90 research outputs found
Constraining the Physical Properties of Near-Earth Object 2009 BD
We report on Spitzer Space Telescope IRAC observations of near-Earth object
(NEO) 2009 BD that were carried out in support of the NASA Asteroid Robotic
Retrieval Mission (ARRM) concept. We did not detect 2009 BD in 25 hrs of
integration at 4.5 micron. Based on an upper-limit flux density determination
from our data, we present a probabilistic derivation of the physical properties
of this object. The analysis is based on the combination of a thermophysical
model with an orbital model accounting for the non-gravitational forces acting
upon the body. We find two physically possible solutions. The first solution
shows 2009 BD as a 2.9+/-0.3 m diameter rocky body (rho = 2.9+/-0.5 g cm-3)
with an extremely high albedo of 0.85(+0.20/-0.10) that is covered with
regolith-like material, causing it to exhibit a low thermal inertia (Gamma =
30(+20/-10) SI units). The second solution suggests 2009 BD to be a 4+/-1 m
diameter asteroid with pV = 0.45(+0.35/-0.15) that consists of a collection of
individual bare rock slabs (Gamma = 2000+/-1000 SI units, rho = 1.7(+0.7/-0.4)
g cm-3). We are unable to rule out either solution based on physical reasoning.
2009 BD is the smallest asteroid for which physical properties have been
constrained, in this case using an indirect method and based on a detection
limit, providing unique information on the physical properties of objects in
the size range smaller than 10 m.Comment: 28 pages, 8 figures, accepted for publication in Ap
Enhanced activity of massive black holes by stellar capture assisted by a self-gravitating accretion disc
We study the probability of close encounters between stars from a nuclear
cluster and a massive black hole. The gravitational field of the system is
dominated by the black hole in its sphere of influence. It is further modified
by the cluster mean field (a spherical term) and a gaseous disc/torus (an
axially symmetric term) causing a secular evolution of stellar orbits via Kozai
oscillations. Intermittent phases of large eccentricity increase the chance
that stars become damaged inside the tidal radius of the central hole. Such
events can produce debris and lead to recurring episodes of enhanced accretion
activity. We introduce an effective loss cone and associate it with tidal
disruptions during the high-eccentricity phases of the Kozai cycle. By
numerical integration of the trajectories forming the boundary of the loss cone
we determine its shape and volume. We also include the effect of relativistic
advance of pericentre. The potential of the disc has the efffect of enlarging
the loss cone and, therefore, the predicted number of tidally disrupted stars
should grow by factor of ~10^2. On the other hand, the effect of the cluster
mean potential together with the relativistic pericentre advance act against
the eccentricity oscillations. In the end we expect the tidal disruption events
to be approximately ten times more frequent in comparison with the model in
which the three effects -- the cluster mean field, the relativistic pericentre
advance, and the Kozai mechanism -- are all ignored. The competition of
different influences suppresses the predicted star disruption rate as the black
hole mass increases. Hence, the process under consideration is more important
for intermediate-mass black holes, M_bh~10^4M_s.Comment: 10 pages, 5 figures; Astronomy & Astrophysics accepte
Light-time computations for the BepiColombo radioscience experiment
The radioscience experiment is one of the on board experiment of the Mercury
ESA mission BepiColombo that will be launched in 2014. The goals of the
experiment are to determine the gravity field of Mercury and its rotation
state, to determine the orbit of Mercury, to constrain the possible theories of
gravitation (for example by determining the post-Newtonian (PN) parameters), to
