479 research outputs found
Statistics of Extreme Gravitational Lensing Events. I.The Zero Shear Case
For a given source and lens pair, there is a thin on-axis tube-like volume
behind the lens in which the radiation flux from the source is greatly
increased due to gravitational lensing. Any objects (such as dust grains) which
pass through such a thin tube will experience strong bursts of radiation, i.e.,
Extreme Gravitational Lensing Events (EGLE). We study the physics and
statistics of EGLE for the case in which finite source size is more important
than shear. One of the several possible significant astrophysical effects is
investigated with an illustrative calculation.Comment: revised and final published version including a new section on the
destruction of dust grains in globular clusters as an exampl
On the Eccentricity Distribution of Exoplanets from Radial Velocity Surveys
We investigate the estimation of orbital parameters by least-
Keplerian fits to radial velocity (RV) data using synthetic data sets. We find
that while the fitted period is fairly accurate, the best-fit eccentricity and
are systematically biased upward from the true values for low
signal-to-noise ratio and moderate number of observations
, leading to a suppression of the number of nearly
circular orbits. Assuming intrinsic distributions of orbital parameters, we
generate a large number of mock RV data sets and study the selection effect on
the eccentricity distribution. We find the overall detection efficiency only
mildly decreases with eccentricity. This is because although high eccentricity
orbits are more difficult to sample, they also have larger RV amplitudes for
fixed planet mass and orbital semi-major axis. Thus the primary source of
uncertainties in the eccentricity distribution comes from biases in Keplerian
fits to detections with low-amplitude and/or small , rather than
from selection effects. Our results suggest that the abundance of
low-eccentricity exoplanets may be underestimated in the current sample and we
urge caution in interpreting the eccentricity distributions of low-amplitude
detections in future RV samples.Comment: Accepted for publication in Ap
Viscoelastic Models of Tidally Heated Exomoons
Tidal heating of exomoons may play a key role in their habitability, since
the elevated temperature can melt the ice on the body even without significant
solar radiation. The possibility of life is intensely studied on Solar System
moons such as Europa or Enceladus, where the surface ice layer covers tidally
heated water ocean. Tidal forces may be even stronger in extrasolar systems,
depending on the properties of the moon and its orbit. For studying the tidally
heated surface temperature of exomoons, we used a viscoelastic model for the
first time. This model is more realistic than the widely used, so-called fixed
Q models, because it takes into account the temperature dependency of the tidal
heat flux, and the melting of the inner material. With the use of this model we
introduced the circumplanetary Tidal Temperate Zone (TTZ), that strongly
depends on the orbital period of the moon, and less on its radius. We compared
the results with the fixed Q model and investigated the statistical volume of
the TTZ using both models. We have found that the viscoelastic model predicts
2.8 times more exomoons in the TTZ with orbital periods between 0.1 and 3.5
days than the fixed Q model for plausible distributions of physical and orbital
parameters. The viscoelastic model gives more promising results in terms of
habitability, because the inner melting of the body moderates the surface
temperature, acting like a thermostat.Comment: accepted for publication in Ap
Gravitational lensing in modified Newtonian dynamics
Modified Newtonian dynamics (MOND) is an alternative theory of gravity that
aims to explain large-scale dynamics without recourse to any form of dark
matter. However the theory is incomplete, lacking a relativistic counterpart,
and so makes no definite predictions about gravitational lensing. The most
obvious form that MONDian lensing might take is that photons experience twice
the deflection of massive particles moving at the speed of light, as in general
relativity (GR). In such a theory there is no general thin-lens approximation
(although one can be made for spherically-symmetric deflectors), but the
three-dimensional acceleration of photons is in the same direction as the
relativistic acceleration would be. In regimes where the deflector can
reasonably be approximated as a single point-mass (specifically low-optical
depth microlensing and weak galaxy-galaxy lensing), this naive formulation is
consistent with observations. Forthcoming galaxy-galaxy lensing data and the
possibility of cosmological microlensing have the potential to distinguish
unambiguously between GR and MOND. Some tests can also be performed with
extended deflectors, for example by using surface brightness measurements of
lens galaxies to model quasar lenses, although the breakdown of the thin-lens
approximation allows an extra degree of freedom. Nonetheless, it seems unlikely
that simple ellipsoidal galaxies can explain both constraints. Further, the
low-density universe implied by MOND must be completely dominated by the
cosmological constant (to fit microwave background observations), and such
models are at odds with the low frequency of quasar lenses. These conflicts
might be resolved by a fully consistent relativistic extension to MOND; the
alternative is that MOND is not an accurate description of the universe.Comment: MNRAS, in press; 11 pages, 10 figure
The Effect of Multiple Heat Sources on Exomoon Habitable Zones
With dozens of Jovian and super-Jovian exoplanets known to orbit their host
stars in or near the stellar habitable zones, it has recently been suggested
that moons the size of Mars could offer abundant surface habitats beyond the
solar system. Several searches for such exomoons are now underway, and the
exquisite astronomical data quality of upcoming space missions and ground-based
extremely large telescopes could make the detection and characterization of
exomoons possible in the near future. Here we explore the effects of tidal
heating on the potential of Mars- to Earth-sized satellites to host liquid
surface water, and we compare the tidal heating rates predicted by tidal
equilibrium model and a viscoelastic model. In addition to tidal heating, we
consider stellar radiation, planetary illumination and thermal heat from the
planet. However, the effects of a possible moon atmosphere are neglected. We
map the circumplanetary habitable zone for different stellar distances in
specific star-planet-satellite configurations, and determine those regions
where tidal heating dominates over stellar radiation. We find that the
`thermostat effect' of the viscoelastic model is significant not just at large
distances from the star, but also in the stellar habitable zone, where stellar
radiation is prevalent. We also find that tidal heating of Mars-sized moons
with eccentricities between 0.001 and 0.01 is the dominant energy source beyond
3--5 AU from a Sun-like star and beyond 0.4--0.6 AU from an M3 dwarf star. The
latter would be easier to detect (if they exist), but their orbital stability
might be under jeopardy due to the gravitational perturbations from the star.Comment: accepted for publication in A&A, 8 pages, 4 figure
- …