768 research outputs found
Formation of Kuiper Belt Binaries
It appears that at least several percent of large Kuiper belt objects are
members of wide binaries. Physical collisions are too infrequent to account for
their formation. Collisionless gravitational interactions are more promising.
These provide two channels for binary formation. In each, the initial step is
the formation of a transient binary when two large bodies penetrate each
other's Hill spheres. Stabilization of a transient binary requires that it lose
energy. Either dynamical friction due to small bodies or the scattering of a
third large body can be responsible. Our estimates favor the former, albeit by
a small margin. We predict that most objects of size comparable to those
currently observed in the Kuiper belt are members of multiple systems. More
specifically, we derive the probability that a large body is a member of a
binary with semi-major axis of order a. The probability depends upon sigma, the
total surface density, Sigma, the surface density of large bodies having radius
R, and theta=10^-4, the angle subtended by the solar radius as seen from the
Kuiper belt. For (sigma/Sigma)R<a< R/theta, the probability is just (Sigma/rho
R)theta^-2, the optical depth of the large bodies divided by the solid angle
subtended by the Sun. For R<a<r_u=(sigma/Sigma)R, it varies inversely with
semimajor axis and reaches (sigma/rho R)theta^-2 at a=R. Based on current
surveys of the Kuiper belt, we estimate Sigma/rho=3 10^-4cm and R=100km. We
obtain sigma/rho=0.3cm by extrapolating the surface density deduced for the
minimum mass solar nebula. Rough predictions are: outside of the critical
separation r_u/a_odot=3'', the binary probability is 0.3%; at separations of
0.2'', comparable to current resolving capabilities, it reaches 5%, in
agreement with results from the HST binary survey by Brown
Stellar encounters as the origin of distant solar system objects in highly eccentric orbits
The discovery of Sedna places new constraints on the origin and evolution of
our solar system. Here we investigate the possibility that a close encounter
with another star produced the observed edge of the Kuiper belt, at roughly 50
AU, and the highly elliptical orbit of Sedna. We show that a passing star
probably scattered Sedna from the Kuiper Belt into its observed orbit. The
likelihood that a planet at 60-80 AU can be scattered into Sedna's orbit is
roughly 50%; this estimate depends critically on the geometry of the flyby.
Even more interesting, though, is the roughly 10% chance that Sedna was
captured from the outer disk of the passing star. Most captures have very high
inclination orbits; detection of these objects would confirm the presence of
extrasolar planets in our own Solar System.Comment: 9 pages, 3 figure
Solar System: Sifting through the debris
A quadrillion previously unnoticed small bodies beyond Neptune have been
spotted as they dimmed X-rays from a distant source. Models of the dynamics of
debris in the Solar System's suburbs must now be reworked.Comment: 3 pages, 1 figure; Nature News and Views on Chang et al. 2006,
Nature, 442, 660-66
Rumen Development of Artificially-Reared Lambs Exposed to Three Different Rearing Regimens
(c) The Author/sPublishe
Growth and Body Composition of Artificially-Reared Lambs Exposed to Three Different Rearing Regimens.
(c) The Author/sThis study was designed to investigate the influence of pellet fibre level, milk replacer composition and age at weaning on growth and body composition of lambs reared artificially. Romney ram lambs were randomly allocated to one of three rearing treatments; HFP57: commercial milk replacer to 57 days of age, and high fibre concentrate pellets; HFP42: commercial milk replacer with early weaning at 42 days of age, and high fibre concentrate pellets; LFP42: high protein milk replacer from 2-16 days of age followed by commercial milk replacer with early weaning at 42 days of age, and low fibre concentrate pellets. Lambs were slaughtered at 57 days of age. Overall average daily liveweight gain of lambs did not differ (p > 0.05) between treatments. Dressing out percentage, carcass weight, empty small intestine and omental fat were higher (p < 0.05) in HFP57 than in both HFP42 and LFP42 lambs. HFP42 and LFP42 lambs had heavier (p < 0.05) empty rumen weights. Whole body protein content was higher (p < 0.05) in HFP42 lambs compared to both HFP57 and LFP42 lambs. Fat content and daily fat deposition were greater (p < 0.05) in HFP57 lambs than HFP42 and LFP42 lambs. Weaning lambs at 42 days of age with provision of either low or high fibre concentrate pellets, resulted in similar growth rates, reduced whole body fat deposition and was a more cost-effective rearing regimen.Published onlin
Effect of Palpable Udder Defects on Milk Yield, Somatic Cell Count, and Milk Composition in Non-Dairy Ewes
(c) The Author/sPublishe
Chandra observations of Cygnus OB2
Cygnus OB2 is the nearest example of a massive star forming region,
containing over 50 O-type stars and hundreds of B-type stars. We have analyzed
two Chandra pointings in Cyg OB2, detecting ~1700 X-ray sources, of which ~1450
are thought to be members of the association. Optical and near-IR photometry
has been obtained for ~90% of these sources from recent deep Galactic plane
surveys. We have performed isochrone fits to the near-IR color-magnitude
diagram, deriving ages of 3.5(+0.75,-1.0) and 5.25(+1.5,-1.0) Myrs for sources
in the two fields, both with considerable spreads around the pre-MS isochrones.
