population III initial mass
function allows stars with low enough mass to survive until z=0 (< 0.8 Msol),
they would be distributed throughout the Galactic halo. On the other hand, the
oldest stars form in halos that collapsed close to the highest density peak of
the final system, and at z=0 they are located preferentially in the central
region of the Galaxy, i.e., in the bulge. According to our models, these trends
are not sensitive to the merger histories of the disk galaxies or the
implementation of supernova feedback. Furthermore, these full hydrodynamics
results are consistent with our N-body results in Paper I, and lend further
weight to the conclusion that surveys of low-metallicity stars in the Galactic
halo can be used to directly constrain the properties of primordial stars. In
particular, they suggest that the current lack of detections of metal-free
stars implies that their lifetimes were shorter than a Hubble time, placing
constraints on the metal-free initial mass function.Comment: Accepted by ApJ. Emulate ApJ styl
C5 Palsy After Cervical Spine Surgery: A Multicenter Retrospective Review of 59 Cases.
STUDY DESIGN: A multicenter, retrospective review of C5 palsy after cervical spine surgery.
OBJECTIVE: Postoperative C5 palsy is a known complication of cervical decompressive spinal surgery. The goal of this study was to review the incidence, patient characteristics, and outcome of C5 palsy in patients undergoing cervical spine surgery.
METHODS: We conducted a multicenter, retrospective review of 13 946 patients across 21 centers who received cervical spine surgery (levels C2 to C7) between January 1, 2005, and December 31, 2011, inclusive. P values were calculated using 2-sample t test for continuous variables and χ(2) tests or Fisher exact tests for categorical variables.
RESULTS: Of the 13 946 cases reviewed, 59 patients experienced a postoperative C5 palsy. The incidence rate across the 21 sites ranged from 0% to 2.5%. At most recent follow-up, 32 patients reported complete resolution of symptoms (54.2%), 15 had symptoms resolve with residual effects (25.4%), 10 patients did not recover (17.0%), and 2 were lost to follow-up (3.4%).
CONCLUSION: C5 palsy occurred in all surgical approaches and across a variety of diagnoses. The majority of patients had full recovery or recovery with residual effects. This study represents the largest series of North American patients reviewed to date
Identifying Contributions to the Stellar Halo from Accreted, Kicked-Out, and In Situ Populations
[Abridged] We present a medium-resolution spectroscopic survey of late-type
giant stars at mid-Galactic latitudes of (30),
designed to probe the properties of this population to distances of 9
kpc. Because M giants are generally metal-rich and we have limited
contamination from thin disk stars by the latitude selection, most of the stars
in the survey are expected to be members of the thick disk
(-0.6) with some contribution from the metal-rich component of
the nearby halo.
Here we report first results for 1799 stars. The distribution of radial
velocity (RV) as a function of l for these stars shows (1) the expected thick
disk population and (2) local metal-rich halo stars moving at high speeds
relative to the disk, that in some cases form distinct sequences in RV-
space. High-resolution echelle spectra taken for 34 of these "RV outliers"
reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: seventeen of the
stars have abundances reminiscent of the populations present in dwarf
satellites of the Milky Way; eight have abundances coincident with those of the
Galactic disk and more metal-rich halo; and nine of the stars fall on the locus
defined by the majority of stars in the halo. The chemical abundance trends of
the RV outliers suggest that this sample consists predominantly of stars
accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV
outlier sample may have been formed in the inner Galaxy and subsequently kicked
to higher eccentricity orbits, but the sample is not large enough to
distinguish conclusively between this interpretation and the alternative that
these stars represent the tail of the velocity distribution of the thick disk.
Our data do not rule out the possibility that a minority of the sample could
have formed from gas {\it in situ} on their current orbits.Comment: 43 pages, 9 figures, 4 tables, published in the Astrophysical Journa
The r-process alliance: First release from the Northern search for r-process enhanced metal-poor stars in the Galactic Halo
This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance
The R -Process Alliance: First Release from the Northern Search for r -process-enhanced Metal-poor Stars in the Galactic Halo
© 2018. The American Astronomical Society. All rights reserved. This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ∼ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H] +1.0). Nineteen stars are found to exhibit a "limited-r" signature ([Sr/Ba] > +0.5, [Ba/Eu] < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, α, and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin