363 research outputs found
Investigating the Metallicity-Mixing Length Relation
Stellar models typically use the mixing length approximation as a way to
implement convection in a simplified manner. While conventionally the value of
the mixing length parameter, , used is the solar calibrated value, many
studies have shown that other values of are needed to properly model
stars. This uncertainty in the value of the mixing length parameter is a major
source of error in stellar models and isochrones. Using asteroseismic data, we
determine the value of the mixing length parameter required to properly model a
set of about 450 stars ranging in , , and
. The relationship between the value of required and
the properties of the star is then investigated. For Eddington atmosphere,
non-diffusion models, we find that the value of can be approximated by
a linear model, in the form of . This process is
repeated using a variety of model physics as well as compared to previous
studies and results from 3D convective simulations.Comment: 20 pages, 17 figures, accepted for publication in Ap
MINESweeper: Spectrophotometric Modeling of Stars in the Gaia Era
We present MINESweeper, a tool to measure stellar parameters by jointly
fitting observed spectra and broadband photometry to model isochrones and
spectral libraries. This approach enables the measurement of spectrophotometric
distances, in addition to stellar parameters such as Teff, log(g), [Fe/H],
[a/Fe], and radial velocity. MINESweeper employs a Bayesian framework and can
easily incorporate a variety of priors, including Gaia parallaxes. Mock data
are fit in order to demonstrate how the precision of derived parameters depends
on evolutionary phase and SNR. We then fit a selection of data in order to
validate the model outputs. Fits to a variety of benchmark stars including
Procyon, Arcturus, and the Sun result in derived stellar parameters that are in
good agreement with the literature. We then fit combined spectra and photometry
of stars in the open and globular clusters M92, M13, M3, M107, M71, and M67.
Derived distances, [Fe/H], [a/Fe], and log(g)-Teff, relations are in overall
good agreement with literature values, although there are trends between
metallicity and log(g), within clusters that point to systematic uncertainties
at the ~0.1 dex level. Finally, we fit a large sample of stars from the H3
Spectroscopic Survey in which high quality Gaia parallaxes are also available.
These stars are fit without the Gaia parallaxes so that the geometric
parallaxes can serve as an independent test of the spectrophotometric
distances. Comparison between the two reveals good agreement within their
formal uncertainties after accounting for the Gaia zero point uncertainties.Comment: 20 pages, 14 figures, Accepted by Ap
Rationale and design for the study of rivaroxaban to reduce thrombotic events, hospitalization and death in outpatients with COVID-19: The PREVENT-HD study
© 2021 Elsevier Inc. Background: COVID-19 is associated with both venous and arterial thrombotic complications. While prophylactic anticoagulation is now widely recommended for hospitalized patients with COVID-19, the effectiveness and safety of thromboprophylaxis in outpatients with COVID-19 has not been established. Study Design: PREVENT-HD is a double-blind, placebo-controlled, pragmatic, event-driven phase 3 trial to evaluate the efficacy and safety of rivaroxaban in symptomatic outpatients with laboratory-confirmed COVID-19 at risk for thrombotic events, hospitalization, and death. Several challenges posed by the pandemic have necessitated innovative approaches to clinical trial design, start-up, and conduct. Participants are randomized in a 1:1 ratio, stratified by time from COVID-19 confirmation, to either rivaroxaban 10 mg once daily or placebo for 35 days. The primary efficacy end point is a composite of symptomatic venous thromboembolism, myocardial infarction, ischemic stroke, acute limb ischemia, non-central nervous system systemic embolization, all-cause hospitalization, and all-cause mortality. The primary safety end point is fatal and critical site bleeding according to the International Society on Thrombosis and Haemostasis definition. Enrollment began in August 2020 and is expected to enroll approximately 4,000 participants to yield the required number of end point events. Conclusions: PREVENT-HD is a pragmatic trial evaluating the efficacy and safety of the direct oral anticoagulant rivaroxaban in the outpatient setting to reduce major venous and arterial thrombotic events, hospitalization, and mortality associated with COVID-19
