1,450 research outputs found
Testing the theory of grain growth and fragmentation by millimeter observations of protoplanetary disks
Context. Observations at sub-millimeter and mm wavelengths will in the near
future be able to resolve the radial dependence of the mm spectral slope in
circumstellar disks with a resolution of around a few AU at the distance of the
closest star-forming regions.
Aims. We aim to constrain physical models of grain growth and fragmentation
by a large sample of (sub-)mm observations of disks around pre-main sequence
stars in the Taurus-Auriga and Ophiuchus star-forming regions.
Methods. State-of-the-art coagulation/fragmentation and disk-structure codes
are coupled to produce steady-state grain size distributions and to predict the
spectral slopes at (sub-)mm wavelengths.
Results. This work presents the first calculations predicting the mm spectral
slope based on a physical model of grain growth. Our models can quite naturally
reproduce the observed mm-slopes, but a simultaneous match to the observed
range of flux levels can only be reached by a reduction of the dust mass by a
factor of a few up to about 30 while keeping the gas mass of the disk the same.
This dust reduction can either be due to radial drift at a reduced rate or
during an earlier evolutionary time (otherwise the predicted fluxes would
become too low) or due to efficient conversion of dust into larger, unseen
bodies.Comment: Accepted for publication in A&A Letters. 5 pages, 3 figure
On model selection forecasting, Dark Energy and modified gravity
The Fisher matrix approach (Fisher 1935) allows one to calculate in advance
how well a given experiment will be able to estimate model parameters, and has
been an invaluable tool in experimental design. In the same spirit, we present
here a method to predict how well a given experiment can distinguish between
different models, regardless of their parameters. From a Bayesian viewpoint,
this involves computation of the Bayesian evidence. In this paper, we
generalise the Fisher matrix approach from the context of parameter fitting to
that of model testing, and show how the expected evidence can be computed under
the same simplifying assumption of a gaussian likelihood as the Fisher matrix
approach for parameter estimation. With this `Laplace approximation' all that
is needed to compute the expected evidence is the Fisher matrix itself. We
illustrate the method with a study of how well upcoming and planned experiments
should perform at distinguishing between Dark Energy models and modified
gravity theories. In particular we consider the combination of 3D weak lensing,
for which planned and proposed wide-field multi-band imaging surveys will
provide suitable data, and probes of the expansion history of the Universe,
such as proposed supernova and baryonic acoustic oscillations surveys. We find
that proposed large-scale weak lensing surveys from space should be able
readily to distinguish General Relativity from modified gravity models.Comment: 6 pages, 2 figure
Bayesian analysis of Friedmannless cosmologies
Assuming only a homogeneous and isotropic universe and using both the 'Gold'
Supernova Type Ia sample of Riess et al. and the results from the Supernova
Legacy Survey, we calculate the Bayesian evidence of a range of different
parameterizations of the deceleration parameter. We consider both spatially
flat and curved models. Our results show that although there is strong evidence
in the data for an accelerating universe, there is little evidence that the
deceleration parameter varies with redshift.Comment: 7 pages, 3 figure
Genotyping-by-sequencing resolves relationships in Polygonaceae tribe Eriogoneae
The resolution of cryptic diversity is essential for understanding the evolutionary diversification of lineages and establishing conservation priorities. We examine relationships in Eriogoneae (Polygonaceae), a diverse lineage in western North America. We ask whether Eriogonum umbellatum, a morphologically and ecologically diverse species, is monophyletic and whether its varieties represent evolutionary lineages. We use genotyping-by-sequencing to assemble a SNP dataset for 51 species in the genera Chorizanthe, Eriogonum and Sidotheca. We report a hierarchical phylogenetic analysis using maximum likelihood to estimate the evolutionary history of Eriogoneae. We illustrate admixture components for 21 populations of E. umbellatum, representing four varieties, and test for lineage structure using TreeMix. We identify strongly supported clades within Eriogoneae. Many relationships in the Eucycla + Oregonium and Latifolia clades are supported, while most relationships within the Eriogonum subg. Oligogonum clade and a clade with most Chorizanthe remain unresolved. Eriogonum congdonii resolves within the main E. umbellatum clade, while populations of three varieties of E. umbellatum are closely related to E. ursinum and are associated with serpentine soils. ADmixture and TreeMix analyses suggest E. umbellatum varieties represent evolutionary lineages. These results from SNP data are largely consistent with previous phylogenetic studies of Eriogoneae based on sequence variation. Structure within Oligogonum suggests consistent environmental association and radiation after initial colonization of serpentine. Morphology is unreliable for the infraspecific taxonomy of E. umbellatum. Additional molecular studies are needed to resolve the evolutionary relationships and ecological diversification within this species, in Oligogonum, and in Eriogoneae. © 2021 The Authors. TAXON published by John Wiley & Sons Ltd on behalf of International Association for Plant Taxonomy.We thank SGIker research support services at the University of the Basque Country, Leioa, Spain for DNA extraction and quality control, and acknowledge Centro Nacional de d-An?lisi Gen?mica in Barcelona, Spain for GBS sequencing. We thank J. Andre, N.J. Jensen and J. Steele for contributing samples and