scholarly journals Li-rich giant stars under scrutiny: binarity, magnetic activity, and the evolutionary status after Gaia DR2

2020 ◽  
Vol 498 (2) ◽  
pp. 2295-2308
Author(s):  
B F O Gonçalves ◽  
J S da Costa ◽  
L de Almeida ◽  
M Castro ◽  
J-D do Nascimento
Keyword(s):  
Magnetic Field ◽  
Radial Velocity ◽  
Magnetic Activity ◽  
Small Sample ◽  
Evolutionary Status ◽  
Surface Magnetic Field ◽  
Giant Stars ◽  
Evolutionary State ◽  
Deconvolution Technique ◽  
Red Giant

ABSTRACT We present a study of the evolutionary state of a few lithium-rich giant stars based on the Gaia Data Release 2 (DR2) parallaxes and photometry. We also investigate the chromospheric activity, the presence of a surface magnetic field, and the radial velocity for our sample stars. We analysed both archive and new data. We gathered archive spectra from several instruments, mainly ELODIE and NARVAL, and we added new data acquired with the spectrograph MUSICOS. We applied the least-squares deconvolution technique to obtain Stokes V and I mean profiles to compute longitudinal magnetic field for a subset. Moreover, for the same subset, we analysed the Ca ii H&K emission lines to calculate the S-index. We also derived atmospheric parameters and Li abundances for all 18 stars of our sample. We found that stars previously classified as red giant branch (RGB) may actually be at a different evolutionary state. Furthermore, we identified that most stars in our sample with detection of surface magnetic field show at least moderate rotation velocities, but none the less, we could not detect a magnetic field in two fast rotators. Because of our small sample of magnetic giants, it is difficult to determine if the presence of surface magnetic field and the Li-rich giant phenomena could be somehow linked. The large variation of the radial velocity of part of our sample indicates that some of them might have a binary companion, which may change the way we look at the Li problem in giant stars.

scholarly journals Basic Parameters Determining X-Ray Emission Level in Stars of Spectral Type Later than F5

1983 ◽  
Vol 71 ◽  
pp. 633-635
Author(s):  
Lucio Paternò ◽  
Francesca Zuccarello
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Dynamo Action ◽  
Average Surface ◽  
X Ray ◽  
Surface Magnetic Field ◽  
Emission Level ◽  
Giant Stars ◽  
Basic Parameters

This is a merely exploratory search for estimating the importance of those stellar parameters which we believe to be relevant to the X-ray emission from stars possessing outer convective envelopes.Belvedere et al.(1981,1982) have shown that a mechanism which converts magnetic into thermal energy is plausible for explaining the X-ray emission level in late type main sequence and giant stars. One of the main conclusions of their work is that the average surface X-ray flux Fx depends on the square of the average surface magnetic field strength B. The surface magnetic activity,in stars possessing outer convective envelopes, is likely due to the interaction of rotation with convection which produces both differential rotation (Belvedere et al. 1980a) and dynamo action by the α-effect (Belvedere et al. 1980b).Therefore we would expect that the level of magnetic field intensity depends on star's rotation Ω and the depth of the convection zone(c.z.) D,because both parameters are very important in determining the strength of the interaction.

scholarly journals Effect of a strong magnetic field on gravity-mode period spacings in red giant stars

2020 ◽  
Vol 496 (3) ◽  
pp. 3829-3840
Author(s):  
Shyeh Tjing Loi
Keyword(s):  
Magnetic Field ◽  
Strong Magnetic Field ◽  
Strong Field ◽  
Average Density ◽  
Stellar Structure ◽  
Frequency Interval ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant ◽  
Gravity Mode