Prospective Evaluation of Pregnancy-Associated Plasma Protein-A and Outcomes in Patients With Acute Coronary Syndromes
ObjectivesThis study sought to investigate whether pregnancy-associated plasma protein-A (PAPP-A) is useful for risk assessment in non–ST-segment elevation acute coronary syndrome (NSTE-ACS).BackgroundPAPP-A is a high molecular weight, zinc-binding metalloproteinase that is associated with vulnerable plaque and may be a predictor of cardiovascular disease and mortality.MethodsWe measured PAPP-A at baseline in 3,782 patients with non NSTE-ACS randomized to ranolazine or placebo in the MERLIN–TIMI 36 (Metabolic Efficiency With Ranolazine for Less Ischemia in Non-ST Elevation Acute Coronary Syndromes) trial and followed for an average of 1 year. A cut point of 6.0 μIU/ml was chosen from pilot work in this cohort.ResultsPAPP-A >6.0 μIU/ml at presentation was associated with higher rates of cardiovascular death (CVD) or myocardial infarction (MI) at 30 days (7.4% vs. 3.7%, hazard ratio [HR]: 2.01; 95% confidence interval [CI]: 1.43 to 2.82; p < 0.001) and at 1 year (14.9% vs. 9.7%, HR: 1.63; 95% CI: 1.29 to 2.05; p < 0.001). PAPP-A was also associated with higher rates of CVD (HR: 1.94; 95% CI: 1.07 to 3.52, p = 0.027) and myocardial infarction (HR: 1.82; 95% CI: 1.22 to 2.71, p = 0.003) individually at 30 days. There was no difference in the risk associated with PAPP-A stratified by baseline cardiac troponin I [Accu-TnI >0.04 μg/l], p interaction = 0.87). After adjustment for cardiac troponin I, ST-segment deviation, age, sex, diabetes, smoking, hypertension, and coronary artery disease, PAPP-A was independently associated with CVD/myocardial infarction at 30 days (adjusted HR: 1.62, 95% CI: 1.15 to 2.29; p = 0.006) and 1 year (adjusted HR: 1.35, 95% CI: 1.07 to 1.71; p = 0.012). PAPP-A also improved the net reclassification for CVD/MI (p = 0.003). There was no significant interaction with ranolazine.ConclusionsPAPP-A was independently associated with recurrent cardiovascular events in patients with NSTE-ACS. This finding supports PAPP-A as a candidate prognostic marker in patients with ACS and supports investigation of its therapeutic implications
A Diffuse Metal-Poor Component of the Sagittarius Stream Revealed by the H3 Survey
The tidal disruption of the Sagittarius dwarf galaxy has generated a
spectacular stream of stars wrapping around the entire Galaxy. We use data from
and the H3 Stellar Spectroscopic Survey to identify 823 high-quality
Sagittarius members based on their angular momenta. The H3 Survey is largely
unbiased in metallicity, and so our sample of Sagittarius members is similarly
unbiased. Stream stars span a wide range in [Fe/H] from to , with a mean overall metallicity of [Fe/H]. We
identify a strong metallicity-dependence to the kinematics of the stream
members. At [Fe/H] nearly all members belong to the well-known cold
( km/s) leading and trailing arms. At intermediate
metallicities ([Fe/H]) a significant population (24)
emerges of stars that are kinematically offset from the cold arms. These stars
also appear to have hotter kinematics. At the lowest metallicities
([Fe/H]), the majority of stars (69) belong to this
kinematically-offset diffuse population. Comparison to simulations suggests
that the diffuse component was stripped from the Sagittarius progenitor at
earlier epochs, and therefore resided at larger radius on average, compared to
the colder metal-rich component. We speculate that this kinematically diffuse,
low metallicity, population is the stellar halo of the Sagittarius progenitor
system.Comment: 18 pages, 12 figures, 1 table. Submitted to Ap
Platelet inhibition with ticagrelor 60 mg versus 90 mg twice daily in the PEGASUS-TIMI 54 trial
Background
The PEGASUS-TIMI 54 (Prevention of Cardiovascular Events in Patients with Prior Heart Attack Using Ticagrelor Compared to Placebo on a Background of Aspirin–Thrombolysis In Myocardial Infarction 54) trial studied 2 doses of ticagrelor, 90 mg twice a day (bid) and 60 mg bid, for long-term prevention of ischemic events in patients with prior myocardial infarction. Both doses similarly reduced the rate of ischemic events versus placebo. The pharmacokinetics and pharmacodynamics of ticagrelor 60 mg bid have not been studied.
Objectives
In this study, the authors sought to study the pharmacokinetics and pharmacodynamics for ticagrelor 60 mg compared with 90 mg bid.
