426 research outputs found
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
Evolutionary Models of Super-Earths and Mini-Neptunes Incorporating Cooling and Mass Loss
We construct models of the structural evolution of super-Earth- and
mini-Neptune-type exoplanets with hydrogen-helium envelopes, incorporating
radiative cooling and XUV-driven mass loss. We conduct a parameter study of
these models, focusing on initial mass, radius, and envelope mass fractions, as
well as orbital distance, metallicity, and the specific prescription for mass
loss. From these calculations, we investigate how the observed masses and radii
of exoplanets today relate to the distribution of their initial conditions.
Orbital distance and initial envelope mass fraction are the most important
factors determining planetary evolution, particular radius evolution. Initial
mass also becomes important below a "turnoff mass," which varies with orbital
distance, with mass-radius curves being approximately flat for higher masses.
Initial radius is the least important parameter we study, with very little
difference between the hot start and cold start limits after an age of 100 Myr.
Model sets with no mass loss fail to produce results consistent with
observations, but a plausible range of mass loss scenarios is allowed. In
addition, we present scenarios for the formation of the Kepler-11 planets. Our
best fit to observations Kepler-11b and Kepler-11c involves formation beyond
the snow line, after which they moved inward, circularized, and underwent a
reduced degree mass loss.Comment: 17 pages, 18 figures, 1 table, Accepted to Ap
Axions and SN1987A
The effect of free-streaming axion emission on numerical models for the cooling of the newly born neutron star associated with SN1987A is considered. It is found that for an axion mass of greater than approximately 10 to the -3 eV, axion emission shortens the duration of the expected neutrino burst so significantly that it would be inconsistent with the neutrino observations made by the Kamiokande II and Irvine-Michigan-Brookhaven detectors. However, the possibility has not been investigated that axion trapping (which should occur for masses greater than or equal to 0.02 eV) sufficiently reduces axion emission so that axion masses greater than approximately 2 eV would be consistent with the neutrino observations
Mass-Radius Relations and Core-Envelope Decompositions of Super-Earths and Sub-Neptunes
Many exoplanets have been discovered with radii of 1-4 Earth radii, between
that of Earth and Neptune. A number of these are known to have densities
consistent with solid compositions, while others are "sub-Neptunes" likely to
have significant hydrogen-helium envelopes. Future surveys will no doubt
significantly expand these populations. In order to understand how the measured
masses and radii of such planets can inform their structures and compositions,
we construct models both for solid layered planets and for planets with solid
cores and gaseous envelopes, exploring a range of core masses, hydrogen-helium
envelope masses, and associated envelope entropies. For planets in the
super-Earth/sub-Neptune regime for which both radius and mass are measured, we
estimate how each is partitioned into a solid core and gaseous envelope,
associating a specific core mass and envelope mass with a given exoplanet. We
perform this decomposition for both "Earth-like" rock-iron cores and pure ice
cores, and find that the necessary gaseous envelope masses for this important
sub-class of exoplanets must range very widely from zero to many Earth masses,
even for a given core mass. This result bears importantly on exoplanet
formation and envelope evaporation processes.Comment: 26 pages, 21 figures, 16 tables, accepted to Ap
Axions and SN 1987A: Axion trapping
If an axion of mass between about 10(exp -3) eV and 1 eV exists, axion emission would have significantly affected the cooling of the nascent neutron star associated with SN 1987A. For an axion of mass less than about 10(exp -2) eV, axions produced deep inside the neutron star simply stream out; in a previous paper this case has been addressed. Remarkably, for an axion of mass greater than about 10(exp -2) eV axions would, like neutrinos, have a mean-free path that is smaller than the size of a neutron star, and thus would become 'trapped' and radiated from an axion sphere. In this paper the trapping regime is treated by using numerical models of the initial cooling of a hot neutron star that incorporate a leakage approximation scheme for axion-energy transport. The axion opacity is computed due to inverse nucleon-nucleon, axion bremsstrahlung, and numerical models are used to calculate the integrated axion luminosity, the temperature of the axion sphere, and the effect of axion emission on the neutrino bursts detected by the Kamiokande 2 (K2) and Irvine-Michigan-Brookhaven (IMB) water-Cherenkov detectors. The larger the axion mass, the stronger the trapping and the smaller the axion luminosity. The earlier estimate is confirmed and refined of the axion mass above which trapping is so strong that axion emission does not significantly affect the neutrino burst. Based upon the neutrino-burst duration--the most sensitive barometer of axion cooling--it is concluded that for an axion mass of greater than about 0.3 eV, axion emission would not have had a significant effect on the neutrino bursts detected by K2 and IMB. The present work, together with the previous work, strongly suggests that an axion with mass in the interval 10(exp -3) eV to 0.3 eV is excluded by SN 1987A
Updated Spitzer Emission Spectroscopy of Bright Transiting Hot Jupiter HD189733b
We analyze all existing secondary eclipse time series spectroscopy of hot
Jupiter HD189733b acquired with the now defunct Spitzer/IRS instrument. We
