29,107 research outputs found
Photoevaporation of protoplanetary discs I: hydrodynamic models
In this paper we consider the effect of the direct ionizing stellar radiation
field on the evolution of protoplanetary discs subject to photoevaporative
winds. We suggest that models which combine viscous evolution with
photoevaporation of the disc (e.g. Clarke, Gendrin & Sotomayor 2001)
incorrectly neglect the direct field after the inner disc has drained, at late
times in the evolution. We construct models of the photoevaporative wind
produced by the direct field, first using simple analytic arguments and later
using detailed numerical hydrodynamics. We find that the wind produced by the
direct field at late times is much larger than has previously been assumed, and
we show that the mass-loss rate scales as (where is the
radius of the instantaneous inner disc edge). We suggest that this result has
important consequences for theories of disc evolution, and go on to consider
the effects of this result on disc evolution in detail in a companion paper
(Alexander, Clarke & Pringle 2006b).Comment: 13 pages, 9 figures. Accepted for publication in MNRA
Photoionization models for extreme Ly 1216 and HeII 1640 ratios in quasar halos, and PopIII vs AGN diagnostics
We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or
to enhance the observed number of Ly-alpha photons per incident ionizing
photon, in extended AGN-photoionized nebulae at high-redshift. Using
photoionization models, we explore the impact of ionization parameter, gas
metallicity, ionizing spectrum, electron energy distribution, and cloud viewing
angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the
observed number of Ly-alpha photons per incident ionizing photon. We find that
low ionization parameter, a relatively soft or filtered ionizing spectrum, low
gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha
photons by HI can all result in significantly enhanced Ly-alpha relative to
other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the
cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing
spectrum, the observed number of Ly-alpha photons per incident ionizing photon
is itself significantly enhanced above the nominal Case B value of 0.66 due to
collisional excitation, reaching values up to 5.3 in our 'extreme case' model.
At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is
predominantly via collisional excitation rather than recombination. In
addition, we find that collisional excitation of Ly-alpha becomes more
efficient if the ionizing continuum is pre-filtered through an optically thin
screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of
the z~3.5 quasars studied by Borisova et al. (2016) are consistent with
AGN-photoionization of gas with moderate to low metallicity and/or low
ionization parameter, without requiring exotic ionization/excitation mechanisms
such as strong line-transfer effects. We also present UV-optical diagnostic
diagrams to distinguish between photoionization by Pop III stars and AGN
photoionization.Comment: Accepted for publication in A&A. 14 pages, 9 figures. Abstract
slightly shortened to meet arxiv character limi
The Opacity of the Intergalactic Medium During Reionization: Resolving Small-Scale Structure
Early in the reionization process, the intergalactic medium (IGM) would have
been quite inhomogeneous on small scales, due to the low Jeans mass in the
neutral IGM and the hierarchical growth of structure in a cold dark matter
Universe. This small-scale structure acted as an important sink during the
epoch of reionization, impeding the progress of the ionization fronts that
swept out from the first sources of ionizing radiation. Here we present results
of high-resolution cosmological hydrodynamics simulations that resolve the
cosmological Jeans mass of the neutral IGM in representative volumes several
Mpc across. The adiabatic hydrodynamics we follow are appropriate in an
unheated IGM, before the gas has had a chance to respond to the photoionization
heating. Our focus is determination of the resolution required in cosmological
simulations in order to sufficiently sample and resolve small-scale structure
regulating the opacity of an unheated IGM. We find that a dark matter particle
mass of m_dm 1 Mpc are required. With our
converged results we show how the mean free path of ionizing radiation and
clumping factor of ionized hydrogen depends upon the ultraviolet background
(UVB) flux and redshift. We find, for example at z = 10, clumping factors
typically of 10 to 20 for an ionization rate of Gamma ~ 0.3 - 3 x 1e-12 s^-1,
with corresponding mean free paths of ~ 3 - 15 Mpc, extending previous work on
the evolving mean free path to considerably smaller scales and earlier times.Comment: Accepted for publication in the Astrophysical Journa
Direct Collapse to Supermassive Black Hole Seeds with Radiation Transfer: Cosmological Halos
We have modeled direct collapse of a primordial gas within dark matter halos
in the presence of radiative transfer, in high-resolution zoom-in simulations
in a cosmological framework, down to the formation of the photosphere and the
central object. Radiative transfer has been implemented in the flux-limited
diffusion (FLD) approximation. Adiabatic models were run for comparison. We
find that (a) the FLD flow forms an irregular central structure and does not
exhibit fragmentation, contrary to adiabatic flow which forms a thick disk,
driving a pair of spiral shocks, subject to Kelvin-Helmholtz shear instability
forming fragments; (b) the growing central core in the FLD flow quickly reaches
~10 Mo and a highly variable luminosity of 10^{38}-10^{39} erg/s, comparable to
the Eddington luminosity. It experiences massive recurrent outflows driven by
radiation force and thermal pressure gradients, which mix with the accretion
flow and transfer the angular momentum outwards; and (c) the interplay between
these processes and a massive accretion, results in photosphere at ~10 AU. We
conclude that in the FLD model (1) the central object exhibits dynamically
insignificant rotation and slower than adiabatic temperature rise with density;
(2) does not experience fragmentation leading to star formation, thus promoting
the fast track formation of a supermassive black hole (SMBH) seed; (3)
inclusion of radiation force leads to outflows, resulting in the mass
accumulation within the central 10^{-3} pc, which is ~100 times larger than
characteristic scale of star formation. The inclusion of radiative transfer
reveals complex early stages of formation and growth of the central structure
in the direct collapse scenario of SMBH seed formation.Comment: 19 pages, 16 figures, MNRAS, accepted for publicatio
Particle identification
Particle IDentification (PID) is fundamental to particle physics experiments.