Methods
A total of 180 patients who received >4 weeks of study medication had blood sampling in the morning pre-maintenance dose and again 2 h post-dose. All patients received aspirin. Plasma levels of ticagrelor and its active metabolite AR-C124910XX were determined. P2Y12 inhibition was assessed by the VerifyNow P2Y12 assay (Accumetrics, Inc., San Diego, California) (P2Y12 reaction units [PRU]), light transmittance aggregometry (adenosine diphosphate 5 and 20 μmol/l and arachidonic acid), and vasodilator-stimulated phosphoprotein phosphorylation assays. VerifyNow Aspirin assays and serum thromboxane B2 measurements were performed.
Results
Mean pre- and post-dose plasma levels of ticagrelor were 35% and 38% lower, respectively, with 60 mg versus 90 mg. Both doses achieved high levels of platelet inhibition pre- and post-dose, with numerically slightly more variability with 60 mg: mean (SD) pre-dose PRU values were 59 ± 63 and 47 ± 43 for ticagrelor 60 and 90 mg, respectively (p = 0.34). High platelet reactivity, determined as PRU >208, was rare with the 60-mg pre-dose and was absent post-dose. Platelet reactivity pre- and post-dose, as measured by light transmittance aggregometry or vasodilator-stimulated phosphoprotein assays, was numerically but not significantly lower with 90 mg than with 60 mg. Aspirin response was not affected by either dose.
Conclusions
Ticagrelor 60 mg bid achieved high levels of peak and trough platelet inhibition in nearly all patients, similar to that with 90 mg bid, helping to explain the efficacy of the lower ticagrelor dose in PEGASUS-TIMI 54
Early Results from APOKASC
Asteroseismology and spectroscopy provide complementary constraints on the
fundamental and chemical properties of stars. I describe the first results from
APOKASC, a collaboration between the Kepler asteroseismic science consortium
(KASC) and the SDSS-III APOGEE survey. These include (1) the first test of
asteroseismic scaling relationships in the metal-poor regime using halo and
thick disk stars identified in the APOKASC sample; and (2) the calibration of
spectroscopic parameters using precise asteroseismic measurements of surface
gravity. I also highlight future research avenues that are made possible by
this unique sample of thousands of well-characterized red giant stars.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
At the survey limits::discovery of the Aquarius 2 dwarf galaxy in the VST ATLAS and the SDSS data
We announce the discovery of the Aquarius~2 dwarf galaxy, a new distant
satellite of the Milky Way, detected on the fringes of the VST ATLAS and the
SDSS surveys. The object was originally identified as an overdensity of Red
Giant Branch stars, but chosen for subsequent follow-up based on the presence
of a strong Blue Horizontal Branch, which was also used to measure its distance
of kpc. Using deeper imaging from the IMACS camera on the 6.5m Baade
and spectroscopy with DEIMOS on Keck, we measured the satellite's half-light
radius arcmin, or pc at this distance, and its stellar
velocity dispersion of km s. With mag
arcsec and , the new satellite lies close to two important
detection limits: one in surface brightness; and one in luminosity at a given
distance, thereby making Aquarius~2 one of the hardest dwarfs to find.Comment: 12 pages, 11 figures, submmited to MNRAS; v2 accepted version with
updated luminosit
Calibrating Convective properties of Solar-like Stars in the Kepler Field of View
Stellar models generally use simple parametrizations to treat convection. The
most widely used parametrization is the so-called "Mixing Length Theory" where
the convective eddy sizes are described using a single number, \alpha, the
mixing-length parameter. This is a free parameter, and the general practice is
to calibrate \alpha using the known properties of the Sun and apply that to all
stars. Using data from NASA's Kepler mission we show that using the
solar-calibrated \alpha is not always appropriate, and that in many cases it
would lead to estimates of initial helium abundances that are lower than the
primordial helium abundance. Kepler data allow us to calibrate \alpha for many
other stars and we show that for the sample of stars we have studied, the
mixing-length parameter is generally lower than the solar value. We studied the
correlation between \alpha and stellar properties, and we find that \alpha
increases with metallicity. We therefore conclude that results obtained by
fitting stellar models or by using population-synthesis models constructed with
solar values of \alpha are likely to have large systematic errors. Our results
also confirm theoretical expectations that the mixing-length parameter should
vary with stellar properties.Comment: 16 pages, 4 figures, accepted for publication in ApJ
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