It is likely that factors other than stenosis severity predispose some atherosclerotic plaques to rupture. Because focal increases in circumferential stress may be an important mechanism of plaque ...rupture, we examined peak circumferential stress of atherosclerotic lesions by using finite element analysis based on idealized two-dimensional cross sections of diseased vessels similar to intravascular ultrasound images. The study was designed to test the hypothesis that subintimal plaque structural features such as thickness of the fibrous cap are more important factors in the distribution of stress in the plaque than stenosis severity. The analysis incorporated equilibrium biomechanical parameters from normal and diseased vessels and determined the stress distribution within the plaque at a mean luminal internal pressure of 110 mm Hg. With a constant luminal area reduction of 70%, maximum circumferential stress (sigma max) normalized to luminal pressure (sigma max/P) increased from 6.0 to 24.8 as the thickness of the lipid pool was increased from 38% to 54% of the plaque thickness because of the thinner fibrous cap over the lipid pool. When the lipid pool thickness was constant, increasing the stenosis severity from 70% to 91% by increasing the fibrous cap thickness decreased sigma max/P from 24.8 to 4.7. When no lipid pool was present and the stenosis severity was increased from 70% to 99%, sigma max/P decreased from 5.3 to 4.7. Thus, reducing the fibrous cap thickness dramatically increases peak circumferential stress in the plaque, whereas increasing the stenosis severity actually decreases peak stress in the plaque.
We present first measurements of the evolution of the differential transverse momentum correlation function, C, with collision centrality in Au+Au interactions at sNN=200 GeV. This observable ...exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations previously reported. We use the observed longitudinal broadening of the near-side peak of C with increasing centrality to estimate the ratio of the shear viscosity to entropy density, η/s, of the matter formed in central Au+Au interactions. We obtain an upper limit estimate of η/s that suggests that the produced medium has a small viscosity per unit entropy.
Abstract
We report the discovery of six ultra-faint Milky Way satellites identified through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second ...data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes (
M
V
≥ −3.2 mag) and old, metal-poor stellar populations (
τ
> 10 Gyr, Fe/H < −1.4 dex). Three of these systems are more extended (
r
1/2
> 15 pc), while the other three are compact (
r
1/2
< 10 pc). From these properties, we infer that the former three systems (Boötes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Boötes V, Leo Minor I, and DELVE 4, and tentatively detect a proper-motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems.
ABSTRACT
We investigate the Fe, C, N, O, Mg, Al, Si, K, Ca, Ce, and Nd abundances of 2283 red giant stars in 31 globular clusters from high-resolution spectra observed in both the Northern and ...Southern hemisphere by the SDSS-IV APOGEE-2 survey. This unprecedented homogeneous data set, largest to date, allows us to discuss the intrinsic Fe spread, the shape, and statistics of Al-Mg and N-C anti-correlations as a function of cluster mass, luminosity, age, and metallicity for all 31 clusters. We find that the Fe spread does not depend on these parameters within our uncertainties including cluster metallicity, contradicting earlier observations. We do not confirm the metallicity variations previously observed in M22 and NGC 1851. Some clusters show a bimodal Al distribution, while others exhibit a continuous distribution as has been previously reported in the literature. We confirm more than two populations in ω Cen and NGC 6752, and find new ones in M79. We discuss the scatter of Al by implementing a correction to the standard chemical evolution of Al in the Milky Way. After correction, its dependence on cluster mass is increased suggesting that the extent of Al enrichment as a function of mass was suppressed before the correction. We observe a turnover in the Mg-Al anticorrelation at very low Mg in ω Cen, similar to the pattern previously reported in M15 and M92. ω Cen may also have a weak K-Mg anticorrelation, and if confirmed, it would be only the third cluster known to show such a pattern.
We use the multi-epoch radial velocities acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to perform a large-scale statistical study of stellar multiplicity for ...field stars in the Milky Way, spanning the evolutionary phases between the main sequence (MS) and the red clump. We show that the distribution of maximum radial velocity shifts (ΔRVmax) for APOGEE targets is a strong function of log g, with MS stars showing ΔRVmax as high as ∼300 , and steadily dropping down to ∼30 for log g ∼ 0, as stars climb up the red giant branch (RGB). Red clump stars show a distribution of ΔRVmax values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high ΔRVmax systems in the RGB is consistent with a lognormal period distribution in the MS and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe overflow during H-shell burning. The ΔRVmax distributions also show that the multiplicity characteristics of field stars are metallicity-dependent, with metal-poor (Fe/H −0.5) stars having a multiplicity fraction a factor of 2-3 higher than metal-rich (Fe/H 0.0) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.
