356 research outputs found
Age consistency between exoplanet hosts and field stars
Transiting planets around stars are discovered mostly through photometric
surveys. Unlike radial velocity surveys, photometric surveys do not tend to
target slow rotators, inactive or metal-rich stars. Nevertheless, we suspect
that observational biases could also impact transiting-planet hosts. This paper
aims to evaluate how selection effects reflect on the evolutionary stage of
both a limited sample of transiting-planet host stars (TPH) and a wider sample
of planet-hosting stars detected through radial velocity analysis. Then, thanks
to uniform derivation of stellar ages, a homogeneous comparison between
exoplanet hosts and field star age distributions is developed. Stellar
parameters have been computed through our custom-developed isochrone placement
algorithm, according to PARSEC models. The notable aspects of our algorithm
include the treatment of element diffusion, activity checks in terms of
and , and the evaluation of the stellar evolutionary
speed in the Hertzsprung-Russel diagram in order to better constrain age.
Working with TPH, the observational stellar mean density allows
us to compute stellar luminosity even if the distance is not available, by
combining with the spectroscopic . The median value of
the TPH ages is Gyr. Even if this sample is not very large, however,
the result is very similar to what we found for the sample of spectroscopic
hosts, whose modal and median values are [3, 3.5) Gyr and Gyr,
respectively. Thus, these stellar samples suffer almost the same selection
effects. We also conclude that the age of our Sun is consistent with the age
distribution of solar neighbourhood MS stars with spectral types from late F to
early K, regardless of whether they harbour planets or not. We considered the
possibility that our selected samples are older than the average disc
population.Comment: 13 pages, 1 longtable, 9 figures. Accepted by A&
Revising the ages of planet-hosting stars
This article aims to measure the age of planet-hosting stars (SWP) through
stellar tracks and isochrones computed with the \textsl{PA}dova \&
T\textsl{R}ieste \textsl{S}tellar \textsl{E}volutionary \textsl{C}ode (PARSEC).
We developed algorithms based on two different techniques for determining the
ages of field stars: \emph{isochrone placement} and \emph{Bayesian estimation}.
Their application to a synthetic sample of coeval stars shows the intrinsic
limits of each method. For instance, the Bayesian computation of the modal age
tends to select the extreme age values in the isochrones grid. Therefore, we
used the isochrone placement technique to measure the ages of 317 SWP. We found
that of SWP have ages lower than 0.5 Gyr. The age distribution peaks
in the interval [1.5, 2) Gyr, then it decreases. However, of the
stars are older than 11 Gyr. The Sun turns out to be a common star that hosts
planets, when considering its evolutionary stage. Our SWP age distribution is
less peaked and slightly shifted towards lower ages if compared with ages in
the literature and based on the isochrone fit. In particular, there are no ages
below 0.5 Gyr in the literature.Comment: 16 pages, 18 figures. Accepted by A&
An efficient tool for the assisted design of SAR ADCs capacitive DACs
The optimal design of SAR ADCs requires the accurate estimate of nonlinearity and parasitic capacitance effects in the feedback charge redistribution DAC. Since both contributions depend on the specific array topology, complex calculations, custom modeling and heavy simulations in common circuit design environments are often required. This paper presents a MATLAB-based numerical environment to assist the design of the charge redistribution DACs adopted in SAR ADCs. The tool performs both parametric and statistical simulations taking into account capacitive mismatch and parasitic capacitances computing both differential and integral nonlinearity (DNL, INL). An excellent agreement is obtained with the results of circuit simulators (e.g. Cadence Spectre) featuring up to 10^4 shorter simulation time, allowing statistical simulations that would be otherwise impracticable. The switching energy and SNDR degradation due to static nonlinear effects are also estimated. Simulations and measurements on three designed and two fabricated prototypes confirm that the proposed tool can be used as a valid instrument to assist the design of a charge redistribution SAR ADC and to predict its static and dynamic metrics
Between the Greater and Lesser Caucasus: An Analysis of the Belts from the Tli Burial Ground and Their Contexts (Graves 40b, 215b, 425)
This article presents a new study of three burial contexts from the important Tli graveyard located in the Southern Caucasus. The three tombs contained bronze belts with features mainly pertaining to an Assyro-Urartian tradition. An analysis of the belts’ iconographic apparatuses is given, together with a new chronological position proposed for the belts and the tombs: our idea is that a possible date for them should be within the period between the 7th and 6th centuries B.C.E. The analysis showed, moreover, that the Tli necropolis must be considered a great example of the complex contacts and cultural exchanges between multiple realities located both in the Caucasus and in the Mesopotamian region
The ages of exoplanet hosts and field stars
The thesis aims at homogeneously determining the ages of exoplanet hosts and generic field stars using stellar evolutionary tracks and isochrones computed with the PAdova & TRieste Stellar Evolutionary Code (PARSEC), v. 1.0.
To achieve this goal, we fully developed two algorithms (MatLab/Octave language) based on two different statistical approaches: one is frequentistic, the other is Bayesian. The algorithms have been tested on several samples of synthetic stars, in order to evaluate their reliability and put in evidence their limits.
After that, we applied our custom-developed algorithms to two different samples of stars, belonging to the same spectral type range. They were essentially main-sequence G type stars located in the solar neighbourhood. One sample is made up of planet-hosting stars, while the other contains field stars that do not harbour planets, as far as we know.
First of all, we found that the age distribution of stars harbouring transiting planets is similar to that of stars whose planets have been discovered through the radial velocity technique. This suggests that these two subsets of stars with planets suffer essentially the same selection effects.
Finally we homogeneously compare planet-hosting stars with field stars. The histograms displaying the two age distributions peaks in the same age bin, i.e. [3, 3.5) Gyr, and have the same median age, that is 4.8 Gyr.
