5,305 research outputs found
Impacts of planet migration models on planetary populations. Effects of saturation, cooling and stellar irradiation
Context: Several recent studies have found that planet migration in adiabatic
discs differs significantly from migration in isothermal discs. Depending on
the thermodynamic conditions, i.e., the effectiveness of radiative cooling, and
the radial surface density profile, planets migrate inward or outward. Clearly,
this will influence the semimajor axis - mass distribution of planets as
predicted by population synthesis simulations. Aims: Our goal is to study the
global effects of radiative cooling, viscous torque desaturation and gap
opening as well as stellar irradiation on the tidal migration of a synthetic
planet population. Methods: We combine results from several analytical studies
and 3D hydrodynamic simulations in a new semi-analytical migration model for
the application in our planet population synthesis calculations. Results: We
find a good agreement of our model with torques obtained in a 3D radiative
hydrodynamic simulations. We find three convergence zones in a typical disc,
towards which planets migrate from the in- and outside, affecting strongly the
migration behavior of low-mass planets. Interestingly, this leads to slow type
II like migration behavior for low-mass planets captured in those zones even
without an ad hoc migration rate reduction factor or a yet to be defined
halting mechanism. This means that the new prescription of migration including
non-isothermal effects makes the preciously widely used artificial migration
rate reduction factor obsolete. Conclusions: Outward migration in parts of a
disc makes some planets survive long enough to become massive. The convergence
zones lead to a potentially observable accumulations of low-mass planets at
certain semimajor axes. Our results indicate that further studies of the mass
where the corotation torque saturates will be needed since its value has a
major impact on the properties of planet populations.Comment: 18 pages, 15 figures. Accepted for A&
The core flux of the brightest 10 micron galaxies in the southern sky
Aims. Near diffraction-limited images have been taken at 8.9, 11.9, and 12.9
micron for the brightest extragalactic sources in the southern sky, in order to
optimally plan N-band observations with MIDI (MID-infrared Interferometric
instrument) at the VLTI. Methods. We have assembled a sample of 21 objects
consisting of all the AGNs observable from Paranal observatory, Chile, plus
three non-AGN objects, with an estimated N-band flux greater than 400mJy. We
used the TIMMI2 Mid Infrared instrument mounted on the ESO's 3.6m telescope to
obtain near diffraction-limited images in order to establish the unresolved
core flux within < 0.5 arscsec. Results. Positions and core total fluxes were
obtained for all sources in our sample and compared with similar investigations
in the literature. We find that 15 AGN and the nuclear starburst in NGC 253
exhibit an unresolved core flux < 300mJy at 11.9 micron, making them promising
targets for MIDI at the VLTI. For extended sources, near diffraction-limited
images are presented and discussed.Comment: Accepted to A&
The effect of stellar feedback on the formation and evolution of gas and dust tori in AGN
Recently, the existence of geometrically thick dust structures in Active
Galactic Nuclei (AGN) has been directly proven with the help of mid-infrared
interferometry. The observations are consistent with a two-component model made
up of a geometrically thin and warm central disk, surrounded by a colder,
fluffy torus component. In an exploratory study, we investigate one possible
physical mechanism, which could produce such a structure, namely the effect of
stellar feedback from a young nuclear star cluster on the interstellar medium
in centres of AGN. The model is realised with the help of the hydrodynamics
code TRAMP. We follow the evolution of the interstellar medium by taking
discrete mass loss and energy ejection due to stellar processes, as well as
optically thin radiative cooling into account. In a post-processing step, we
calculate observable quantities (spectral energy distributions and images) with
the help of the radiative transfer code MC3D. The interplay between injection
of mass, supernova explosions and radiative cooling leads to a two-component
structure made up of a cold geometrically thin, but optically thick and very
turbulent disk residing in the vicinity of the angular momentum barrier,
surrounded by a filamentary structure. The latter consists of cold long radial
filaments flowing towards the disk and a hot tenuous medium in between, which
shows both inwards and outwards directed motions. This modelling is able to
reproduce the range of observed neutral hydrogen column densities of a sample
of Seyfert galaxies as well as the relation between them and the strength of
the silicate 10 micron spectral feature. Despite being quite crude, our mean
Seyfert galaxy model is even able to describe the SEDs of two intermediate type
Seyfert galaxies observed with the Spitzer Space Telescope.Comment: 16 pages, 11 figures, accepted by MNRAS, high resolution version can
be downloaded from:
