Context. The 16 Cygni system is composed of two solar analogues with similar masses and ages. A red dwarf is in orbit around 16 Cygni A, and 16 Cygni B hosts a giant planet. The abundances of heavy ...elements are similar in the two stars, but lithium is much more depleted in 16 Cygni B than in 16 Cygni A, by a factor of at least 4.7. Aims. The interest of studying the 16 Cygni system is that the two star have the same age and the same initial composition. The differences currently observed must be due to their different evolution, related to the fact that one of them hosts a planet while the other does not. Methods. We computed models of the two stars that precisely fit the observed seismic frequencies. We used the Toulouse Geneva Evolution Code (TGEC), which includes complete atomic diffusion (including radiative accelerations). We compared the predicted surface abundances with the spectroscopic observations and confirm that another mixing process is needed. We then included the effect of accretion-induced fingering convection. Results. The accretion of planetary matter does not change the metal abundances but leads to lithium destruction, which depends upon the accreted mass. A fraction of the Earth’s mass is enough to explain the lithium surface abundances of 16 Cygni B. We also checked the beryllium abundances. Conclusions. In the case of accretion of heavy matter onto stellar surfaces, the accreted heavy elements do not remain in the outer convective zones, but are mixed downwards by fingering convection induced by the unstable μ-gradient. Depending on the accreted mass, this mixing process may transport lithium down to its nuclear destruction layers and lead to an extra lithium depletion at the surface. A fraction of the Earth’s mass is enough to explain a lithium ratio of 4.7 in the 16 Cygni system. In this case beryllium is not destroyed. Such a process may be frequent in planet-hosting stars and should be studied in other cases in the future.
Abstract
The analysis of the available Transiting Exoplanet Survey Satellite (TESS) light curves of
α
Sex (HD 87887) reveals low-frequency pulsations with a period of about 9.1 hr in this ...spectroscopic A0 III standard star. The IUE observations in 1992 December reveal large flux variations both in the far-UV and in the mid-UV, which are accompanied by variations of the brightness in the
V
band recorded by the the Fine Error Sensor on board IUE. The ultraviolet variability could be due to an eclipse by an hitherto undetected companion of smaller radius, possibly 2.5
R
⊙
, but this needs confirmation by further monitoring possibly with TESS. An abundance determination yields solar abundances for most elements. Only carbon and strontium are underabundant and titanium, vanadium, and baryum mildly overabundant. Identification is provided for most of the lines absorbing more than 2% in the optical spectrum of
α
Sex. Stellar evolution modeling shows that
α
Sex is near the terminal-age main sequence, and its mass, radius, and age are estimated to be
M
= 2.57 ± 0.32
M
⊙
,
R
= 3.07 ± 0.90
R
⊙
,
A
= 385 ± 77 Myr, respectively.
Opacities and Atomic Diffusion Alecian, Georges; Deal, Morgan
Galaxies,
06/2023, Letnik:
11, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Opacity is a fundamental quantity for stellar modeling, and it plays an essential role throughout the life of stars. After gravity drives the collapse of interstellar matter into a protostar, the ...opacity determines how this matter is structured around the stellar core. The opacity explains how the radiation field interacts with the matter and how a major part of the energy flows through the star. It results from all the microscopic interactions of photons with atoms. Part of the momentum exchange between photons and atoms gives rise to radiative accelerations (specific to each type of atom), which are strongly involved in a second-order process: atomic diffusion. Although this process is a slow one, it can have a significant impact on stellar structure and chemical composition measurements. In this review, we discuss the way opacities are presently computed and used in numerical codes. Atomic diffusion is described, and the current status of the consideration of this process is presented.
Abstract
We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), ...and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars
μ
Lep and
β
Scl, and to the superficially normal star
ν
Cap. In the spectra of the six stars, the Hg
ii
line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of
P
orb
= 26.12022 ± 0.00004 day.
