Auroral ion velocity distributions using a relaxation model

For application to the auroral ionosphere we have calculated ion velocity distributions for a weakly-ionized plasma subjected to crossed electric and magnetic fields. By replacing the Boltzmann collision integral with a simple relaxation model, we have been able to obtain an exact solution to Boltzmann's equation. This solution has the advantage over a series expansion in that all the higher order velocity moments are inherent in it. The exact solution is particularly advantageous when studying large departures of the distribution from its Maxwellian form because these departures are caused by the higher velocity moments. In general, however, a simple relaxation model can only be used to obtain qualitative information on the distribution function. Consequently, we can determine when the higher order velocity moments affect the ion velocity distribution and the nature of their effect, but we cannot obtain accurate quantitative results. The higher velocity moments have their greatest effect on the distribution function above about 120 km, where the ion-neutral collision frequency is less than the ion cyclotron frequency. As the magnitude of the electric field increases, these higher moments act to decrease the number of ions at the peak of the distribution function. Peak densities are reduced by a few per cent for perpendicular electric fields of about 20 mV m^?1.     

Auroral ion velocity distribution function: Generalized polynomial solution of Boltzmann's equation

The ion distribution function is calculated for the E and the F regions of the auroral latitudes. In these regions the plasma is weakly ionized and there exist convective electric fields which may attain very significant intensities. Boltzmann's equation is solved in the limit where the ion-neutral collision frequency is much lower than the ion gyrofrequency. This solution is obtained in the form of a generalized polynomial series expansion starting from a good zeroth-order approximation. With this weight function, and considering a development to the order of the 32 moments, good approximations are obtained for high electric fields. The resonant charge exchange model and the polarization model are successively considered. The Post-Rosenbluth instability threshold is discussed for the above two models.     

Auroral ion velocity distributions for a polarization collision model

We have calculated the effect that convection electric fields have on the velocity distribution of auroral ions at the altitudes where the plasma is weakly-ionized and where the various ion-neutral collision frequencies are much smaller than the ion cyclotron frequencies, i.e. between about 130 and 300 km. The appropriate Boltzmann equation has been solved by expanding the ion velocity distribution function in a generalized orthogonal polynomial series about a bi-Maxwellian weight factor. We have retained enough terms in the series expansion to enable us to obtain reliable quantitative results for electric field strengths as large as 90 mV m^?1. Although we have considered a range of ion-neutral scattering mechanisms, our main emphasis has been devoted to the long-range polarization interaction. In general, we have found that to lowest order the ion velocity distribution is better represented by a two-temperature or bi-Maxwellian distribution than by a one-temperature Maxwellian, with there being different ion temperatures parallel and perpendicular to the geomagnetic field. However, the departures from this zeroth-order bi-Maxwellian distribution become significant when the ion drift velocity approaches (or exceeds) the neutral thermal speed.     

The velocity field of UGC 6697 revisited

The edge-on galaxy UGC 6697 has a peculiar morphological appearance: a high surface brightness SE component and what appears to be a tidal tail with blue color and low surface brightness extending toward the NW, as dramatically evidenced in subarcsecond broad band images taken with the VLT and in a deep Hα frame. Long-slit spectroscopy shows a 500 km/s rigid-body rotation curve with a sudden central velocity jump, of more than 200 km/s amplitude. Fabry-Perot observations of the Hα emission line enable us to trace a detailed 2D velocity field, confirming the complex kinematical behaviour, particularly in the circumnuclear region where multiple velocity components are clearly superposed on the same line of sight. This region hosts a double nucleus and shows sudden color and metallicity gradients, which can be reconciled with the complex velocity field assuming the presence of a second galaxy hidden behind the main body of UGC 6697. This revised version was published online in September 2006 with corrections to the Cover Date.     

The effect of a non-maxwellian electron velocity distribution on Be-like ion diagnostics in the Sun

Non-Maxwellian collision rates of the Be-sequence ions Ciii and Ov in the solar transition region are derived for quiet Sun and flare conditions using the electron velocity distribution functions of Shoub. The rates are found to be enhanced relative to the Maxwellian values but only at low temperatures where the fractional abundances of these species is very small. This implies that the electron density and temperature diagnostics used by previous authors for the transition region will be unchanged by non-Maxwellian effects. It is noted that such effects will only be important for species that are formed at low temperatures and have large transition energies such as Hei.     

