240 research outputs found
The Evolution and Internal Structure of Jupiter and Saturn with Compositional Gradients
The internal structure of gas giant planets may be more complex than the
commonly assumed core-envelope structure with an adiabatic temperature profile.
Different primordial internal structures as well as various physical processes
can lead to non-homogenous compositional distributions. A non-homogenous
internal structure has a significant impact on the thermal evolution and final
structure of the planets. In this paper, we present alternative structure and
evolution models for Jupiter and Saturn allowing for non-adiabatic primordial
structures and the mixing of heavy elements by convection as these planets
evolve. We present the evolution of the planets accounting for various initial
composition gradients, and in the case of Saturn, include the formation of a
helium-rich region as a result of helium rain. We investigate the stability of
regions with composition gradients against convection, and find that the helium
shell in Saturn remains stable and does not mix with the rest of the envelope.
In other cases, convection mixes the planetary interior despite the existence
of compositional gradients, leading to the enrichment of the envelope with
heavy elements. We show that non-adiabatic structures (and cooling histories)
for both Jupiter and Saturn are feasible. The interior temperatures in that
case are much higher that for standard adiabatic models. We conclude that the
internal structure is directly linked to the formation and evolution history of
the planet. These alternative internal structures of Jupiter and Saturn should
be considered when interpreting the upcoming Juno and Cassini data.Comment: accepted for publication in Ap
A Thousand and One Nova Outbursts
Multicycle nova evolution models have been calculated over the past twenty
years, the number being limited by numerical constraints. Here we present a
long-term evolution code that enables a continuous calculation through an
unlimited number of nova cycles for an unlimited evolution time, even up to (or
exceeding) a Hubble time. Starting with two sets of the three independent nova
parameters -- the white dwarf mass, the temperature of its isothermal core, and
the rate of mass transfer on to it -- we have followed the evolution of two
models, with initial masses of 1 and 0.65 solar masses, accretion rates
(constant throughout each calculation) of 1e-11 and 1e-9 solar-masses/yr, and
relatively high initial temperatures (as they are likely to be at the onset of
the outburst phase), through over 1000 and over 3000 cycles, respectively. The
results show that although on the short-term consecutive outbursts are almost
identical, on the long-term scale the characteristics change. This is mainly
due to the changing core temperature, which decreases very similarly to that of
a cooling white dwarf for a time, but at a slower rate thereafter. As the white
dwarf's mass continually decreases, since both models lose more mass than they
accrete, the central pressure decreases accordingly. The outbursts on the
massive white dwarf change gradually from fast to moderately fast, and the
other characteristics (velocity, abundance ratios, isotopic ratios) change,
too. Very slowly, a steady state is reached, where all characteristics, both in
quiescence and in outburst, remain almost constant. For the less massive white
dwarf accreting at a high rate, outbursts are similar throughout the evolution.Comment: To be published in MNRA
Defining the Termination of the Asymptotic Giant Branch
I suggest a theoretical quantitative definition for the termination of the
asymptotic giant branch (AGB) phase and the beginning of the post-AGB phase. I
suggest that the transition will be taken to occur when the ratio of the
dynamical time scale to the the envelope thermal time scale, Q, reaches its
maximum value. Time average values are used for the different quantities, as
the criterion does not refer to the short time-scale variations occurring on
the AGB and post-AGB, e.g., thermal pulses (helium shell flashes) and magnetic
activity. Along the entire AGB the value of Q increases, even when the star
starts to contract. Only when a rapid contraction starts does the value of Q
start to decrease. This criterion captures the essence of the transition from
the AGB to the post AGB phase, because Q is connected to the stellar effective
temperature, reaching its maximum value at T~4000-6000 K, it is related to the
mass loss properties, and it reaches its maximum value when rapid contraction
starts and envelope mass is very low.Comment: Submitted to ApJ Letter
The Electrodynamics of Inhomogeneous Rotating Media and the Abraham and Minkowski Tensors II: Applications
Applications of the covariant theory of drive-forms are considered for a
class of perfectly insulating media. The distinction between the notions of
"classical photons" in homogeneous bounded and unbounded stationary media and
in stationary unbounded magneto-electric media is pointed out in the context of
the Abraham, Minkowski and symmetrized Minkowski electromagnetic
stress-energy-momentum tensors. Such notions have led to intense debate about
the role of these (and other) tensors in describing electromagnetic
interactions in moving media. In order to address some of these issues for
material subject to the Minkowski constitutive relations, the propagation of
harmonic waves through homogeneous and inhomogeneous, isotropic plane-faced
slabs at rest is first considered. To motivate the subsequent analysis on
accelerating media two classes of electromagnetic modes that solve Maxwell's
equations for uniformly rotating homogeneous polarizable media are enumerated.
Finally it is shown that, under the influence of an incident monochromatic,
circularly polarized, plane electromagnetic wave, the Abraham and symmetrized
Minkowski tensors induce different time-averaged torques on a uniformly
rotating materially inhomogeneous dielectric cylinder. We suggest that this
observation may offer new avenues to explore experimentally the covariant
electrodynamics of more general accelerating media.Comment: 29 pages, 4 figures. Accepted for publication in Proc. Roy. Soc.
How Sensitive is the CMB to a Single Lens?
We study the imprints of a single lens, that breaks statistical isotropy, on
the CMB and calculate the signal to noise ratio (S/N) for its detection. We
emphasize the role of non-Gaussianities induced by LCDM weak lensing in this
calculation and show that typically the S/N is much smaller than expected. In
particular we find that the hypothesis that a void (texture) is responsible for
the WMAP cold spot can barely (cannot) be tested via weak lensing of the CMB.Comment: Accepted for publication in JCAP, 24 pages, 5 figure
Inflection Point Inflation and Time Dependent Potentials in String Theory
We consider models of inflection point inflation. The main drawback of such
models is that they suffer from the overshoot problem. Namely the initial
condition should be fine tuned to be near the inflection point for the universe
to inflate. We show that stringy realizations of inflection point inflation are
common and offer a natural resolution to the overshoot problem.Comment: 15 pages, 2 figures, refs. adde
Cosmological Imprints of Pre-Inflationary Particles
We study some of the cosmological imprints of pre-inflationary particles. We
show that each such particle provides a seed for a spherically symmetric cosmic
defect. The profile of this cosmic defect is fixed and its magnitude is linear
in a single parameter that is determined by the mass of the pre-inflationary
particle. We study the CMB and peculiar velocity imprints of this cosmic defect
and suggest that it could explain some of the large scale cosmological
anomalies.Comment: 31 pages, 7 figure
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