1,619 research outputs found
Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12
Explaining the origin of the orbit of 2000 CR105 (a ~ 230AU, q ~ 45AU) is a
major test for our understanding of the primordial evolution of the outer Solar
System. Gladman et al. (2001) showed that this objects could not have been a
normal member of the scattered disk that had its perihelion distance increased
by chaotic diffusion. In this paper we explore four seemingly promising
mechanisms for explaining the origin of the orbit of this peculiar object: (i)
the passage of Neptune through a high-eccentricity phase, (ii) the past
existence of massive planetary embryos in the Kuiper belt or the scattered
disk, (iii) the presence of a massive trans-Neptunian disk at early epochs
which exerted tides on scattered disk objects, and (iv) encounters with other
stars. Of all these mechanisms, the only one giving satisfactory results is the
passage of a star. Indeed, our simulations show that the passage of a solar
mass star at about 800 AU only perturbs objects with semi-major axes larger
than roughly 200 AU to large perihelion distances. This is in good agreement
with the fact that 2000 CR105 has a semi-major axis of 230AU and no other
bodies with similar perihelion distances but smaller semi-major axes have yet
been discovered. The discovery of 2003 VB12, (a=450AU, q=75AU) announced a few
days before the submission of this paper, strengthen our conclusions.Comment: AJ submitted. 27 pages, 6 figure
WHAM Observations of H-alpha Emission from High Velocity Clouds in the M, A, and C Complexes
The first observations of the recently completed Wisconsin H-Alpha Mapper
(WHAM) facility include a study of emission lines from high velocity clouds in
the M, A, and C complexes, with most of the observations on the M I cloud. We
present results including clear detections of H-alpha emission from all three
complexes with intensities ranging from 0.06 R to 0.20 R. In every observed
direction where there is significant high velocity H I gas seen in the 21 cm
line we have found associated ionized hydrogen emitting the H-alpha line. The
velocities of the H-alpha and 21 cm emission are well correlated in every case
except one, but the intensities are not correlated. There is some evidence that
the ionized gas producing the H-alpha emission envelopes the 21 cm emitting
neutral gas but the H-alpha "halo", if present, is not large. If the H-alpha
emission arises from the photoionization of the H I clouds, then the implied
Lyman continuum flux F_{LC} at the location of the clouds ranges from 1.3 to
4.2 x 10^5 photons cm^{-2} s^{-1}. If, on the other hand, the ionization is due
to a shock arising from the collision of the high-velocity gas with an ambient
medium in the halo, then the density of the pre-shocked gas can be constrained.
We have also detected the [S II] 6716 angstrom line from the M I cloud and have
evidence that the [S II] to H-alpha ratio varies with location on the cloud.Comment: 32 pages, 18 figures, to appear in ApJ (Sept. 10, 1998
Astrometric Microlensing Constraints on a Massive Body in the Outer Solar System with Gaia
A body in Solar orbit beyond the Kuiper belt exhibits an annual parallax that
exceeds its apparent proper motion by up to many orders of magnitude. Apparent
motion of this body along the parallactic ellipse will deflect the angular
position of background stars due to astrometric microlensing ("induced
parallax"). By synoptically sampling the astrometric position of background
stars over the entire sky, constraints on the existence (and basic properties)
of a massive nearby body may be inferred. With a simple simulation, we estimate
the signal-to-noise for detecting such a body -- as function of mass,
heliocentric distance, and ecliptic latitude -- using the anticipated
sensitivity and temporal cadences from Gaia (launch 2011). A Jupiter-mass
(M_Jup) object at 2000 AU is detectable by Gaia over the whole sky above
5-sigma, with even stronger constraints if it lies near the ecliptic plane.
