1,466 research outputs found
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Shocks and a Giant Planet in the Disk Orbiting BP Piscium?
Spitzer IRS spectroscopy supports the interpretation that BP Piscium, a gas
and dust enshrouded star residing at high Galactic latitude, is a first-ascent
giant rather than a classical T Tauri star. Our analysis suggests that BP
Piscium's spectral energy distribution can be modeled as a disk with a gap that
is opened by a giant planet. Modeling the rich mid-infrared emission line
spectrum indicates that the solid-state emitting grains orbiting BP Piscium are
primarily composed of ~75 K crystalline, magnesium-rich olivine; ~75 K
crystalline, magnesium-rich pyroxene; ~200 K amorphous, magnesium-rich
pyroxene; and ~200 K annealed silica ('cristobalite'). These dust grains are
all sub-micron sized. The giant planet and gap model also naturally explains
the location and mineralogy of the small dust grains in the disk. Disk shocks
that result from disk-planet interaction generate the highly crystalline dust
which is subsequently blown out of the disk mid-plane and into the disk
atmosphere.Comment: 25 pages, 4 figures, 1 table. Accepted to Ap
Dust-grain processing in circumbinary discs around evolved binaries. The RV Tauri spectral twins RU Cen and AC Her
Context: We study the structure and evolution of circumstellar discs around
evolved binaries and their impact on the evolution of the central system. Aims:
To study in detail the binary nature of RUCen and ACHer, as well as the
structure and mineralogy of the circumstellar environment. Methods: We combine
multi-wavelength observations with a 2D radiative transfer study. Our radial
velocity program studies the central stars, while our Spitzer spectra and
broad-band SEDs are used to constrain mineralogy, grain sizes and physical
structure of the circumstellar environment. Results: We determine the orbital
elements of RUCen showing that the orbit is highly eccentric with a rather long
period of 1500 days. The infrared spectra of both objects are very similar and
the spectral dust features are dominated by Mg-rich crystalline silicates. The
small peak-to-continuum ratios are interpreted as being due to large grains.
Our model contains two components with a cold midplain dominated by large
grains, and the near- and mid-IR which is dominated by the emission of smaller
silicates. The infrared excess is well modelled assuming a hydrostatic passive
irradiated disc. The profile-fitting of the dust resonances shows that the
grains must be very irregular. Conclusions: These two prototypical RVTauri
pulsators with circumstellar dust are binaries where the dust is trapped in a
stable disc. The mineralogy and grain sizes show that the dust is highly
processed, both in crystallinity and grain size. The cool crystals show that
either radial mixing is very efficient and/or that the thermal history at grain
formation has been very different from that in outflows. The physical processes
governing the structure of these discs are similar to those observed in
protoplanetary discs around young stellar objects.Comment: 11 pages, 12 figures, accepted for publication by A&
Solidification of liquid metal drops during impact
Hot liquid metal drops impacting onto a cold substrate solidify during their
subsequent spreading. Here we experimentally study the influence of
solidification on the outcome of an impact event. Liquid tin drops are impacted
onto sapphire substrates of varying temperature. The impact is visualised both
from the side and from below, which provides a unique view on the
solidification process. During spreading an intriguing pattern of radial
ligaments rapidly solidifies from the centre of the drop. This pattern
determines the late-time morphology of the splat. A quantitative analysis of
the drop spreading and ligament formation is supported by scaling arguments.
Finally, a phase diagram for drop bouncing, deposition and splashing as a
function of substrate temperature and impact velocity is provided
IRAS\,11472-0800: an extremely depleted pulsating binary post-AGB star
We focus here on one particular and poorly studied object, IRAS11472-0800. It
is a highly evolved post-Asymptotic Giant Branch (post-AGB) star of spectral
type F, with a large infrared excess produced by thermal emission of
circumstellar dust. We deploy a multi-wavelength study which includes the
analyses of optical and IR spectra as well as a variability study based on
photometric and spectroscopic time-series. The spectral energy distribution
(SED) properties as well as the highly processed silicate N-band emission show
that the dust in IRAS11472-0800 is likely trapped in a stable disc. The
energetics of the SED and the colour variability show that our viewing angle is
close to edge-on and that the optical flux is dominated by scattered light.
