3,785 research outputs found
Multi-wavelength modeling of the spatially resolved debris disk of HD 107146
(abridged) We aim to constrain the location, composition, and dynamical state
of planetesimal populations and dust around the young, sun-like (G2V) star HD
107146}. We consider coronagraphic observations obtained with the Advanced
Camera for Surveys (HST/ACS) onboard the HST in broad V and broad I filters, a
resolved 1.3mm map obtained with the Combined Array for Research in
Millimeter-Wave Astronomy (CARMA), Spitzer/IRS low resolution spectra, and the
spectral energy distribution (SED) of the object at wavelengths ranging from
3.5micron to 3.1mm. We complement these data with new coronagraphic high
resolution observations of the debris disk using the Near Infrared Camera and
Multi-Object Spectrometer (HST/NICMOS) aboard the HST in the F110W filter. The
SED and images of the disk in scattered light as well as in thermal reemission
are combined in our modeling using a parameterized model for the disk density
distribution and optical properties of the dust. A detailed analytical model of
the debris disk around HD 107146 is presented that allows us to reproduce the
almost entire set of spatially resolved and unresolved multi-wavelength
observations. Considering the variety of complementary observational data, we
are able to break the degeneracies produced by modeling SED data alone. We find
the disk to be an extended ring with a peak surface density at 131AU.
Furthermore, we find evidence for an additional, inner disk probably composed
of small grains released at the inner edge of the outer disk and moving inwards
due to Poynting-Robertson drag. A birth ring scenario (i.e., a more or less
broad ring of planetesimals creating the dust disk trough collisions) is found
to be the most likely explanation of the ringlike shape of the disk.Comment: 15 pages, 9 figures, accepted for publication in A&
Reducing complexity: An iterative strategy for parameter determination in biological networks
AbstractThe dynamics of biological networks are fundamental to a variety of processes in many areas of biology and medicine. Understanding of such networks on a systemic level is facilitated by mathematical models describing these networks. However, since mathematical models of signalling networks commonly aim to describe several highly connected biological quantities and many model parameters cannot be measured directly, quantitative dynamic models often present challenges with respect to model calibration. Here, we propose an iterative fitting routine to decompose the problem of fitting a system of coupled ordinary differential equations describing a signalling network into smaller subproblems. Parameters for each differential equation are estimated separately using a Differential Evolution algorithm while all other dynamic quantities in the model are treated as input to the system. The performance of this algorithm is evaluated on artificial networks with known structure and known model parameters and compared to a conventional optimisation procedure for the same problem. Our analysis indicates that the procedure results in a significantly higher quality of fit and more efficient reconstruction of the true parameters than the conventional algorithm
The dust, planetesimals and planets of HD 38529
HD 38529 is a post-main sequence G8III/IV star (3.5 Gyr old) with a planetary
system consisting of at least two planets having Msin(i) of 0.8 MJup and 12.2
MJup, semimajor axes of 0.13 AU and 3.74 AU, and eccentricities of 0.25 and
0.35, respectively. Spitzer observations show that HD 38529 has an excess
emission above the stellar photosphere, with a signal-to-noise ratio (S/N) at
70 micron of 4.7, a small excess at 33 micron (S/N=2.6) and no excess <30
micron. We discuss the distribution of the potential dust-producing
planetesimals from the study of the dynamical perturbations of the two known
planets, considering in particular the effect of secular resonances. We
identify three dynamically stable niches at 0.4-0.8 AU, 20-50 AU and beyond 60
AU. We model the spectral energy distribution of HD 38529 to find out which of
these niches show signs of harboring dust-producing plantesimals. The secular
analysis, together with the SED modeling resuls, suggest that the planetesimals
responsible for most of the dust emission are likely located within 20-50 AU, a
configuration that resembles that of the Jovian planets + Kuiper Belt in our
Solar System. Finally, we place upper limits (8E-6 lunar masses of 10 micron
particles) to the amount of dust that could be located in the dynamically
stable region that exists between the two planets (0.25--0.75 AU).Comment: 23 pages, including 1 table and 5 figures. Accepted for publication
in Ap
A simple test for the existence of two accretion modes in Active Galactic Nuclei
By analogy to the different accretion states observed in black-hole X-ray
binaries (BHXBs), it appears plausible that accretion disks in active galactic
nuclei (AGN) undergo a state transition between a radiatively efficient and
inefficient accretion flow. If the radiative efficiency changes at some
critical accretion rate, there will be a change in the distribution of black
hole masses and bolometric luminosities at the corresponding transition
luminosity. To test this prediction, I consider the joint distribution of AGN
black hole masses and bolometric luminosities for a sample taken from the
literature. The small number of objects with low Eddington-scaled accretion
rates mdot < 0.01 and black hole masses Mbh < 10^9 Msun constitutes tentative
evidence for the existence of such a transition in AGN. Selection effects, in
particular those associated with flux-limited samples, systematically exclude
objects in particular regions of the black hole mass-luminosity plane.