Behaviour of ion velocity distributions for a simple collision model

For application to studies of the high latitude ionosphere, we have calculated ion velocity distributions for a weekly-ionized plasma subjected to crossed electric and magnetic fields. An exact solution to Boltzmann's equation has been obtained by replacing the Boltzmann collision integral with a simple relaxation model. At altitudes above about 150 km, where the ion collision frequency is much less than the ion cyclotron frequency, the ion distribution takes the shape of a torus in velocity space for electric fields greater than 40 mV m^?1. This shape persists for 1–2 hr after application of the electric field. At altitudes where the ion collision and cyclotron frequencies are approximately equal (about 120 km), the ion velocity distribution is shaped like a bean for large electric field strengths. This bean-shaped distribution persists throughout the lifetime of ionospheric electric fileds. These highly non-Maxwellian ion velocity distributions may have an appreciable affect on the interpretation of ion temperature measurements.     

The redshift distribution law of quasars revisited

Can observational selection effects, tied with the existence of strong lines, explain the observed log (1 + z) periodicity of the quasar's histogram, as sometimes claimed? A first approach shows that one must distinguish radio from optical quasars. In the former case spectroscopic selection due to strong lines is investigated from the study of homogeneous samples. Then, the role played by pairs of strong lines in the redshift's determination is evaluated and it shows that teh spectroscopic selection cannot explain the observed periodicity of the radio quasar's histogram. The study of the complete sample confirms this conslusion.     

The initial–final mass relationship of white dwarfs revisited: effect on the luminosity function and mass distribution

The initial–final mass relationship connects the mass of a white dwarf with the mass of its progenitor in the main sequence. Although this function is of fundamental importance to several fields in modern astrophysics, it is not well constrained either from the theoretical or from the observational points of view. In this work, we revise the present semi-empirical initial–final mass relationship by re-evaluating the available data. The distribution obtained from grouping all our results presents a considerable dispersion, which is larger than the uncertainties. We have carried out a weighted least-squares linear fit of these data and a careful analysis to give some clues on the dependence of this relationship on some parameters such as metallicity or rotation. The semi-empirical initial–final mass relationship arising from our study covers the range of initial masses from 1.0 to  6.5 M_⊙  , including in this way the low-mass domain, poorly studied until recently. Finally, we have also performed a test of the initial–final mass relationship by studying its effect on the luminosity function and on the mass distribution of white dwarfs. This was done by using different initial–final mass relationships from the literature, including the expression derived in this work, and comparing the results obtained with the observational data from the Palomar Green Survey and the Sloan Digital Sky Survey. We find that the semi-empirical initial–final mass relationship derived here gives results in good agreement with the observational data, especially in the case of the white dwarf mass distribution.     

The cooling function of HD molecule revisited

We report new calculations of the cooling rate of primordial gas by the HD molecule, taking into account its ro-vibrational structure. The HD cooling function is calculated including radiative and collisional transitions for   J ≤ 8  rotational levels, and for the vibrational levels v = 0, 1, 2 and 3. The ro-vibrational level population is calculated from the balance equation assuming steady state. The cooling function is evaluated in the ranges of the kinetic temperatures, T _k, from 10² to  2 × 10⁴ K  and the number densities, n _H, from 1 to  10⁸ cm⁻³  . We find that the inclusion of collisional ro-vibrational transitions increases significantly the HD cooling efficiency, in particular for high densities and temperatures. For   n _H≳ 10⁵  and   T _k∼ 10⁴ K  the cooling function becomes more than an order of magnitude higher than previously reported. We give also the HD cooling rate in the presence of the cosmic microwave radiation field for radiation temperatures of 30, 85 and 276 K (redshifts of 10, 30 and 100). The tabulated cooling functions are available at http://www.cifus.uson.mx/Personal_Pages/anton/DATA/HD_cooling/HD_cool.html . We discuss the relevance to explore the effects of including our results into models and simulations of galaxy formation, especially in the regime when gas cools down from temperatures above ∼3000 K.     

Analysis of the H ring velocity distribution function near the Martian and Venusian bow shocks by an analytical model

We have studied the H⁺ velocity distribution function at Mars and Venus near the bow shock both in the solar wind and in the magnetosheath by a simple analytical one-dimensional model. We found that over half of the ions in the ring velocity distribution which moved towards the magnetosheath were scattered back into the bow shock. The original ring distribution is destroyed in less than an ion gyro period. Ions contained in the magnetosphere which hit the bow shock bounce back into the solar wind with a maximum energy exceeding twice the energy of solar wind protons. The ions finite gyroradius causes an asymmetric flow in the magnetosheath with respect to the direction of the convective electric field, which can be observed already a few ion gyroradius downstream of the bow shock.     

