2,645 research outputs found
Starspots and spin-orbit alignment for Kepler cool host stars
The angle between the spin axis of the host star and the orbit of its planets
(i.e., the stellar obliquity) is precious information about the formation and
evolution of exoplanetary systems. Measurements of the Rossiter-McLaughlin
effect revealed that many stars that host a hot-Jupiter have high obliquities,
suggesting that hot-Jupiter formation involves excitation of orbital
inclinations. In this contribution we show how the passage of the planet over
starspots can be used to measure the obliquity of exoplanetary systems. This
technique is used to obtain - for the first time - the obliquity of a system
with several planets that lie in a disk, Kepler-30, with the result that the
star has an obliquity smaller than 10 degrees. The implications for the
formation of exoplanetary systems, in particular the hot-Jupiter population,
are also discussed.Comment: To appear in special edition of AN, proceedings of the Cool Stars 17
conference, Barcelona June 201
Dynamical Aspects of Generalized Palatini Theories of Gravity
We study the field equations of modified theories of gravity in which the
lagrangian is a general function of the Ricci scalar and Ricci-squared terms in
Palatini formalism. We show that the independent connection can be expressed as
the Levi-Civita connection of an auxiliary metric which, in particular cases of
interest, is related with the physical metric by means of a disformal
transformation. This relation between physical and auxiliary metric boils down
to a conformal transformation in the case of f(R) theories. We also show with
explicit models that the inclusion of Ricci squared terms in the action can
impose upper bounds on the accessible values of pressure and density, which
might have important consequences for the early time cosmology and black hole
formation scenarios. Our results indicate that the phenomenology of
f(R_{ab}R^{ab}) theories is much richer than that of f(R) and f(R_{ab}R^{ab})
theories and that they also share some similarities with Bekenstein's
relativistic theory of MOND.Comment: 8 pages, no figure
A Study of the Shortest-Period Planets Found With Kepler
We present the results of a survey aimed at discovering and studying
transiting planets with orbital periods shorter than one day
(ultra--short-period, or USP, planets), using data from the {\em Kepler}
spacecraft. We computed Fourier transforms of the photometric time series for
all 200,000 target stars, and detected transit signals based on the presence of
regularly spaced sharp peaks in the Fourier spectrum. We present a list of 106
USP candidates, of which 18 have not previously been described in the
literature. In addition, among the objects we studied, there are 26 USP
candidates that had been previously reported in the literature which do not
pass our various tests. All 106 of our candidates have passed several standard
tests to rule out false positives due to eclipsing stellar systems. A low false
positive rate is also implied by the relatively high fraction of candidates for
which more than one transiting planet signal was detected. By assuming these
multi-transit candidates represent coplanar multi-planet systems, we are able
to infer that the USP planets are typically accompanied by other planets with
periods in the range 1-50 days, in contrast with hot Jupiters which very rarely
have companions in that same period range. Another clear pattern is that almost
all USP planets are smaller than 2 , possibly because gas giants in
very tight orbits would lose their atmospheres by photoevaporation when subject
to extremely strong stellar irradiation. Based on our survey statistics, USP
planets exist around approximately of G-dwarf stars, and
of K-dwarf stars.Comment: 20 pages, 10 figures. Submitted to ApJ. This version has been
reviewed by a refere
Bouncing Cosmologies in Palatini Gravity
We consider the early time cosmology of f(R) theories in Palatini formalism
and study the conditions that guarantee the existence of homogeneous and
isotropic models that avoid the Big Bang singularity. We show that for such
models the Big Bang singularity can be replaced by a cosmic bounce without
violating any energy condition. In fact, the bounce is possible even for
pressureless dust. We give a characterization of such models and discuss their
dynamics in the region near the bounce. We also find that power-law lagrangians
with a finite number of terms may lead to non-singular universes, which
contrasts with the infinite-series Palatini f(R) lagrangian that one needs to
fully capture the effective dynamics of Loop Quantum Cosmology. We argue that
these models could also avoid the formation of singularities during stellar
gravitational collapse.Comment: 8 pages, 4 figures; added references and a short comment in sec.I
Transits and Occultations of an Earth-Sized Planet in an 8.5-Hour Orbit
We report the discovery of an Earth-sized planet () in
an 8.5-hour orbit around a late G-type star (KIC 8435766, Kepler-78). The
object was identified in a search for short-period planets in the {\it Kepler}
database and confirmed to be a transiting planet (as opposed to an eclipsing
stellar system) through the absence of ellipsoidal light variations or
substantial radial-velocity variations. The unusually short orbital period and
the relative brightness of the host star ( = 11.5) enable robust
detections of the changing illumination of the visible hemisphere of the
planet, as well as the occultations of the planet by the star. We interpret
these signals as representing a combination of reflected and reprocessed light,
with the highest planet dayside temperature in the range of 2300 K to 3100 K.
Follow-up spectroscopy combined with finer sampling photometric observations
will further pin down the system parameters and may even yield the mass of the
planet.Comment: Accepted for publication, ApJ, 10 pages and 6 figure
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