Main-sequence, solar-like stars (M 1.5 M ) have outer convective envelopes that are sufficiently thick to affect significantly their overall structure. The radii of these stars, in particular, are ...sensitive to the details of inefficient, superadiabatic convection occurring in their outermost layers. The standard treatment of convection in stellar evolution models, based on the mixing-length theory (MLT), provides only a very approximate description of convection in the superadiabatic regime. Moreover, it contains a free parameter, MLT, whose standard calibration is based on the Sun and is routinely applied to other stars, ignoring the differences in their global parameters (e.g., effective temperature, gravity, chemical composition) and previous evolutionary history. In this paper, we present a calibration of MLT based on 3D radiation hydrodynamics (RHD) simulations of convection. The value of MLT is adjusted to match the specific entropy in the deep, adiabatic layers of the convective envelope to the corresponding value obtained from the 3D RHD simulations, as a function of the position of the star in the plane and its chemical composition. We have constructed a model of the present-day Sun using such entropy-based calibration. We find that its past luminosity evolution is not affected by the entropy calibration. The predicted solar radius, however, exceeds that of the standard model during the past several billion years, resulting in a lower surface temperature. This illustrative calculation also demonstrates the viability of the entropy approach for calibrating the radii of other late-type stars.
Context. Average stellar radii in open clusters can be estimated from rotation periods and projected rotational velocities under the assumption that the spin axis has a random orientation. These ...estimates are independent of distance, interstellar absorption, and models, but their validity can be limited by lacking data (truncation) or data that only represent upper or lower limits (censoring). Aims. We present a new statistical analysis method to estimate average stellar radii in the presence of censoring and truncation. Methods. We used theoretical distribution functions of the projected stellar radius Rsini to define a likelihood function in the presence of censoring and truncation. Average stellar radii in magnitude bins were then obtained by a maximum likelihood parametric estimation procedure. Results. This method is capable of recovering the average stellar radius within a few percent with as few as aboutten measurements. Here we apply this for the first time to the dataset available for the Pleiades. We find an agreement better than ≈10 percent between the observed R vs. MK relationship and current standard stellar models for 1.2 ≥ M/M⊙ ≥ 0.85 with no evident bias. Evidence of a systematic deviation at 2σ level are found for stars with 0.8 ≥ M/M⊙ ≥ 0.6 that approach the slow-rotator sequence. Fast rotators (P < 2 d) agree with standard models within 15 percent with no systematic deviations in the whole 1.2 ≳ M/M⊙ ≳ 0.5 range. Conclusions. The evidence of a possible radius inflation just below the lower mass limit of the slow-rotator sequence indicates a possible connection with the transition from the fast- to the slow-rotator sequence.
Context. Magnetic cycles analogous to the solar cycle have been detected in tens of solar-like stars by analyzing long-term time series of different magnetic activity indexes. The relationship ...between the cycle properties and global stellar parameters is not fully understood yet. One reason for this is the lack of long-term time series for stars covering a wide range of stellar parameters. Aims. We searched for activity cycles in a sample of 90 young solar-like stars with ages between 4 and 95 Myr with the aim to investigate the properties of activity cycles in this age range. Methods. We measured the length Pcyc of a given cycle by analyzing the long-term time series of three different activity indexes: the period of rotational modulation, the amplitude of the rotational modulation and the median magnitude in the V band. For each star, we also computed the global magnetic activity index ⟨ IQR ⟩ that is proportional to the amplitude of the rotational modulation and can be regarded as a proxy of the mean level of the surface magnetic activity. Results. We detected activity cycles in 67 stars. Secondary cycles were also detected in 32 stars of the sample. The lack of correlation between Pcyc and Prot and the position of our targets in the Pcyc/Prot−Ro-1 diagram suggest that these stars belong to the so-called transitional branch and that the dynamo acting in these stars is different from the solar dynamo and from that acting in the older Mt. Wilson stars. This statement is also supported by the analysis of the butterfly diagrams whose patterns are very different from those seen in the solar case. We computed the Spearman correlation coefficient rS between Pcyc, ⟨ IQR ⟩ and various stellar parameters. We found that Pcyc in our sample is uncorrelated with all the investigated parameters. The ⟨ IQR ⟩ index is positively correlated with the convective turnover timescale, the magnetic diffusivity timescale τdiff, and the dynamo number DN, whereas it is anti-correlated with the effective temperature Teff, the photometric shear ΔΩphot and the radius RC at which the convective zone is located. We investigated how Pcyc and ⟨ IQR ⟩ evolve with the stellar age. We found that Pcyc is about constant and that ⟨ IQR ⟩ decreases with the stellare age in the range 4–95 Myr. Finally we investigated the magnetic activity of the star AB Dor A by merging All Sky Automatic Survey (ASAS) time series with previous long-term photometric data. We estimated the length of the AB Dor A primary cycle as Pcyc = 16.78 ± 2 yr and we also found shorter secondary cycles with lengths of 400 d, 190 d, and 90 d, respectively.
