3,840 research outputs found
Pushing the limits: K2 observations of the trans-Neptunian objects 2002 GV31 and (278361) 2007 JJ43
We present the first photometric observations of trans-Neptunian objects
(TNOs) taken with the Kepler space telescope, obtained in the course of the K2
ecliptic survey. Two faint objects have been monitored in specifically designed
pixel masks that were centered on the stationary points of the objects, when
their daily motion was the slowest. In the design of the experiment, only the
apparent path of these objects were retrieved from the detectors, i.e. the
costs in terms of Kepler pixels were minimized. Because of the faintness of the
targets we employ specific reduction techniques and co-added images. We measure
rotational periods and amplitudes in the unfiltered Kepler band as follows: for
(278361) 2007 JJ43 and 2002 GV31 we get P_rot=12.097 h and P_rot=29.2 h while
0.10 and 0.35 mag for the total amplitudes, respectively. Future space
missions, like TESS and PLATO are not well suited to this kind of observations.
Therefore, we encourage to include the brightest TNOs around their stationary
points in each observing campaign to exploit this unique capability of the K2
Mission -- and therefore to provide unbiased rotational, shape and albedo
characteristics of many objects.Comment: Accepted for publication in ApJ Letters, 5.2 pages in emulateapj
style, misspelled 2007 JJ43 designation correcte
Mapping a star with transits: orbit precession effects in the Kepler-13 system
Kepler-13b (KOI-13.01) is a most intriguing exoplanet system due to the rapid
precession rate, exhibiting several exotic phenomena. We analyzed
Short Cadence data up to Quarter 14, with a total time-span of 928 days, to
reveal changes in transit duration, depth, asymmetry, and identify the possible
signals of stellar rotation and low-level activity. We investigated long-term
variations of transit light curves, testing for duration, peak depth and
asymmetry. We also performed cluster analysis on quarters. We computed
the autocorrelation function of the out-of-transit light variations. Transit
duration, peak depth, and asymmetry evolve slowly, due to the slowly drifting
transit path through the stellar disk. The detected transit shapes will map the
stellar surface on the time scale of decades. We found a very significant
clustering pattern with 3-orbit period. Its source is very probably the
rotating stellar surface, in the 5:3 spin-orbit resonance reported in a
previous study. The autocorrelation function of the out-of-transit light
variations, filtered to 25.4 hours and harmonics, shows slow variations and a
peak around 300--360 day period, which could be related to the activity cycle
of the host star.Comment: 7 pages, 7 figures, accepted in MNRA
Phase transition and selection in a four-species cyclic Lotka-Volterra model
We study a four species ecological system with cyclic dominance whose
individuals are distributed on a square lattice. Randomly chosen individuals
migrate to one of the neighboring sites if it is empty or invade this site if
occupied by their prey. The cyclic dominance maintains the coexistence of all
the four species if the concentration of vacant sites is lower than a threshold
value. Above the treshold, a symmetry breaking ordering occurs via growing
domains containing only two neutral species inside. These two neutral species
can protect each other from the external invaders (predators) and extend their
common territory. According to our Monte Carlo simulations the observed phase
transition is equivalent to those found in spreading models with two equivalent
absorbing states although the present model has continuous sets of absorbing
states with different portions of the two neutral species. The selection
mechanism yielding symmetric phases is related to the domain growth process
whith wide boundaries where the four species coexist.Comment: 4 pages, 5 figure
Main-Belt Asteroids in the K2 Engineering Field of View
Unlike NASA's original Kepler Discovery Mission, the renewed K2 Mission will
stare at the plane of the Ecliptic, observing each field for approximately 75
days. This will bring new opportunities and challenges, in particular the
presence of a large number of main-belt asteroids that will contaminate the
photometry. The large pixel size makes K2 data susceptible to the effect of
apparent minor planet encounters. Here we investigate the effects of asteroid
encounters on photometric precision using a sub-sample of the K2 Engineering
data taken in February, 2014. We show examples of asteroid contamination to
facilitate their recognition and distinguish these events from other error
sources. We conclude that main-belt asteroids will have considerable effects on
K2 photometry of a large number of photometric targets during the Mission, that
will have to be taken into account. These results will be readily applicable
for future space photometric missions applying large-format CCDs, such as TESS
and PLATO.Comment: accepted for publication in AJ, 6 page
CHEOPS performance for exomoons: The detectability of exomoons by using optimal decision algorithm
Many attempts have already been made for detecting exomoons around transiting
exoplanets but the first confirmed discovery is still pending. The experience
that have been gathered so far allow us to better optimize future space
telescopes for this challenge, already during the development phase. In this
paper we focus on the forthcoming CHaraterising ExOPlanet Satellite
(CHEOPS),describing an optimized decision algorithm with step-by-step
evaluation, and calculating the number of required transits for an exomoon
detection for various planet-moon configurations that can be observable by
CHEOPS. We explore the most efficient way for such an observation which
minimizes the cost in observing time. Our study is based on PTV observations
(photocentric transit timing variation, Szab\'o et al. 2006) in simulated
CHEOPS data, but the recipe does not depend on the actual detection method, and
it can be substituted with e.g. the photodynamical method for later
applications. Using the current state-of-the-art level simulation of CHEOPS
data we analyzed transit observation sets for different star-planet-moon
configurations and performed a bootstrap analysis to determine their detection
statistics. We have found that the detection limit is around an Earth-sized
moon. In the case of favorable spatial configurations, systems with at least
such a large moon and with at least Neptune-sized planet, 80\% detection chance
requires at least 5-6 transit observations on average. There is also non-zero
chance in the case of smaller moons, but the detection statistics deteriorates
rapidly, while the necessary transit measurements increase fast. (abridged)Comment: 32 pages, 14 figures, accepted for publication in PAS
Frequentist coverage of adaptive nonparametric Bayesian credible sets
We investigate the frequentist coverage of Bayesian credible sets in a
nonparametric setting. We consider a scale of priors of varying regularity and
choose the regularity by an empirical Bayes method. Next we consider a central
set of prescribed posterior probability in the posterior distribution of the
chosen regularity. We show that such an adaptive Bayes credible set gives
correct uncertainty quantification of "polished tail" parameters, in the sense
of high probability of coverage of such parameters. On the negative side, we
show by theory and example that adaptation of the prior necessarily leads to
gross and haphazard uncertainty quantification for some true parameters that
are still within the hyperrectangle regularity scale.Comment: Published at http://dx.doi.org/10.1214/14-AOS1270 in the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Causation, Measurement Relevance and No-conspiracy in EPR
In this paper I assess the adequacy of no-conspiracy conditions employed in
the usual derivations of the Bell inequality in the context of EPR
correlations. First, I look at the EPR correlations from a purely
phenomenological point of view and claim that common cause explanations of
these cannot be ruled out. I argue that an appropriate common cause explanation
requires that no-conspiracy conditions are re-interpreted as mere common
cause-measurement independence conditions. In the right circumstances then,
violations of measurement independence need not entail any kind of conspiracy
(nor backwards in time causation). To the contrary, if measurement operations
in the EPR context are taken to be causally relevant in a specific way to the
experiment outcomes, their explicit causal role provides the grounds for a
common cause explanation of the corresponding correlations.Comment: 20 pages, 1 figur
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