9,734 research outputs found
On the Orbits of Low-mass Companions to White Dwarfs and the Fates of the Known Exoplanets
The ultimate fates of binary companions to stars (including whether the
companion survives and the final orbit of the binary) are of interest in light
of an increasing number of recently discovered, low-mass companions to white
dwarfs (WDs). In this Letter, we study the evolution of a two-body system
wherein the orbit adjusts due to structural changes in the primary, dissipation
of orbital energy via tides, and mass loss during the giant phases; previous
studies have not incorporated changes in the primary's spin. For companions
ranging from Jupiter's mass to ~0.3 Msun and primaries ranging from 1-3 Msun,
we determine the minimum initial semimajor axis required for the companion to
avoid engulfment by the primary during post-main-sequence evolution, and
highlight the implications for the ultimate survival of the known exoplanets.
We present regions in secondary mass and orbital period space where an engulfed
companion might be expected to survive the common envelope phase (CEP), and
compare with known M dwarf+WD short-period binaries. Finally, we note that
engulfed Earth-like planets cannot survive a CEP. Detection of a
first-generation terrestrial planet in the white dwarf habitable zone requires
scattering from a several-AU orbit to a high-eccentricity orbit (with a
periastron of ~Rsun) from which it is damped into a circular orbit via tidal
friction, possibly rendering it an uninhabitable, charred ember.Comment: Replaced with version in Journa
The formation of high-field magnetic white dwarfs from common envelopes
The origin of highly-magnetized white dwarfs has remained a mystery since
their initial discovery. Recent observations indicate that the formation of
high-field magnetic white dwarfs is intimately related to strong binary
interactions during post-main-sequence phases of stellar evolution. If a
low-mass companion, such as a planet, brown dwarf, or low-mass star is engulfed
by a post-main-sequence giant, the hydrodynamic drag in the envelope of the
giant leads to a reduction of the companion's orbit. Sufficiently low-mass
companions in-spiral until they are shredded by the strong gravitational tides
near the white dwarf core. Subsequent formation of a super-Eddington accretion
disk from the disrupted companion inside a common envelope can dramatically
amplify magnetic fields via a dynamo. Here, we show that these disk-generated
fields are sufficiently strong to explain the observed range of magnetic field
strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary
analogue may also contribute to the origin of magnetar fields.Comment: Accepted to Proceedings of the National Academy of Sciences. Under
PNAS embargo until time of publicatio
Higher-order Continuum Approximation for Rarefied Gases
The Hilbert-Chapman-Enskog expansion of the kinetic equations in mean flight
times is believed to be asymptotic rather than convergent. It is therefore
inadvisable to use lower order results to simplify the current approximation as
is done in the traditional Chapman-Enskog procedure, since that is an iterative
method. By avoiding such recycling of lower order results, one obtains
macroscopic equations that are asymptotically equivalent to the ones found in
the Chapman-Enskog approach. The new equations contain higher order terms that
are discarded in the Chapman-Enskog method. These make a significant impact on
the results for such problems as ultrasound propagation. In this paper, it is
shown that these results turn out well with relatively little complication when
the expansions are carried to second order in the mean free time, for the
example of the relaxation or BGK model of kinetic theory.Comment: 20 pages, 2 figures, RevTeX 4 macro
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
The tachocline revisited
The solar tachocline is a shear layer located at the base of the solar
convection zone. The horizontal shear in the tachocline is likely turbulent,
and it is often assumed that this turbulence would be strongly anisotropic as a
result of the local stratification. What role this turbulence plays in the
tachocline dynamics, however, remains to be determined. In particular, it is
not clear whether it would result in a turbulent eddy diffusivity, or
anti-diffusivity, or something else entirely. In this paper, we present the
first direct numerical simulations of turbulence in horizontal shear flows at
low Prandtl number, in an idealized model that ignores rotation and magnetic
fields. We find that several regimes exist, depending on the relative
importance of the stratification, viscosity and thermal diffusivity. Our
results suggest that the tachocline is in the stratified turbulence regime,
which has very specific properties controlled by a balance between buoyancy,
inertia, and thermal diffusion.Comment: Invited review for the meeting Dynamics of the Sun and Stars:
Honoring the Life and Work of Michael J. Thompson (Boulder, Colorado, 24-26
September 2019
Amplitude Modulation and Relaxation-Oscillation of Counterpropagating Rolls within a Broken-Symmetry Laser-Induced Electroconvection Strip
We report a liquid-crystal pattern-formation experiment in which we break the
lateral (translational) symmetry of a nematic medium with a laser-induced
thermal gradient. The work is motivated by an improved measurement (reported
here) of the temperature dependence of the electroconvection threshold voltage
in planar-nematic 4-methoxybenzylidene-4-butylaniline (MBBA). In contrast with
other broken-symmetry-pattern studies that report a uniform drift, we observe a
strip of counterpropagating rolls that collide at a sink point, and a strong
temporally periodic amplitude modulation within a width of 3-4 rolls about the
sink point. The time dependence of the amplitude at a fixed position is
periodic but displays a nonsinusoidal relaxation-oscillation profile. After
reporting experimental results based on spacetime contours and wavenumber
profiles, along with a measurement of the change in the drift frequency with
applied voltage at a fixed control parameter, we propose some potential
guidelines for a theoretical model based on saddle-point solutions for
Eckhaus-unstable states and coupled complex Ginzburg-Landau equations.
Published in PRE 73, 036317 (2006).Comment: Published in Physical Review E in March 200
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