Abstract
We report the discovery of two ultra-faint satellites in the vicinity of the Large Magellanic Cloud (LMC) in data from the Magellanic Satellites Survey (MagLiteS ). Situated 18 deg (∼20 kpc) ...from the LMC and separated from each other by only 18 arcmin, Carina II and III form an intriguing pair. By simultaneously modelling the spatial and the colour–magnitude stellar distributions, we find that both Carina II and Carina III are likely dwarf galaxies, although this is less clear for Carina III. There are in fact several obvious differences between the two satellites. While both are well described by an old and metal poor population, Carina II is located at ∼36 kpc from the Sun, with MV ∼ −4.5 and rh ∼ 90 pc, and it is further confirmed by the discovery of 3 RR Lyrae at the right distance. In contrast, Carina III is much more elongated, measured to be fainter (MV ∼ −2.4), significantly more compact (rh ∼ 30 pc), and closer to the Sun, at ∼28 kpc, placing it only 8 kpc away from Car II. Together with several other systems detected by the Dark Energy Camera, Carina II and III form a strongly anisotropic cloud of satellites in the vicinity of the Magellanic Clouds.
APOGEE-2 is a high-resolution, near-infrared spectroscopic survey observing ∼3 × 105 stars across the entire sky. It is the successor to APOGEE and is part of the Sloan Digital Sky Survey IV ...(SDSS-IV). APOGEE-2 is expanding on APOGEE's goals of addressing critical questions of stellar astrophysics, stellar populations, and Galactic chemodynamical evolution using (1) an enhanced set of target types and (2) a second spectrograph at Las Campanas Observatory in Chile. APOGEE-2 is targeting red giant branch and red clump stars, RR Lyrae, low-mass dwarf stars, young stellar objects, and numerous other Milky Way and Local Group sources across the entire sky from both hemispheres. In this paper, we describe the APOGEE-2 observational design, target selection catalogs and algorithms, and the targeting-related documentation included in the SDSS data releases.
We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with broad-band photometric data from several sources as well as parallaxes from
Gaia
Data Release 2 (DR2). ...Using the Bayesian isochrone-fitting code
StarHorse
, we derived the distances, extinctions, and astrophysical parameters for around 388 815 APOGEE stars. We achieve typical distance uncertainties of ∼6% for APOGEE giants, ∼2% for APOGEE dwarfs, and extinction uncertainties of ∼0.07 mag, when all photometric information is available, and ∼0.17 mag if optical photometry is missing.
StarHorse
uncertainties vary with the input spectroscopic catalogue, available photometry, and parallax uncertainties. To illustrate the impact of our results, we show that thanks to
Gaia
DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane. We thereby provide an unprecedented coverage of the disc close to the Galactic mid-plane (|
Z
Gal
| < 1 kpc) from the Galactic centre out to
R
Gal
∼ 20 kpc. The improvements in statistics as well as distance and extinction uncertainties unveil the presence of the bar in stellar density and the striking chemical duality in the innermost regions of the disc, which now clearly extend to the inner bulge. We complement this paper with distances and extinctions for stars in other public released spectroscopic surveys: 324 999 in GALAH DR2, 4 928 715 in LAMOST DR5, 408 894 in RAVE DR6, and 6095 in GES DR3.
Abstract
We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a ...compact, ultra-faint stellar system (
r
1
/
2
=
41
−
6
+
8
pc;
M
V
= −4.25 ± 0.2 mag) located at a heliocentric distance of
90
−
6
+
4
kpc
. Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring
σ
v
=
3.3
−
1.1
+
1.7
km s
−1
(after excluding three velocity outliers); this implies a mass-to-light ratio of
M
1
/
2
/
L
V
,
1
/
2
=
167
−
99
+
224
M
⊙
/
L
⊙
for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of Fe/H =
−
2.63
−
0.30
+
0.26
dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV’s proper motion using data from Gaia Early Data Release 3, finding (
μ
α
*
,
μ
δ
) = (0.33 ± 0.07, −0.21 ± 0.08) mas yr
−1
. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system’s centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.
Data from the SDSS-IV/Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have been released as part of SDSS Data Releases 13 (DR13) and 14 (DR14). These include high-resolution H-band ...spectra, radial velocities, and derived stellar parameters and abundances. DR13, released in 2016 August, contained APOGEE data for roughly 150,000 stars, and DR14, released in 2017 August, added about 110,000 more. Stellar parameters and abundances have been derived with an automated pipeline, the APOGEE Stellar Parameter and Chemical Abundance Pipeline (ASPCAP). We evaluate the performance of this pipeline by comparing the derived stellar parameters and abundances to those inferred from optical spectra and analysis for several hundred stars. For most elements-C, Na, Mg, Al, Si, S, Ca, Cr, Mn, Ni-the DR14 ASPCAP analyses have systematic differences with the comparisons samples of less than 0.05 dex (median), and random differences of less than 0.15 dex (standard deviation). These differences are a combination of the uncertainties in both the comparison samples as well as the ASPCAP analysis. Compared to the references, magnesium is the most accurate alpha-element derived by ASPCAP, and shows a very clear thin/thick disk separation, while nickel is the most accurate iron-peak element (besides iron itself).
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