Premartensite to martensite transition and its implications on the origin of modulation in Ni2MnGa ferromagnetic shape memory alloy

We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector q shows smooth analytic behaviour with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phases. The existence of the incommensurate 7M like modulated structure down to 5K suggests that the incommensurate 7M like modulation is the ground state of Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model

Grain Physics and Rosseland Mean Opacities

Tables of mean opacities are often used to compute the transfer of radiation in a variety of astrophysical simulations from stellar evolution models to proto-planetary disks. Often tables, such as Ferguson et al. (2005), are computed with a predetermined set of physical assumptions that may or may not be valid for a specific application. This paper explores the effects of several assumptions of grain physics on the Rosseland mean opacity in an oxygen rich environment. We find that changing the distribution of grain sizes, either the power-law exponent or the shape of the distribution, has a marginal effect on the total mean opacity. We also explore the difference in the mean opacity between solid homogenous grains and grains that are porous or conglomorations of several species. Changing the amount of grain opacity included in the mean by assuming a grain-to-gas ratio significantly affects the mean opacity, but in a predictable way.Comment: 19 pages, 6 figures, accepted for publication in Ap

Direct Imaging of Multiple Planets Orbiting the Star HR 8799

Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step towards imaging Earth-like planets. Imaging detections are challenging due to the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter-clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our Solar System.Comment: 30 pages, 5 figures, Research Article published online in Science Express Nov 13th, 200

Picosecond magnetization dynamics in nanomagnets: Crossover to nonuniform precession

Copyright © 2005 The American Physical SocietyThe picosecond magnetization dynamics of arrays of square Ni88Fe12∕Co80Fe20 bilayer nanoelements were studied by optical pump-probe measurements. Experimentally observed modes were found to fall upon two branches, with a crossover from the high- to low-frequency regime as the element size was reduced to less than 220 nm. Micromagnetic simulations revealed that the branches are associated with center and edge modes. The edge mode is found to dominate as the element size is reduced so that the magnetic response to a pulsed field becomes less spatially uniform

Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574

While following up L dwarf candidates selected photometrically from the Two Micron All Sky Survey, we uncovered an unusual object designated 2MASS J01415823-4633574. Its optical spectrum exhibits very strong bands of vanadium oxide but abnormally weak absorptions by titanium oxide, potassium, and sodium. Morphologically such spectroscopic characteristics fall intermediate between old, field early-L dwarfs (log(g)~5) and very late M giants (log(g)~0), leading us to favor low gravity as the explanation for the unique spectral signatures of this L dwarf. Such a low gravity can be explained only if this L dwarf is much lower in mass than a typical old field L dwarf of similar temperature and is still contracting to its final radius. These conditions imply a very young age. Further evidence of youth is found in the near-infrared spectrum, including a triangular-shaped H-band continuum reminiscent of young brown dwarf candidates discovered in the Orion Nebula Cluster. Using the above information along with comparisons to brown dwarf atmospheric and interior models, our current best estimate is that this L dwarf has an age of 1-50 Myr and a mass of 6-25 M_Jupiter. The location of 2MASS 0141-4633 on the sky coupled with a distance estimate of ~35 pc and the above age estimate suggests that this object may be a brown dwarf member of either the 30-Myr-old Tucana/Horologium Association or the ~12-Myr-old beta Pic Moving Group.Comment: Accepted for publication in the 10 March 2006 issue (volume 639) of the Astrophysical Journa

Characterizing exoplanetary atmospheres through infrared polarimetry

Planets can emit polarized thermal radiation, just like brown dwarfs. We present calculated thermal polarization signals from hot exoplanets, using an advanced radiative transfer code that fully includes all orders of scattering by gaseous molecules and cloud particles. The code spatially resolves the disk of the planet, allowing simulations for horizontally inhomogeneous planets. Our results show that the degree of linear polarization, P, of an exoplanet's thermal radiation is expected to be highest near the planet's limb and that this P depends on the temperature and its gradient, the scattering properties and the distribution of the cloud particles. Integrated over the disk of a spherically symmetric planet, P of the thermal radiation equals zero. However, for planets that appear spherically asymmetric, e.g. due to flattening, cloud bands or spots in their atmosphere, differences in their day and night sides, and/or obscuring rings, P is often larger than 0.1 %, in favorable cases even reaching several percent at near-infrared wavelengths. Detection of thermal polarization signals can give access to planetary parameters that are otherwise hard to obtain: it immediately confirms the presence of clouds, and P can then constrain atmospheric inhomogeneities and the flattening due to the planet's rotation rate. For zonally symmetric planets, the angle of polarization will yield the components of the planet's spin axis normal to the line-of-sight. Finally, our simulations show that P is generally more sensitive to variability in a cloudy planet's atmosphere than the thermal flux is, and could hence better reveal certain dynamical processes.Comment: 9 pages, 5 figures, accepted for publication in Ap

Limits on the 2.2-μm contrast ratio of the close-orbiting planet HD 189733b

We obtained 238 spectra of the close-orbiting extrasolar giant planet HD 189733b with resolution R∼ 15 000 during one night of observations with the Near-Infrared High-Resolution Spectrograph (NIRSPEC), at the Keck II Telescope. We have searched for planetary absorption signatures in the 2.0-2.4 μm region where H2O and CO are expected to be the dominant atmospheric opacities. We employ a phase-dependent orbital model and tomographic techniques to search for the planetary absorption signatures in the combined stellar and planetary spectra. Because potential absorption signatures are hidden in the noise of each single exposure, we use a model list of lines to apply a spectral deconvolution. The resulting mean profile possesses a signal-to-noise ratio (S/N) that is 20 times greater than that found in individual lines. Our spectral time series thus yields spectral signatures with a mean S/N = 2720. We are unable to detect a planetary signature at a contrast ratio of log10(Fp/F*) =−3.40, with 63.8 per cent confidence. Our findings are not consistent with model predictions which nevertheless give a good fit to mid-infrared observations of HD 189733b. The 1σ result is a factor of 1.7 times less than the predicted 2.185-μm planet/star flux ratio of log10(Fp/F*) ∼−3.1