631 research outputs found
Parameter extraction techniques for the analysis and modeling of resistive memories
A revision of the different numerical techniques employed to extract resistive switching (RS) and modeling parameters is presented. The set and reset voltages, commonly used for variability estimation, are calculated for different resistive memory technologies. The methodologies to extract the series resistance and the parameters linked to the charge-flux memristive modeling approach are also described. It is found that the obtained cycle-to-cycle (C2C) variability depends on the numerical technique used. This result is important, and it implies that when analyzing C2C variability, the extraction technique should be described to perform fair comparisons between different resistive memory technologies. In addition to the use of extensive experimental data for different types of resistive memories, we have also included kinetic Monte Carlo (kMC) simulations to study the formation and rupture events of the percolation paths that constitute the conductive filaments (CF) that allow resistive switching operation in filamentary unipolar and bipolar devices.Consejería de Conocimiento,
Investigaci ́on y Universidad, Junta de Andalucía (Spain) and the FEDER
program for the projects A.TIC.117.UGR18, B-TIC-624-UGR20 and
IE2017-5414Ramón y Cajal grant No. RYC2020-030150-IFunding for open access charge: Universidad de
Granada/CBU
Inflammatory markers and bone mass in children with overweight/obesity: the role of muscular fitness
Objectives
To examine which inflammatory markers are associated with bone mass and whether this association varies according to muscular fitness in children with overweight/obesity.
Methods
Plasma interleukin-1β (IL-1β), IL-6, tumor necrosis factor-α (TNF-α), epidermal growth factor, vascular endothelial growth factor A (VEGF), and C-reactive protein were analyzed in 55 children aged 8–11 years. A muscular fitness score was computed. Bone mineral content (BMC) of the total body-less head (TBLH) and lumbar spine (LS) were assessed using dual-energy x-ray absorptiometry.
Results
IL-6 (β = −0.136) and VEGF (β = −0.099) were associated with TBLH BMC, while TNF-α (β = −0.345) and IL-1β (β = 0.212) were associated with LS BMC (P < 0.05). The interaction effect of muscular fitness showed a trend in the association of VEGF with TBLH BMC (P = 0.122) and TNF-α with LS BMC (P = 0.057). Stratified analyses by muscular fitness levels showed an inverse association of VEGF with TBLH BMC (β = −0.152) and TNF-α with LS BMC (β = −0.491) in the low-fitness group, while no association was found in the high-fitness group.
Conclusion
IL-6, VEGF, TNF-α, and IL-1β are significantly associated with bone mass. Higher muscular fitness may attenuate the adverse effect of high VEGF and TNF-α on bone mass
HADES RV Programme with HARPS-N at TNG. VII. Rotation and activity of M-Dwarfs from time-series high-resolution spectroscopy of chromospheric indicators
We aim to investigate the presence of signatures of magnetic cycles and
rotation on a sample of 71 early M-dwarfs from the HADES RV programme using
high-resolution time-series spectroscopy of the Ca II H & K and Halpha
chromospheric activity indicators, the radial velocity series, the parameters
of the cross correlation function and the V-band photometry. We used mainly
HARPS-N spectra, acquired over four years, and add HARPS spectra from the
public ESO database and ASAS photometry light-curves as support data, extending
the baseline of the observations of some stars up to 12 years. We provide
log(R'hk) measurements for all the stars in the sample, cycle length
measurements for 13 stars, rotation periods for 33 stars and we are able to
measure the semi-amplitude of the radial velocity signal induced by rotation in
16 stars. We complement our work with previous results and confirm and refine
the previously reported relationships between the mean level of chromospheric
emission, measured by the log(R'hk), with the rotation period, and with the
measured semi-amplitude of the activity induced radial velocity signal for
early M-dwarfs. We searched for a possible relation between the measured
rotation periods and the lengths of the magnetic cycle, finding a weak
correlation between both quantities. Using previous v sin i measurements we
estimated the inclinations of the star's poles to the line of sight for all the
stars in the sample, and estimate the range of masses of the planets GJ 3998 b
and c (2.5 - 4.9 Mearth and 6.3 - 12.5 Mearth), GJ 625 b (2.82 Mearth), GJ 3942
b (7.1 - 10.0 Mearth) and GJ 15A b (3.1 - 3.3 Mearth), assuming their orbits
are coplanar with the stellar rotation.Comment: 19 pages, 16 figures, 10 table
The HADES RV Programme with HARPS-N@TNG IV. Time resolved analysis of the Ca ii H&K and H{\alpha} chromospheric emission of low-activity early-type M dwarfs
M dwarfs are prime targets for planet search programs, particularly of those
focused on the detection and characterization of rocky planets in the habitable
zone. Understanding their magnetic activity is important because it affects our
ability to detect small planets, and it plays a key role in the
characterization of the stellar environment. We analyze observations of the Ca
II H&K and H{\alpha} lines as diagnostics of chromospheric activity for
low-activity early-type M dwarfs. We analyze the time series of spectra of 71
early-type M dwarfs collected for the HADES project for planet search purposes.