This paper reviews PID strategies and methods used by the large LHC
experiments, which provide outstanding examples of the state-of-the-art. The
first part focuses on the general design of these experiments with respect to
PID and the technologies used. Three PID techniques are discussed in more
detail: ionization measurements, time-of-flight measurements and Cherenkov
imaging. Four examples of the implementation of these techniques at the LHC are
given, together with selections of relevant examples from other experiments and
short overviews on new developments. Finally, the Alpha Magnetic Spectrometer
(AMS 02) experiment is briefly described as an impressive example of a
space-based experiment using a number of familiar PID techniques.Comment: 61 pages, 30 figure
Direct Collapse to Supermassive Black Hole Seeds with Radiative Transfer: Isolated Halos
Direct collapse within dark matter (DM) halos is a promising path to form
supermassive black hole (SMBH) seeds at high redshifts. The outer part of this
collapse remains optically thin, and has been studied intensively using
numerical simulations. However, the innermost region of the collapse is
expected to become optically thick and requires us to follow the radiation
field in order to understand its subsequent evolution. So far, the adiabatic
approximation has been used exclusively for this purpose. We apply radiative
transfer in the flux-limited diffusion (FLD) approximation to solve the
evolution of coupled gas and radiation, for isolated halos. For direct collapse
within isolated DM halos, we find that (1) the photosphere forms at ~10^{-6} pc
and rapidly expands outward. (2) A central core forms, with a mass of ~1 Mo,
supported by thermal gas pressure gradients and rotation. (3) Growing thermal
gas and radiation pressure gradients dissolve it. (4) This process is
associated with a strong anisotropic outflow, and another core forms nearby and
grows rapidly. (5) Typical radiation luminosity emerging from the photosphere
encompassing these cores is ~5 x 10^{37}-5 x 10^{38} erg/s, of order the
Eddington luminosity. (6) Two variability timescales are associated with this
process: a long one, which is related to the accretion flow within the central
~10^{-4}-10^{-3} pc, and ~0.1 yr, which is related to radiation diffusion. (7)
Adiabatic models have been run for comparison and their evolution differs
profoundly from that of the FLD models, by forming a central
geometrically-thick disk. Overall, an adiabatic equation of state is not a good
approximation to the advanced stage of direct collapse, mainly because the
radiation in the FLD is capable of escaping due to anisotropy in the optical
depth and associated gradients.Comment: 19 pages, 17 figures, MNRAS, in press; correcting typo
Spartan Daily, October 12, 1983
Volume 81, Issue 31https://scholarworks.sjsu.edu/spartandaily/7080/thumbnail.jp
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
The resurrection of Antoni Gaudà in post-war media : a critical chronology: 1945–1965
The Post-war time coincides with the rehabilitation of Antoni GaudĂ, a process closely linked to the spread of his architecture on a global scale. Although GaudĂ’s historiography has paid specific attention to some outstanding episodes of this rehabilitation, these have not been shared from the outlook of the media apparatus that sustains them, following a temporal cadence that favours their relational reading from a critical perspective. The Post-war “resurrection” of GaudĂ cannot be separated from the large number of publications, exhibitions, photographic series and even films that shaped his figure according to the interests of time: a media operation, not neutral at all, which explains, to a great extent, the treatment that the architect and his work still receive today.Peer ReviewedPostprint (published version
Mock galaxy redshift catalogues from simulations: implications for Pan-STARRS1
We describe a method for constructing mock galaxy catalogues which are well
suited for use in conjunction with large photometric surveys. We use the
semi-analytic galaxy formation model of Bower et al. implemented in the
Millennium simulation. We apply our method to the specific case of the surveys
soon to commence with PS1, the first of 4 telescopes planned for the Pan-STARRS
system. PS1 has 5 photometric bands (grizy), and will carry out an all-sky 3pi
survey and a medium deep survey (MDS) over 84 sq.deg. We calculate the expected
magnitude limits for extended sources in the two surveys. We find that, after 3
years, the 3pi survey will have detected over 10^8 galaxies in all 5 bands, 10
million of which will lie at redshift z>0.9, while the MDS will have detected
over 10^7 galaxies with 0.5 million lying at z>2. These numbers at least double
if detection in the shallowest band, y is not required. We then evaluate the
accuracy of photometric redshifts estimated using an off-the-shelf photo-z
code. With the grizy bands alone it is possible to achieve an accuracy in the
3pi survey of Delta z/(1+z)~0.06 for 0.25<z<0.8, which could be reduced by
about 15% using near infrared photometry from the UKIDDS survey, but would
increase by about 25% for the deeper sample without the y band photometry. For
the MDS an accuracy of Delta z/(1+z)~0.05 is achievable for 0.02<z<1.5 using
grizy. A dramatic improvement in accuracy is possible by selecting only red
galaxies. In this case, Delta z/(1+z)~0.02-0.04 is achievable for ~100 million
galaxies at 0.4<z<1.1 in the 3pi survey and for 30 million galaxies in the MDS
at 0.4<z<2. We investigate the effect of using photo-z in the estimate of the
baryonic acoustic oscillation scale. We find that PS1 will achieve a similar
accuracy in this estimate as a spectroscopic survey of 20 million galaxies.Comment: 23 pages, 18 figures, accepted by MNRA
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