18,212 research outputs found
Stability Boundaries for Resonant Migrating Planet Pairs
Convergent migration allows pairs of planet to become trapped into mean
motion resonances. Once in resonance, the planets' eccentricities grow to an
equilibrium value that depends on the ratio of migration time scale to the
eccentricity damping timescale, , with higher values of
equilibrium eccentricity for lower values of . For low equilibrium
eccentricities, . The stability of a planet pair
depends on eccentricity so the system can become unstable before it reaches its
equilibrium eccentricity. Using a resonant overlap criterion that takes into
account the role of first and second order resonances and depends on
eccentricity, we find a function that defines the lowest
value for , as a function of the ratio of total planet mass to stellar mass
() and the period ratio of the resonance defined as ,
that allows two convergently migrating planets to remain stable in resonance at
their equilibrium eccentricities. We scaled the functions for each
resonance of the same order into a single function . The function
for planet pairs in first order resonances is linear with increasing planet
mass and quadratic for pairs in second order resonances with a coefficient
depending on the relative migration rate and strongly on the planet to planet
mass ratio. The linear relation continues until the mass approaches a critical
mass defined by the 2/7 resonance overlap instability law and .
We compared our analytic boundary with an observed sample of resonant two
planet systems. All but one of the first order resonant planet pair systems
found by radial velocity measurements are well inside the stability region
estimated by this model. We calculated for Kepler systems without
well-constrained eccentricities and found only weak constraints on .Comment: 11 pages, 7 figure
MANAGING THE INCONCEIVABLE: PARTICIPATORY ASSESSMENTS OF IMPACTS AND RESPONSES TO EXTREME CLIMATE CHANGE
A comprehensive understanding of the implications of extreme climate change requires an in-depth exploration of the perceptions and reactions of the affected stakeholder groups and the lay public. The project on âAtlantic sea level rise: Adaptation to imaginable worst-case climate changeâ (Atlantis) has studied one such case, the collapse of the West Antarctic Ice Sheet and a subsequent 5-6 meter sea-level rise. Possible methods are presented for assessing the societal consequences of impacts and adaptation options in selected European regions by involving representatives of pertinent stakeholders. Results of a comprehensive review of participatory integrated assessment methods with a view to their applicability in climate impact studies are summarized including Simulation-Gaming techniques, the Policy Exercise method, and the Focus Group technique. Succinct presentations of these three methods are provided together with short summaries of relevant earlier applications to gain insights into the possible design options. Building on these insights, four basic versions of design procedures suitable for use in the Atlantis project are presented. They draw on design elements of several methods and combine them to fit the characteristics and fulfill the needs of addressing the problem of extreme sea-level rise. The selected participatory techniques and the procedure designs might well be useful in other studies assessing climate change impacts and exploring adaptation options.sea level rise, West Antarctic ice sheet, climate change
Purveyors of fine halos: Re-assessing globular cluster contributions to the Milky Way halo build-up with SDSS-IV
There is ample evidence in the Milky Way for globular cluster (GC)
disruption. Hence one may expect that also part of the Galactic halo field
stars may once have formed in GCs. We quantify the fraction of halo stars
donated by GCs by searching for stars that bear the unique chemical
fingerprints typical for a subset of GC stars often dubbed `second-generation
stars'. These are stars showing light element abundance anomalies such as a
pronounced CN-band strength accompanied by weak CH-bands. Based on this
indicator, past studies have placed the fraction of halo stars with a GC origin
between a few to up to 50%. Using low-resolution spectra from the most recent
data release of the latest extension of the Sloan Digital Sky Survey (SDSS-IV),
we were able to identify 118 metal-poor ([Fe/H]) CN-strong
stars in a sample of 4470 halo giant stars out to 50 kpc. This results in an
observed fraction of these stars of 2.60.2%. Using an updated formalism to
account for the fraction of stars lost early on in the GCs' evolution we
estimate the fraction of the halo that stems from disrupted clusters to be