The HARPS-N spectra provide simultaneously the H&K doublet and the H{\alpha}
line. We develop a reduction scheme able to correct the HARPS-N spectra for
instrumental and atmospheric effects, and to provide flux-calibrated spectra in
units of flux at the stellar surface. The H&K and H{\alpha} fluxes are compared
with each other, and their variability is analyzed. We find that the H and K
flux excesses are strongly correlated with each other, while the H{\alpha} flux
excess is generally less correlated with the H&K doublet. We also find that
H{\alpha} emission does not increase monotonically with the H&K line flux,
showing some absorption before being filled in by chromospheric emission when
H&K activity increases. Analyzing the time variability of the emission fluxes,
we derive a tentative estimate of the rotation period (of the order of a few
tens of days) for some of the program stars, and the typical lifetime of
chromospheric active regions (a few stellar rotations). Our results are in good
agreement with previous studies. In particular, we find evidence that the
chromospheres of early-type M dwarfs could be characterized by different
filaments coverage, affecting the formation mechanism of the H{\alpha} line. We
also show that chromospheric structure is likely related to spectral type
Using non–invasive genetic techniques to assist in maned wolf conservation in a remnant fragment of the Brazilian Cerrado
The maned wolf is a South American canid considered a keystone species of the Cerrado. We performed a genetic assessment of maned wolves that inhabit a small remnant fragment of the Cerrado in Brazil. We collected 84 fecal samples over a year and also included two tissue samples from road–killed animals. We successfully identified the species, sex, and individuals using molecular markers. Using microsatellite loci analysis we identified 13 different individuals, eight females and five males. The genetic variability level found and the high number of individuals detected indicates the presence of an open population
Variability estimation in resistive switching devices, a numerical and kinetic Monte Carlo perspective
Acknowledgments
The authors thank the support of the Spanish Ministry of Science, Innovation and Universities and the FEDER program through projects TEC2017-84321-C4-1-R, TEC2017-84321-C4-3-R, and projects A.TIC.117.UGR18, IE2017-5414 and B.TIC.624.UGR20 funded by the Consejería de Conocimiento, Investigación y Universidad, Junta de Andalucía (Spain) and the FEDER program. Funding for open access charge: Universidad de Granada/CBUAWe have analyzed variability in resistive memories (Resistive Random Access Memories, RRAMs) making use of advanced numerical techniques to process experimental measurements and simulations based on the kinetic Monte Carlo technique. The devices employed in the study were fabricated using the TiN/Ti/HfO2/W stack. The switching parameters were obtained making use of new developed extraction methods. The appropriateness of the advanced parameter extraction methodologies has been checked by comparison to kinetic Monte Carlo simulations; in particular, the reset and set events have been studied and detected. The data obtained were employed to shed light on the resistive switching operation and the cycle-to-cycle variability. It has been shown that variability depends on the numerical technique employed to obtain the set and reset voltages, therefore, this issue must be taken into consideration in RS characterization and modeling studies. The proposed techniques are complementary and depending on the technology and the curves shape the features of a particular method could make it to be the most appropriate.Spanish Ministry of Science, Innovation and Universities and the FEDER program through projects TEC2017-84321-C4-1-R, TEC2017-84321-C4-3-RConsejería de Conocimiento, Investigación y Universidad, Junta de Andalucía (Spain) and the FEDER program, projects A.TIC.117.UGR18, IE2017-5414 and B.TIC.624.UGR20Funding for open access charge: Universidad de Granada/CBU
The HADES RV Programme with HARPS-N at TNG XI. GJ 685 b: a warm super-Earth around an active M dwarf
Small rocky planets seem to be very abundant around low-mass M-type stars.
Their actual planetary population is however not yet precisely understood.
