9,449 research outputs found
Exponential Mixing for a Stochastic PDE Driven by Degenerate Noise
We study stochastic partial differential equations of the reaction-diffusion
type. We show that, even if the forcing is very degenerate (i.e. has not full
rank), one has exponential convergence towards the invariant measure. The
convergence takes place in the topology induced by a weighted variation norm
and uses a kind of (uniform) Doeblin condition.Comment: 10 pages, 1 figur
Chemical evolution of the metal poor Globular Cluster NGC 6809
We present the abundances analysis for a sample of 11 red giant branch stars
in the metal-poor globular cluster NGC 6809 based on high-resolution spectra.
Our main goals are to characterize its chemical composition and analyze this
cluster's behavior associated with the Multiple Population (MPs) phenomenon. In
our work we obtained the stellar parameters and chemical abundances of 24
elements (O, Na, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Sc, Ni, Cu, Zn, Y, Zr,
Ba, La, Ce, Eu, Nd and Dy). We found a radial velocity of 174.7 3.2 km
and a mean iron content of [Fe/H]=-2.01 0.02 in good agreement
with other studies. Moreover, we found a large spread in the abundances of the
light elements O, Na and Al confirming the presence of a Na-O anti-correlation
a Na-Al correlation. The Mg-Al anti-correlation is also present in our cluster.
The and iron-peak elements show good agreement with the halo field
star trend. The heavy elements are dominated by the r-process.Comment: 13 pages, 11 figures, 3 tables, accepted for publication in MNRA
Game-theoretic analysis of development practices: Challenges and opportunities
Developers continuously invent new practices, usually grounded in hard-won experience, not theory. Game theory studies cooperation and conflict; its use will speed the development of effective processes. A survey of game theory in software engineering finds highly idealised models that are rarely based on process data. This is because software processes are hard to analyse using traditional game theory since they generate huge game models. We are the first to show how to use game abstractions, developed in artificial intelligence, to produce tractable game-theoretic models of software practices. We present Game-Theoretic Process Improvement (GTPI), built on top of empirical game-theoretic analysis. Some teams fall into the habit of preferring âquick-and-dirtyâ code to slow-to-write, careful code, incurring technical debt. We showcase GTPIâs ability to diagnose and improve such a development process. Using GTPI, we discover a lightweight intervention that incentivises developers to write careful code: add a singlecode reviewer who needs to catch only 25% of kludges. This 25% accuracy is key; it means that a reviewer does not need to examine each commit in depth, making this process intervention cost-effective
A submillimeter search for pre- and proto-brown dwarfs in Chamaeleon II
Context. Chamaeleon II molecular cloud is an active star forming region that
offers an excellent opportunity for studying the formation of brown dwarfs in
the southern hemisphere. Aims. Our aims are to identify a population of pre-
and proto- brown dwarfs (5 sigma mass limit threshold of ~0.015 Msun) and
provide information on the formation mechanisms of substellar objects. Methods.
We performed high sensitivity observations at 870 microns using the LABOCA
bolometer at the APEX telescope towards an active star forming region in
Chamaeleon II. The data are complemented with an extensive multiwavelength
catalogue of sources from the optical to the far-infrared to study the nature
of the LABOCA detections. Results. We detect fifteen cores at 870 microns, and
eleven of them show masses in the substellar regime. The most intense objects
in the surveyed field correspond to the submillimeter counterparts of the well
known young stellar objects DK Cha and IRAS 12500-7658. We identify a possible
proto-brown dwarf candidate (ChaII-APEX-L) with IRAC emission at 3.6 and 4.5
microns. Conclusions. Our analysis indicates that most of the spatially
resolved cores are transient, and that the point-like starless cores in the
sub-stellar regime (with masses between 0.016 Msun and 0.066 Msun) could be
pre-brown dwarfs cores gravitationally unstable if they have radii smaller than
220 AU to 907 AU (1.2" to 5" at 178 pc) respectively for different masses. ALMA
observations will be the key to reveal the energetic state of these pre-brown
dwarfs candidates.Comment: 11 pages, 6 figure
Electrical transport and optical studies of ferromagnetic Cobalt doped ZnO nanoparticles exhibiting a metal-insulator transition
The observed correlation of oxygen vacancies and room temperature
ferromagnetic ordering in Co doped ZnO1-o nanoparticles reported earlier (Naeem
et al Nanotechnology 17, 2675-2680) has been further explored by transport and
optical measurements. In these particles room temperature ferromagnetic
ordering had been observed to occur only after annealing in forming gas. In the
current work the optical properties have been studied by diffuse reflection
spectroscopy in the UV-Vis region and the band gap of the Co doped compositions
has been found to decrease with Co addition. Reflections minima are observed at
the energies characteristic of Co+2 d-d (tethrahedral symmetry) crystal field
transitions, further establishing the presence of Co in substitutional sites.