111%. This number represents the case that stars lost from GCs were
entirely from the first generation and is thus merely an upper limit. Our
conclusions are sensitive to our assumptions of the mass lost early on from the
first generation formed in the GCs, the ratio of first-to-second generation
stars, and other GC parameters. We carefully test the influence of varying
these parameters on the final result and find that, under realistic scenarios,
the above fraction depends on the main assumptions at less than 10%. We further
recover a flat trend in this fraction with Galactocentric radius, with a
marginal indication of a rise beyond 30 kpc that could reflect the ex-situ
origin of the outer halo. (abridged)Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Cyanoacetylene in IC 342: An Evolving Dense Gas Component with Starburst Age
We present the first images of the J=5-4 and J=16-15 lines of the dense gas
tracer, cyanoacetylene, HC_3N, in an external galaxy. The central 200 pc of the
nearby star-forming spiral galaxy, IC 342, was mapped using the VLA and the
Plateau de Bure Interferometer. HC_3N(5-4) line emission is found across the
nuclear mini-spiral, but is very weak towards the starburst site, the location
of the strongest mid-IR and radio emission. The J=16-15 and 10-9 lines are also
faint near the large HII region complex, but are brighter relative to the 5-4
line, consistent with higher excitation. The brightest HC_3N emission is
located in the northern arm of the nuclear minispiral, 100 pc away from the
radio/IR source to the southwest of the nucleus. This location appears less
affected by ultraviolet radiation, and may represent a more embedded, earlier
stage of star formation. HC_3N excitation temperatures are consistent with
those determined from C^{18}O; the gas is dense, 10^{4-5}/cc, and cool, T_K ~<
40 K. So as to not violate limits on the total H_2 mass determined from
C^{18}O, at least two dense components are required to model IC 342's giant
molecular clouds. These observations suggest that HC_3N(5-4) is an excellent
probe of the dense, quiescent gas in galaxies. The high excitation combined
with faint emission towards the dense molecular gas at the starburst indicates
that it currently lacks large masses of very dense gas. We propose a scenario
where the starburst is being caught in the act of dispersing or destroying its
dense gas in the presence of the large HII region. This explains the high star
formation efficiency seen in the dense component. The little remaining dense
gas appears to be in pressure equilibrium with the starburst HII region.Comment: Accepted, AJ. 12 pages, 5 figure
Retrofitting O'Raifeartaigh Models with Dynamical Scales
We provide a method for obtaining simple models of supersymmetry breaking,
with all small mass scales generated dynamically, and illustrate it with
explicit examples. We start from models of perturbative supersymmetry breaking,
such as O'Raifeartaigh and Fayet models, that would respect an symmetry if
their small input parameters transformed as the superpotential does. By
coupling the system to a pure supersymmetric Yang-Mills theory (or a more
general supersymmetric gauge theory with dynamically small vacuum expectation
values), these parameters are replaced by powers of its dynamical scale in a
way that is naturally enforced by the symmetry. We show that supersymmetry
breaking in these models may be straightforwardly mediated to the
supersymmetric Standard Model, obtain complete models of direct gauge
mediation, and comment on related model building strategies that arise in this
simple framework.Comment: 15 pages, harvmac bi
A note on non-homogeneous deformations with homogeneous Cauchy stress for a strictly rank-one convex energy in isotropic hyperelasticity
It has recently been shown that for a Cauchy stress response induced by a
strictly rank-one convex hyperelastic energy potential, a homogeneous Cauchy
stress tensor field cannot correspond to a non-homogeneous deformation if the
deformation gradient has discrete values, i.e. if the deformation is piecewise
affine linear and satisfies the Hadamard jump condition. In this note, we
expand upon these results and show that they do not hold for arbitrary
deformations by explicitly giving an example of a strictly rank-one convex
energy and a non-homogeneous deformation such that the induced Cauchy stress
tensor is constant. In the planar case, our example is related to another
previous result concerning criteria for generalized convexity properties of
conformally invariant energy functions, which we extend to the case of strict
rank-one convexity
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