Aggressive behavior complicates the presentation of many psychiatric illnesses, and is associated with significant morbidity. Antipsychotic medications are used to treat this symptom dimension across ...multiple diagnoses. In this meta-analysis we sought to identify the effect size of antipsychotic medications for the treatment of reactive-impulsive aggression in adults, and identify differences across underlying diagnosis and specific agent. A search was conducted of four databases, MEDLINE, PsychINFO, Embase and the Cochrane Library to end date of August 10, 2016. The search terms included “aggression”, “irritable mood”, “anger”, “hostility” and “antipsychotic agents” or “dopamine antagonists”. 505 results were found, of which 47 were reviewed in detail and 21 ultimately included in the analysis. Antipsychotics were broadly effective for the treatment of aggression, but with effect sizes similar to those for non-pharmacologic interventions (standard mean difference=0.29, 95% confidence interval 0.22–0.36, z=8.5, p<0.001). There was no evidence for differences according to choice of agent (χ2=2.7, df=6, p=0.85), or conclusive evidence as to the importance of the underlying diagnosis (χ2=3.2, df=3, p=0.36). A small but significant dose effect was identified (β=0.0002, 95% CI 0.0001–0.0004, p=0.038). Although antipsychotics appear to be effective for treatment of aggression, their small effect sizes in the context of their significant side-effects should be taken into account when making clinical decisions about their use.
•Antipsychotics are frequently prescribed for aggression.•They appear to have a modest effect for this symptom dimension.•Available clinical evidence does not support a difference in effect size between different antipsychotic medications.
Abstract
Asteroseismology is playing an increasingly important role in the characterization of red giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of ...the evolved red giant branch (RGB) hosts KOI-3886 and
ι
Draconis, making use of end-of-mission Kepler (KOI-3886) and multisector TESS (
ι
Draconis) time-series photometry. We also model the benchmark star KIC 8410637, a member of an eclipsing binary, thus providing a direct test to the seismic determination. We test the impact of adopting different sets of observed modes as seismic constraints. Inclusion of
ℓ
= 1 and 2 modes improves the precision of the stellar parameters, albeit marginally, compared to adopting radial modes alone, with 1.9%–3.0% (radius), 5%–9% (mass), and 19%–25% (age) reached when using all
p
-dominated modes as constraints. Given the very small spacing of adjacent dipole mixed modes in evolved RGB stars, the sparse set of observed
g
-dominated modes is not able to provide extra constraints, further leading to highly multimodal posteriors. Access to multiyear time-series photometry does not improve matters, with detailed modeling of evolved RGB stars based on (lower-resolution) TESS data sets attaining a precision commensurate with that based on end-of-mission Kepler data. Furthermore, we test the impact of varying the atmospheric boundary condition in our stellar models. We find the mass and radius estimates to be insensitive to the description of the near-surface layers, at the expense of substantially changing both the near-surface structure of the best-fitting models and the values of associated parameters like the initial helium abundance,
Y
i
. Attempts to measure
Y
i
from seismic modeling of red giants may thus be systematically dependent on the choice of atmospheric physics.
Aims. Atomic diffusion, including the effect of radiative accelerations on individual elements, leads to important variations of the chemical composition inside the stars. The accumulation in ...specific layers of the elements, which are the main contributors of the local opacity, leads to hydrodynamical instabilities that modify the internal stellar structure and surface abundances. Our aim is to study these effects and compare the resulting surface abundances with spectroscopic observations Methods. We computed the detailed structure of A-type stars including these effects. We used the Toulouse-Geneva Evolution Code (TGEC), where radiative accelerations are computed using the single valued parameter (SVP) method, and we added double-diffusive convection with mixing coefficients deduced from three-dimensional (3D) simulations. Results. We show that the modification of the initial chemical composition has important effects on the internal stellar mixing and leads to different surface abundances of the elements. The results fit the observed surface chemical composition well if the layers, which are individually mixed by double-diffusive convection, are connected.
Elemental abundances of Sun-like stars have been shown to be crucial for understanding the detailed properties of planets surrounding them. However, accurately measuring elemental abundances of M ...stars, the most abundant class of stars in the solar neighbourhood, is challenging due to their faintness and pervasive molecular features in optical photospheric spectra. As a result, elemental abundances of Sun-like stars have been proposed to constrain those of M stars, particularly by scaling X/H given measured Fe/H This work aims to test the robustness of this convenient practice based on two selected sets of M- and GK-dwarf stellar abundances and a set of rigorous statistical methods. We compiled the elemental abundances of a sample of up to 43 M dwarfs for ten major rock-forming elements (Fe, C, O, Mg, Si, Al, Ca, Na, Ni, and Ti) from high-resolution near-infrared stellar surveys including APOGEE, CARMENES, and Subaru. We carried out bootstrap-based linear regressions on the selected sample of M dwarfs to constrain the statistical trends of X/H versus Fe/H and then compare them with those of GK dwarfs (sampled from the GALAH database). We then applied a two-sample, multivariate Mahalanobis Distance test to assess the significance of the differences in the X/H Fe/H trends for individual elemental pairs between M and GK dwarfs. We find that the null hypothesis -- that is, no significant difference in the chemical trends of X/H versus Fe/H between M and GK dwarfs -- is strongly rejected for all elements except for Si, for which the rejection is marginal ($p$-value close to 0.05), and Na and Ni, for which the results are inconclusive. This finding suggests that assuming no difference may result in biased results, and thus inaccurate constraints on characterising rocky planets around M dwarfs by scaling the (unmeasured) chemical abundances of planet-hosting M dwarfs from the chemical trends of X/H Fe/H determined by GK dwarfs. It is therefore crucial for both the stellar and exoplanet communities to be aware of these observed differences. To better understand these differences, we advocate for dedicated modelling techniques for M-dwarf atmospheres and an increasing set of benchmark, homogeneous abundance analyses. Intermediately, our statistically constrained trends of X/H Fe/H for M dwarfs provide a new constraint on estimating M-dwarf elemental abundances given measured Fe/H and further on characterising the detailed properties of M-dwarf-hosted rocky worlds.