http://www.mpe.mpg.de/~mschartm/papers/schartmann_2008b.pd
First Time-dependent Study of H2 and H3+ Ortho-Para Chemistry in the Diffuse Interstellar Medium: Observations Meet Theoretical Predictions
The chemistry in the diffuse interstellar medium initiates the gradual
increase of molecular complexity during the life cycle of matter. A key
molecule that enables build-up of new molecular bonds and new molecules via
proton-donation is H3+. Its evolution is tightly related to molecular hydrogen
and thought to be well understood. However, recent observations of ortho and
para lines of H2 and H3+ in the diffuse ISM showed a puzzling discrepancy in
nuclear spin excitation temperatures and populations between these two key
species. H3+, unlike H2, seems to be out of thermal equilibrium, contrary to
the predictions of modern astrochemical models. We conduct the first
time-dependent modeling of the para-fractions of H2 and H3+ in the diffuse ISM
and compare our results to a set of line-of-sight observations, including new
measurements presented in this study. We isolate a set of key reactions for H3+
and find that the destruction of the lowest rotational states of H3+ by
dissociative recombination largely control its ortho/para ratio. A plausible
agreement with observations cannot be achieved unless a ratio larger than 1:5
for the destruction of (1,1)- and (1,0)-states of H3+ is assumed. Additionally,
an increased CR ionization rate to 10(-15) 1/s further improves the fit whereas
variations of other individual physical parameters, such as density and
chemical age, have only a minor effect on the predicted ortho/para ratios. Thus
our study calls for new laboratory measurements of the dissociative
recombination rate and branching ratio of the key ion H3+ under interstellar
conditions.Comment: 27 pages, 6 figures, 3 table
Application of recent results on the orbital migration of low mass planets: convergence zones
Previous models of the combined growth and migration of protoplanets needed
large ad hoc reduction factors for the type I migration rate as found in the
isothermal approximation. In order to eliminate these factors, a simple
semi-analytical model is presented that incorporates recent results on the
migration of low mass planets in non-isothermal disks. It allows for outward
migration. The model is used to conduct planetary populations synthesis
calculations. Two points with zero torque are found in the disks. Planets
migrate both in- and outward towards these convergence zones. They could be
important for accelerating planetary growth by concentrating matter in one
point. We also find that the updated type I migration models allow the
formation of both close-in low mass planets, but also of giant planets at large
semimajor axes. The problem of too rapid migration is significantly mitigated.Comment: 4 pages, 3 figures. Proceedings of the IAU Symposium 276, 2010: The
Astrophysics of Planetary Systems: Formation, Structure, and Dynamical
Evolution, ed. A. Sozzetti, M. G. Lattanzi, and A. P. Bos
NGC 2264 IRS1: The central engine and its cavity
We present a high-resolution study of NGC 2264 IRS1 in CS(2-1) and in the
3-mm continuum using the IRAM Plateau de Bure Interferometer. We complement
these radio data with images taken at 2.2, 4.6, and 11.9 micron. The combined
information allow a new interpretation of the closest environment of NGC 2264
IRS1. No disk around the B-type star IRS1 was found. IRS1 and its low-mass
companions are located in a low-density cavity which is surrounded by the
remaining dense cloud core which has a clumpy shell-like structure. Strong
evidence for induced on-going star formation was found in the surroundings of
IRS1. A deeply embedded very young stellar object 20 arcsec to the north of
IRS1 is powering a highly collimated bipolar outflow. The object 8 in the
closer environment of IRS1 is a binary surrounded by dusty circumbinary
material and powering two bipolar outflows.Comment: 17 pages, 6 figures, The paper is accepted and will appear in the
Astrophysical Journal, Vol 599, No 1 (issue December 10). A high-resolution
postscript version of this paper is available here (
http://www.astro.uni-jena.de/Users/martin/publi.html). Furthermore, you can
find a high resolution PDF file here (
http://www.tls-tautenburg.de/research/tls-research/pub2003.html
Effects of On-Farm Diets for Organic Pig Production on Performance and Carcass Quality
An experiment was conducted to evaluate the effects of a restriction to home-grown feedstuffs and abstinence from supplementation with synthetic amino acids (AA), as ideal objectives in organic pig production according to the IFOAM standards, on growth performance and carcass characteristics. One hundred individually housed pigs were allocated to four dietary treatments and fed from growing through finishing to compare three organic barley/wheat-based diets with an isocaloric conventional diet supplemented with synthetic AA. Protein sources in the organic treatments were either faba beans, supplemented with potato protein to the same AA level as the control diet (FA+PO), peas and lupines (PE+LU) or faba beans and lupines (FA+LU), both without further supplementation, leading to a lower level of limited AA.
