585 research outputs found
Status of the 24Mg( alpha , gamma ) 28Si reaction rate at stellar temperatures
International audienceBackground: The Mg24(α,γ)Si28 reaction influences the production of magnesium and silicon isotopes during carbon burning and is one of eight reaction rates found to significantly impact the shape of calculated x-ray burst light curves. The reaction rate is based on measured resonance strengths and known properties of levels in Si28. Purpose: It is necessary to update the astrophysical reaction rate for Mg24(α,γ)Si28 incorporating recent modifications to the nuclear level data for Si28, and to determine if any additional as-yet unobserved resonances could contribute to the Mg24(α,γ)Si28 reaction rate. Methods: The reaction rate has been recalculated incorporating updated level assignments from Si28(α,α′)Si28 data using the ratesmc Monte Carlo code. Evidence from the Si28(p,p′)Si28 reaction suggests that there are no further known resonances which could increase the reaction rate at astrophysically important temperatures, though some resonances do not yet have measured resonance strengths. Results: The reaction rate is substantially unchanged from previously calculated rates, especially at astrophysically important temperatures. However, the reaction rate is now constrained to better than 20% across the astrophysically relevant energy range, with 95% confidence. Calculations of the x-ray burst light curve show no appreciable variations when varying the reaction rate within the uncertainty from the Monte Carlo calculations. Conclusion: The Mg24(α,γ)Si28 reaction rate, at temperatures relevant to carbon burning and Type I x-ray bursts, is well constrained by the available experimental data. This removes one reaction from the list of eight previously found to cause variations in x-ray burst light-curve calculations
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Pulse profile modelling of thermonuclear burst oscillations - II: Handling variability
Pulse profile modelling is a relativistic ray-tracing technique that can be used to infer masses, radii, and geometric parameters of neutron stars. In a previous study, we looked at the performance of this technique when applied to thermonuclear burst oscillations from accreting neutron stars. That study showed that ignoring the variability associated with burst oscillation sources resulted in significant biases in the inferred mass and radius, particularly for the high count rates that are nominally required to obtain meaningful constraints. In this follow-on study, we show that the bias can be mitigated by slicing the bursts into shorter segments where variability can be neglected, and jointly fitting the segments. Using this approach, the systematic uncertainties on the mass and radius are brought within the range of the statistical uncertainty. With about 106 source counts, this yields uncertainties of approximately 10 per cent for both the mass and radius. However, this modelling strategy requires substantial computational resources. We also confirm that the posterior distributions of the mass and radius obtained from multiple bursts of the same source can be merged to produce outcomes comparable to that of a single burst with an equivalent total number of counts
Lipocalin-2 as an Infection-Related Biomarker to Predict Clinical Outcome in Ischemic Stroke
Objectives From previous data in animal models of cerebral ischemia,
lipocalin-2 (LCN2), a protein related to neutrophil function and cellular iron
homeostasis, is supposed to have a value as a biomarker in ischemic stroke
patients. Therefore, we examined LCN2 expression in the ischemic brain in an
animal model and measured plasma levels of LCN2 in ischemic stroke patients.
Methods In the mouse model of transient middle cerebral artery occlusion
(tMCAO), LCN2 expression in the brain was analyzed by immunohistochemistry and
correlated to cellular nonheme iron deposition up to 42 days after tMCAO. In
human stroke patients, plasma levels of LCN2 were determined one week after
ischemic stroke. In addition to established predictive parameters such as age,
National Institutes of Health Stroke Scale and thrombolytic therapy, LCN2 was
included into linear logistic regression modeling to predict clinical outcome
at 90 days after stroke. Results Immunohistochemistry revealed expression of
LCN2 in the mouse brain already at one day following tMCAO, and the amount of
LCN2 subsequently increased with a maximum at 2 weeks after tMCAO.
Accumulation of cellular nonheme iron was detectable one week post tMCAO and
continued to increase. In ischemic stroke patients, higher plasma levels of
LCN2 were associated with a worse clinical outcome at 90 days and with the
occurrence of post-stroke infections. Conclusions LCN2 is expressed in the
ischemic brain after temporary experimental ischemia and paralleled by the
accumulation of cellular nonheme iron. Plasma levels of LCN2 measured in
patients one week after ischemic stroke contribute to the prediction of
clinical outcome at 90 days and reflect the systemic response to post-stroke
infections
Association of grade ≥3 neutropenia (NP) with outcomes in patients with metastatic castration-resistant prostate cancer (mCRPC) receiving cabazitaxel
Time-of-flight mass measurements of neutron-rich chromium isotopes up to N = 40 and implications for the accreted neutron star crust
We present the mass excesses of 59-64Cr, obtained from recent time-of-flight
nuclear mass measurements at the National Superconducting Cyclotron Laboratory
at Michigan State University. The mass of 64Cr is determined for the first
time, with an atomic mass excess of -33.48(44) MeV. We find a significantly
different two-neutron separation energy S2n trend for neutron-rich isotopes of
chromium, removing the previously observed enhancement in binding at N=38.
Additionally, we extend the S2n trend for chromium to N=40, revealing behavior
consistent with the previously identified island of inversion in this region.
We compare our results to state-of-the-art shell-model calculations performed
with a modified Lenzi-Nowacki-Poves-Sieja interaction in the fp shell,
including the g9/2 and d5/2 orbits for the neutron valence space. We employ our
result for the mass of 64Cr in accreted neutron star crust network calculations
and find a reduction in the strength and depth of electron-capture heating from
the A=64 isobaric chain, resulting in a cooler than expected accreted neutron
star crust. This reduced heating is found to be due to the >1-MeV reduction in
binding for 64Cr with respect to values from commonly used global mass models.Comment: Accepted to Physical Review
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