4,770 research outputs found
Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations
We study lithium depletion in low-mass and solar-like stars as a function of
time, using a new diffusion coefficient describing extra-mixing taking place at
the bottom of a convective envelope. This new form is motivated by
multi-dimensional fully compressible, time implicit hydrodynamic simulations
performed with the MUSIC code. Intermittent convective mixing at the convective
boundary in a star can be modeled using extreme value theory, a statistical
analysis frequently used for finance, meteorology, and environmental science.
In this letter, we implement this statistical diffusion coefficient in a
one-dimensional stellar evolution code, using parameters calibrated from
multi-dimensional hydrodynamic simulations of a young low-mass star. We propose
a new scenario that can explain observations of the surface abundance of
lithium in the Sun and in clusters covering a wide range of ages, from
50 Myr to 4 Gyr. Because it relies on our physical model of convective
penetration, this scenario has a limited number of assumptions. It can explain
the observed trend between rotation and depletion, based on a single additional
assumption, namely that rotation affects the mixing efficiency at the
convective boundary. We suggest the existence of a threshold in stellar
rotation rate above which rotation strongly prevents the vertical penetration
of plumes and below which rotation has small effects. In addition to providing
a possible explanation for the long standing problem of lithium depletion in
pre-main sequence and main sequence stars, the strength of our scenario is that
its basic assumptions can be tested by future hydrodynamic simulations.Comment: 7 pages, 3 figures, Accepted for publication in ApJ Letter
Evidence for magnetic clusters in NiV close to the quantum critical concentration
The d-metal alloy NiV undergoes a quantum phase transition from
a ferromagnetic ground state to a paramagnetic ground state as the vanadium
concentration is increased. We present magnetization, ac-susceptibility and
muon-spin relaxation data at several vanadium concentrations near the critical
concentration at which the onset of ferromagnetic order is
suppressed to zero temperature. Below , the muon data reveal a broad
magnetic field distribution indicative of long-range ordered ferromagnetic
state with spatial disorder. We show evidence of magnetic clusters in the
ferromagnetic phase and close to the phase boundary in this disordered
itinerant system as an important generic ingredient of a disordered quantum
phase transition. In contrast, the temperature dependence of the magnetic
susceptibility above is best described in terms of a magnetic quantum
Griffiths phase with a power-law distribution of fluctuation rates of dynamic
magnetic clusters. At the lowest temperatures, the onset of a short-range
ordered cluster-glass phase is recognized by an increase in the muon
depolarization in transverse fields and maxima in ac-susceptibility.Comment: 6 pages, 5 figures, submitted to Proceedings of SCES 201
Multi-boson effects and the normalization of the two-pion correlation function
The two-pion correlation function can be defined as a ratio of either the
measured momentum distributions or the normalized momentum space probabilities.
We show that the first alternative avoids certain ambiguities since then the
normalization of the two-pion correlator contains important information on the
multiplicity distribution of the event ensemble which is lost in the second
alternative. We illustrate this explicitly for specific classes of event
ensembles.Comment: 6 pages, three figures,submit to PR
Muon-spin relaxation and heat capacity measurements on the magnetoelectric and multiferroic pyroxenes LiFeSi2O6 and NaFeSi2O6
The results of muon-spin relaxation and heat capacity measurements on two
pyroxene compounds LiFeSi2O6 and NaFeSi2O6 demonstrate that despite their
underlying structural similarity the magnetic ordering is considerably
different. In LiFeSi2O6 a single muon precession frequency is observed below
TN, consistent with a single peak at TN in the heat capacity and a commensurate
magnetic structure. In applied magnetic fields the heat capacity peak splits in
two. In contrast, for natural NaFeSi2O6, where multiferroicity has been
observed in zero-magnetic-field, a rapid Gaussian depolarization is observed
showing that the magnetic structure is more complex. Synthetic NaFeSi2O6 shows
a single muon precession frequency but with a far larger damping rate than in
the lithium compound. Heat capacity measurements reproduce the phase diagrams
previously derived from other techniques and demonstrate that the magnetic
entropy is mostly associated with the build up of correlations in the
quasi-one-dimensional Fe3+ chains
Comparison of two- and three-dimensional compressible convection in a pre-main sequence star
Extending our recent studies of two-dimensional stellar convection to 3D, we
compare three-dimensional hydrodynamic simulations to identically set-up
two-dimensional simulations, for a realistic pre-main sequence star. We compare
statistical quantities related to convective flows including: average velocity,
vorticity, local enstrophy, and penetration depth beneath a convection zone.
