8,097 research outputs found
Possible climates on terrestrial exoplanets
What kind of environment may exist on terrestrial planets around other stars?
In spite of the lack of direct observations, it may not be premature to
speculate on exoplanetary climates, for instance to optimize future telescopic
observations, or to assess the probability of habitable worlds. To first order,
climate primarily depends on 1) The atmospheric composition and the volatile
inventory; 2) The incident stellar flux; 3) The tidal evolution of the
planetary spin, which can notably lock a planet with a permanent night side.
The atmospheric composition and mass depends on complex processes which are
difficult to model: origins of volatile, atmospheric escape, geochemistry,
photochemistry. We discuss physical constraints which can help us to speculate
on the possible type of atmosphere, depending on the planet size, its final
distance for its star and the star type. Assuming that the atmosphere is known,
the possible climates can be explored using Global Climate Models analogous to
the ones developed to simulate the Earth as well as the other telluric
atmospheres in the solar system. Our experience with Mars, Titan and Venus
suggests that realistic climate simulators can be developed by combining
components like a "dynamical core", a radiative transfer solver, a
parametrisation of subgrid-scale turbulence and convection, a thermal ground
model, and a volatile phase change code. On this basis, we can aspire to build
reliable climate predictors for exoplanets. However, whatever the accuracy of
the models, predicting the actual climate regime on a specific planet will
remain challenging because climate systems are affected by strong positive
destabilizing feedbacks (such as runaway glaciations and runaway greenhouse
effect). They can drive planets with very similar forcing and volatile
inventory to completely different states.Comment: In press, Proceedings of the Royal Society A 31 pages, 6 figure
Comment on " Gain coefficient method for amplified spontaneous emission in thin waveguided film of a conjugated polymer " [APL 93, 163307 (2008)]
Comment on " Gain coefficient method for amplified spontaneous emission in
thin waveguided film of a conjugated polymer " [APL 93, 163307 (2008)
Longitudinal magnetoresistance in Co-doped BaFe2As2 and LiFeAs single crystals: Interplay between spin fluctuations and charge transport in iron-pnictides
The longitudinal in-plane magnetoresistance (LMR) has been measured in
different Ba(Fe_(1-x)Co_x)2As2 single crystals and in LiFeAs. For all these
compounds, we find a negative LMR in the paramagnetic phase whose magnitude
increases as H^2. We show that this negative LMR can be readily explained in
terms of suppression of the spin fluctuations by the magnetic field. In the
Co-doped samples, the absolute value of the LMR coefficient is found to
decrease with doping content in the paramagnetic phase. The analysis of its T
dependence in an itinerant nearly antiferromagnetic Fermi liquid model
evidences that the LMR displays a qualitative change of T variation with
increasing Co content. The latter occurs at optimal doping for which the
antiferromagnetic ground state is suppressed. The same type of analysis for the
negative LMR measured in LiFeAs suggests that this compound is on the verge of
magnetism.Comment: 6 pages, 6 figure
Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields
Electric polarization loops are measured at room temperature on highly pure
BiFeO3 single crystals synthesized by a flux growth method. Because the
crystals have a high electrical resistivity, the resulting low leakage currents
allow us to measure a large spontaneous polarization reaching 100
microC.cm^{-2}, a value never reported in the bulk. During electric cycling,
the slow degradation of the material leads to an evolution of the hysteresis
curves eventually preventing full saturation of the crystals.Comment: 8 pages, 3 figure
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