Electrical transport measurements on palletized samples of the nanoparticles
show that the effect of a forming gas is to strongly decrease the resistivity
with increasing Co concentration. For the air annealed and non-ferromagnetic
samples the variation in the resistivity as a function of Co content are
opposite to those observed in the particles prepared in forming gas. The
ferromagnetic samples exhibit an apparent change from insulator to metal with
increasing temperatures for T>380K and this change becomes more pronounced with
increasing Co content. The magnetic and resistive behaviors are correlated by
considering the model by Calderon et al [M. J. Calderon and S. D. Sarma, Annals
of Physics 2007 (Accepted doi: 10.1016/j.aop.2007.01.010] where the
ferromagnetism changes from being mediated by polarons in the low temperature
insulating region to being mediated by the carriers released from the weakly
bound states in the higher temperature metallic region.Comment: 7 pages, 6 figure
Short Communication. Physiological effects of Rhizopogon Roseolus on Pinus halepensis seedlings
Aim of study: The inoculation of forest seedlings with ectomycorrhizal fungi can improve the morphological and physiological qualities of plants, especially those used for regeneration of arid areas. Rhizopogon roseolus is an ectomycorrhizal fungus (ECM) commonly used for reforestation. In this study, the specific objectives were to know some morphophysiological effects of Rhizopogon Roseolus on Pinus halepensis seedlings under standard nursery conditionsArea of study: ETSI Montes and EUIT Forestal, Madrid.Material and Methods: In nursery, under well watered conditions and peat growing substrates, Aleppo pine seedlings were inoculated with R. roseolus. Five months after the inoculations, we examined the growth, water parameters (osmotic potential at full turgor [ΚÏfull], osmotic potential at zero turgor [ΚÏ0], and the tissue modulus of elasticity near full turgor [Emax]), mycorrhizal colonization, and concentration and content of macronutrients in the seedlings. Subsequently, a trial was conducted to assess the root growth potential.Main results: The mycorrhization decreased the height and diameter of mycorrhizal seedlings but increased the root weight and root branching. R. roseolus did not cause any significant effect on the regeneration of new roots or on any of the tested hydric parameters, but it did improve N uptake of the seedlings.Research highlights: The mycorrhizal inoculation increased the N uptake. The mycorrhizal inoculation caused opposite effects on some growth parametersKeywords: Osmotic adjustment; elastic adjustment; mineral nutrition; root growth potential; nursery; Rhizopogon roseolus;  Pinus halepensis.
Spitzer Infrared Spectrograph Observations of Magellanic Cloud Planetary Nebulae: the nature of dust in low metallicity circumstellar ejecta
We present 5 - 40 micron spectroscopy of 41 planetary nebulae (PNe) in the
Magellanic Clouds, observed with the Infrared Spectrograph on board the Spitzer
Space Telescope. The spectra show the presence of a combination of nebular
emission lines and solid-state features from dust, superimposed on the thermal
IR continuum.
By analyzing the 25 LMC and 16 SMC PNe in our sample we found that the IR
spectra of 14 LMC and 4 SMC PNe are dominated by nebular emission lines, while
the other spectra show solid-state features. We observed that the solid-state
features are compatible with carbon-rich dust grains (SiC, polycyclic aromatic
hydrocarbons (PAHs), etc.) in most cases, except in three PNe showing
oxygen-rich dust features. The frequency of carbonaceous dust features is
generally higher in LMC than in SMC PNe.
The spectral analysis allowed the correlations of the dust characteristics
with the gas composition and morphology, and the properties of the central
stars. We found that: 1) all PNe with carbonaceous dust features have C/O>1,
none of these being bipolar or otherwise highly asymmetric; 2) all PNe with
oxygen-rich dust features have C/O<1, with probable high mass progenitors if
derived from single-star evolution (these PNe are either bipolar or highly
asymmetric); 3) the dust temperature tracks the nebular and stellar evolution;
and 4) the dust production efficiency depends on metallicity, with low
metallicity environments not favoring dust production.Comment: The Astrophysical Journal, in pres
Panchromatic observations and modeling of the HV Tau C edge-on disk
We present new high spatial resolution (<~ 0.1") 1-5 micron adaptive optics
images, interferometric 1.3 mm continuum and 12CO 2-1 maps, and 350 micron, 2.8
and 3.3 mm fluxes measurements of the HV Tau system. Our adaptive optics images
reveal an unusually slow orbital motion within the tight HV Tau AB pair that
suggests a highly eccentric orbit and/or a large deprojected physical
separation. Scattered light images of the HV Tau C edge-on protoplanetary disk
suggest that the anisotropy of the dust scattering phase function is almost
independent of wavelength from 0.8 to 5 micron, whereas the dust opacity
decreases significantly over the same range. The images further reveal a marked
lateral asymmetry in the disk that does not vary over a timescale of 2 years.
We further detect a radial velocity gradient in the disk in our 12CO map that
lies along the same position angle as the elongation of the continuum emission,
which is consistent with Keplerian rotation around an 0.5-1 Msun central star,
suggesting that it could be the most massive component in the triple system. We
use a powerful radiative transfer model to compute synthetic disk observations
and use a Bayesian inference method to extract constraints on the disk
properties. Each individual image, as well as the spectral energy distribution,
of HV Tau C can be well reproduced by our models with fully mixed dust provided
grain growth has already produced larger-than-interstellar dust grains.
However, no single model can satisfactorily simultaneously account for all
observations. We suggest that future attempts to model this source include more
complex dust properties and possibly vertical stratification. (Abridged)Comment: 26 pages, 11 figures, editorially accepted for publication in Ap
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