describe the novel approaches we develop to remove the systematic effects and
extract accurate secondary eclipse depths as a function of wavelength in order
to construct the emission spectrum of the exoplanet. We compare our results to
a previous study by Grillmair et al. that did not examine all data sets
available to us. We are able to confirm the detection of a water feature near
6{\mu}m claimed by Grillmair et al. We compare the planetary emission spectrum
to three model families -- based on isothermal atmosphere, gray atmosphere, and
two realizations of the complex radiative transfer model by Burrows et al.,
adopted in Grillmair et al.'s study. While we are able to reject the simple
isothermal and gray models based on the data at the 97% level just from the IRS
data, these rejections hinge on eclipses measured within relatively narrow
wavelength range, between 5.5 and 7{\mu}m. This underscores the need for
observational studies with broad wavelength coverage and high spectral
resolution, in order to obtain robust information on exoplanet atmospheres.Comment: 16 pages, 13 figures and 3 tables. Accepted for publication in Ap
The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
There is no universally acknowledged criterion to distinguish brown dwarfs
from planets. Numerous studies have used or suggested a definition based on an
object's mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of
deuterium. Here, we investigate various deuterium-burning masses for a range of
models. We find that, while 13 M_J is generally a reasonable rule of thumb, the
deuterium fusion mass depends on the helium abundance, the initial deuterium
abundance, the metallicity of the model, and on what fraction of an object's
initial deuterium abundance must combust in order for the object to qualify as
having burned deuterium. Even though, for most proto-brown dwarf conditions,
50% of the initial deuterium will burn if the object's mass is ~(13.0 +/-
0.8)M_J, the full range of possibilities is significantly broader. For models
ranging from zero-metallicity to more than three times solar metallicity, the
deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity,
10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of
initial deuterium burned).Comment: "Models" section expanded, references added, accepted by Ap
Criteria for Core-Collapse Supernova Explosions by the Neutrino Mechanism
We investigate the criteria for successful core-collapse supernova explosions
by the neutrino mechanism. We find that a
critical-luminosity/mass-accretion-rate condition distinguishes non-exploding
from exploding models in hydrodynamic one-dimensional (1D) and two-dimensional
(2D) simulations. We present 95 such simulations that parametrically explore
the dependence on neutrino luminosity, mass accretion rate, resolution, and
dimensionality. While radial oscillations mediate the transition between 1D
accretion (non-exploding) and exploding simulations, the non-radial standing
accretion shock instability characterizes 2D simulations. We find that it is
useful to compare the average dwell time of matter in the gain region with the
corresponding heating timescale, but that tracking the residence time
distribution function of tracer particles better describes the complex flows in
multi-dimensional simulations. Integral quantities such as the net heating
rate, heating efficiency, and mass in the gain region decrease with time in
non-exploding models, but for 2D exploding models, increase before, during, and
after explosion. At the onset of explosion in 2D, the heating efficiency is
2% to 5% and the mass in the gain region is 0.005 M_{\sun}
to 0.01 M_{\sun}. Importantly, we find that the critical luminosity for
explosions in 2D is 70% of the critical luminosity required in 1D. This
result is not sensitive to resolution or whether the 2D computational domain is
a quadrant or the full 180. We suggest that the relaxation of the
explosion condition in going from 1D to 2D (and to, perhaps, 3D) is of a
general character and is not limited by the parametric nature of this study.Comment: 32 pages in emulateapj, including 17 figures, accepted for
publication in ApJ, included changes suggested by the refere
Kicks and Induced Spins of Neutron Stars at Birth
Using simulations of non-rotating supernova progenitors, we explore the kicks
imparted to and the spins induced in the compact objects birthed in core
collapse. We find that the recoil due to neutrino emissions can be a factor
affecting core recoil, comparable to and at times larger than the corresponding
kick due to matter recoil. This result would necessitate a revision of the
general model of the origin of pulsar proper motions. In addition, we find that
the sign of the net neutrino momentum can be opposite to the sign of the
corresponding matter recoil. As a result, at times the pulsar recoil and ejecta
can be in the same direction. Moreover, our results suggest that the duration
of the dipole in the neutrino emissions can be shorter than the duration of the
radiation of the neutron-star binding energy. This allows a larger dipole
asymmetry to arise, but for a shorter time, resulting in kicks in the observed
pulsar range. Furthermore, we find that the spin induced by the aspherical
accretion of matter can leave the residues of collapse with spin periods
comparable to those inferred for radio pulsars and that there seems to be a
slight anti-correlation between the direction of the induced spin and the net
kick direction. This could explain such a correlation among observed radio
pulsars. Finally, we find that the kicks imparted to black holes are due to the
neutrino recoil alone, resulting in birth kicks 100 km s most of
the time.Comment: 29 pages, 24 figures, accepted for publication in MNRA
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