5,931 research outputs found
A possible contribution to CMB anisotropies at high l from primordial voids
We present preliminary results of an analysis into the effects of primordial
voids on the cosmic microwave background (CMB). We show that an inflationary
bubble model of void formation predicts excess power in the CMB angular power
spectrum that peaks between 2000 < l < 3000. Therefore, voids that exist on or
close to the last scattering surface at the epoch of decoupling can contribute
significantly to the apparent rise in power on these scales recently detected
by the Cosmic Background Imager (CBI).Comment: 5 pages, 3 figures. MNRAS accepted versio
First order resonance overlap and the stability of close two planet systems
Motivated by the population of multi-planet systems with orbital period
ratios 1<P2/P1<2, we study the long-term stability of packed two planet
systems. The Hamiltonian for two massive planets on nearly circular and nearly
coplanar orbits near a first order mean motion resonance can be reduced to a
one degree of freedom problem (Sessin & Ferraz Mello (1984), Wisdom (1986),
Henrard et al. (1986)). Using this analytically tractable Hamiltonian, we apply
the resonance overlap criterion to predict the onset of large scale chaotic
motion in close two planet systems. The reduced Hamiltonian has only a weak
dependence on the planetary mass ratio, and hence the overlap criterion is
independent of the planetary mass ratio at lowest order. Numerical integrations
confirm that the planetary mass ratio has little effect on the structure of the
chaotic phase space for close orbits in the low eccentricity (e <~0.1) regime.
We show numerically that orbits in the chaotic web produced primarily by first
order resonance overlap eventually experience large scale erratic variation in
semimajor axes and are Lagrange unstable. This is also true of the orbits in
this overlap region which are Hill stable. As a result, we can use the first
order resonance overlap criterion as an effective stability criterion for pairs
of observed planets. We show that for low mass (<~10 M_Earth) planetary systems
with initially circular orbits the period ratio at which complete overlap
occurs and widespread chaos results lies in a region of parameter space which
is Hill stable. Our work indicates that a resonance overlap criterion which
would apply for initially eccentric orbits needs to take into account second
order resonances. Finally, we address the connection found in previous work
between the Hill stability criterion and numerically determined Lagrange
instability boundaries in the context of resonance overlap.Comment: Accepted for publication in Ap
The evolution and star formation of dwarf galaxies in the Fornax Cluster
We present the results of a spectroscopic survey of 675 bright (16.5<Bj<18)
galaxies in a 6 degree field centred on the Fornax cluster with the FLAIR-II
spectrograph on the UK Schmidt Telescope. We measured redshifts for 516
galaxies of which 108 were members of the Fornax Cluster. Nine of these are new
cluster members previously misidentified as background galaxies. The cluster
dynamics show that the dwarf galaxies are still falling into the cluster
whereas the giants are virialised. Our spectral data reveal a higher rate of
star formation among the dwarf galaxies than suggested by morphological
classification: 35 per cent have H-alpha emission indicative of star formation
but only 19 per cent were morphologically classified as late-types. The
distribution of scale sizes is consistent with evolutionary processes which
transform late-type dwarfs to early-type dwarfs. The fraction of dwarfs with
active star formation drops rapidly towards the cluster centre. The
star-forming dwarfs are concentrated in the outer regions of the cluster, the
most extreme in an infalling subcluster. We estimate gas depletion time scales
for 5 dwarfs with detected HI emission: these are long (of order 10 Gyr),
indicating that active gas removal must be involved if they are transformed
into gas-poor dwarfs as they fall further into the cluster. In agreement with
our previous results, we find no compact dwarf elliptical (M32-like) galaxies
in the Fornax Cluster.Comment: To appear in Monthly Notices of the Royal Astronomical Societ
Stability of Satellites in Closely Packed Planetary Systems
We perform numerical integrations of four-body (star, planet, planet,
satellite) systems to investigate the stability of satellites in planetary
Systems with Tightly-packed Inner Planets (STIPs). We find that the majority of
closely-spaced stable two-planet systems can stably support satellites across a
range of parameter-space which is only slightly decreased compared to that seen
for the single-planet case. In particular, circular prograde satellites remain
stable out to (where is the Hill Radius) as opposed to
in the single-planet case. A similarly small restriction in the
stable parameter-space for retrograde satellites is observed, where planetary
close approaches in the range 2.5 to 4.5 mutual Hill radii destabilize most
satellites orbits only if . In very close planetary pairs (e.g.
the 12:11 resonance) the addition of a satellite frequently destabilizes the
entire system, causing extreme close-approaches and the loss of satellites over
a range of circumplanetary semi-major axes. The majority of systems
investigated stably harbored satellites over a wide parameter-space, suggesting
that STIPs can generally offer a dynamically stable home for satellites, albeit
with a slightly smaller stable parameter-space than the single-planet case. As
we demonstrate that multi-planet systems are not a priori poor candidates for
hosting satellites, future measurements of satellite occurrence rates in
multi-planet systems versus single-planet systems could be used to constrain
either satellite formation or past periods of strong dynamical interaction
between planets.Comment: 11 pages, 5 figures. Accepted for publication, ApJ
Techniques for achieving magnetic cleanliness on deep-space missions
Techniques for obtaining magnetic cleanliness on deep space missions to allow interplanetary magnetic field mappin
Canadian national sport organisations’ use of the web for relationship marketing in promoting sport participation
Sport participation development requires a systematic process which involves knowledge creation, dissemination and interactions between National Sport Organisations, participants, clubs and associations as well as other agencies. Using a relationship marketing approach (Grönroos, 1997, Gummesson, 2002, Olkkonen, 1999), this paper addressed the question ‘How do Canadian NSOs use the Web, in terms of functionality and services offered, to create and maintain relationships with sport participants and their sport delivery partners?’ Ten Canadian NSOs’ websites were examined: functionality was analysed using Burgess and Cooper’s (2000) eMICA model, while NSOs’ utilisation of the Internet to establish and maintain relationships with sport participants was analysed using Wang, Head and Archer’s (2000) relationship-building process model for the Web. It was found that Canadian NSOs were receptive to the use of the Web, but their information-gathering and dissemination activities, which make-up the relationship-building process, appear sparse, and in some cases are lagging behind the voluntary sector in the country
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