Therefore, they require particular attention in the analysis of distributions
of black hole mass, bolometric luminosity, and derived quantities like the
accretion rate. I suggest further observational tests of the BHXB-AGN
unification scheme which are based on the jet domination of the energy output
of BHXBs in the hard state, and on the possible equivalence of BHXB in the very
high (or "steep power-law") state showing ejections and efficiently accreting
quasars and radio galaxies with powerful radio jets.Comment: Accepted by ApJ; 14 pages, 4 figures, uses emulateap
PCCH-Arctic – Polar Climate and Cultural Heritage – Preservation and Restoration Management
publishedVersio
Are Debris Disks and Massive Planets Correlated?
Using data from the Spitzer Space Telescope Legacy Science Program
``Formation and Evolution of Planetary Systems'' (FEPS), we have searched for
debris disks around 9 FGK stars (2-10 Gyr), known from radial velocity (RV)
studies to have one or more massive planets. Only one of the sources, HD 38529,
has excess emission above the stellar photosphere; at 70 micron the
signal-to-noise ratio in the excess is 4.7 while at wavelengths < 30 micron
there is no evidence of excess. The remaining sources show no excesses at any
Spitzer wavelengths. Applying survival tests to the FEPS sample and the results
for the FGK survey published in Bryden et al. (2006), we do not find a
significant correlation between the frequency and properties of debris disks
and the presence of close-in planets. We discuss possible reasons for the lack
of a correlation.Comment: 24 pages, 3 figures. Accepted to Astrophysical Journa
The Magic Number Problem for Subregular Language Families
We investigate the magic number problem, that is, the question whether there
exists a minimal n-state nondeterministic finite automaton (NFA) whose
equivalent minimal deterministic finite automaton (DFA) has alpha states, for
all n and alpha satisfying n less or equal to alpha less or equal to exp(2,n).
A number alpha not satisfying this condition is called a magic number (for n).
It was shown in [11] that no magic numbers exist for general regular languages,
while in [5] trivial and non-trivial magic numbers for unary regular languages
were identified. We obtain similar results for automata accepting subregular
languages like, for example, combinational languages, star-free, prefix-,
suffix-, and infix-closed languages, and prefix-, suffix-, and infix-free
languages, showing that there are only trivial magic numbers, when they exist.
For finite languages we obtain some partial results showing that certain
numbers are non-magic.Comment: In Proceedings DCFS 2010, arXiv:1008.127
Azobenzene/Ag(111)
The adsorption structure of the molecular switch azobenzene on Ag(111) is
investigated by a combination of normal incidence x-ray standing waves and
dispersion-corrected density functional theory. The inclusion of nonlocal
collective substrate response (screening) in the dispersion correction
improves the description of dense monolayers of azobenzene, which exhibit a
substantial torsion of the molecule. Nevertheless, for a quantitative
agreement with experiment explicit consideration of the effect of vibrational
mode anharmonicity on the adsorption geometry is crucial
Biochar standardization and legislation harmonization
It is a relatively new concept to use biochar as soil amendment and for climate change mitigation. For this reason, the national and supranational legislation in the EU is not yet adequately prepared to regulate both the production and the application of biochar. Driven by this “regulatory gap”, voluntary biochar quality standards have been formed in Europe with the European Biochar Certificate, in the UK with the Biochar Quality Mandate and in the USA with the IBI Standard which is intended to be used internationally. In parallel to this, biochar producers and biochar users in a number of EU countries were partly successful in fitting the new biochar product into the existing national legislation for fertilisers, soil improvers and composts. The intended revision of the EC Regulation 2003/2003 on fertilisers offers the opportunity to regulate the use of biochar at the EU level. This publication summarizes the efforts on biochar standardization which have been carried out by voluntary products standards and illustrates existing legislation in EU member states, which apply to the production and use of biochar. It describes existing and planned EU regulations, which impact biochar applications and it develops recommendations on the harmonization of biochar legislation in the EU.
First published online: 24 Jan 201
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