With photospheric abundances of [Fe/H] = -2.7 and [Sc/H]=-4.2, we discovered
that IRAS11472-0800 is one of the most chemically-depleted objects known to
date. Moreover, IRAS11472-0800 is a pulsating star with a period of 31.16 days
and a peak-to-peak amplitude of 0.6 mag in V. The radial velocity variability
is strongly influenced by the pulsations, but the significant cycle-to-cycle
variability is systematic on a longer time scale, which we interpret as
evidence for binary motion. We conclude that IRAS11472-0800 is a pulsating
binary star surrounded by a circumbinary disc. The line-of-sight towards the
object lies close the the orbital plane making that the optical light is
dominated by scattered light. IRAS11472-0800 is one of the most
chemically-depleted objects known so far and links the dusty RV\,Tauri stars to
the non-pulsating class of strongly depleted objects.Comment: 12 pages, 14 figures Accepted for publication in A&A Main Journa
Selective elimination in dialkoxy-PPV precursors yielding polymers with isolated tetraalkoxy-stilbene units
Needle age-related and seasonal photosynthetic capacity variation is negligible for modelling yearly gas exchange of a sparse temperate Scots pine forest
In this study, we quantified the predictive accuracy loss involved with
omitting photosynthetic capacity variation for a Scots pine (<i>Pinus sylvestris</i> L.) stand in
Flanders, Belgium. Over the course of one phenological year, we measured the
maximum carboxylation capacity at 25 °C (<i>V</i><sub>m25</sub>), the maximum electron
transport capacity at 25 °C (<i>J</i><sub>m25</sub>), and the leaf area index (LAI) of
different-aged needle cohorts in the upper and lower canopy. We used these
measurements as input for a process-based multi-layer canopy model with the
objective to quantify the difference in yearly gross ecosystem productivity
(GEP) and canopy transpiration (<i>E</i><sub>can</sub>) simulated under scenarios in which
the observed needle age-related and/or seasonal variation of <i>V</i><sub>m25</sub> and
<i>J</i><sub>m25</sub> was omitted. We compared simulated GEP with estimations obtained from
eddy covariance measurements. Additionally, we measured summer needle N
content to investigate the relationship between photosynthetic capacity
parameters and needle N content along different needle ages.
<br><br>
Results show that <i>V</i><sub>m25</sub> and <i>J</i><sub>m25</sub> were, respectively, 27% and
13% higher in current-year than in one-year old needles. A significant
seasonality effect was found on <i>V</i><sub>m25</sub>, but not on <i>J</i><sub>m25</sub>. Summer needle
N content was considerably lower in current-year than in one-year-old
needles. As a result, the correlations between <i>V</i><sub>m25</sub> and needle N content
and <i>J</i><sub>m25</sub> and needle N content were negative and non-significant,
respectively. Some explanations for these unexpected correlations were
brought forward. Yearly GEP was overestimated by the canopy model by ±15% under all scenarios. The inclusion and omission of the observed
needle age-related <i>V</i><sub>m25</sub> and <i>J</i><sub>m25</sub> variation in the model simulations
led to statistically significant but ecologically irrelevant differences in
simulated yearly GEP and <i>E</i><sub>can</sub>. Omitting seasonal variation did not yield
significant simulation differences. Our results indicate that intensive
photosynthetic capacity measurements over the full growing season and
separate simulation of needle age classes were no prerequisites for accurate
simulations of yearly canopy gas exchange. This is true, at least, for the
studied stand, which has a very sparse canopy and is exposed to high N
deposition and, hence, is not fully representative for temperate Scots pine
stands. Nevertheless, we believe well-parameterized process-based canopy
models – as applied in this study – are a useful tool to quantify losses of
predictive accuracy involved with canopy simplification in modelling
On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants
We investigate the occurrence of crystalline silicates in oxygen-rich evolved
stars across a range of metallicities and mass-loss rates. It has been
suggested that the crystalline silicate feature strength increases with
increasing mass-loss rate, implying a correlation between lattice structure and
wind density. To test this, we analyse Spitzer IRS and Infrared Space
Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98
red supergiants in the Milky Way, the Large and Small Magellanic Clouds and
Galactic globular clusters. These encompass a range of spectral morphologies
from the spectrally-rich which exhibit a wealth of crystalline and amorphous
silicate features to 'naked' (dust-free) stars. We combine spectroscopic and
photometric observations with the GRAMS grid of radiative transfer models to
derive (dust) mass-loss rates and temperature. We then measure the strength of
the crystalline silicate bands at 23, 28 and 33 microns. We detect crystalline
silicates in stars with dust mass-loss rates which span over 3 dex, down to
rates of ~10^-9 solar masses/year. Detections of crystalline silicates are more
prevalent in higher mass-loss rate objects, though the highest mass-loss rate
objects do not show the 23-micron feature, possibly due to the low temperature
of the forsterite grains or it may indicate that the 23-micron band is going
into absorption due to high column density. Furthermore, we detect a change in
the crystalline silicate mineralogy with metallicity, with enstatite seen
increasingly at low metallicity.Comment: Accepted for publication in MNRAS, 24 pages, 16 figure
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