Currently several surveys aim to expand the statistics with intensive detection
campaigns, both photometric and spectroscopic. We analyse 106 spectroscopic
HARPS-N observations of the active M0-type star GJ 685 taken over the past five
years. We combine these data with photometric measurements from different
observatories to accurately model the stellar rotation and disentangle its
signals from genuine Doppler planetary signals in the RV data. We run an MCMC
analysis on the RV and activity indexes time series to model the planetary and
stellar signals present in the data, applying Gaussian Process regression
technique to deal with the stellar activity signals. We identify three periodic
signals in the RV time series, with periods of 9, 24, and 18 d. Combining the
analyses of the photometry of the star with the activity indexes derived from
the HARPS-N spectra, we identify the 18 d and 9 d signals as activity-related,
corresponding to the stellar rotation period and its first harmonic
respectively. The 24 d signals shows no relations with any activity proxy, so
we identify it as a genuine planetary signal. We find the best-fit model
describing the Doppler signal of the newly-found planet, GJ 685\,b,
corresponding to an orbital period d and a
minimum mass M. We also study a
sample of 70 RV-detected M-dwarf planets, and present new statistical evidence
of a difference in mass distribution between the populations of single- and
multi-planet systems, which can shed new light on the formation mechanisms of
low-mass planets around late-type stars.Comment: 18 pages, 13 figures, accepted for publication in A&
HADES RV Programme with HARPS-N at TNG VI. GJ 3942 b behind dominant activity signals
Short- to mid-term magnetic phenomena on the stellar surface of M-type stars
cannot only resemble the effects of planets in radial velocity data, but also
may hide them. We analyze 145 spectroscopic HARPS-N observations of GJ 3942
taken over the past five years and additional photometry to disentangle stellar
activity effects from genuine Doppler signals as a result of the orbital motion
of the star around the common barycenter with its planet. To achieve this, we
use the common methods of pre-whitening, and treat the correlated red noise by
a first-order moving average term and by Gaussian-process regression following
an MCMC analysis. We identify the rotational period of the star at 16.3 days
and discover a new super-Earth, GJ 3942 b, with an orbital period of 6.9 days
and a minimum mass of 7.1 Me. An additional signal in the periodogram of the
residuals is present but we cannot claim it to be related to a second planet
with sufficient significance at this point. If confirmed, such planet candidate
would have a minimum mass of 6.3 Me and a period of 10.4 days, which might
indicate a 3:2 mean-motion resonance with the inner planet
Multi-band high resolution spectroscopy rules out the hot Jupiter BD+20 1790b - First data from the GIARPS Commissioning
Context. Stellar activity is currently challenging the detection of young
planets via the radial velocity (RV) technique. Aims. We attempt to
definitively discriminate the nature of the RV variations for the young active
K5 star BD+20 1790, for which visible (VIS) RV measurements show divergent
results on the existence of a substellar companion. Methods. We compare VIS
data with high precision RVs in the near infrared (NIR) range by using the
GIANO - B and IGRINS spectrographs. In addition, we present for the first time
simultaneous VIS-NIR observations obtained with GIARPS (GIANO - B and HARPS -
N) at Telescopio Nazionale Galileo (TNG). Orbital RVs are achromatic, so the RV
amplitude does not change at different wavelengths, while stellar activity
induces wavelength-dependent RV variations, which are significantly reduced in
the NIR range with respect to the VIS. Results. The NIR radial velocity
measurements from GIANO - B and IGRINS show an average amplitude of about one
quarter with respect to previously published VIS data, as expected when the RV
jitter is due to stellar activity. Coeval multi-band photometry surprisingly
shows larger amplitudes in the NIR range, explainable with a mixture of cool
and hot spots in the same active region. Conclusions. In this work, the claimed
massive planet around BD+20 1790 is ruled out by our data. We exploited the
crucial role of multi- wavelength spectroscopy when observing young active
stars: thanks to facilities like GIARPS that provide simultaneous observations,
this method can reach its maximum potential.Comment: 12 pages, 7 figure
Climate variability and human impact in South America during the last 2000 years: synthesis and perspectives from pollen records
An improved understanding of present-day climate variability and change relies on high-quality data sets from the past 2 millennia. Global efforts to model regional climate modes are in the process of being validated against, and integrated with, records of past vegetation change. For South America, however, the full potential of vegetation records for evaluating and improving climate models has hitherto not been sufficiently acknowledged due to an absence of information on the spatial and temporal coverage of study sites. This paper therefore serves as a guide to high-quality pollen records that capture environmental variability during the last 2 millennia. We identify 60 vegetation (pollen) records from across South America which satisfy geochronological requirements set out for climate modelling, and we discuss their sensitivity to the spatial signature of climate modes throughout the continent. Diverse patterns of vegetation response to climate change are observed, with more similar patterns of change in the lowlands and varying intensity and direction of responses in the highlands. Pollen records display local-scale responses to climate modes; thus, it is necessary to understand how vegetation–climate interactions might diverge under variable settings. We provide a qualitative translation from pollen metrics to climate variables. Additionally, pollen is an excellent indicator of human impact through time. We discuss evidence for human land use in pollen records and provide an overview considered useful for archaeological hypothesis testing and important in distinguishing natural from anthropogenically driven vegetation change. We stress the need for the palynological community to be more familiar with climate variability patterns to correctly attribute the potential causes of observed vegetation dynamics. This manuscript forms part of the wider LOng-Term multi-proxy climate REconstructions and Dynamics in South America – 2k initiative that provides the ideal framework for the integration of the various palaeoclimatic subdisciplines and palaeo-science, thereby jump-starting and fostering multidisciplinary research into environmental change on centennial and millennial timescales
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