Amino acid supplementation with potato protein showed the same performance compared with the conventional diet, supplemented with synthetic AA, although crude protein levels differed markedly. Pigs fed the organic diets without AA supplementation grew more slowly (P < .05), had a decreased feed intake in the grower period (P < .05) but nearly the same feed efficiency (P .05) compared to conventional or organic diets with AA supplementation. Carcass characteristics differed in percentage of lean meat and longissimus area, being lower in the treatments without AA supplementation (P < .05). However, the intramuscular fat was higher without AA supplementation (2.9% fat) than with supplementation (1.2% fat) (P < .01). The data show that the exclusion of AA supplementation resulted in a reduction in pig performance but in an increase in intramuscular fat content (IMF), the latter is an important aspect of eating quality characteristics.
(SUNDRUM, A., L. BĂTFERING, M. HENNING and K.-H. HOPPENBROCK (2000): Effects of On-Farm Diets for Organic Pig Production on Performance and Carcass Quality. J. Animal Sci. 78, 1199-1205.
Polarization of Thermal Emission from Aligned Dust Grains Under an Anisotropic Radiation Field
If aspherical dust grains are immersed in an anisotropic radiation field,
their temperature depends on the cross-sections projected in the direction of
the anisotropy.It was shown that the temperature difference produces polarized
thermal emission even without alignment, if the observer looks at the grains
from a direction different from the anisotropic radiation. When the dust grains
are aligned, the anisotropy in the radiation makes various effects on the
polarization of the thermal emission, depending on the relative angle between
the anisotropy and alignment directions. If the both directions are parallel,
the anisotropy produces a steep increase in the polarization degree at short
wavelengths. If they are perpendicular, the polarization reversal occurs at a
wavelength shorter than the emission peak. The effect of the anisotropic
radiation will make a change of more than a few % in the polarization degree
for short wavelengths and the effect must be taken into account in the
interpretation of the polarization in the thermal emission. The anisotropy in
the radiation field produces a strong spectral dependence of the polarization
degree and position angle, which is not seen under isotropic radiation. The
dependence changes with the grain shape to a detectable level and thus it will
provide a new tool to investigate the shape of dust grains. This paper presents
examples of numerical calculations of the effects and demonstrates the
importance of anisotropic radiation field on the polarized thermal emission.Comment: 13pages, 7figure
High Spatial Resolution Observations of Two Young Protostars in the R Corona Australis Region
We present multi-wavelength, high spatial resolution imaging of the IRS 7
region in the R Corona Australis molecular cloud. Our observations include 1.1
mm continuum and HCO^+ J = images from the SMA, ^{12}CO J =
outflow maps from the DesertStar heterodyne array receiver on the HHT, 450
m and 850 m continuum images from SCUBA, and archival Spitzer IRAC
and MIPS 24 \micron images. The accurate astrometry of the IRAC images allow us
to identify IRS 7 with the cm source VLA 10W (IRS 7A) and the X-ray source X_W.
The SMA 1.1 mm image reveals two compact continuum sources which are also
distinguishable at 450 m. SMA 1 coincides with X-ray source CXOU
J190156.4-365728 and VLA cm source 10E (IRS 7B) and is seen in the IRAC and
MIPS images. SMA 2 has no infrared counterpart but coincides with cm source VLA
9. Spectral energy distributions constructed from SMA, SCUBA and Spitzer data
yield bolometric temperatures of 83 K for SMA 1 and 70 K for SMA 2. These
temperatures along with the submillimeter to total luminosity ratios indicate
that SMA 2 is a Class 0 protostar, while SMA 1 is a Class 0/Class I
transitional object (L= \Lsun). The ^{12}CO J = outflow map
shows one major and possibly several smaller outflows centered on the IRS 7
region, with masses and energetics consistent with previous work. We identify
the Class 0 source SMA 2/VLA 9 as the main driver of this outflow. The complex
and clumpy spatial and velocity distribution of the HCO^+ J =
emission is not consistent with either bulk rotation, or any known molecular
outflow activity.Comment: 31 pages, 8 figures, Accepted to Ap
Spin-flip model of spin-polarized vertical-cavity surface-emitting lasers: Asymptotic analysis, numerics, and experiments
The spin-flip model describing optically pumped spin-polarized vertical-cavity surface-emitting lasers is considered. The steady-state solutions of the model for elliptically polarized fields are studied. Asymptotic analysis for the existence and stability of the steady-state solutions is developed, particularly in the presence of pump polarization ellipticity. The expansion is with respect to small parameters representing the ellipticity and the difference between the total pump power and the lasing threshold. The analytical results are then confirmed numerically, where it is obtained that generally one of the steady-state solutions is stable while the other is not. The theoretical results are shown to be in qualitative agreement with the experiments
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