The NGC 7742 star cluster luminosity function: a population analysis revisited

We re-examine the properties of the star cluster population in the circumnuclear starburst ring in the face-on spiral galaxy NGC 7742, whose young cluster mass function has been reported to exhibit significant deviations from the canonical power law. We base our reassessment on the clusters' luminosities(an observational quantity) rather than their masses(a derived quantity), and confirm conclusively that the galaxy's starburst-ring clusters—and particularly the youngest subsample, log(t yr~(-1)) ≤ 7.2—show evidence of a turnover in the cluster luminosity function well above the 90% completeness limit adopted to ensure the reliability of our results. This confirmation emphasizes the unique conundrum posed by this unusual cluster population.     

Color diversity among Kuiper belt objects: The collisional resurfacing model revisited

A re-evaluation of the collisional resurfacing model based on up-to-date Kuiper belt objects size distribution and a more precise treatment of the cosmic-ray environment at the outer Solar System is presented. The result of the irradiation due to cosmic-rays with different energies altered in a different way the material of the objects, producing, under certain conditions, a thick irradiation mantle. Since the collisional resurfacing model is based in the competition between darkening by cosmic-rays and resurfacing due to impacts, the color of objects in different regions of the belt could vary if the projectile populations in those regions are truncated at a different radius.     

The velocity correlation function in cosmic-ray diffusion theory

The concept of velocity correlation functions is introduced and applied to the calculation of cosmic ray spatial diffusion coefficients. It is assumed that the pitch angle scattering coefficient is already known from some other theory, and is reasonably well-behaved. Previous results for the coefficient for diffusion parallel to the mean field are recovered when the velocity-changing mechanism is artificially restricted to pitch angle scattering. The velocity correlation method is then applied to the more general case where there are fluctuations in the local mean field. It is found that the parallel diffusion coefficient is reduced in proportion to the amplitude of the field fluctuations, and that the ratio of the perpendicular to parallel diffusion coefficients cannot be greater than B _x ² /B ₀ ² . It is shown in the appendix that the Liouville form of the scattering equation implies that the Fokker-Planck coefficients ²/t=2D and /t=D /, and that all higher-order coefficients are identically zero.     

The peculiar velocity field in a quintessence model

We investigate the evolution of matter density perturbations and some properties of the peculiar velocity field for a special class of exponential potentials in a scalar field model for quintessence, for which a general exact solution is known. The data from the 2-degree Field Galaxy Redshift Survey (2dFGRS) suggest a value of the present-day pressureless matter density  Ω_M0= 0.18 ± 0.05  .     

The quasar-frame velocity distribution of narrow C iv absorbers

We report on a survey for narrow (full widths at half-minimum <600 km s⁻¹) C  iv absorption lines in a sample of bright quasars at redshifts  1.8 ≤ z < 2.25  in the Sloan Digital Sky Survey. Our main goal is to understand the relationship of narrow C  iv absorbers to quasar outflows and, more generally, to quasar environments. We determine velocity zero-points using the broad Mg  ii emission line, and then measure the absorbers' quasar-frame velocity distribution. We examine the distribution of lines arising in quasar outflows by subtracting model fits to the contributions from cosmologically intervening absorbers and absorption due to the quasar host galaxy or cluster environment. We find that a substantial number (  ≥43 ± 6  per cent) of absorbers with   W ^λ1548₀ > 0.3  Å in the velocity range  +750 ≲ v ≲+ 12 000  km s⁻¹ are intrinsic to the active galactic nucleus outflow. This 'outflow fraction' peaks near   v =+2000  km s⁻¹ with a value of   f _outflow≃ 0.81 ± 0.13  . At velocities below   v ≈+ 2000  km s⁻¹, the incidence of outflowing systems drops, possibly due to geometric effects or to the over-ionization of gas that is nearer the accretion disc. Furthermore, we find that outflow absorbers are on average broader and stronger than cosmologically intervening systems. Finally, we find that ∼14 per cent of the quasars in our sample exhibit narrow, outflowing C  iv absorption with   W ^λ1548₀ > 0.3  Å, slightly larger than that for broad absorption line systems.