5,469 research outputs found
The Radial Distribution of the Kuiper Belt
We examine the radial distribution of the Kuiper Belt objects (KBOs) using a method that is insensitive to observational bias effects. This technique allows the use of the discovery distances of all KBOs, independent of orbital classification or discovery circumstance. We verify the presence of an outer edge to the Kuiper Belt, as reported in other works, and we measure this edge to be at R = 47 ± 1 AU given any physically plausible model of the size distribution. We confirm that this outer edge is due to the classical KBOs, the most numerically dominant observationally. In addition, we find that current surveys do not preclude the presence of a second, unobserved Kuiper Belt beyond R = 76 AU
Discovery of a Candidate Inner Oort Cloud Planetoid
We report the discovery of the minor planet (90377) Sedna, the most distant object ever seen in the solar system. Prediscovery images from 2001, 2002, and 2003 have allowed us to refine the orbit sufficiently to conclude that Sedna is on a highly eccentric orbit that permanently resides well beyond the Kuiper Belt with a semimajor axis of 480 ± 40 AU and a perihelion of 76 ± 4 AU. Such an orbit is unexpected in our current understanding of the solar system but could be the result of scattering by a yet-to-be-discovered planet, perturbation by an anomalously close stellar encounter, or formation of the solar system within a cluster of stars. In all of these cases a significant additional population is likely present, and in the two most likely cases Sedna is best considered a member of the inner Oort Cloud, which then extends to much smaller semimajor axes than previously expected. Continued discovery and orbital characterization of objects in this inner Oort Cloud will verify the genesis of this unexpected population
Isotropic inelastic and superelastic collisional rates in a multiterm atom
The spectral line polarization of the radiation emerging from a magnetized
astrophysical plasma depends on the state of the atoms within the medium, whose
determination requires considering the interactions between the atoms and the
magnetic field, between the atoms and photons (radiative transitions), and
between the atoms and other material particles (collisional transitions). In
applications within the framework of the multiterm model atom (which accounts
for quantum interference between magnetic sublevels pertaining either to the
same J-level or to different J-levels within the same term) collisional
processes are generally neglected when solving the master equation for the
atomic density matrix. This is partly due to the lack of experimental data
and/or of approximate theoretical expressions for calculating the collisional
transfer and relaxation rates (in particular the rates for interference between
sublevels pertaining to different J-levels, and the depolarizing rates due to
elastic collisions). In this paper we formally define and investigate the
transfer and relaxation rates due to isotropic inelastic and superelastic
collisions that enter the statistical equilibrium equations of a multiterm
atom. Under the hypothesis that the atom-collider interaction can be described
by a dipolar operator, we provide expressions that relate the collisional rates
for interference between different J-levels to the usual collisional rates for
J-level populations. Finally, we apply the general equations to the case of a
two-term atom with unpolarized lower term, illustrating the impact of inelastic
and superelastic collisions on scattering polarization through radiative
transfer calculations in a slab of stellar atmospheric plasma anisotropically
illuminated by the photospheric radiation field.Comment: Accepted for publication in Astronomy & Astrophysic
Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations represent
an important physical aspect that, however, is often neglected in practical
applications by adopting the simplifying approximation of complete
redistribution on velocity. In this paper, we present an application of the
Non-LTE problem for polarized radiation taking such correlations into account
through the velocity-space density matrix formalism. We consider a two-level
atom with infinitely sharp upper and lower levels, and we derive the
corresponding statistical equilibrium equations neglecting the contribution of
velocity-changing collisions. Coupling such equations with the radiative
transfer equations for polarized radiation, we derive a set of coupled
equations for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The equations
presented in this paper provide a complete and solid description of the physics
of pure Doppler redistribution, a phenomenon generally described within the
framework of the redistribution matrix formalism. The redistribution matrix
corresponding to this problem (generally referred to as R_I) is derived
starting from the statistical equilibrium equations for the velocity-space
density matrix and from the radiative transfer equations for polarized
radiation, thus showing the equivalence of the two approaches.Comment: Accepted for publication in Astronomy & Astrophysic
A Correlation Between Inclination and Color in the Classical Kuiper Belt
We have measured broadband optical BVR photometry of 24 Classical and
Scattered Kuiper belt objects (KBOs), approximately doubling the published
sample of colors for these classes of objects. We find a statistically
significant correlation between object color and inclination in the Classical
Kuiper belt using our data. The color and inclination correlation increases in
significance after the inclusion of additional data points culled from all
published works. Apparently, this color and inclination correlation has not
been more widely reported because the Plutinos show no such correlation, and
thus have been a major contaminant in previous samples. The color and
inclination correlation excludes simple origins of color diversity, such as the
presence of a coloring agent without regard to dynamical effects.
Unfortunately, our current knowledge of the Kuiper belt precludes us from
understanding whether the color and inclination trend is due to environmental
factors, such as collisional resurfacing, or primordial population effects. A
perihelion and color correlation is also evident, although this appears to be a
spurious correlation induced by sampling bias, as perihelion and inclination
are correlated in the observed sample of KBOs.Comment: Accepted to Astrophysical Journal Letter
Polynomial Approximants for the Calculation of Polarization Profiles in the \ion{He}{1} 10830 \AA Multiplet
The \ion{He}{1} multiplet at 10830 \AA is formed in the incomplete
Paschen-Back regime for typical conditions found in solar and stellar
atmospheres. The positions and strengths of the various components that form
the Zeeman structure of this multiplet in the Paschen-Back regime are
approximated here by polynomials. The fitting errors are smaller than
m\AA in the component positions and in the relative
strengths. The approximant polynomials allow for a very fast implementation of
the incomplete Paschen-Back regime in numerical codes for the synthesis and
inversion of polarization profiles in this important multiplet.Comment: ApJ Supplements (in press
- âŠ