The flux of positrons and electrons (e+ + e−) has been measured by the Fermi Large Area Telescope (LAT) in the energy range between 7 GeV and 2 TeV. We discuss a number of interpretations of Pass 8 ...Fermi-LAT e+ + e− spectrum, combining electron and positron emission from supernova remnants (SNRs) and pulsar wind nebulae (PWNe), or produced by the collision of cosmic rays (CRs) with the interstellar medium. We find that the Fermi-LAT spectrum is compatible with the sum of electrons from a smooth SNR population, positrons from cataloged PWNe, and a secondary component. If we include in our analysis constraints from the AMS-02 positron spectrum, we obtain a slightly worse fit to the e+ + e− Fermi-LAT spectrum, depending on the propagation model. As an additional scenario, we replace the smooth SNR component within 0.7 kpc with the individual sources found in Green's catalog of Galactic SNRs. We find that separate consideration of far and near sources helps to reproduce the e+ + e− Fermi-LAT spectrum. However, we show that the fit degrades when the radio constraints on the positron emission from Vela SNR (which is the main contributor at high energies) are taken into account. We find that a break in the power-law injection spectrum at about 100 GeV can also reproduce the measured e+ + e− spectrum and, among the CR propagation models that we consider, no reasonable break of the power-law dependence of the diffusion coefficient can modify the electron flux enough to reproduce the observed shape.
•Two long-acting somatostatin analogs (SSA) are available in clinical practice for NET.•Octreotide and lanreotide are approved for clinical syndrome and tumor growth control.•However, some clinical ...indications of approved SSA remain controversial.•A group of NET-specialists, critically addressed ten challenging questions on SSA use.
Octreotide and lanreotide are the two somatostatin analogs (SSA) currently available in clinical practice. They have been approved first to control the clinical syndrome (mainly carcinoid syndrome) associated with functioning neuroendocrine tumors (NET) and later for tumor growth control in advanced low/intermediate grade NET. Although evidence regarding their role, especially as antiproliferative therapy, has been increasing over the years some clinical indications remain controversial. Solicited by AIOM (Italian Association of Medical Oncology) a group of clinicians from various specialties, including medical oncology, endocrinology, and gastroenterology, deeply involved in NET for their clinical and research activity, addressed eight open questions, critically reviewing evidence and guidelines and sharing clinical take-home messages. The questions regarded the use of long-acting octreotide and lanreotide in the following settings: functioning and non-functioning NET refractory to label dose, first-line metastatic pulmonary NET, combination with other therapy with an antiproliferative intent, maintenance in NET responding to other therapies, adjuvant treatment, Ki-67-related cut-off, somatostatin receptor imaging, safety, and feasibility. The level of evidence is not absolute for the majority of these clinical contexts, so it is recommended to distinguish routine versus sporadic utilization in very selected cases. Mention of such specific issues by the main European guidelines (ENETS, European Neuroendocrine Tumor Society, and ESMO, European Society for Medical Oncology) was explored and their position reported. However, different clinical decisions on single patients could be made if the case is carefully discussed within a NET-dedicated multidisciplinary team.
