539 research outputs found
The Frequency Distribution of Semi-major Axis of Wide Binaries. Cosmogony and Dynamical Evolution
The frequency distribution f(a) of semi-major axis of double and multiple
systems, as well as their eccentricities and mass ratios, contain valuable
fossil information about the process of star formation and the dynamical
history of the systems. In order to advance in the understanding of these
questions, we have made an extensive analysis of the frequency distribution f
(a) for wide binaries (a>25 AU) in the various published catalogues, as well as
in our own (Poveda et al., 1994; Allen et al., 2000; Poveda & Hernandez, 2003).
Based upon all these studies we have established that the frequency f(a) is
function of the age of the system and follows Oepik's distribution f(a) ~ 1/a
in the range of 100 AU < a < a[c](t), where a[c](t) is a critical semi-major
axis beyond which binaries have dissociated by encounters with massive objects.
We argue that the physics behind the distribution f(a) ~ 1/a is a process of
energy relaxation, analogous to that present in stellar clusters (secular
relaxation) or in spherical galaxies (violent relaxation). The frequency
distribution of mass ratios in triple systems as well as the existence of
runaway stars, indicate that both types of relaxation are important in the
process of binary and multiple star formation.Comment: International Astronomical Union. Symposium no. 240, held 22-25
August, 2006 in Prague, Czech Republi
Dust and Gas as Seeds for Metal-Poor Star Formation
I address the issue of dust and gas as seeds for metal-poor star formation by
reviewing what we know about star formation in nearby dwarf galaxies and its
relationship to the gas and dust. I (try to) speculate on the extent to which
processes in nearby galaxies mimic star formation in the early universe.Comment: To be published in "Low Metallicity Star Formation: From the First
Stars to Dwarf Galaxies," Proceedings of IAU Symposium No. 255, eds. L.K.
Hunt, S. Madden, & R. Schneider (Cambridge: Cambridge Univ Press
Planetary migration in evolving planetesimals discs
In the current paper, we further improved the model for the migration of
planets introduced in Del Popolo et al. (2001) and extended to time-dependent
planetesimal accretion disks in Del Popolo and Eksi (2002). In the current
study, the assumption of Del Popolo and Eksi (2002), that the surface density
in planetesimals is proportional to that of gas, is released. In order to
obtain the evolution of planetesimal density, we use a method developed in
Stepinski and Valageas (1997) which is able to simultaneously follow the
evolution of gas and solid particles for up to 10^7 yrs. Then, the disk model
is coupled to migration model introduced in Del Popolo et al. (2001) in order
to obtain the migration rate of the planet in the planetesimal. We find that
the properties of solids known to exist in protoplanetary systems, together
with reasonable density profiles for the disk, lead to a characteristic radius
in the range 0.03-0.2 AU for the final semi-major axis of the giant planet.Comment: IJMP A in prin
Orbital Configurations and Magnetic Properties of Double-Layered Antiferromagnet CsCuClBr
We report the single-crystal X-ray analysis and magnetic properties of a new
double-layered perovskite antiferromagnet, CsCuClBr. This
structure is composed of CuClBr double layers with elongated
CuClBr octahedra and is closely related to the SrTiO
structure. An as-grown crystal has a singlet ground state with a large
excitation gap of K, due to the strong
antiferromagnetic interaction between the two layers. CsCuClBr
undergoes a structural phase transition at K accompanied
by changes in the orbital configurations of Cu ions. Once a
CsCuClBr crystal is heated above , its magnetic
susceptibility obeys the Curie-Weiss law with decreasing temperature even below
and does not exhibit anomalies at . This implies that in
the heated crystal, the orbital state of the high-temperature phase remains
unchanged below , and thus, this orbital state is the metastable
state. The structural phase transition at is characterized as an
order-disorder transition of Cu orbitals.Comment: 6pages. 6figures, to appear in J. Phys. Soc. Jpn. Vol.76 No.
