31,522 research outputs found
An analytic approximation of the feasible space of metabolic networks
Assuming a steady-state condition within a cell, metabolic fluxes satisfy an
under-determined linear system of stoichiometric equations. Characterizing the
space of fluxes that satisfy such equations along with given bounds (and
possibly additional relevant constraints) is considered of utmost importance
for the understanding of cellular metabolism. Extreme values for each
individual flux can be computed with Linear Programming (as Flux Balance
Analysis), and their marginal distributions can be approximately computed with
Monte-Carlo sampling. Here we present an approximate analytic method for the
latter task based on Expectation Propagation equations that does not involve
sampling and can achieve much better predictions than other existing analytic
methods. The method is iterative, and its computation time is dominated by one
matrix inversion per iteration. With respect to sampling, we show through
extensive simulation that it has some advantages including computation time,
and the ability to efficiently fix empirically estimated distributions of
fluxes
Self-consistent modelling of line-driven hot-star winds with Monte Carlo radiation hydrodynamics
Radiative pressure exerted by line interactions is a prominent driver of
outflows in astrophysical systems, being at work in the outflows emerging from
hot stars or from the accretion discs of cataclysmic variables, massive young
stars and active galactic nuclei. In this work, a new radiation hydrodynamical
approach to model line-driven hot-star winds is presented. By coupling a Monte
Carlo radiative transfer scheme with a finite-volume fluid dynamical method,
line-driven mass outflows may be modelled self-consistently, benefiting from
the advantages of Monte Carlo techniques in treating multi-line effects, such
as multiple scatterings, and in dealing with arbitrary multidimensional
configurations. In this work, we introduce our approach in detail by
highlighting the key numerical techniques and verifying their operation in a
number of simplified applications, specifically in a series of self-consistent,
one-dimensional, Sobolev-type, hot-star wind calculations. The utility and
accuracy of our approach is demonstrated by comparing the obtained results with
the predictions of various formulations of the so-called CAK theory and by
confronting the calculations with modern sophisticated techniques of predicting
the wind structure. Using these calculations, we also point out some useful
diagnostic capabilities our approach provides. Finally we discuss some of the
current limitations of our method, some possible extensions and potential
future applications.Comment: 15 pages, 8 figures; accepted for publication in MNRA
A scheme for radiation pressure and photon diffusion with the M1 closure in RAMSES-RT
We describe and test an updated version of radiation-hydrodynamics (RHD) in
the RAMSES code, that includes three new features: i) radiation pressure on
gas, ii) accurate treatment of radiation diffusion in an unresolved optically
thick medium, and iii) relativistic corrections that account for Doppler
effects and work done by the radiation to first order in v/c. We validate the
implementation in a series of tests, which include a morphological assessment
of the M1 closure for the Eddington tensor in an astronomically relevant
setting, dust absorption in a optically semi-thick medium, direct pressure on
gas from ionising radiation, convergence of our radiation diffusion scheme
towards resolved optical depths, correct diffusion of a radiation flash and a
constant luminosity radiation, and finally, an experiment from Davis et al. of
the competition between gravity and radiation pressure in a dusty atmosphere,
and the formation of radiative Rayleigh-Taylor instabilities. With the new
features, RAMSES-RT can be used for state-of-the-art simulations of radiation
feedback from first principles, on galactic and cosmological scales, including
not only direct radiation pressure from ionising photons, but also indirect
pressure via dust from multi-scattered IR photons reprocessed from
higher-energy radiation, both in the optically thin and thick limits.Comment: 25 pages, 13 figures, accepted for publication in MNRAS. Revised to
match published versio
On the Dynamics of Supermassive Black Holes in Gas-Rich, Star-Forming Galaxies: the Case for Nuclear Star Cluster Coevolution
We introduce a new model for the formation and evolution of supermassive
black holes (SMBHs) in the RAMSES code using sink particles, improving over
previous work the treatment of gas accretion and dynamical evolution. This new
model is tested against a suite of high-resolution simulations of an isolated,
gas-rich, cooling halo. We study the effect of various feedback models on the
SMBH growth and its dynamics within the galaxy.
In runs without any feedback, the SMBH is trapped within a massive bulge and
is therefore able to grow quickly, but only if the seed mass is chosen larger
than the minimum Jeans mass resolved by the simulation. We demonstrate that, in
the absence of supernovae (SN) feedback, the maximum SMBH mass is reached when
Active Galactic Nucleus (AGN) heating balances gas cooling in the nuclear
region.
When our efficient SN feedback is included, it completely prevents bulge
formation, so that massive gas clumps can perturb the SMBH orbit, and reduce
the accretion rate significantly. To overcome this issue, we propose an
observationally motivated model for the joint evolution of the SMBH and a
parent nuclear star cluster (NSC), which allows the SMBH to remain in the
nuclear region, grow fast and resist external perturbations. In this scenario,
however, SN feedback controls the gas supply and the maximum SMBH mass now
depends on the balance between AGN heating and gravity. We conclude that
SMBH/NSC co-evolution is crucial for the growth of SMBH in high-z galaxies, the
progenitors of massive elliptical today.Comment: accepted for publication in MNRA
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