The potential energy curves (PECs) for the interaction of 3CH2 with 3O2 in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CH2OO) are ...studied using electronic structure calculations. The bonding mechanism is interpreted by analyzing the ground state multireference configuration interaction (MRCI) wave function of the reacting species and at all points along the PES. The interaction of 3CH2 with 3O2 on the singlet surface leads to a flat long-range attractive PEC lacking any maxima or minima along the curve. The triplet surface stems into a maximum along the curve resulting in a transition state with an energy barrier of 5.3 kcal/mol at CASSCF(4,4)/cc-pVTZ level. The resulting 3CH2OO is less stable than the 1CH2OO. In this study, the biradical character (β) is used as a measure to understand the difference in the topology of the singlet and triplet PECs and the relation of the biradical nature of the species with their structures. The 3CH2OO has a larger biradical character than 1CH2OO, and because of the larger bond order of 1CH2OO, the C–O covalent bond becomes harder to break, thereby stabilizing 1CH2OO. Thus, this study provides insights into the shape of the PEC obtained from the reaction between 3CH2 and 3O2 in terms of their bonding nature and from the shape of the curves, the temperature dependence or independence of the rate of the reaction is discussed.
Context. Surface differential rotation (SDR) plays a key role in dynamo models and determines a lower limit on the accuracy of stellar rotation period measurements. SDR estimates are therefore ...essential to constrain theoretical models and infer realistic rotation period uncertainties. Aims. We measure a lower limit to SDR in a sample of solar-like stars belonging to young loose stellar associations with the aim of investigating how SDR depends on global stellar parameters in the age range (4 − 95 Myr). Methods. 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 employed long-term photometry to measure the amplitude of such a modulation and to compute the quantity ΔΩphot = 2π/Pmin - 2π/Pmax that is a lower limit to SDR. Results. We find that ΔΩphot increases with the stellar effective temperature and with the global convective turn-over timescale τc, which is the characteristic time for the rise of a convective element through the stellar convection zone. We find that ΔΩphot is proportional to Teff2.18±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 investigate how ΔΩphot changes in time in a ~1 M⊙ star. We find that ΔΩphot steeply increases between 4 and 30 Myr and that it is almost constant between 30 and 95 Myr. 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 ΔΩphot 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.
Tunneling Enhancement of the Gas-Phase CH + CO2 Reaction at Low Temperature Nuñez-Reyes, Dianailys; Hickson, Kevin M; Loison, Jean-Christophe ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
12/2020, Letnik:
124, Številka:
51
Journal Article
Recenzirano
Odprti dostop
The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving ...molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO2 → HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C–O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO2 reaction accelerates at low temperature as zero-point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO2 reaction, while the equivalent effect for the CD + CO2 reaction results instead from a submerged barrier with respect to reactants.
Thermochemical and Kinetics of the CH3OH + (4S)N Reactional System Ferreira, Rhayla Mendes; Roberto-Neto, Orlando; Machado, Francisco B. C ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
07/2018, Letnik:
122, Številka:
28
Journal Article
Recenzirano
The reaction of methanol (CH3OH) with atomic nitrogen was studied considering three elementary reactions, the hydrogen abstractions from the hydroxyl or methyl groups (R1 and R3, respectively) and ...the C–O bond break (R2). Thermochemical properties were obtained using ab initio methods and density functional theory approximations with aug-cc-pVXZ (X = T and Q) basis sets. The minimum energy path was built with a dual-level methodology using the BB1K functional as the low-level and the CCSD(T) as the high-level. This surface was used to calculate the thermal rate constants in the frame of variational transitional state theory considering the tunneling effects. Our results indicate the dehydrogenation of the methyl group (R3) as the dominant path with k R3 = 7.5 × 10–27 cm3·molecule–1·s–1 at 300 K. The thermal rate constants were fitted to a modified Arrhenius equation for use in mechanism studies of the methanol decomposition.
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