12,300 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
Thermal Processes Governing Hot-Jupiter Radii
There have been many proposed explanations for the larger-than-expected radii
of some transiting hot Jupiters, including either stellar or orbital energy
deposition deep in the atmosphere or deep in the interior. In this paper, we
explore the important influences on hot-Jupiter radius evolution of (i)
additional heat sources in the high atmosphere, the deep atmosphere, and deep
in the convective interior; (ii) consistent cooling of the deep interior
through the planetary dayside, nightside, and poles; (iii) the degree of heat
redistribution to the nightside; and (iv) the presence of an upper atmosphere
absorber inferred to produce anomalously hot upper atmospheres and inversions
in some close-in giant planets. In particular, we compare the radius expansion
effects of atmospheric and deep-interior heating at the same power levels and
derive the power required to achieve a given radius increase when night-side
cooling is incorporated. We find that models that include consistent day/night
cooling are more similar to isotropically irradiated models when there is more
heat redistributed from the dayside to the nightside. In addition, we consider
the efficacy of ohmic heating in the atmosphere and/or convective interior in
inflating hot Jupiters. Among our conclusions are that (i) the most highly
irradiated planets cannot stably have uB > (10 km/s Gauss) over a large
fraction of their daysides, where u is the zonal wind speed and B is the
dipolar magnetic field strength in the atmosphere, and (ii) that ohmic heating
cannot in and of itself lead to a runaway in planet radius.Comment: Accepted by ApJ., 20 pages, 11 figure
Signatures of X-rays in the early Universe
[abridged] With their long mean free paths and efficient heating of the
intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal
and ionization history of the Universe. We explore this in various signals: (i)
Reionization history: including X-rays results in an earlier, more extended
reionization. Efficient thermal feedback from X-ray heating could yield an
extended, ~10% ionized epoch. (ii) Reionization morphology: a sizable (~10%)
contribution of X-rays to reionization results in a more uniform morphology,
though the impact is modest when compared at the same global neutral fraction,
xH. However, changes in morphology cannot be countered by increasing the bias
of the ionizing sources, making them a robust signature. (iii) The kinetic
Sunyaev-Zel'dovich (kSZ) effect: at a fixed reionization history, X-rays
decrease the kSZ power at l=3000 by ~0.5 microK^2. Our extreme model in which
X-rays dominate reionization is the only one that is marginally consistent with
upper limits from the South Pole Telescope, assuming no thermal
Sunyaev-Zel'dovich (tSZ) - dusty galaxy correlation. Since this extreme model
is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy
signal. (iv) The cosmic 21cm signal: the impact of X-rays on the 21cm power
spectrum during the advanced stages of reionization (xH<0.7) is modest, except
in extreme, X-ray dominated models. The largest impact of X-rays is to govern
IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray
heating likely overlaps with the beginning of reionization (xH>0.9). This
results in a 21cm power spectrum which is ~ 10-100 times higher than obtained
from naive estimates ignoring this overlap. However, if thermal feedback is
efficient, the resulting extended epoch between X-ray heating and reionization
could provide a clean probe of the matter power spectrum in emission.Comment: 17 pages, 12 figures, MNRAS in-pres
A prospectus for a theory of variable variability
It is proposed that the kind of stellar variability exhibited by the Sun in its magnetic activity cycle should be considered as a prototype of a class of stellar variability. The signature includes long 'periods' (compared to that of the radial fundamental model), erratic behavior, and intermittency. As other phenomena in the same variability class we nominate the liminosity fluctuations of ZZ Ceti stars and the solar 160 m oscillation. We discuss the possibility that analogous physical mechanisms are at work in all these cases, namely instabilities driven in a thin layer. These instabilities should be favorable to grave modes (in angle) and should arise in conditions that may allow more than one kind of instability to occur at once. The interaction of these competing instabilities produces complicated temporal variations. Given suitable idealizations, it is shown how to begin to compute solutions of small, but finite, amplitude
The Structure of Exoplanets
The hundreds of exoplanets that have been discovered in the past two decades
offer a new perspective on planetary structure. Instead of being the archetypal
examples of planets, those of our Solar System are merely possible outcomes of
planetary system formation and evolution, and conceivably not even terribly
common outcomes (although this remains an open question). Here, we review the
diverse range of interior structures that are known to, and speculated to,
exist in exoplanetary systems -- from mostly degenerate objects that are more
than 10 times as massive as Jupiter, to intermediate-mass Neptune-like objects
with large cores and moderate hydrogen/helium envelopes, to rocky objects with
roughly the mass of the Earth.Comment: To be published in PNAS special issue on exoplanets. 6 pages, 3
figure
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