Hypotheses for the mass (~3M_Jup), distance (~20,000 AU) and location of the
proposed perturber ("Planet X") which gives rise to long-period comets may be
testable.Comment: 17 pages, 6 figures. Figures revised, new figure added, minor text
revisions. Accepted to ApJ, to appear in the Dec 10, 2005 issue (v635
A Model for the Moving `Wisps' in the Crab Nebula
I propose that the moving `wisps' near the center of the Crab Nebula result
from nonlinear Kelvin-Helmholtz instabilities in the equatorial plane of the
shocked pulsar wind. Recent observations suggest that the wisps trace out
circular wavefronts in this plane, expanding radially at speeds approximately
less than c/3. Instabilities could develop if there is sufficient velocity
shear between a faster-moving equatorial zone and a slower moving shocked
pulsar wind at higher latitudes. The development of shear could be related to
the existence of a neutral sheet -- with weak magnetic field -- in the
equatorial zone, and could also be related to a recent suggestion by Begelman
that the magnetic field in the Crab pulsar wind is much stronger than had been
thought. I show that plausible conditions could lead to the growth of
instabilities at the radii and speeds observed, and that their nonlinear
development could lead to the appearance of sharp wisplike features.Comment: 7 pages; 3 postscript figures; LaTex, uses emulateapj.sty; to Appear
in the Astrophysical Journal, Feb. 20, 1999, Vol. 51
Local Surface Density of the Galactic Disk from a 3-D Stellar Velocity Sample
We have re-estimated the surface density of the Galactic disk in the solar
neighborhood within 0.4 kpc of the Sun using parallaxes and proper
motions of a kinematically and spatially unbiased sample of 1476 old bright red
giant stars from the Hipparcos catalog with measured radial velocities from
Barbier-Brossat & Figon (2000). We determine the vertical distribution of the
red giants as well as the vertical velocity dispersion of the sample, (14.4
0.26 km/sec), and combine these to derive the surface density of
gravitating matter in the Galactic disk as a function of the galactic
coordinate . The surface density of the disk increases from 10.5 0.5
/ pc within 50 pc to 42 6 / pc
within 350 pc. The estimated volume density of the galactic disk within
50 pc is about 0.1 / pc which is close to the volume
density estimates of the observed baryonic matter in the solar neighborhood.Comment: 24 pages, 15 figures, AJ in pres
До проблеми вивчення весілля Шевченкового краю (за матеріалами передвесільної обрядовості)
While businesses may attract potential offenders and thus be conducive to
disorder, the number of employees could offset this by exercising social
control on offenders. This study uses data from different sources to test
this expectation across 278 Dutch neighborhoods in the four largest cities
of the Netherlands, using multivariate multilevel analysis to disentangle
individual perception differences of disorder and neighborhood effects.
Attention is paid to traditional explanations of disorder (i.e., poverty, residential
mobility, and ethnic heterogeneity). Results show a positive relationship
between business presence and neighborhood disorder. We do not find
consistent results of the number of employees (i.e., bigger businesses are
not always better or worse). Our research demonstrates that the presence of neighborhood businesses could rival the effects of social disorganization
theory
Picosecond fluorescence of intact and dissolved PSI-LHCI crystals
Over the last years many crystal structures of photosynthetic pigment-protein complexes have been determined, and used extensively to model spectroscopic results obtained on the same proteins in solution. However, the crystal structure is not necessarily identical to the structure of the protein in solution. Here we studied picosecond fluorescence of Photosystem I-Light Harvesting Complex I (PSI-LHCI), a multisubunit pigment protein complex that catalyzes the first steps of photosynthesis. The ultrafast fluorescence of PSI-LHCI crystals is identical to that of dissolved crystals, but differs considerably from most kinetics presented in literature. In contrast to most studies, the present data can be modeled quantitatively with only 2 compartments: PSI core and LHCI. This yields the rate of charge separation from an equilibrated core (22.5+/-2.5 ps) and rates of excitation energy transfer from LHCI to core (kLC) and vice versa (kCL). The ratio R=kCL/kLC between these rates appears to be wavelength-dependent and scales with the ratio of the absorption spectra of LHCI and core, indicating the validity of a detailed balance relation between both compartments. kLC depends slightly but non systematically on detection wavelength, averaging (9.4+/-4.9 ps)(-1). R ranges from 0.5 (below 690 nm) to around 1.3 above 720 nm
A direct empirical proof of the existence of dark matter
We present new weak lensing observations of 1E0657-558 (z=0.296), a unique
cluster merger, that enable a direct detection of dark matter, independent of
assumptions regarding the nature of the gravitational force law. Due to the
collision of two clusters, the dissipationless stellar component and the
fluid-like X-ray emitting plasma are spatially segregated. By using both
wide-field ground based images and HST/ACS images of the cluster cores, we
create gravitational lensing maps which show that the gravitational potential
does not trace the plasma distribution, the dominant baryonic mass component,
but rather approximately traces the distribution of galaxies. An 8-sigma
significance spatial offset of the center of the total mass from the center of
the baryonic mass peaks cannot be explained with an alteration of the
gravitational force law, and thus proves that the majority of the matter in the
system is unseen.Comment: Accepted for publication in ApJ
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