861 research outputs found
The Detectability of Transit Depth Variations due to Exoplanetary Oblateness and Spin Precession
Knowledge of an exoplanet's oblateness and obliquity would give clues about
its formation and internal structure. In principle, a light curve of a
transiting planet bears information about the planet's shape, but previous work
has shown that the oblateness-induced signal will be extremely difficult to
detect. Here we investigate the potentially larger signals due to planetary
spin precession. The most readily detectable effects are transit depth
variations (TV) in a sequence of light curves. For a planet as oblate
as Jupiter or Saturn, the transit depth will undergo fractional variations of
order 1%. The most promising systems are those with orbital periods of
approximately 15--30 days, which is short enough for the precession period to
be less than about 40 years, and long enough to avoid spin-down due to tidal
friction. The detectability of the TV signal would be enhanced by moons
(which would decrease the precession period) or planetary rings (which would
increase the amplitude). The Kepler mission should find several planets for
which precession-induced TV signals will be detectable. Due to modeling
degeneracies, Kepler photometry would yield only a lower bound on oblateness.
The degeneracy could be lifted by observing the oblateness-induced asymmetry in
at least one transit light curve, or by making assumptions about the planetary
interior.Comment: Accepted for publication in The Astrophysical Journa
Are Tidal Effects Responsible for Exoplanetary Spin-Orbit Alignment?
The obliquities of planet-hosting stars are clues about the formation of
planetary systems. Previous observations led to the hypothesis that for
close-in giant planets, spin-orbit alignment is enforced by tidal interactions.
Here, we examine two problems with this hypothesis. First, Mazeh and coworkers
recently used a new technique -- based on the amplitude of starspot-induced
photometric variability -- to conclude that spin-orbit alignment is common even
for relatively long-period planets, which would not be expected if tides were
responsible. We re-examine the data and find a statistically significant
correlation between photometric variability and planetary orbital period that
is qualitatively consistent with tidal interactions. However it is still
difficult to explain quantitatively, as it would require tides to be effective
for periods as long as tens of days. Second, Rogers and Lin argued against a
particular theory for tidal re-alignment by showing that initially retrograde
systems would fail to be re-aligned, in contradiction with the observed
prevalence of prograde systems. We investigate a simple model that overcomes
this problem by taking into account the dissipation of inertial waves and the
equilibrium tide, as well as magnetic braking. We identify a region of
parameter space where re-alignment can be achieved, but it only works for
close-in giant planets, and requires some fine tuning. Thus, while we find both
problems to be more nuanced than they first appeared, the tidal model still has
serious shortcomings.Comment: 12 pages, 9 figures. Accepted for publication in Ap
The Oblique Orbit of WASP-107b from K2 Photometry
Observations of nine transits of WASP-107 during the {\it K2} mission reveal
three separate occasions when the planet crossed in front of a starspot. The
data confirm the stellar rotation period to be 17 days --- approximately three
times the planet's orbital period --- and suggest that large spots persist for
at least one full rotation. If the star had a low obliquity, at least two
additional spot crossings should have been observed. They were not observed,
giving evidence for a high obliquity. We use a simple geometric model to show
that the obliquity is likely in the range 40-140, i.e., both spin-orbit
alignment and anti-alignment can be ruled out. WASP-107 thereby joins the small
collection of relatively low-mass stars hosting a giant planet with a high
obliquity. Most such stars have been observed to have low obliquities; all the
exceptions, including WASP-107, involve planets with relatively wide orbits
("warm Jupiters", with ). This demonstrates a
connection between stellar obliquity and planet properties, in contradiction to
some theories for obliquity excitation.Comment: Submitted to AAS journal
Evidence for the Tidal Destruction of Hot Jupiters by Subgiant Stars
Tidal transfer of angular momentum is expected to cause hot Jupiters to
spiral into their host stars. Although the timescale for orbital decay is very
uncertain, it should be faster for systems with larger and more evolved stars.
Indeed, it is well established that hot Jupiters are found less frequently
around subgiant stars than around main-sequence stars. However, the
interpretation of this finding has been ambiguous, because the subgiants are
also thought to be more massive than the F- and G-type stars that dominate the
main-sequence sample. Consequently it has been unclear whether the absence of
hot Jupiters is due to tidal destruction, or inhibited formation of those
planets around massive stars. Here we show that the Galactic space motions of
the planet-hosting subgiant stars demand that on average they be similar in
mass to the planet-hosting main-sequence F- and G-type stars. Therefore the two
samples are likely to differ only in age, and provide a glimpse of the same
exoplanet population both before and after tidal evolution. As a result, the
lack of hot Jupiters orbiting subgiants is clear evidence for their tidal
destruction. Questions remain, though, about the interpretation of other
reported differences between the planet populations around subgiants and
main-sequence stars, such as their period and eccentricity distributions and
overall occurrence rates.Comment: 12 pages and 6 figures in emulateapj format; accepted for publication
in Ap
Obliquities of Kepler Stars: Comparison of Single- and Multiple-Transit Systems
The stellar obliquity of a transiting planetary system can be constrained by
combining measurements of the star's rotation period, radius, and projected
rotational velocity. Here we present a hierarchical Bayesian technique for
recovering the obliquity distribution of a population of transiting planetary
systems, and apply it to a sample of 70 Kepler Objects of Interest. With ~95%
confidence we find that the obliquities of stars with only a single detected
transiting planet are systematically larger than those with multiple detected
transiting planets. This suggests that a substantial fraction of Kepler's
single-transiting systems represent dynamically hotter, less orderly systems
than the "pancake-flat" multiple-transiting systems.Comment: 8 pages, 7 figures, accepted to Ap
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