2,148 research outputs found
Benzene formation in the inner regions of protostellar disks
Benzene (c-C6H6) formation in the inner 3 AU of a protostellar disk can be
efficient, resulting in high abundances of benzene in the midplane region. The
formation mechanism is different to that found in interstellar clouds and in
protoplanetary nebulae, and proceeds mainly through the reaction between allene
(C3H4) and its ion. This has implications for PAH formation, in that some
fraction of PAHs seen in the solar system could be native rather than inherited
from the interstellar medium.Comment: 9 pages, 2 colour figures, to be published in the Astrophysical
Journal Letter
A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres
The atmospheres of substellar objects contain clouds of oxides, iron,
silicates, and other refractory condensates. Water clouds are expected in the
coolest objects. The opacity of these `dust' clouds strongly affects both the
atmospheric temperature-pressure profile and the emergent flux. Thus any
attempt to model the spectra of these atmospheres must incorporate a cloud
model. However the diversity of cloud models in atmospheric simulations is
large and it is not always clear how the underlying physics of the various
models compare. Likewise the observational consequences of different modeling
approaches can be masked by other model differences, making objective
comparisons challenging. In order to clarify the current state of the modeling
approaches, this paper compares five different cloud models in two sets of
tests. Test case 1 tests the dust cloud models for a prescribed L, L--T, and
T-dwarf atmospheric (temperature T, pressure p, convective velocity
vconv)-structures. Test case 2 compares complete model atmosphere results for
given (effective temperature Teff, surface gravity log g). All models agree on
the global cloud structure but differ in opacity-relevant details like grain
size, amount of dust, dust and gas-phase composition. Comparisons of synthetic
photometric fluxes translate into an modelling uncertainty in apparent
magnitudes for our L-dwarf (T-dwarf) test case of 0.25 < \Delta m < 0.875 (0.1
< \Delta m M 1.375) taking into account the 2MASS, the UKIRT WFCAM, the Spitzer
IRAC, and VLT VISIR filters with UKIRT WFCAM being the most challenging for the
models. (abr.)Comment: 22 pages, 17 figures, MNRAS 2008, accepted, (minor grammar/typo
corrections
Complexity of links in 3-manifolds
We introduce a natural-valued complexity c(X) for pairs X=(M,L), where M is a
closed orientable 3-manifold and L is a link contained in M. The definition
employs simple spines, but for well-behaved X's we show that c(X) equals the
minimal number of tetrahedra in a triangulation of M containing L in its
1-skeleton. Slightly adapting Matveev's recent theory of roots for graphs, we
carefully analyze the behaviour of c under connected sum away from and along
the link. We show in particular that c is almost always additive, describing in
detail the circumstances under which it is not. To do so we introduce a certain
(0,2)-root for a pair X, we show that it is well-defined, and we prove that X
has the same complexity as its (0,2)-root. We then consider, for links in the
3-sphere, the relations of c with the crossing number and with the hyperbolic
volume of the exterior, establishing various upper and lower bounds. We also
specialize our analysis to certain infinite families of links, providing rather
accurate asymptotic estimates.Comment: 24 pages, 6 figure
Hamiltonian and physical Hilbert space in polymer quantum mechanics
In this paper, a version of polymer quantum mechanics, which is inspired by
loop quantum gravity, is considered and shown to be equivalent, in a precise
sense, to the standard, experimentally tested, Schroedinger quantum mechanics.
The kinematical cornerstone of our framework is the so called polymer
representation of the Heisenberg-Weyl (H-W) algebra, which is the starting
point of the construction. The dynamics is constructed as a continuum limit of
effective theories characterized by a scale, and requires a renormalization of
the inner product. The result is a physical Hilbert space in which the
continuum Hamiltonian can be represented and that is unitarily equivalent to
the Schroedinger representation of quantum mechanics. As a concrete
implementation of our formalism, the simple harmonic oscillator is fully
developed.Comment: 19 pages, 2 figures. Comments and references added. Version to be
published in CQ
Comparison of cloud models for Brown Dwarfs
A test case comparison is presented for different dust cloud model approaches
applied in brown dwarfs and giant gas planets. We aim to achieve more
transparency in evaluating the uncertainty inherent to theoretical modelling.
