212 research outputs found
Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry
The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing
PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan
has been a subject of controversy for a long time. An analysis of the
atmospheric emission band observed by the Visible and Infrared Mapping
Spectrometer (VIMS) at 3.28 micrometer suggests the presence of neutral
polycyclic aromatic species in the upper atmosphere of Titan. These molecules
are seen as the counter part of negative and positive aromatics ions suspected
by the Plasma Spectrometer onboard the Cassini spacecraft, but the low
resolution of the instrument hinders any molecular speciation.
In this work we investigate the specific aromatic content of Titan's
atmospheric aerosols through laboratory simulations. We report here the
selective detection of aromatic compounds in tholins, Titan's aerosol
analogues, produced with a capacitively coupled plasma in a N2:CH4 95:5 gas
mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight
Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced
analogues. This analytical technique is based on the ionization of molecules by
Resonance Enhanced Multi-Photon Ionization (REMPI) using a {\lambda}=248 nm
wavelength laser which is selective for aromatic species. This allows for the
selective identification of compounds having at least one aromatic ring. Our
experiments show that tholins contain a trace amount of small PAHs with one to
three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as
constituents of tholins. Molecules relevant to astrobiology are detected as is
the case of the substituted DNA base adenine
Nonlinear Absorptions
AbstractLet (Ω,Bμ) be a σ-finite measure space, A be an m-completely accretive operator in L1(Ω) (A is m-accretive with resolvant Jλ= (I+λA)−1 satisfying u≤û+k ⇒Jλ≤Jλû + k for k≥0), and j: Ω × R → [0,∞] be measurable in x ∈ Ω. convex, and l.s.c. in r ∈ R with j(x, 0) = 0. We consider the operator A + B where B is the operator in L1(Ω) defined by w ∈ Bu iff j(r) ≥j(u(x)) + (r − u(x)) w(x) for any r ∈ R, a.e. x ∈ Ω.We define natural m-completely accretive extensions of A + B in L1(Ω) and study their dependence with respect to j and different cases where A + B is itself m-completely accretive; we consider also the evolution problem du/dt + Au + Bu ∋ ƒ, u(0)= u0
Probabilistic representation for solutions of an irregular porous media type equation: the degenerate case
We consider a possibly degenerate porous media type equation over all of
with , with monotone discontinuous coefficients with linear
growth and prove a probabilistic representation of its solution in terms of an
associated microscopic diffusion. This equation is motivated by some singular
behaviour arising in complex self-organized critical systems. The main idea
consists in approximating the equation by equations with monotone
non-degenerate coefficients and deriving some new analytical properties of the
solution
Existence results for a Cauchy-Dirichlet parabolic problem with a repulsive gradient term
We study the existence of solutions of a nonlinear parabolic problem of
Cauchy-Dirichlet type having a lower order term which depends on the gradient.
The model we have in mind is the following: \begin{cases}\begin{split} &
u_t-\text{div}(A(t,x)\nabla u|\nabla u|^{p-2})=\gamma |\nabla u|^q+f(t,x)
&\qquad\text{in } Q_T,\\ & u=0 &\qquad\text{on }(0,T)\times \partial \Omega,\\
& u(0,x)=u_0(x) &\qquad\text{in } \Omega, \end{split}\end{cases} where
, is a bounded domain of , , , the matrix is coercive and with measurable bounded
coefficients, the r.h.s. growth rate satisfies the superlinearity condition and the initial
datum is an unbounded function belonging to a suitable Lebesgue space
. We point out that, once we have fixed , there exists a
link between this growth rate and exponent which allows
one to have (or not) an existence result. Moreover, the value of deeply
influences the notion of solution we can ask for. The sublinear growth case
with is dealt at the end of the paper for what concerns
small value of , namely
A new infrared band in the Interstellar and Circumstellar Clouds: C_4 or C_4H?