ABSTRACT When a star evolves into a red giant, the enhanced coupling between core-based gravity modes and envelope-based pressure modes forms mixed modes, allowing its deep interior to be probed by asteroseismology. The ability to obtain information about stellar interiors is important for constraining theories of stellar structure and evolution, for which the origin of various discrepancies between prediction and observation is still under debate. Ongoing speculation surrounds the possibility that some red giant stars may harbour strong (dynamically significant) magnetic fields in their cores, but interpretation of the observational data remains controversial. In part, this is tied to shortfalls in our understanding of the effects of strong fields on the seismic properties of gravity modes, which lies beyond the regime of standard perturbative methods. Here, we seek to investigate the effect of a strong magnetic field on the asymptotic period spacings of gravity modes. We use a Hamiltonian ray approach to measure the volume of phase space occupied by mode-forming rays, this being roughly proportional to the average density of modes (number of modes per unit frequency interval). A strong field appears to systematically increase this by about 10 per cent, which predicts a ∼10 per cent smaller period spacing. Evidence of near integrability in the ray dynamics hints that the gravity-mode spectrum may still exhibit pseudo-regularities under a strong field.

scholarly journals Interplay between magnetic fields and differential rotation in a stably stratified stellar radiative zone

2020 ◽  
Vol 641 ◽  
pp. A13 ◽  
Author(s):  
L. Jouve ◽  
F. Lignières ◽  
M. Gaurat
Keyword(s):  
Magnetic Field ◽  
Differential Rotation ◽  
Large Scale ◽  
Stable Stratification ◽  
Rotational Instability ◽  
Intermediate Mass ◽  
Poloidal Magnetic Field ◽  
Giant Stars ◽  
Radiative Zone ◽  
Red Giant

Context. The interactions between magnetic fields and differential rotation in stellar radiative interiors could play a major role in achieving an understanding of the magnetism of intermediate-mass and massive stars and of the differential rotation profile observed in red-giant stars. Aims. The present study is aimed at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We investigate in detail the effects of the stable stratification and thermal diffusion and we aim, in particular, to assess the role of the stratification at stabilising the system. Methods. We performed 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfvén frequency, we built a magnetic configuration strongly dominated by its toroidal component. We then perturbed this configuration to observe the development of non-axisymmetric instabilities. Results. The parameters of the simulations were chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Brunt-Väisälä frequency, the rotation, and the initial poloidal field strength. After a transient time and using a suitable adimensionalisation, we find that the axisymmetric state only depends on tes/tAp the ratio between the Eddington–Sweet circulation time scale and the Alfvén time scale. A scale analysis of the Boussinesq magnetohydrodynamical equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows. Conclusions. Non-axisymmetric instabilities are likely to exist in stellar radiative zones despite the stable stratification. They could be at the origin of the magnetic dichotomy observed in intermediate-mass and massive stars. They are also unavoidable candidates for the transport of angular momentum in red giant stars.

scholarly journals Abundances of C, N and O in Planetary Nebulae

1989 ◽  
Vol 131 ◽  
pp. 218-218 ◽  
Author(s):  
A. A. Nikitin ◽  
A. F. Kholtygin ◽  
A. A. Sapar ◽  
T. H. Feklistova
Keyword(s):  
Planetary Nebulae ◽  
Abundance Ratio ◽  
Evolutionary Status ◽  
Spectral Features ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant ◽  
Abundance Determination

The abundances of C, N and O in planetary nebulae must correspond to the evolutionary status of their progenitor red giant stars. The best spectral features for abundance determination of these elements are the recombination lines, which depend weakly on the variations of Te and ne. The abundance ratio of the ions A+ and H+ can be given by [1–3].

scholarly journals Magnetic fields in single late-type giants in the Solar vicinity: How common is magnetic activity on the giant branches?