Context.
Chemical composition is an important factor that affects stellar evolution. The element abundance on the stellar surface evolves along the lifetime of the star because of transport ...processes, including atomic diffusion. However, models of stars with masses higher than about 1.2
M
⊙
predict unrealistic variations at the stellar surface. This indicates the need for competing transport processes that are mostly computationally expensive for large grids of stellar models.
Aims.
The purpose of this study is to implement turbulent mixing in stellar models and assess the possibility of reproducing the effect of radiative accelerations with turbulent mixing for elements like iron in order to make the computation of large grids possible.
Methods.
We computed stellar models with the Module for Experiments in Stellar Astrophysics code and assessed the effects of atomic diffusion (with radiative acceleration) in the presence of turbulent mixing. Starting from a turbulent mixing prescription already calibrated on helium surface abundances of F-type stars as a reference, we parametrised the effect of radiative accelerations on iron with a turbulent diffusion coefficient. Finally, we tested this parametrisation by modelling two F-type stars of the
Kepler
Legacy sample.
Results.
We found that, for iron, a parametrisation of turbulent mixing that simulates the effect of radiative acceleration is possible. This leads to an increase in the efficiency of the turbulent mixing to counteract the effect of gravitational settling. This approximation does not affect significantly the surface abundances of the other elements we studied, except for oxygen and calcium. We demonstrate that this parametrisation has a negligible impact on the accuracy of the seismic properties inferred with these models. Moreover, turbulent mixing makes the computation of realistic F-type star models including the effect atomic diffusion possible. This leads to differences of about 10% in the inferred ages compared to results obtained with models that neglect these processes.
Conclusions.
The inclusion of turbulent mixing and atomic diffusion with radiative accelerations allows a more realistic characterisation of F-type stars. The parametrisation of the effect of radiative acceleration on iron opens the possibility to compute larger grids of stellar models in a reasonable amount of time, which is currently difficult when the different chemical transport mechanisms, especially radiative accelerations, are considered, although this parametrisation cannot simulate the evolution of abundances of all elements (e.g. calcium).
Context. The modelling of chemical transport mechanisms is crucial for accurate stellar characterisations. Atomic diffusion is one of these processes and is commonly included in stellar models. ...However, it is usually neglected for F-type or more massive stars because it produces surface abundance variations that are unrealistic. Additional mechanisms to counteract atomic diffusion must therefore be considered. It has been demonstrated that turbulent mixing can prevent excessive variation in surface abundances, and can also be calibrated to mimic the effects of radiative accelerations on iron. Aims. We aim to evaluate the effect of calibrated turbulent mixing on the characterisation of a sample of F-type stars, and how the estimates compare with those obtained when chemical transport mechanisms are neglected. Methods. We selected stars from two samples: one from the Kepler LEGACY sample and the other from a sample of Kepler planet-hosting stars. We inferred their stellar properties using two grids. The first grid considers atomic diffusion only in models that do not show excessive variation in chemical abundances at the stellar surface. The second grid includes atomic diffusion in all the stellar models and calibrated turbulent mixing to avoid unrealistic surface abundances. Results. Comparing the derived results from the two grids, we find that the results for the more massive stars in our sample show greater dispersion in the inferred values of mass, radius, and age due to the absence of atomic diffusion in one of the grids. This can lead to relative uncertainties for individual stars of up to 5% on masses, 2% on radii, and 20% on ages. Conclusions. This work shows that a proper modelling of the microscopic transport processes is crucial for the accurate estimation of their fundamental properties – not only for G-type stars but also for F-type stars.
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