Comparison between polar regions of Mars from HEND/Odyssey data

In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO₂ seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO₂ deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×¹⁰¹⁵ kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO₂ occurred 10-15 degrees of Ls later than is predicted by the GCM.     

Beam Mode Expansion of Corrugated Conical Horns with Phase Correcting Lens: Application to Radioastronomy Receivers

A classical radioastronomy receiver is fed with a corrugated horn and an independent lens, both placed in a cryostat to lower the noise temperature. The beam is focused and directed using a combination of elliptical and plane mirrors. This paper proposes modifying the initial feeding system by placing the lens onto the horn aperture, thereby allowing a size reduction of the horn and lens, and a simplification of their mechanical design. The profiled lens is shaped to correct the phase error on the horn aperture. A quasi-optical model of the horn-plus-lens system has been developed using a Beam Mode Expansion (BME). Results using both a hyperbolic-planar lens and a spherical-elliptical lens, as well as results obtained by using Geometrical Optics (GO) with a Kirchoff–Huygens integration to get the far-field pattern, have been compared with measurements. As a direct application, a full focusing system for the new 40-m radiotelescope at the “Centro Astronómico de Yebes” is presented for the 22, 30 and 45 GHz bands. This paper has developed a QO model for a corrugated conical horn with a phase-correcting lens. This revised version was published online in July 2006 with corrections to the Cover Date.     

Searching for an intrinsic stellar population in compact high-velocity clouds

We are investigating the hypothesis that Compact High-Velocity Clouds (CHVCs) are the left-over building blocks of Local Group galaxies. To this end, we are searching for their embedded stellar populations using FORS at the VLT. The search is done with single-star photometry in V and I bands, which is sensitive to both, young and old, stellar populations. Five CHVCs of our sample have been observed so far down to I=24. We pointed the VLT towards the highest HI column density regions, as determined in Effelsberg radio data. In an alternate approach, we searched 2MASS public data towards those 5 CHVCs down to K=16. While the VLT data probe the central regions out to distance moduli of about 27, the 2MASS data are sensitive to a population of red giant stars to distance moduli of about 20. The 2MASS data, on the other hand, cover a much wider field of view than the VLT data (radius of 1 degree versus FORS field of 6.8 arcmin). We did not find a stellar population intrinsic to the CHVCs in either data. In this paper, we illustrate our search methods. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quintessential inflation models with a nonminimally coupled scalar field

The evolution of a homogeneous, isotropic cosmological model driven by a nonminimally coupled scalar field is studied. The potential for the quintessential inflation model proposed by Peebles and Vilenkin is selected as a scalar potential. Possible scenarios for the cosmological dynamics are described in the conformal Einstein and Jordan representations. It is shown that, unlike in models with a minimal scalar field, here a class of solutions exists for which the scalar field is fixed at finite values during cosmological expansion. __________ Translated from Astrofizika, Vol. 49, No. 3, pp. 487–498 (August 2006).     

On the influence of magnetic fields on level populations

The influence of magnetic fields on the energy level populations of atoms is firstly studied by analysing the Stokes profiles of Fe  i 6302.5 forming in the solar magnetized atmosphere, with the aid of a departure factor defined to evaluate the deviation from the normal Boltzmann distribution without a magnetic field. This factor is directly related to the ratio of line-source function to the continuum one. The relationship between the departure and the magnetic field reveals an effect that the magnetic field induces an exponential increase in the level population of the lower level of Fe  i 6302.5 (Landé factor   g = 2.5  ) with the field strength. This indicates that the magnetic field can cause the redistribution of populations of those levels whose Landé factors are non-zero. Therefore, this effect should be included in the calculation of the statistical equilibrium. Secondly, an experiment utilizing the Hg5461 line in the laboratory on the Earth is carried out to reveal that the exponential relation is independent of variations in temperature, and the excitation is completely magneto-induced. Finally, the exponential relation is explained by taking account of the magnetic energy in the Boltzmann distribution.     

Radiation Products in Processed Ices Relevant to Edgeworth-Kuiper-Belt Objects

Near the inner edge of the Edgeworth-Kuiper Belt (EKB) are Pluto and Charon, which are known to have N₂- and H₂O-dominated surface ices, respectively. Such non-polar and polar ices, and perhaps mixtures of them, also may be present on other trans-Neptunian objects. Pluto, Charon, and all EKB objects reside in a weak, but constant UV-photon and energetic ion radiation environment that drives chemical reactions in their surface ices. Effects of photon and ion processing include changes in ice composition, volatility, spectra, and albedo, and these have been studied in a number of laboratories. This paper focuses on ice processing by ion irradiation and is aimed at understanding the volatiles, ions, and residues that may exist on outer solar system objects. We summarize radiation chemical products of N₂-rich and H₂O-rich ices containing CO or CH₄, including possible volatiles such as alcohols, acids, and bases. Less-volatile products that could accumulate on EKB objects are observed to form in the laboratory from acid-base reactions, reactions promoted by warming, or reactions due to radiation processing of a relatively pure ice (e.g., CO → C₃O₂). New IR spectra are reported for the 1–5 mu;m region, along with band strengths for the stronger features of carbon suboxide, carbonic acid, the ammonium and cyanate ions, polyoxymethylene, and ethylene glycol. These six materials are possible contributors to EKB surfaces, and will be of interest to observers and future missions.