provide the spacecraft position for geodesy experiments and to contribute to
planetary ephemerides improvement. This is possible thanks to a new technology
which allows to reach great accuracies in the observables range and range rate;
it is well known that a similar level of accuracy requires studying a suitable
model taking into account numerous relativistic effects. In this paper we deal
with the modelling of the space-time coordinate transformations needed for the
light-time computations and the numerical methods adopted to avoid rounding-off
errors in such computations.Comment: 14 pages, 7 figures, corrected reference
Detection of Semi-Major Axis Drifts in 54 Near-Earth Asteroids: New Measurements of the Yarkovsky Effect
We have identified and quantified semi-major axis drifts in Near-Earth
Asteroids (NEAs) by performing orbital fits to optical and radar astrometry of
all numbered NEAs. We focus on a subset of 54 NEAs that exhibit some of the
most reliable and strongest drift rates. Our selection criteria include a
Yarkovsky sensitivity metric that quantifies the detectability of semi-major
axis drift in any given data set, a signal-to-noise metric, and orbital
coverage requirements. In 42 cases, the observed drifts (~10^-3 AU/Myr) agree
well with numerical estimates of Yarkovsky drifts. This agreement suggests that
the Yarkovsky effect is the dominant non-gravitational process affecting these
orbits, and allows us to derive constraints on asteroid physical properties. In
12 cases, the drifts exceed nominal Yarkovsky predictions, which could be due
to inaccuracies in our knowledge of physical properties, faulty astrometry, or
modeling errors. If these high rates cannot be ruled out by further
observations or improvements in modeling, they would be indicative of the
presence of an additional non-gravitational force, such as that resulting from
a loss of mass of order a kilogram per second. We define the Yarkovsky
efficiency f_Y as the ratio of the change in orbital energy to incident solar
radiation energy, and we find that typical Yarkovsky efficiencies are ~10^-5.Comment: Accepted for publication by The Astronomical Journal. 42 pages, 8
figure
On highly eccentric stellar trajectories interacting with a self-gravitating disc in Sgr A*
We propose that Kozai's phenomenon is responsible for the long-term evolution
of stellar orbits near a supermassive black hole. We pursue the idea that this
process may be driven by a fossil accretion disc in the centre of our Galaxy,
causing the gradual orbital decay of stellar trajectories, while setting some
stars on highly elliptic orbits. We evolve model orbits that undergo repetitive
transitions across the disc over the period of ~10^7 years. We assume that the
disc mass is small compared to the central black hole, and its gravitational
field comparatively weak, yet non-zero, and we set the present values of
orbital parameters of the model star consistent with those reported for the S2
star in Sagittarius A*. We show how a model trajectory decays and circularizes,
but at some point the mean eccentricity is substantially increased by Kozai's
resonance. In consequence the orbital decay of highly eccentric orbits is
accelerated. A combination of an axially symmetric gravitational field and
dissipative environment can provide a mechanism explaining the origin of stars
on highly eccentric orbits tightly bound to the central black hole. In the
context of other S-stars, we can conclude that an acceptable mass of the disc
(i.e., M_d<=1 percent of the black hole mass) is compatible with their
surprisingly young age and small pericentre distances, provided these stars
were formed at r<=10^5 gravitational radii.Comment: Accepted for publication in A&A; 9 pages, 6 figures. Revised version
with minor language corrections (no change in content
Conservation laws for systems of extended bodies in the first post-Newtonian approximation.