We report on the detection with the Infrared Space Observatory (ISO) of a
molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and
in Sgr B2. Taking into account the chemistry of these objects the most
likelihood carrier is a carbon chain. We tentatively assign the band to the
nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been
derived in matrix experiments (Withey et al. 1991). An alternate carrier might
be C_4H, although the frequency of its lowest energy vibrational bending mode,
nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived
maximum abundance is nearly similar to that found for C_3 in the interstellar
and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence,
tetra-atomic carbon could be one of the most abundant carbon chain molecules in
these media.Comment: 11 pages, 1 figure, accepted in ApJ Letter
Existence of weak solutions for the generalized Navier-Stokes equations with damping
In this work we consider the generalized Navier-Stokes equations with the presence of a damping term in the momentum equation. The problem studied here derives from the set of equations which govern isothermal flows of incompressible and homogeneous non-Newtonian fluids. For the generalized Navier-Stokes problem with damping, we prove the existence of weak solutions by using regularization techniques, the theory of monotone operators and compactness arguments together with the local decomposition of the pressure and the Lipschitz-truncation method. The existence result proved here holds for any and any sigma > 1, where q is the exponent of the diffusion term and sigma is the exponent which characterizes the damping term.MCTES, Portugal [SFRH/BSAB/1058/2010]; FCT, Portugal [PTDC/MAT/110613/2010]info:eu-repo/semantics/publishedVersio
A kinetic formulation for multidimensional scalar conservation laws with boundary conditions and applications
International audienceWe state a kinetic formulation of weak entropy solutions of a general multidimensional scalar conservation law with initial and boundary conditions. We first associate with any weak entropy solution a entropy defect measure; the analysis of this measure at the boundary of the domain relies on the study of weak entropy sub and supersolutions and implies the introduction of the notion of sided boundary defect measures. As a first application, we prove that any weak entropy subsolution of the initial-boundary value problem is bounded above by any weak entropy supersolution (Comparison Theorem). We next study a BGK-like kinetic model that approximates the scalar conservation law. We prove that such a model converges by adapting the proof of the Comparison Theorem
Mathematical analysis of a model of river channel formation.
The study of overland flow of water over an erodible sediment leads to a coupled model describing the evolution of the topographic elevation and the depth of the overland water film. The spatially uniform solution of this model is unstable, and this instability corresponds to the formation of rills, which in reality then grow and coalesce to form large-scale river channels. In this paper we consider the deduction and mathematical analysis of a deterministic model describing river channel formation and the evolution of its depth. The model involves a degenerate nonlinear parabolic equation (satisfied on the interior of the support of the solution) with a super-linear source term and a prescribed constant mass. We propose here a global formulation of the problem (formulated in the whole space, beyond the support of the solution) which allows us to show the existence of a solution and leads to a suitable numerical scheme for its approximation. A particular novelty of the model is that the evolving channel self-determines its own width, without the need to pose any extra conditions at the channel margin
The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres
We are now on a clear trajectory for improvements in exoplanet observations
that will revolutionize our ability to characterize their atmospheric
structure, composition, and circulation, from gas giants to rocky planets.
However, exoplanet atmospheric models capable of interpreting the upcoming
observations are often limited by insufficiencies in the laboratory and
theoretical data that serve as critical inputs to atmospheric physical and
chemical tools. Here we provide an up-to-date and condensed description of
areas where laboratory and/or ab initio investigations could fill critical gaps
in our ability to model exoplanet atmospheric opacities, clouds, and chemistry,
building off a larger 2016 white paper, and endorsed by the NAS Exoplanet
Science Strategy report. Now is the ideal time for progress in these areas, but
this progress requires better access to, understanding of, and training in the
production of spectroscopic data as well as a better insight into chemical
reaction kinetics both thermal and radiation-induced at a broad range of
temperatures. Given that most published efforts have emphasized relatively
Earth-like conditions, we can expect significant and enlightening discoveries
as emphasis moves to the exotic atmospheres of exoplanets.Comment: Submitted as an Astro2020 Science White Pape
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