Power Density Spectra of Gamma-Ray Bursts in the Internal Shock Model

We simulate Gamma-Ray Bursts arising from internal shocks in relativistic winds, calculate their power density spectrum (PDS), and identify the factors to which the PDS is most sensitive: the wind ejection features, which determine the wind dynamics and its optical thickness, and the energy release parameters, which give the pulse 50-300 keV radiative efficiency. For certain combinations of ejection features and wind parameters the resulting PDS exhibits the features observed in real bursts. We found that the upper limit on the efficiency of conversion of wind kinetic energy into 50-300 keV photons is $\sim$ 1%. Winds with a modulated Lorentz factor distribution of the ejecta yield PDSs in accord with current observations and have efficiencies closer to $10^{-3}$, while winds with a random, uniform Lorentz factor ejection must be optically thick to the short duration pulses to produce correct PDSs, and have an overall efficiency around $10^{-4}$.Comment: 6 pages, 4 figures, Latex, submitted to The Astrophysical Journal (05/04/99

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 $P_{ cyc}$ 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 $P_{ cyc}$ and $P_{ rot}$ 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 $r_{ S}$ between $P_{ cyc}$, and different stellar parameters. We found that $P_{ cyc}$ is uncorrelated with all the investigated parameters. The index is positively correlated with the convective turn-over time-scale, the magnetic diffusivity time-scale $\tau_{ diff}$, and the dynamo number $D_{ N}$, whereas it is anti-correlated with the effective temperature $T_{ eff}$, the photometric shear $\Delta\Omega_{\rm phot}$ and the radius $R_{ C}$ at which the convective zone is located. We found that $P_{ cyc}$ 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 $P_{ cyc} = 16.78 \pm 2 \rm yr$.Comment: 19 pages , 15 figures, accepte

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

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 ($\le 1.4 M_{\odot}$) 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 $P \le 2$ d. The low amplitude branch, in turn, shows a period bimodality with a main clustering at periods $P \approx$ 5 - 10 d and a secondary clustering of ultra-fast rotators at $P \le 0.5$ 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 $\sim$ 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

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&

Analysis of Temporal Features of Gamma Ray Bursts in the Internal Shock Model

In a recent paper we have calculated the power density spectrum of Gamma-Ray Bursts arising from multiple shocks in a relativistic wind. The wind optical thickness is one of the factors to which the power spectrum is most sensitive, therefore we have further developed our model by taking into account the photon down-scattering on the cold electrons in the wind. For an almost optically thick wind we identify a combination of ejection features and wind parameters that yield bursts with an average power spectrum in agreement with the observations, and with an efficiency of converting the wind kinetic energy in 50-300 keV emission of order 1%. For the same set of model features the interval time between peaks and pulse fluences have distributions consistent with the log-normal distribution observed in real bursts.Comment: ApJ in press, 2000; with slight revisions; 12 pag, 6 fi

G protein mutations in endocrine diseases

This review summarizes the pathogenetic role of naturally occurring mutations of G protein genes in endocrine diseases. Although in vitro mutagenesis and transfection assays indicate that several G proteins have mitogenic potential, to date only two G proteins have been identified which harbor naturally occurring mutations, Gs\u3b1, the activator of adenylyl cyclase and Gi2\u3b1, which is involved in several functions, including adenylyl cyclase inhibition and ion channel modulation. The gene encoding Gs\u3b1 (GNAS1) may be altered by loss or gain of function mutations. Indeed, heterozygous inactivating germ line mutations in this gene cause pseudohypoparathyroidism type Ia, in which physical features of Albright hereditary osteodystrophy (AHO) are associated with resistance to several hormones, i.e. PTH, TSH and gonadotropins, that activate Gs-coupled receptors or pseudopseudohypoparathyroidism in which AHO is the only clinical manifestation. Evidence suggests that the variable and tissue-specific hormone resistance observed in PHP Ia may result from tissue-specific imprinting of the GNAS1 gene, although the Gs\u3b1 knockout model only in part reproduces the human AHO phenotype. Activating somatic Gs\u3b1 mutations leading to cell proliferation have been identified in endocrine tumors constituted by cells in which cAMP is a mitogenic signal, i.e. GH-secreting pituitary adenomas, hyperfunctioning thyroid adenomas and Leydig cell tumors. When the same mutations occur very early in embryogenesis they cause McCune-Albright syndrome. Although these mutations would in principle confer growth advantage, studies failed to detect differences in the clinical and hormonal phenotypes, suggesting the existence of mechanisms able to counteract the activation of the cAMP pathway. Activating mutations of Gi2\u3b1 have been identified in a subset of ovarian, adrenal and pituitary tumors, but their prevalence and significance are still controversial. Finally, although G\u3b1 subunits are the only components of the heterotrimeric GTP binding proteins which harbor known mutations, \u3b2/\u3b3 subunits should be considered possible targets of genetic alterations as suggested by the frequent presence of \u3b23 subunit variants in patients with essential hypertension

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