Structure and rotations of the Hoyle state
The excited state of the 12C nucleus known as the "Hoyle state" constitutes
one of the most interesting, difficult and timely challenges in nuclear
physics, as it plays a key role in the production of carbon via fusion of three
alpha particles in red giant stars. In this letter, we present ab initio
lattice calculations which unravel the structure of the Hoyle state, along with
evidence for a low-lying spin-2 rotational excitation. For the 12C ground state
and the first excited spin-2 state, we find a compact triangular configuration
of alpha clusters. For the Hoyle state and the second excited spin-2 state, we
find a "bent-arm" or obtuse triangular configuration of alpha clusters. We also
calculate the electromagnetic transition rates between the low-lying states of
12C.Comment: 4 pages, 3 figures, 4 tables, version to be published in Physical
Review Letter
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
Five More Massive Binaries in the Cygnus OB2 Association
We present the orbital solutions for four OB spectroscopic binaries, MT145,
GSC 03161-00815, 2MASS J20294666+4105083, and Schulte 73, and the partial
orbital solution to the B spectroscopic binary, MT372, as part of an ongoing
study to determine the distribution of orbital parameters for massive binaries
in the Cygnus OB2 Association. MT145 is a new, single-lined, moderately
eccentric (e=0.291+/-0.009) spectroscopic binary with period of 25.140+/-0.008
days. GSC 03161-00815 is a slightly eccentric (e=0.10+/-0.01), eclipsing,
interacting and double-lined spectroscopic binary with a period of
4.674+/-0.004 days. 2MASS J20294666+4105083 is a moderately eccentric
(e=0.273+/-0.002) double-lined spectroscopic binary with a period of
2.884+/-0.001 days. Schulte 73 is a slightly eccentric (e=0.169+/-0.009),
double-lined spectroscopic binary with a period of 17.28+/-0.03 days and the
first "twin" in our survey with a mass ratio of q=0.99+/-0.02. MT372 is a
single-lined, eclipsing system with a period of 2.228 days and low eccentricity
(e~0). Of the now 18 known OB binaries in Cyg OB2, 14 have periods and mass
ratios. Emerging evidence also shows that the distribution of log(P) is flat
and consistent with Oepik's Law.Comment: Accepted to Astronomical Journa
Topological Phases near a Triple Degeneracy
We study the pattern of three state topological phases that appear in systems
with real Hamiltonians and wave functions. We give a simple geometric
construction for representing these phases. We then apply our results to
understand previous work on three state phases. We point out that the ``mirror
symmetry'' of wave functions noticed in microwave experiments can be simply
understood in our framework.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let
Spin Driven Jahn-Teller Distortion in a Pyrochlore system
The ground-state properties of the spin-1 antiferromagnetic Heisenberg model
on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By
breaking up each spin into a pair of 1/2-spins, the problem is reduced to the
equivalent one of the spin-1/2 tetrahedral network in analogy with the valence
bond solid state in one dimension. The twofold degeneracy of the spin-singlets
of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to
tetragonal structural transition. It is proposed that the present mechanism is
responsible for the phase transition observed in the spin-1 spinel compounds
ZnVO and MgVO.Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Let
Finding the trigger to Iapetus' odd global albedo pattern: Dynamics of dust from Saturn's irregular satellites
The leading face of Saturn's moon Iapetus, Cassini Regio, has an albedo only
one tenth that on its trailing side. The origin of this enigmatic dichotomy has
been debated for over forty years, but with new data, a clearer picture is
emerging. Motivated by Cassini radar and imaging observations, we investigate
Soter's model of dark exogenous dust striking an originally brighter Iapetus by
modeling the dynamics of the dark dust from the ring of the exterior retrograde
satellite Phoebe under the relevant perturbations. In particular, we study the
particles' probabilities of striking Iapetus, as well as their expected spatial
distribution on the Iapetian surface. We find that, of the long-lived particles
(greater than about 5 microns), most particle sizes (greater than about 10
microns) are virtually certain to strike Iapetus, and their calculated
distribution on the surface matches up well with Cassini Regio's extent in its
longitudinal span. The satellite's polar regions are observed to be bright,
presumably because ice is deposited there. Thus, in the latitudinal direction
we estimate polar dust deposition rates to help constrain models of thermal
migration invoked to explain the bright poles (Spencer & Denk 2010). We also
analyze dust originating from other irregular outer moons, determining that a
significant fraction of that material will eventually coat Iapetus--perhaps
explaining why the spectrum of Iapetus' dark material differs somewhat from
that of Phoebe. Finally we track the dust particles that do not strike Iapetus,
and find that most land on Titan, with a smaller fraction hitting Hyperion. As
has been previously conjectured, such exogenous dust, coupled with Hyperion's
chaotic rotation, could produce Hyperion's roughly isotropic, moderate-albedo
surface.Comment: Accepted for publication in Icaru
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