The median age is very similar to that of the Sun. On the one hand, this suggests that our star is in the typical evolutionary stage characterizing G-type stars of the solar neighbourhood. On the other hand, a median age of 4.8 Gyr results to be older than the typical age that is assumed for the thin disc, where the analyzed stars are located.
The selection we made in spectral type does introduce a bias. Moreover, it is likely that the sampling of stars limited only to the solar neighbourhood is not representative of the entire thin disc. By extending the analysis to farther distances, so that to include young star-forming regions such as the Taurus-Auriga complex or the Orion Nebula, we expect to collect much more young stars
A 64-Channel 965-μW Neural Recording SoC with UWB Wireless Transmission in 130-nm CMOS
This brief presents a 64-channel neural recording system-on-chip (SoC) with a 20-Mb/s wireless telemetry. Each channel of the analog front end consists of a low-noise bandpass amplifier, featuring a noise efficiency factor of 3.11 with an input-referred noise of 5.6 μVrms in a 0.001- to 10-kHz band and a 31.25-kSps 6-fJ/conversion-step 10-bit SAR analog-to-digital converter. The recorded signals are multiplexed in the digital domain and transmitted via an 11.7% efficiency pulse-position modulation ultrawideband transmitter, reaching a transmission range in excess of 7.5 m. The chip has been fabricated in a 130-nm CMOS process, measures 25 mm2, and dissipates 965 μW from a 0.5-V supply. This SoC features the lowest power per channel (15 μW) and the lowest energy per bit (48.2 pJ) among state-of-the-art wireless neural recording systems with a number of channels larger than 32. The proposed circuit is able to transmit the raw neural signal in a large bandwidth (up to 10 kHz) without performing any data compression or losing vital information, such as local field potentials
On the Hyperparameters influencing a PINN's generalization beyond the training domain
Physics-Informed Neural Networks (PINNs) are Neural Network architectures
trained to emulate solutions of differential equations without the necessity of
solution data. They are currently ubiquitous in the scientific literature due
to their flexible and promising settings. However, very little of the available
research provides practical studies that aim for a better quantitative
understanding of such architecture and its functioning. In this paper, we
analyze the performance of PINNs for various architectural hyperparameters and
algorithmic settings based on a novel error metric and other factors such as
training time. The proposed metric and approach are tailored to evaluate how
well a PINN generalizes to points outside its training domain. Besides, we
investigate the effect of the algorithmic setup on the outcome prediction of a
PINN, inside and outside its training domain, to explore the effect of each
hyperparameter. Through our study, we assess how the algorithmic setup of PINNs
influences their potential for generalization and deduce the settings which
maximize the potential of a PINN for accurate generalization. The study that we
present returns insightful and at times counterintuitive results on PINNs.
These results can be useful in PINN applications when defining the model and
evaluating it
MCMCI: A code to fully characterise an exoplanetary system
Useful information can be retrieved by analysing the transit light curve of a
planet-hosting star or induced radial velocity oscillations. However, inferring
the physical parameters of the planet, such as mass, size, and semi-major axis,
requires preliminary knowledge of some parameters of the host star, especially
its mass or radius, which are generally inferred through theoretical
evolutionary models. We seek to present and test a whole algorithm devoted to
the complete characterisation of an exoplanetary system thanks to the global
analysis of photometric or radial velocity time series combined with
observational stellar parameters derived either from spectroscopy or
photometry. We developed an integrated tool called MCMCI. This tool combines
the Markov chain Monte Carlo (MCMC) approach of analysing photometric or radial
velocity time series with a proper interpolation within stellar evolutionary
isochrones and tracks, known as isochrone placement, to be performed at each
chain step, to retrieve stellar theoretical parameters such as age, mass, and
radius. We tested the MCMCI on the HD 219134 multi-planetary system hosting two
transiting rocky super Earths and on WASP-4, which hosts a bloated hot Jupiter.
Even considering different input approaches, a final convergence was reached
within the code, we found good agreement with the results already stated in the
literature and we obtained more precise output parameters, especially
concerning planetary masses. The MCMCI tool offers the opportunity to perform
an integrated analysis of an exoplanetary system without splitting it into the
preliminary stellar characterisation through theoretical models. Rather this
approach favours a close interaction between light curve analysis and
isochrones, so that the parameters recovered at each step of the MCMC enter as
inputs for purposes of isochrone placement.Comment: 18 pages, 6 figures, 6 tables. Accepted for publication in A&A.
Source code link: https://github.com/Bonfanti88/MCMC
The Antecedents of the Technology Acceptance Model in Microentrepreneurs’ Intention to Use Social Networking Sites
Social media platforms offer significant growth opportunities for enterprises, particularly microenterprises, due to the chance to establish direct contact with customers. Drawing on the Technology-Acceptance Model (TAM), in the present study, we investigate the psychological reasons that lead microentrepreneurs to use Social Networking Sites (SNSs) for their business. In doing so, we also extend TAM by taking into account entrepreneurs’ personalities (e.g., extraversion and openness to experience) and their perceived risk. We collected data by examining 247 microentrepreneurs engaged in the production of handmade objects. Our results confirm that of all the TAM behavioral antecedents tested, perceived usefulness and attitude toward SNSs’ usage for business proved to be the best predictors of the intention to use SNSs for business activity. The results also indicate that extraversion, openness to experience, and perceived risk, as external factors, significantly affect the TAM constructs. We discuss implications and suggestions for future research
Suppression of Flicker Noise Up-Conversion in a 65-nm CMOS VCO in the 3.0-to-3.6 GHz Band
- …