These statistics are produced during stationary, steady-state compressible
convection in the star's convection zone. Our simulations with the MUSIC code
confirm the common result that two-dimensional simulations of stellar
convection have a higher magnitude of velocity on average than
three-dimensional simulations. Boundary conditions and the extent of the
spherical shell can affect the magnitude and variability of convective
velocities. The difference between 2D and 3D velocities is dependent on these
background points; in our simulations this can have an effect as large as the
difference resulting from the dimensionality of the simulation. Nevertheless,
radial velocities near the convective boundary are comparable in our 2D and 3D
simulations. The average local enstrophy of the flow is lower for
two-dimensional simulations than for three-dimensional simulations, indicating
a different shape and structuring of 3D stellar convection. We perform a
statistical analysis of the depth of convective penetration below the
convection zone, using the model proposed in our recent study (Pratt et al.
2017). Here we analyze the convective penetration in three dimensional
simulations, and compare the results to identically set-up 2D simulations. In
3D the penetration depth is as large as the penetration depth calculated from
2D simulations.Comment: 13 pages, 8 figure
Extreme value statistics for two-dimensional convective penetration in a pre-Main Sequence star
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.We examine a penetration layer formed between a central radiative zone and a large convection zone in the deep interior of a young low-mass star. Using the Multidimensional Stellar Implicit Code (MUSIC) to simulate two-dimensional compressible stellar convection in a spherical geometry over long times, we produce statistics that characterize the extent and impact of convective penetration in this layer. We apply extreme value theory to the maximal extent of convective penetration at any time. We compare statistical results from simulations which treat non-local convection, throughout a large portion of the stellar radius, with simulations designed to treat local convection in a small region surrounding the penetration layer. For each of these situations, we compare simulations of different resolution, which have different velocity magnitudes. We also compare statistical results between simulations that radiate energy at a constant rate to those that allow energy to radiate from the stellar surface according to the local surface temperature. Based on the frequency and depth of penetrating convective structures, we observe two distinct layers that form between the convection zone and the stable radiative zone. We show that the probability density function of the maximal depth of convective penetration at any time corresponds closely in space with the radial position where internal waves are excited. We find that the maximal penetration depth can be modeled by a Weibull distribution with a small shape parameter. Using these results, and building on established scalings for diffusion enhanced by large-scale convective motions, we propose a new form for the diffusion coefficient that may be used for one-dimensional stellar evolution calculations in the large P\'eclet number regime. These results should contribute to the 321D link.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework (FP7/2007-2013)/ERC grant agreement no. 32047
Two-dimensional simulations of internal gravity waves in a 5 Zero-Age-Main-Sequence model
Main-sequence intermediate-mass stars present a radiative envelope that
supports internal gravity waves (IGWs). Excited at the boundary with the
convective core, IGWs propagate towards the stellar surface and are suspected
to impact physical processes such as rotation and chemical mixing. Using the
fully compressible time-implicit code MUSIC, we study IGWs in two-dimensional
simulations of a zero-age-main-sequence 5 solar mass star model up to 91\% of
the stellar radius with different luminosity and radiative diffusivity
enhancements. Our results show that low frequency waves excited by core
convection are strongly impacted by radiative effects as they propagate. This
impact depends on the radial profile of radiative diffusivity which increases
by almost 5 orders of magnitude between the centre of the star and the top of
the simulation domain. In the upper layers of the simulation domain, we observe
an increase of the temperature. Our study suggests that this is due to heat
