3,320 research outputs found
Activity cycles in members of young loose stellar associations
Magnetic cycles have been detected in tens of solar-like stars. The
relationship between the cycle properties and global stellar parameters is not
fully understood yet.
We searched for activity cycles in 90 solar-like stars with ages between 4
and 95 Myr aiming to investigate the properties of activity cycles in this age
range.
We measured the length of a given cycle by analyzing the long-term
time-series of three activity indexes. For each star, we computed also the
global magnetic activity index that is proportional to the amplitude of
the rotational modulation and is a proxy of the mean level of the surface
magnetic activity. We detected activity cycles in 67 stars. Secondary cycles
were also detected in 32 stars. The lack of correlation between and
suggest that these stars belong to the Transitional Branch and that
the dynamo acting in these stars is different from the solar one. This
statement is also supported by the analysis of the butterfly diagrams.
We computed the Spearman correlation coefficient between ,
and different stellar parameters. We found that is
uncorrelated with all the investigated parameters. The index is
positively correlated with the convective turn-over time-scale, the magnetic
diffusivity time-scale , and the dynamo number , whereas
it is anti-correlated with the effective temperature , the
photometric shear and the radius at which
the convective zone is located.
We found that is about constant and that decreases with the
stellare age in the range 4-95 Myr. We investigated the magnetic activity of AB
Dor A by merging ASAS time-series with previous long-term photometric data. We
estimated the length of the AB Dor A primary cycle as .Comment: 19 pages , 15 figures, accepte
Evidence of New Magnetic Transitions in Late-Type Dwarfs from Gaia DR2
The second Gaia data release contains the identification of 147 535 low-mass
() rotational modulation variable candidates on (or close
to) the main sequence, together with their rotation period and modulation
amplitude. The richness, the period and amplitude range, and the photometric
precision of this sample make it possible to unveil, for the first time,
signatures of different surface inhomogeneity regimes in the amplitude-period
density diagram. The modulation amplitude distribution shows a clear
bimodality, with an evident gap at periods d. The low amplitude
branch, in turn, shows a period bimodality with a main clustering at periods 5 - 10 d and a secondary clustering of ultra-fast rotators at d. The amplitude-period multimodality is correlated with the position in
the period-absolute magnitude (or period-color) diagram, with the low- and
high-amplitude stars occupying different preferential locations. Here we argue
that such a multimodality represents a further evidence of the existence of
different regimes of surface inhomogeneities in young and middle-age low-mass
stars and we lay out possible scenarios for their evolution, which manifestly
include rapid transitions from one regime to another. In particular, the data
indicate that stars spinning up close to break-up velocity undergo a very rapid
change in their surface inhomogeneities configuration, which is revealed here
for the first time. The multimodality can be exploited to identify field stars
of age 100 -- 600 Myr belonging to the slow-rotator low-amplitude
sequence, for which age can be estimated from the rotation period via
gyrochronology relationships.Comment: 15 pages, 6 figures, Accepted by Ap
Lower limit for differential rotation in members of young loose stellar associations
Surface differential rotation (SDR) plays a key role in dynamo models. SDR
estimates are therefore essential for constraining theoretical models. We
measure a lower limit to SDR in a sample of solar-like stars belonging to young
associations with the aim of investigating how SDR depends on global stellar
parameters in the age range (4-95 Myr). The rotation period of a solar-like
star can be recovered by analyzing the flux modulation caused by dark spots and
stellar rotation. The SDR and the latitude migration of dark-spots induce a
modulation of the detected rotation period. We employ long-term photometry to
measure the amplitude of such a modulation and to compute the quantity
DeltaOmega_phot =2p/P_min -2pi/P_max that is a lower limit to SDR. We find that
DeltaOmega_phot increases with the stellar effective temperature and with the
global convective turn-over time-scale tau_c. We find that DeltaOmega_phot is
proportional to Teff^2.18pm 0.65 in stars recently settled on the ZAMS. This
power law is less steep than those found by previous authors, but closest to
recent theoretical models. We find that DeltaOmega_phot steeply increases
between 4 and 30 Myr and that itis almost constant between 30 and 95 Myr in a 1
M_sun star. We find also that the relative shear increases with the Rossby
number Ro. Although our results are qualitatively in agreement with
hydrodynamical mean-field models, our measurements are systematically higher
than the values predicted by these models. The discrepancy between
DeltaOmega_phot measurements and theoretical models is particularly large in
stars with periods between 0.7 and 2 d. Such a discrepancy, together with the
anomalous SDR measured by other authors for HD 171488 (rotating in 1.31 d),
suggests that the rotation period could influence SDR more than predicted by
the models.Comment: 23 pages, 15 figures, 5 tables,accepted by Astronomy and Astrophysic
Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits
Current three-dimensional (3D) hydrodynamical model atmospheres together with
NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical
abundances from high-resolution stellar spectra. Not much is known about the
presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not
to mention its production in magnetically active targets like HD123351. From
fits of the observed CFHT spectrum with synthetic line profiles based on 1D and
3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and
to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic
ratio by fitting different synthetic spectra to the Li-line region of a
high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are
computed with four different line lists, using in parallel 3D hydrodynamical
CO5BOLD and 1D LHD model atmospheres and treating the line formation of the
lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and
6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al.