We show in how far model results for characteristic dust quantities vary due to
different assumptions. We also demonstrate differences in the spectral energy
distributions resulting from our individual cloud modelling in 1D substellar
atmosphere simulationsComment: 5 pages, Proceeding to "Exoplantes: Detection, Formation, Dynamics",
eds. Ferraz-Mello et
Dynamic mineral clouds on HD 189733b. II. Monte Carlo radiative transfer for 3D cloudy exoplanet atmospheres : combining scattering and emission spectra
G.L. and Ch.H. highlight the financial support of the European community under the FP7 ERC starting grant 257431.Context. As the 3D spatial properties of exoplanet atmospheres are being observed in increasing detail by current and new generations of telescopes, the modelling of the 3D scattering effects of cloud forming atmospheres with inhomogeneous opacity structures becomes increasingly important to interpret observational data. Aims. We model the scattering and emission properties of a simulated cloud forming, inhomogeneous opacity, hot Jupiter atmosphere of HD 189733b. We compare our results to available Hubble Space Telescope (HST) and Spitzer data and quantify the effects of 3D multiple scattering on observable properties of the atmosphere. We discuss potential observational properties of HD 189733b for the upcoming Transiting Exoplanet Survey Satellite (TESS) and CHaracterising ExOPlanet Satellite (CHEOPS) missions. Methods. We developed a Monte Carlo radiative transfer code and applied it to post-process output of our 3D radiative-hydrodynamic, cloud formation simulation of HD 189733b. We employed three variance reduction techniques, i.e. next event estimation, survival biasing, and composite emission biasing, to improve signal to noise of the output. For cloud particle scattering events, we constructed a log-normal area distribution from the 3D cloud formation radiative-hydrodynamic results, which is stochastically sampled in order to model the Rayleigh and Mie scattering behaviour of a mixture of grain sizes. Results. Stellar photon packets incident on the eastern dayside hemisphere show predominantly Rayleigh, single-scattering behaviour, while multiple scattering occurs on the western hemisphere. Combined scattered and thermal emitted light predictions are consistent with published HST and Spitzer secondary transit observations. Our model predictions are also consistent with geometric albedo constraints from optical wavelength ground-based polarimetry and HST B band measurements. We predict an apparent geometric albedo for HD 189733b of 0.205 and 0.229, in the TESS and CHEOPS photometric bands respectively. Conclusions. Modelling the 3D geometric scattering effects of clouds on observables of exoplanet atmospheres provides an important contribution to the attempt to determine the cloud properties of these objects. Comparisons between TESS and CHEOPS photometry may provide qualitative information on the cloud properties of nearby hot Jupiter exoplanets.Publisher PDFPeer reviewe
Dynamic mineral clouds on HD 189733b : I. 3D RHD with kinetic, non-equilibrium cloud formation
Funding: European community under the FP7 ERC starting grant 257431 (G.L., Ch. H.).Context. Observations of exoplanet atmospheres have revealed the presence of cloud particles in their atmospheres. 3D modelling of cloud formation in atmospheres of extrasolar planets coupled to the atmospheric dynamics has long been a challenge. Aims. We investigate the thermo-hydrodynamic properties of cloud formation processes in the atmospheres of hot Jupiter exoplanets. Methods. We simulate the dynamic atmosphere of HD 189733b with a 3D model that couples 3D radiative-hydrodynamics with a kinetic, microphysical mineral cloud formation module designed for RHD/GCM exoplanet atmosphere simulations. Our simulation includes the feedback effects of cloud advection and settling, gas phase element advection and depletion/replenishment and the radiative effects of cloud opacity. We model the cloud particles as a mix of mineral materials which change in size and composition as they travel through atmospheric thermo-chemical environments. All local cloud properties such as number density, grain size and material composition are time-dependently calculated. Gas phase element depletion as a result of cloud formation is included in the model. In situ effective medium theory and Mie theory is applied to calculate the wavelength dependent opacity of the cloud component. Results. We present a 3D cloud structure of a chemically complex, gaseous atmosphere of the hot Jupiter HD 189733b. Mean cloud particle sizes are typically sub-micron (0.01â0.5 ÎŒm) at pressures less than 1 bar with hotter equatorial regions containing the smallest grains. Denser cloud structures occur near terminator regions and deeper (~1 bar) atmospheric layers. Silicate materials such as MgSiO3[s] are found to be abundant at mid-high latitudes, while TiO2[s] and SiO2[s] dominate the equatorial regions. Elements involved in the cloud formation can be depleted by several orders of magnitude. Conclusions. The interplay between radiative-hydrodynamics and cloud kinetics leads to an inhomogeneous, wavelength dependent opacity cloud structure with properties differing in longitude, latitude and depth. This suggests that transit spectroscopy would sample a variety of cloud particles properties (sizes, composition, densities).Publisher PDFPeer reviewe
Infrared spectra of TiO2 clusters for hot Jupiter atmospheres
Context. Clouds seem unavoidable in cool and dense environments, and hence,
are necessary to explain observations of exoplanet atmospheres, most recently
of WASP 96b with JWST. Understanding the formation of cloud condensation nuclei
in non-terrestrial environments is therefore crucial to develop accurate models
to interpret present and future observations. Aims. The goal of the paper is to
support observations with infrared spectra for (TiO2)N clusters in order to
study cloud formation in exoplanet atmospheres. Methods. Vibrational
frequencies are derived from quantum-chemical calculations for 123
(TiO2)-clusters and their isomers, and line-broadening mechanisms are
evaluated. Cluster spectra are calculated for several atmospheric levels for
two example exoplanet atmospheres (WASP 121b-like and WASP 96b-like) to
identify possible spectral fingerprints for cloud formation. Results.