The general form of the global conservation laws for -body systems in the
first post-Newtonian approximation of general relativity is considered. Our
approach applies to the motion of an isolated system of arbitrarily
composed and shaped, weakly self-gravitating, rotating, deformable bodies and
uses a framework recently introduced by Damour, Soffel and Xu (DSX). We succeed
in showing that seven of the first integrals of the system (total mass-energy,
total dipole mass moment and total linear momentum) can be broken up into a sum
of contributions which can be entirely expressed in terms of the basic
quantities entering the DSX framework: namely, the relativistic individual
multipole moments of the bodies, the relativistic tidal moments experienced by
each body, and the positions and orientations with respect to the global
coordinate system of the local reference frames attached to each body. On the
other hand, the total angular momentum of the system does not seem to be
expressible in such a form due to the unavoidable presence of irreducible
nonlinear gravitational effects.Comment: 18 pages, Revte
Orbital decay of satellites crossing an accretion disc
Motion of stellar-mass satellites is studied around a massive compact body
which is surrounded by a gaseous slab of a stationary accretion disc. The
satellites suffer an orbital decay due to hydrodynamical interaction with the
disc medium (transitions across the disc, gap opening in the disc, density
waves) and gravitational radiation. Arbitrary orbital eccentricities and
inclinations are considered, and it is observed how the competing effects
depend on the parameters of the model, namely, the mass and compactness of the
orbiters, the osculating elements of their trajectories, and surface density of
the disc. These effects have a visible impact on the satellites long-term
motion, and they can produce observational consequences with respect to
galactic central clusters. It is shown that the satellite-disc collisions do
not impose serious restrictions on the results of gravitational wave
experiments if the disc medium is diluted and the orbiter is compact but they
are important in the case of environments with relatively high density. We thus
concentrate on application to accretion flows in which the density is not
negligible. We discuss the expected quasi-stationary structure of the cluster
that is established on sub-parsec scales within the sphere of gravitational
influence of the central object. Relevant to this region, we give the power-law
slopes defining the radial profile of modified clusters and we show that their
values are determined by satellite interaction with the accretion flow rather
than their initial distribution.Comment: Astronomy & Astrophysics, in press; 11 pages and 6 figures, LaTeX2e
(aa501.cls
MarcoPolo-R: Near-Earth Asteroid sample return mission selected for the assessment study phase of the ESA program cosmic vision
This paper presents the sample return mission to a primitive Near-Earth Asteroid (NEA) MarcoPolo-R proposed to the European Space Agency in December 2010. MarcoPolo-R was selected in February 2011 with three other missions addressing different science objectives for the two-year Assessment Phase of the Medium-Class mission competition of the Cosmic Vision 2 program for launch in 2022. The baseline target of MarcoPolo-R is the binary NEA (175706) 1996 FG3, which offers an efficient operational and technical mission profile. A binary target also provides enhanced science return. The choice of a binary target allows several scientific investigations to occur more easily than through a single object, in particular regarding the fascinating geology and geophysics of asteroids. MarcoPolo-R will rendezvous with a primitive, organic-rich NEA, scientifically characterize it at multiple scales, and return a bulk sample to Earth for laboratory analyses. The MarcoPolo-R sample will provide a representative sample from the surface of a known asteroid with known geologic context, and will contribute to the inventory of primitive material that is probably missing from the meteorite collection. The MarcoPolo-R samples will thus contribute to the exploration of the origin of planetary materials and initial stages of habitable planet formation, to the identification and characterization of the organics and volatiles in a primitive asteroid and to the understanding of the unique geomorphology, dynamics and evolution of a binary asteroid that belongs to the Potentially Hazardous Asteroid (PHA) population
Orbital effects of a monochromatic plane gravitational wave with ultra-low frequency incident on a gravitationally bound two-body system
We analytically compute the long-term orbital variations of a test particle
orbiting a central body acted upon by an incident monochromatic plane
gravitational wave. We assume that the characteristic size of the perturbed
two-body system is much smaller than the wavelength of the wave. Moreover, we
also suppose that the wave's frequency is much smaller than the particle's
orbital one. We make neither a priori assumptions about the direction of the
wavevector nor on the orbital geometry of the planet. We find that, while the
semi-major axis is left unaffected, the eccentricity, the inclination, the
longitude of the ascending node, the longitude of pericenter and the mean
anomaly undergo non-vanishing long-term changes. They are not secular trends
because of the slow modulation introduced by the tidal matrix coefficients and
by the orbital elements themselves. They could be useful to indepenedently
constrain the ultra-low frequency waves which may have been indirectly detected
in the BICEP2 experiment. Our calculation holds, in general, for any
gravitationally bound two-body system whose characteristic frequency is much
larger than the frequency of the external wave. It is also valid for a generic
perturbation of tidal type with constant coefficients over timescales of the
order of the orbital period of the perturbed particle.Comment: LaTex2e, 24 pages, no figures, no tables. Changes suggested by the
referees include
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