D. Marino for collaboration in nucleic acid extractions. We acknowledge the contributions of O. Lao Grueso to the?ADmixture and TreeMix analyses. We thank U.S. Forest Service employees D. Austin, J. Fedorchuk, M. Friend, J. Haas, D. Ikeda, L. Janeway, J. Nelson, D. Netz, A. Sanger, and S. Weis among others for facilitating permitting and collecting. This work was supported by funds from the Basque Government in support of the Terrestrial Plant Diversity group of the Department of Plant Biology and Ecology, University of the Basque Country, and an ERC Advanced Grant, FP7-IDEAS-ERC, ?ADAPT?, project 339941 awarded to T. Brown. We thank SGIker research support services at the University of the Basque Country, Leioa, Spain for DNA extraction and quality control, and acknowledge Centro Nacional de dâAnĂ lisi GenĂČmica in Barcelona, Spain for GBS sequencing. We thank J. Andre, N.J. Jensen and J. Steele for contributing samples and D. Marino for collaboration in nucleic acid extractions. We acknowledge the contributions of O. Lao Grueso to the ADmixture and TreeMix analyses. We thank U.S. Forest Service employees D. Austin, J. Fedorchuk, M. Friend, J. Haas, D. Ikeda, L. Janeway, J. Nelson, D. Netz, A. Sanger, and S. Weis among others for facilitating permitting and collecting. This work was supported by funds from the Basque Government in support of the Terrestrial Plant Diversity group of the Department of Plant Biology and Ecology, University of the Basque Country, and an ERC Advanced Grant, FP7âIDEASâERC, âADAPTâ, project 339941 awarded to T. Brown
The ecosystem carbon sink implications of mountain forest expansion into abandoned grazing land: The role of subsoil and climatic factors
Woody encroachment is a widespread phenomenon resulting from the abandonment of mountain agricultural and pastoral practices during the last century. As a result, forests have expanded, increasing biomass and necromass carbon (C) pools. However, the impact on soil organic carbon (SOC) is less clear. The main aim of this study was to investigate the effect of woody encroachment on SOC stocks and ecosystem C pools in six chronosequences located along the Italian peninsula, three in the Alps and three in the Apennines. Five stages along the chronosequences were identified in each site. Considering the topsoil (0 30 cm), subsoil (30 cm-bedrock) and whole soil profile, the temporal trend in SOC stocks was similar in all sites, with an initial increment and subsequent decrement in the intermediate phase. However, the final phase of the woody encroachment differed significantly between the Alps (mainly conifers) and the Apennines (broadleaf forests) sites, with a much more pronounced increment in the latter case. Compared to the previous pastures, after mature forest (>62 years old) establishment, SOC stocks increased by: 2.1(mean) ± 18.1(sd) and 50.1 ± 25.2 Mg C·ha -1 in the topsoil, 7.3 ± 17.4 and 93.2 ± 29.7 Mg C·ha -1 in the subsoil, and 9.4 ± 24.4 and 143.3 ± 51.0 Mg C·ha -1 in the whole soil profile in Alps and Apennines, respectively. Changes in SOC stocks increased with mean annual air temperature and average minimum winter temperature, and were negatively correlated with the sum of summer precipitation. At the same time, all other C pools (biomass and necromass) increased by 179.1 ± 51.3 and 304.2 ± 67.6 Mg C·ha -1 in the Alps and the Apennines sites, respectively. This study highlights the importance of considering both the subsoil, since deep soil layers contributed 38% to the observed variations in carbon stocks after land use change, and the possible repercussions for the carbon balance of large areas where forests are expanding, especially under pressing global warming scenarios. © 2019 Elsevier B.V.The project of this work is part of the research activities of the PhD in science, technology and biotechnology for sustainability. The first author received a fully founded scholarship partially by the University of Tuscia (Viterbo - Italy) and partially by the University of Molise . Tommaso Chiti participated in the project by conducting his work with the funding obtained through the LIFE MediNet project (grant number LIFE15 PRE IT/732295 ). Jorge Curiel Yuste was financed in part by the Basque Government through the BERC 2018-2021 (grant code) program and by Spanish Ministry of Economy and Competitiveness (MINECO) through BC3 MarĂa de Maeztu excellence accreditation MDM-2017-0714. I.C (grant code)
Engineering of quantum dot photon sources via electro-elastic fields
The possibility to generate and manipulate non-classical light using the
tools of mature semiconductor technology carries great promise for the
implementation of quantum communication science. This is indeed one of the main
driving forces behind ongoing research on the study of semiconductor quantum
dots. Often referred to as artificial atoms, quantum dots can generate single
and entangled photons on demand and, unlike their natural counterpart, can be
easily integrated into well-established optoelectronic devices. However, the
inherent random nature of the quantum dot growth processes results in a lack of
control of their emission properties. This represents a major roadblock towards
the exploitation of these quantum emitters in the foreseen applications. This
chapter describes a novel class of quantum dot devices that uses the combined
action of strain and electric fields to reshape the emission properties of
single quantum dots. The resulting electro-elastic fields allow for control of
emission and binding energies, charge states, and energy level splittings and
are suitable to correct for the quantum dot structural asymmetries that usually
prevent these semiconductor nanostructures from emitting polarization-entangled
photons. Key experiments in this field are presented and future directions are
discussed.Comment: to appear as a book chapter in a compilation "Engineering the
Atom-Photon Interaction" published by Springer in 2015, edited by A.