Rotational motion of and transitions in the clusters cause significant line
broadening, so that individual vibrational lines are broadened beyond the
spectral resolution of the medium resolution mode of the JWST mid-infrared
instrument MIRI at R = 3000. However, each individual cluster isomer exhibits a
"fingerprint" IR spectrum. In particular, larger (TiO2)-clusters have
distinctly different spectra from smaller clusters. Morning and evening
terminator for the same planet can exhibit different total absorbances due to
different cluster sizes being more abundant. Conclusions. The largest
(TiO2)-clusters are not necessarily the most abundant (TiO2)-clusters in the
high-altitude regions of ultra-hot Jupiters, and the different cluster isomers
will contribute to the local absorbance. Planets with a considerable day-night
asymmetry will be most suitable to search for (TiO2)-cluster isomers in order
to improve cloud formation modelling.Comment: 8 pages, 8 figures, 1 table, accepted for publication in A&
M dwarf stars in the light of (future) exoplanet searches
We present a brief overview of a splinter session on M dwarf stars as planet
hosts that was organized as part of the Cool Stars 17 conference. The session
was devoted to reviewing our current knowledge of M dwarf stars and exoplanets
in order to prepare for current and future exoplanet searches focusing in low
mass stars. We review the observational and theoretical challenges to
characterize M dwarf stars and the importance of accurate fundamental
parameters for the proper characterization of their exoplanets and our
understanding on planet formation.Comment: 4 pages, 1 figure. Summary of the splinter session "M dwarf stars in
the light of (future) exoplanet searches" held at the 17th Cambridge Workshop
on Cool Stars, Stellar Systems, and the Sun, June 28th 2012, Barcelona,
Spain. Submitted for publication in Astronomische Nachrichten - Astronomical
Notes (AN) 334, Issue 1-2, Eds Klaus Strassmeier and Mercedes L\'opez-Morale
Intense Mass Loss from C-rich AGB Stars at low Metallicity?
We argue that the energy injection of pulsations may be of greater importance
to the mass-loss rate of AGB stars than metallicity, and that the mass-loss
trend with metallicity is not as simple as sometimes assumed. Using our
detailed radiation hydrodynamical models that include dust formation, we
illustrate the effects of pulsation energy on wind properties. We find that the
mass-loss rate scales with the kinetic energy input by pulsations as long as a
dust-saturated wind does not occur, and all other stellar parameters are kept
constant. This includes the absolute abundance of condensible carbon (not bound
in CO), which is more relevant than keeping the C/O-ratio constant when
comparing stars of different metallicity. The pressure and temperature
gradients in the atmospheres of stars, become steeper and flatter,
respectively, when the metallicity is reduced, while the radius where the
atmosphere becomes opaque is typically associated with a higher gas pressure.
This effect can be compensated for by adjusting the velocity amplitude of the
variable inner boundary (piston), which is used to simulate the effects of
pulsation, to obtain models with comparable kinetic-energy input. Hence, it is
more relevant to compare models with similar energy-injections than of similar
velocity amplitude. Since there is no evidence for weaker pulsations in
low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity
C-stars have a lower mass-loss rate, than their more metal-rich counterparts
with similar stellar parameters, as long as they have a comparable amount of
condensible carbon.Comment: 4 pages, 3 figures. Accepted for publication in A&A. Updated after
language editing. Additional typos fixe
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