The presence of a second population in the region, somewhat older than the
present-day O-type stars, has been suggested by other authors and fits with the
ages derived here. The fraction of sources with inner circumstellar disks (as
traced by the K-band excess) is found to be very low, but appropriate for a
population of age ~5 Myrs. We measure the stellar mass functions and find a
power-law slope of Gamma = -1.09 +/- 0.13, in good agreement with the global
mean value estimated by Kroupa. A steepening of the mass function at high
masses is observed and we suggest this is due to the presence of the previous
generation of stars that have lost their most massive members. Finally,
combining our mass function and an estimate of the radial density profile of
the association suggests a total mass of Cyg OB2 of ~30,000 Msun, similar to
that of many of our Galaxy's most massive star forming regions.Comment: 6 pages, 4 figures, conference proceedings for JENAM 2010: Star
Clusters in the Era of Large Surveys, Editors: A.Moitinho and J. Alve
The formation of Kuiper-belt Binaries through Exchange Reactions
Recent observations have revealed an unexpectedly high binary fraction among
the Trans-Neptunian Objects (TNOs) that populate the Kuiper-belt. The
discovered binaries have four characteristics they comprise a few percent of
the TNOs, the mass ratio of their components is close to unity, their internal
orbits are highly eccentric, and the orbits are more than 100 times wider than
the primary's radius. In contrast, theories of binary asteroid formation tend
to produce close, circular binaries. Therefore, a new approach is required to
explain the unique characteristics of the TNO binaries. Two models have been
proposed. Both, however, require extreme assumptions on the size distribution
of TNOs. Here we show a mechanism which is guaranteed to produces binaries of
the required type during the early TNO growth phase, based on only one
plausible assumption, namely that initially TNOs were formed through
gravitational instabilities of the protoplanetary dust layer.Comment: 12pages, 4 figure
Rumen development of artificially-reared lambs exposed to three different rearing regimens
The objective of this study was to examine the effect of three different rearing regimens on rumen development in lambs reared artificially. Romney ram lambs were randomly allocated to one of three treatments: commercial milk replacer fed to 57 d of age and high fibre concentrate pellets (HFP57); commercial milk replacer, high fibre concentrate pellets, and early weaning from milk replacer at 42 d of age (HFP42); high protein milk replacer from 2–16 d of age followed by commercial milk replacer, low fibre concentrate pellets, and early weaning from milk replacer at 42 d of age (LFP42). Lambs were slaughtered at 57 d of age. Volatile fatty acid content in rumen fluid at slaughter was analysed and rumen tissue samples were collected for histological examination. The rumen n-butyric content was greater (p < 0.05) in both LFP42 and HFP42 treatment lambs compared to HFP57 lambs. The n-valeric content was greater (p < 0.05) in LFP42 lambs compared to both HFP57 and HFP42 treatment lambs. Thickness of the rumen dorsal wall determined by ultrasound scanning at 49 d was greater (p < 0.05) in both HFP42 and LFP42 lambs compared to HFP57 lambs. There was an interaction (p < 0.05) between treatment and site of rumen tissue sampling on papillae width, density, and rumen muscular layer thickness. Collectively, early weaning and the provision of a low fibre pellet leads to improved rumen function and physical development
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