added in these layers by IGWs damped by radiative diffusion. We show that
non-linear effects linked to large amplitude IGWs may be relevant just above
the convective core. Both these effects are intensified by the artificial
enhancement of the luminosity and radiative diffusivity, with enhancement
factors up to times the realistic values. Our results also highlight
that direct comparison between numerical simulations with enhanced luminosity
and observations must be made with caution. Finally, our work suggests that
thermal effects linked to the damping of IGWs could have a non-negligible
impact on stellar structure.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Benchmarking the Multi-dimensional Stellar Implicit Code MUSIC
11 pages, 11 figures, accepted for publication in A&AWe present the results of a numerical benchmark study for the MUlti-dimensional Stellar Implicit Code (MUSIC) based on widely applicable two- and three-dimensional compressible hydrodynamics problems relevant to stellar interiors. MUSIC is an implicit large eddy simulation code that uses implicit time integration, implemented as a Jacobian-free Newton Krylov method. A physics based preconditioning technique which can be adjusted to target varying physics is used to improve the performance of the solver. The problems used for this benchmark study include the Rayleigh-Taylor and Kelvin-Helmholtz instabilities, and the decay of the Taylor-Green vortex. Additionally we show a test of hydrostatic equilibrium, in a stellar environment which is dominated by radiative effects. In this setting the flexibility of the preconditioning technique is demonstrated. This work aims to bridge the gap between the hydrodynamic test problems typically used during development of numerical methods and the complex flows of stellar interiors. A series of multi-dimensional tests are performed and analysed. Each of these test cases is analysed with a simple, scalar diagnostic, with the aim of enabling direct code comparisons. As the tests performed do not have analytic solutions we verify MUSIC by comparing to established codes including ATHENA and the PENCIL code. MUSIC is able to both reproduce behaviour from established and widely-used codes as well as results expected from theoretical predictions. This benchmarking study concludes a series of papers describing the development of the MUSIC code and provides confidence in the future applications.This project has received funding from the European
Unions Seventh Framework Programme for research, technological development
and demonstration under grant agreement no 320478. The calculations for this
paper were performed on the DiRAC Complexity machine, jointly funded by
STFC and the Large Facilities Capital Fund of BIS, and the University of Exeter
Super- computer, a DiRAC Facility jointly funded by STFC, the Large Facilities
Capital Fund of BIS and the University of Exeter. We are very thankful to Colin
McNally for providing his results for the Kelvin-Helmholtz test
Two-photon correlations as a sign of sharp transition in quark-gluon plasma
The photon production arising due to time variation of the medium has been
considered. The Hamilton formalism for photons in time-variable medium (plasma)
has been developed with application to inclusive photon production. The results
have been used for calculation of the photon production in the course of
transition from quark-gluon phase to hadronic phase in relativistic heavy ion
collisions. The relative strength of the effect as well as specific two- photon
correlations have been evaluated. It has been demonstrated that the opposite
side two-photon correlations are indicative of the sharp transition from the
quark-gluon phase to hadrons.Comment: 23 pages, 2 figure
Pion interferometry with pion-source-medium interactions
An extended pion source, which can be temporarily created by a high energy
nuclear collision, will also absorb and distort the outgoing pions. We discuss
how this effect alters the interferometric pattern of the two-pion momentum
correlation function. In particular, we show that the two-pion correlation
function decreases rapidly when the opening angle between the pions increases.
The opening-angle dependence should serve as a new means of obtaining
information about the pion source in the analysis of experimental data.Comment: 14 pages (revtex) and 9 figures (uuencoded), Caltech preprint
MAP-175, Indiana Univ. preprint IU/NTC 914-1
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