6,986 research outputs found
Detecting transit signatures of exoplanetary rings using SOAP3.0
CONTEXT. It is theoretically possible for rings to have formed around
extrasolar planets in a similar way to that in which they formed around the
giant planets in our solar system. However, no such rings have been detected to
date.
AIMS: We aim to test the possibility of detecting rings around exoplanets by
investigating the photometric and spectroscopic ring signatures in
high-precision transit signals.
METHODS: The photometric and spectroscopic transit signals of a ringed planet
is expected to show deviations from that of a spherical planet. We used these
deviations to quantify the detectability of rings. We present SOAP3.0 which is
a numerical tool to simulate ringed planet transits and measure ring
detectability based on amplitudes of the residuals between the ringed planet
signal and best fit ringless model.
RESULTS: We find that it is possible to detect the photometric and
spectroscopic signature of near edge-on rings especially around planets with
high impact parameter. Time resolution 7 mins is required for the
photometric detection, while 15 mins is sufficient for the spectroscopic
detection. We also show that future instruments like CHEOPS and ESPRESSO, with
precisions that allow ring signatures to be well above their noise-level,
present good prospects for detecting rings.Comment: 13 pages, 16 figures, 2 tables , accepted for publication in A&
Classification of areas using pixel-by-pixel and sample classifiers
There are no author-identified significant results in this report
Distinguishing the albedo of exoplanets from stellar activity
Light curves show the flux variation from the target star and its orbiting
planets as a function of time. In addition to the transit features created by
the planets, the flux also includes the reflected light component of each
planet, which depends on the planetary albedo. This signal is typically
referred to as phase curve and could be easily identified if there were no
additional noise. As well as instrumental noise, stellar activity, such as
spots, can create a modulation in the data, which may be very difficult to
distinguish from the planetary signal. We analyze the limitations imposed by
the stellar activity on the detection of the planetary albedo, considering the
limitations imposed by the predicted level of instrumental noise and the short
duration of the observations planned in the context of the CHEOPS mission. As
initial condition, we have assumed that each star is characterized by just one
orbiting planet. We built mock light curves that included a realistic stellar
activity pattern, the reflected light component of the planet and an
instrumental noise level, which we have chosen to be at the same level as
predicted for CHEOPS. We then fit these light curves to try to recover the
reflected light component, assuming the activity patterns can be modeled with a
Gaussian process.We estimate that at least one full stellar rotation is
necessary to obtain a reliable detection of the planetary albedo. This result
is independent of the level of noise, but it depends on the limitation of the
Gaussian process to describe the stellar activity when the light curve
time-span is shorter than the stellar rotation. Finally, in presence of typical
CHEOPS gaps in the simulations, we confirm that it is still possible to obtain
a reliable albedo.Comment: Accepted for publication in A&A, 14 pages, 12 figure
Measuring the orbit shrinkage rate of hot Jupiters due to tides
A tidal interaction between a star and a close-in exoplanet leads to
shrinkage of the planetary orbit and eventual tidal disruption of the planet.
