14,764 research outputs found
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
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
Legendrian and transverse twist knots
In 1997, Chekanov gave the first example of a Legendrian nonsimple knot type:
the knot. Epstein, Fuchs, and Meyer extended his result by showing
that there are at least different Legendrian representatives with maximal
Thurston--Bennequin number of the twist knot with crossing number
. In this paper we give a complete classification of Legendrian and
transverse representatives of twist knots. In particular, we show that
has exactly Legendrian representatives with
maximal Thurston--Bennequin number, and transverse
representatives with maximal self-linking number. Our techniques include convex
surface theory, Legendrian ruling invariants, and Heegaard Floer homology.Comment: 27 pages, v3: added figure, other minor changes, to appear in JEM
Interior Structures and Tidal Heating in the TRAPPIST-1 Planets
With seven planets, the TRAPPIST-1 system has the largest number of
exoplanets discovered in a single system so far. The system is of
astrobiological interest, because three of its planets orbit in the habitable
zone of the ultracool M dwarf. Assuming the planets are composed of
non-compressible iron, rock, and HO, we determine possible interior
structures for each planet. To determine how much tidal heat may be dissipated
within each planet, we construct a tidal heat generation model using a single
uniform viscosity and rigidity for each planet based on the planet's
composition. With the exception of TRAPPIST-1c, all seven of the planets have
densities low enough to indicate the presence of significant HO in some
form. Planets b and c experience enough heating from planetary tides to
maintain magma oceans in their rock mantles; planet c may have eruptions of
silicate magma on its surface, which may be detectable with next-generation
instrumentation. Tidal heat fluxes on planets d, e, and f are lower, but are
still twenty times higher than Earth's mean heat flow. Planets d and e are the
most likely to be habitable. Planet d avoids the runaway greenhouse state if
its albedo is 0.3. Determining the planet's masses within
to 0.5 Earth masses would confirm or rule out the presence of HO and/or
iron in each planet, and permit detailed models of heat production and
transport in each planet. Understanding the geodynamics of ice-rich planets f,
g, and h requires more sophisticated modeling that can self-consistently
balance heat production and transport in both rock and ice layers.Comment: 34 pages, 3 tables, 4 figures. Accepted for publication in Astronomy
& Astrophysics -- final version including corrections made in proof stag
Possibility for albedo estimation of exomoons: Why should we care about M dwarfs?
Occultation light curves of exomoons may give information on their albedo and
hence indicate the presence of ice cover on the surface. Icy moons might have
subsurface oceans thus these may potentially be habitable. The objective of our
paper is to determine whether next generation telescopes will be capable of
albedo estimations for icy exomoons using their occultation light curves. The
success of the measurements depends on the depth of the moon's occultation in
the light curve and on the sensitivity of the used instruments. We applied
simple calculations for different stellar masses in the V and J photometric
bands, and compared the flux drop caused by the moon's occultation and the
estimated photon noise of next generation missions with 5 confidence.
We found that albedo estimation by this method is not feasible for moons of
solar-like stars, but small M dwarfs are better candidates for such
measurements. Our calculations in the J photometric band show that E-ELT
MICADO's photon noise is just about 4 ppm greater than the flux difference
caused by a 2 Earth-radii icy satellite in a circular orbit at the snowline of
an 0.1 stellar mass star. However, considering only photon noise underestimates
the real expected noise, because other noise sources, such as CCD read-out and
dark signal become significant in the near infrared measurements. Hence we
conclude that occultation measurements with next generation missions are far
too challenging, even in the case of large, icy moons at the snowline of small
M dwarfs. We also discuss the role of the parameters that were neglected in the
calculations, e.g. inclination, eccentricity, orbiting direction of the moon.
We predict that the first albedo estimations of exomoons will probably be made
for large icy moons around the snowline of M4 -- M9 type main sequence stars.Comment: 13 pages, 6 figures, accepted for publication in A&
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