5,033 research outputs found
Spectator Behavior in a Quantum Hall Antidot with Multiple Bound Modes
We theoretically study Aharonov-Bohm resonances in an antidot system with
multiple bound modes in the integer quantum Hall regime, taking capacitive
interactions between the modes into account. We find the spectator behavior
that the resonances of some modes disappear and instead are replaced by those
of other modes, due to internal charge relaxation between the modes. This
behavior is a possible origin of the features of previous experimental data
which remain unexplained, spectator behavior in an antidot molecule and
resonances in a single antidot with three modes.Comment: 4 pages, 3 figures, to be published in Physical Review Letter
Spectral modeling of type II supernovae. I. Dilution factors
We present substantial extensions to the Monte Carlo radiative transfer code
TARDIS to perform spectral synthesis for type II supernovae. By incorporating a
non-LTE ionization and excitation treatment for hydrogen, a full account of
free-free and bound-free processes, a self-consistent determination of the
thermal state and by improving the handling of relativistic effects, the
improved code version includes the necessary physics to perform spectral
synthesis for type II supernovae to high precision as required for the reliable
inference of supernova properties. We demonstrate the capabilities of the
extended version of TARDIS by calculating synthetic spectra for the
prototypical type II supernova SN1999em and by deriving a new and independent
set of dilution factors for the expanding photosphere method. We have
investigated in detail the dependence of the dilution factors on photospheric
properties and, for the first time, on changes in metallicity. We also compare
our results with two previously published sets of dilution factors by Eastman
et al. (1996) and by Dessart & Hillier (2005), and discuss the potential
sources of the discrepancies between studies.Comment: 16 pages, 12 figures, 2 tables, accepted for publication in A&
Spectral sequences of Type Ia supernovae. I. Connecting normal and sub-luminous SN Ia and the presence of unburned carbon
Type Ia supernovae are generally agreed to arise from thermonuclear
explosions of carbon-oxygen white dwarfs. The actual path to explosion,
however, remains elusive, with numerous plausible parent systems and explosion
mechanisms suggested. Observationally, type Ia supernovae have multiple
subclasses, distinguished by their lightcurves and spectra. This raises the
question whether these reflect that multiple mechanisms occur in nature, or
instead that explosions have a large but continuous range of physical
properties. We revisit the idea that normal and 91bg-like supernovae can be
understood as part of a spectral sequence, in which changes in temperature
dominate. Specifically, we find that a single ejecta structure is sufficient to
provide reasonable fits of both the normal type Ia supernova SN~2011fe and the
91bg-like SN~2005bl, provided that the luminosity and thus temperature of the
ejecta are adjusted appropriately. This suggests that the outer layers of the
ejecta are similar, thus providing some support of a common explosion
mechanism. Our spectral sequence also helps to shed light on the conditions
under which carbon can be detected in pre-maximum SN~Ia spectra -- we find that
emission from iron can "fill in" the carbon trough in cool SN~Ia. This may
indicate that the outer layers of the ejecta of events in which carbon is
detected are relatively metal poor compared to events where carbon is not
detected
The reverberation signatures of rotating disc winds in active galactic nuclei
The broad emission lines (BELs) in active galactic nuclei (AGN) respond to
ionizing continuum variations. The time and velocity dependence of their
response depends on the structure of the broad-line region: its geometry,
kinematics and ionization state. Here, we predict the reverberation signatures
of BELs formed in rotating accretion disc winds. We use a Monte Carlo radiative
transfer and ionization code to predict velocity-delay maps for representative
high- (C) and low-ionization (H) emission lines in both high- and
moderate-luminosity AGN. Self-shielding, multiple scattering and the ionization
structure of the outflows are all self-consistently taken into account, while
small-scale structure in the outflow is modelled in the micro-clumping
approximation. Our main findings are: (1) The velocity-delay maps of
smooth/micro-clumped outflows often contain significant negative responses.
(2)~The reverberation signatures of disc wind models tend to be rotation
dominated and can even resemble the classic "red-leads-blue" inflow signature.
(3) Traditional "blue-leads-red" outflow signatures can usually only be
observed in the long-delay limit. (4) Our models predict lag-luminosity
relationships similar to those inferred from observations, but systematically
underpredict the observed centroid delays. (5) The ratio between "virial
product" and black hole mass predicted by our models depends on viewing angle.
Our results imply that considerable care needs to be taken in interpreting data
obtained by observational reverberation mapping campaigns. In particular, basic
signatures such as "red-leads-blue", "blue-leads-red" and "blue and red vary
jointly" are not always reliable indicators of inflow, outflow or rotation.
This may help to explain the perplexing diversity of such signatures seen in
observational campaigns to date.Comment: 15 pages, 17 figures, 2 tables. Accepted by MNRAS 20/7/201
Type Ia Supernovae and Accretion Induced Collapse
Using the population synthesis binary evolution code StarTrack, we present
theoretical rates and delay times of Type Ia supernovae arising from various
formation channels. These channels include binaries in which the exploding
white dwarf reaches the Chandrasekhar mass limit (DDS, SDS, and helium-rich
donor scenario) as well as the sub-Chandrasekhar mass scenario, in which a
white dwarf accretes from a helium-rich companion and explodes as a SN Ia
before reaching the Chandrasekhar mass limit. We find that using a common
envelope parameterization employing energy balance with alpha=1 and lambda=1,
the supernova rates per unit mass (born in stars) of sub-Chandrasekhar mass SNe
Ia exceed those of all other progenitor channels at epochs t=0.7 - 4 Gyr for a
burst of star formation at t=0. Additionally, the delay time distribution of
the sub-Chandrasekhar model can be divided in to two distinct evolutionary
channels: the `prompt' helium-star channel with delay times < 500 Myr, and the
`delayed' double white dwarf channel with delay times > 800 Myr spanning up to
a Hubble time. These findings are in agreement with recent
observationally-derived delay time distributions which predict that a large
number of SNe Ia have delay times < 1 Gyr, with a significant fraction having
delay times < 500 Myr. We find that the DDS channel is also able to account for
the observed rates of SNe Ia. However, detailed simulations of white dwarf
mergers have shown that most of these mergers will not lead to SNe Ia but
rather to the formation of a neutron star via accretion-induced collapse. If
this is true, our standard population synthesis model predicts that the only
progenitor channel which can account for the rates of SNe Ia is the
sub-Chandrasekhar mass scenario, and none of the other progenitors considered
can fully account for the observed rates.Comment: 6 pages, 1 figure, 1 table, to appear in proceedings for "Binary Star
Evolution: Mass Loss, Accretion and Mergers
Postfledging Survival, Movements, and Dispersal of Ring Ouzels (Turdus torquatus)
We thank Invercauld Estate for cooperation with access to Glen Clunie. S. Redpath, J. Wilson, and S. Roos provided valuable comments on the manuscript. This study was funded by the Royal Society for the Protection of Birds, Scottish Natural Heritage, and the Cairngorms National Park Authority. J.L.L. was supported by the Natural Environment Research Council.Peer reviewedPublisher PD
- …