Measuring the shrinkage of the orbits will allow for the tidal quality
parameter of the star () to be measured, which is an important
parameter to obtain information about stellar interiors. We analyse data from
TESS for two targets known to host close-in hot Jupiters, WASP-18 and WASP-19,
to measure the current limits on orbital period variation and provide new
constrains on . We modelled the transit shape using all the available
TESS observations and fitted the individual transit times of each transit. We
used previously published transit times together with our results to fit two
models, a constant period model, and a quadratic orbital decay model, MCMC
algorithms. We find period change rates of for
WASP-18b and for WASP-19b and we do not find
significant evidence of orbital decay in these targets. We obtain new lower
limits for of in WASP-18 and
in WASP-19, corresponding to upper limits of the
orbital decay rate of and ,
respectively, with a 95% confidence level. We compare our results with other
relevant targets for tidal decay studies. We find that the orbital decay rate
in both WASP-18b and WASP-19b appears to be smaller than the measured orbital
decay of WASP-12b. We show that the minimum value of in WASP-18 is
two orders of magnitude higher than that of WASP-12, while WASP-19 has a
minimum value one order of magnitude higher, which is consistent with other
similar targets. Further observations are required to constrain the orbital
decay of WASP-18 and WASP-19.Comment: 10 pages plus 5-page appendix. To be published in Astronomy and
Astrophysic
Optimal switching of a nanomagnet assisted by microwaves
We develop an efficient and general method for optimizing the microwave field
that achieves magnetization switching with a smaller static field. This method
is based on optimal control and renders an exact solution for the 3D microwave
field that triggers the switching of a nanomagnet with a given anisotropy and
in an oblique static field. Applying this technique to the particular case of
uniaxial anisotropy, we show that the optimal microwave field, that achieves
switching with minimal absorbed energy, is modulated both in frequency and in
magnitude. Its role is to drive the magnetization from the metastable
equilibrium position towards the saddle point and then damping induces the
relaxation to the stable equilibrium position. For the pumping to be efficient,
the microwave field frequency must match at the early stage of the switching
process the proper precession frequency of the magnetization, which depends on
the magnitude and direction of the static field. We investigate the effect of
the static field (in amplitude and direction) and of damping on the
characteristics of the microwave field. We have computed the switching curves
in the presence of the optimal microwave field. The results are in qualitative
agreement with micro-SQUID experiments on isolated nanoclusters. The strong
dependence of the microwave field and that of the switching curve on the
damping parameter may be useful in probing damping in various nanoclusters.Comment: 9 pages, 8 figure
Flashing annihilation term of a logistic kinetic as a mechanism leading to Pareto distributions
It is shown analytically that the flashing annihilation term of a Verhulst
kinetic leads to the power--law distribution in the stationary state. For the
frequency of switching slower than twice the free growth rate this provides the
quasideterministic source of a Levy noises at the macroscopic level.Comment: 1 fi
An integrated approach for analysing and assessing the performance of virtual learning groups
Collaborative distance learning involves a variety of elements and factors that have to be considered and measured in order to analyse and assess group and individual performance more effectively and objectively. This paper presents an approach that integrates qualitative, social network analysis (SNA) and quantitative techniques for evaluating online collaborative learning interactions. Integration of various different data sources, tools and techniques provides a more complete and robust framework for group modelling and guarantees a more efficient evaluation of group effectiveness and individual competence. Our research relies on the analysis of a real, long-term, complex collaborative experience, which is initially evaluated in terms of principled criteria and a basic qualitative process. At the end of the experience, the coded student interactions are further analysed through the SNA technique to assess participatory aspects, identify the most effective groups and the most prominent actors. Finally, the approach is contrasted and completed through a statistical technique which sheds more light on the results obtained that far. The proposal draws a well-founded line toward the development of a principled framework for the monitoring and analysis of group interaction and group scaffolding which can be considered a major issue towards the actual application of the CSCL proposals to real classrooms.Peer ReviewedPostprint (author's final draft
Soil Phosphorus Availability and Eucalypt Phosphorus Uptake from Soluble and Insoluble Sources of Phosphorus
P recovery efficiency from natural rock phosphates and a concentrated phosphate by Eucalyptus grandis plantation in relation to triple superphosphate was evaluated in field trials conducted in the cerrado area of Brazil. Two experiments were carried out in two sites of the Savanna area of Minas Gerais State, Brazil. The rates of the natural phosphates (RP) Araxa and Patos de Minas(P2O5 24% and Ca 25%), were 500, 1000, 2000, and 4000 kg/ha and Arafertil (33% -P2O5 and 33% Ca)concentrated rock was tested using 1000kg/ha rate only. The triple superphosphate (TP = 45% -P2O5 and 13% Ca) was applied at 250, 500, 1000, and 2000 kg/ha. Mehlich -1 and Bray -1 extractants were used to extract P from the soil. The concentration of P extracted varied with the reagent used. Mehlich-1 extracted about 38.8% more P over the Bray -1 extractant at both sites, although, in one of the sites the extraction was 18% higher than the other. On both experimental sites, application of phosphate from both natural and concentrated forms increased tree height, stem volume and above ground dry matter production, but there was no difference between them at the rate of 1000 kg/ha. P-fixing capacity by trees increased with increasing soil P utilization efficiency. P recovery by the trees varied from 3 to 11% depending on rates and source. But P fixing capacity was higher for TP than for RP. Keywords: Araxa rock, concentrated arafertil; recovery efficiency, P-fixing capacity and eucalypt
- …