(2012), updated with new atomic data for V I, which results in the best fit of
the lithium line profile of HD123351. Two other line lists lead to similar
results but with inferior fit qualities. Our 2-sigma detection of the 6Li
isotope is the result of a careful statistical analysis and the visual
inspection of each achieved fit. Since the presence of a significant amount of
6Li in the atmosphere of a cool evolved star is not expected in the framework
of standard stellar evolution theory, non-standard, external lithium production
mechanisms, possibly related to stellar activity or a recent accretion of rocky
material, need to be invoked to explain the detection of 6Li in HD123351.Comment: 16 pages, 11 figures. Accepted for publication in A&
The Impact of Education and Culture on Poverty Reduction: Evidence from Panel Data of European Countries
The 2030 Agenda has among its key objectives the poverty eradication through increasing the level of education. A good level of education and investment in culture of a country is in fact necessary to guarantee a sustainable economy, in which coexists satisfactory levels of quality of life and an equitable distribution of income. There is a lack of studies in particular on the relations between some significant dimensions, such as education, culture and poverty, considering time lags for the measurement of impacts. Therefore, this study aims to fill this gap by focusing on the relationship between education, culture and poverty based on a panel of data from 34 European countries, over a 5-year period, 2015–2019. For this purpose, after applying principal component analysis to avoid multicollinearity problems, the authors applied three different approaches: pooled-ordinary least squares model, fixed effect model and random effect model. Fixed-effects estimator was selected as the optimal and most appropriate model. The results highlight that increasing education and culture levels in these countries reduce poverty. This opens space to new research paths and policy strategies that can start from this connection to implement concrete actions aimed at widening and improving educational and cultural offer
A Sea Level Equation for seismic perturbations
Large earthquakes are a potentially important source of relative sea level variations, since they can drive global deformation and simultaneously perturb the gravity field of the Earth. For the first time, we formalize a gravitationally self-consistent, integral sea level equation suitable for earthquakes, in which we account both for direct effects by the seismic dislocation and for the feedback from water loading associated with sea level changes. Our approach builds upon the well-established theory first proposed in the realm of glacio-isostatic adjustment modelling. The seismic sea level equation is numerically implemented to model sea level signals following the 2004 Sumatra–Andaman earthquake, showing that surface loading from ocean water redistribution (so far ignored in post-seismic deformation modelling) may account for a significant fraction of the total computed post-seismic sea level variatio
A three-wave longitudinal study on the underlying metacognitive mechanism between depression and Internet gaming disorder
AbstractBackground and aims: Internet Gaming Disorder (IGD) and depression have negative consequences on individuals' mental health, but their relationships are complex. This three-wave longitudinal study aimed to detect the metacognitive mechanisms underlying the association between IGD tendency and depression based on the self-regulatory executive function model. Methods: A total of 1,243 Chinese undergraduate student gamers (57% female, M = 19.77, SD = 1.29) were recruited at the baseline survey (Wave 1 [W1]), with 622 and 574 of them taking part in the two follow-up surveys (Wave 2 [W2] at 6 and Wave 3 [W3] at 12 months later), respectively. Results: The three-wave path model demonstrated, after controlling for the autoregressive effect of each variable, that depression consistently predicted IGD tendency but not vice versa, while negative but not positive metacognitions about online gaming (MOG) significantly predicted both depression and IGD tendency. Moreover, two statistically significant mediation paths: (i) negative MOG [W1] → depression [W2] → IGD tendency [W3]; and (ii) depression [W1] → negative MOG [W2] → IGD tendency [W3] were identified. Discussion and conclusions: These findings extend the understanding of the associations among depression, IGD tendency, and MOG, highlighting how negative MOG has a stronger prospective effect than positive MOG on depression and IGD tendency, and also reveal the mutual mediation effects of depression and negative MOG on IGD tendency. Integrated programmes with both emotional regulation training and Metacognitive Therapy are recommended for IGD treatment
Efficiency and spectrum of internal gamma-ray burst shocks
We present an analysis of the Internal Shock Model of GRBs, where gamma-rays
are produced by internal shocks within a relativistic wind. We show that
observed GRB characteristics impose stringent constraints on wind and source
parameters. We find that a significant fraction, of order 20 %, of the wind
kinetic energy can be converted to radiation, provided the distribution of
Lorentz factors within the wind has a large variance and provided the minimum
Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s
is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy
breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time
R/c < 1 ms, suggesting a possible explanation for the observed clustering of
spectral break energies in this range. The lower limit to wind Lorenz factor
and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies
are set by Thomson optical depth due to electron positron pairs produced by
synchrotron photons. Natural consequences of the model are absence of bursts
with peak emission energy significantly exceeding 1 MeV, and existence of low
luminosity bursts with low, 1 keV to 10 keV, break energies.Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and
electron energy fraction. Accepted for publication in Astrophysical Journa
- …