Predojevic and M. W. Mitchel
Observational signatures of Jordan-Brans-Dicke theories of gravity
We analyze the Jordan-Brans-Dicke model (JBD) of gravity, where deviations
from General Relativity (GR) are described by a scalar field non-minimally
coupled to gravity. The theory is characterized by a constant coupling
parameter, ; GR is recovered in the limit . In such theories, gravity modifications manifest at early times,
so that one cannot rely on the usual approach of looking for inconsistencies in
the expansion history and perturbations growth in order to discriminate between
JBD and GR. However, we show that a similar technique can be successfully
applied to early and late times observables instead. Cosmological parameters
inferred extrapolating early-time observations to the present will match those
recovered from direct late-time observations only if the correct gravity theory
is used. We use the primary CMB, as will be seen by the Planck satellite, as
the early-time observable; and forthcoming and planned Supernov{\ae}, Baryonic
Acoustic Oscillations and Weak Lensing experiments as late-time observables. We
find that detection of values of as large as 500 and 1000 is
within reach of the upcoming (2010) and next-generation (2020) experiments,
respectively.Comment: minor revision, references added, matching version published in JCA
Fusion reaction of halo nuclei: A real-time wave-packet method for three-body tunneling dynamics
We investigate fusion cross section of a nucleus with a valence neutron,
using the time-dependent wave-packet method. For a stable projectile, in which
the valence neutron is tightly bound (e_n < -3 MeV), the neutron could enhance
the fusion probability when the matching condition of orbital energies are
satisfied. In contrast, for a halo nucleus, in which the binding energy of the
neutron is very small (e_n>-1 MeV), the fusion probability is hindered by the
presence of the weakly bound neutron.Comment: Talk at Internaitonal Conference on "Reaction Mechanisms and Nuclear
Structure at the Coulomb Barrier" (FUSION06), Venice, Italy, March 19-23,
200
Constraints on cosmic-ray propagation models from a global Bayesian analysis
Research in many areas of modern physics such as, e.g., indirect searches for
dark matter and particle acceleration in SNR shocks, rely heavily on studies of
cosmic rays (CRs) and associated diffuse emissions (radio, microwave, X-rays,
gamma rays). While very detailed numerical models of CR propagation exist, a
quantitative statistical analysis of such models has been so far hampered by
the large computational effort that those models require. Although statistical
analyses have been carried out before using semi-analytical models (where the
computation is much faster), the evaluation of the results obtained from such
models is difficult, as they necessarily suffer from many simplifying
assumptions, The main objective of this paper is to present a working method
for a full Bayesian parameter estimation for a numerical CR propagation model.
For this study, we use the GALPROP code, the most advanced of its kind, that
uses astrophysical information, nuclear and particle data as input to
self-consistently predict CRs, gamma rays, synchrotron and other observables.
We demonstrate that a full Bayesian analysis is possible using nested sampling
and Markov Chain Monte Carlo methods (implemented in the SuperBayeS code)
despite the heavy computational demands of a numerical propagation code. The
best-fit values of parameters found in this analysis are in agreement with
previous, significantly simpler, studies also based on GALPROP.Comment: 19 figures, 3 tables, emulateapj.sty. A typo is fixed. To be
published in the Astrophysical Journal v.728 (February 10, 2011 issue).
Supplementary material can be found at
http://www.g-vo.org/pub/GALPROP/GalpropBayesPaper
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