2013 ◽  
Vol 9 (S302) ◽  
pp. 373-376 ◽  
Author(s):  
Renada Konstantinova-Antova ◽  
Michel Aurière ◽  
Corinne Charbonnel ◽  
Natalia Drake ◽  
Gregg Wade ◽  
...  
Keyword(s):  
Large Scale ◽  
Mass Range ◽  
Magnetic Activity ◽  
Asymptotic Giant Branch ◽  
Evolutionary Status ◽  
Fundamental Parameters ◽  
First Results ◽  
Red Giant ◽  
Hertzsprung Gap ◽  
The Magnetic Field

AbstractWe present our first results on a new sample containing all single G, K and M giants down to V = 4 mag in the Solar vicinity, suitable for spectropolarimetric (Stokes V) observations with Narval at TBL, France. For detection and measurement of the magnetic field (MF), the Least Squares Deconvolution (LSD) method was applied (Donati et al. 1997) that in the present case enables detection of large-scale MFs even weaker than the solar one (the typical precision of our longitudinal MF measurements is 0.1-0.2 G). The evolutionary status of the stars is determined on the basis of the evolutionary models with rotation (Lagarde et al. 2012; Charbonnel et al., in prep.) and fundamental parameters given by Massarotti et al. (1998). The stars appear to be in the mass range 1-4 M⊙, situated at different evolutionary stages after the Main Sequence (MS), up to the Asymptotic Giant Branch (AGB).The sample contains 45 stars. Up to now, 29 stars are observed (that is about 64% of the sample), each observed at least twice. For 2 stars in the Hertzsprung gap, one is definitely Zeeman detected. Only 5 G and K giants, situated mainly at the base of the Red Giant Branch (RGB) and in the He-burning phase are detected. Surprisingly, a lot of stars ascending towards the RGB tip and in early AGB phase are detected (8 of 13 observed stars). For all Zeeman detected stars v sin i is redetermined and appears in the interval 2-3 km/s, but few giants with MF possess larger v sin i.

scholarly journals Geomagnetic Kp Index and Planetary Magnetosphere Size Relationship: for Mercury and Jupiter During two Types of Geomagnetic Conditions

2020 ◽  
Vol 17 (3) ◽  
pp. 0806
Author(s):  
Mays M. Al-Gbory ◽  
Najat Mohamed Ameen
Keyword(s):  
Magnetic Field ◽  
Solar Activity ◽  
Solar System ◽  
Weak Magnetic Field ◽  
Geomagnetic Storms ◽  
Magnetic Activity ◽  
The Sun ◽  
Planetary Surface ◽  
Surface Magnetic Field ◽  
The Many

     Kp index correlates with the many magnetosphere properties, which are used to measure the level of magnetic activity. In the solar system, the two different planets, Mercury with weak magnetic field and Jupiter with strong magnetic field, are selected for this study to calculate the planet's magnetosphere radius (RMP) which represents the size of magnetosphere compared with solar activity through Kp index,  through two types of geomagnetic conditions; quiet and strong for the period (2016-2018). From the results, we found that there are reversible relations between them during strong geomagnetic storms, while there are direct relations during quiet geomagnetic conditions. Also it is found that there is a reduction in the size of magnetosphere during the strong geomagnetic storms as compared to the magnetosphere size during geomagnetic quiet conditions for the two planets: Mercury and Jupiter. We can conclude from these results that the relation between storm type and magnetosphere size is independent of the strength of planetary surface magnetic field and their distance from the Sun.

scholarly journals A new method for the asteroseismic determination of the evolutionary state of red-giant stars

2016 ◽  
Vol 466 (3) ◽  
pp. 3344-3352 ◽  
Author(s):  
Yvonne Elsworth ◽  
Saskia Hekker ◽  
Sarbani Basu ◽  
Guy R. Davies
Keyword(s):  
New Method ◽  
Giant Stars ◽  
Evolutionary State ◽  
Red Giant Stars ◽  
Red Giant

scholarly journals Exoplanet Surveys at Universidad de Chile

2012 ◽  
Vol 8 (S293) ◽  
pp. 454-459
Author(s):  
Patricio Rojo ◽  
James Jenkins ◽  
Sergio Hoyer ◽  
Matías Jones
Keyword(s):  
Radial Velocity ◽  
Extrasolar Planet ◽  
The South ◽  
Transiting Planets ◽  
Giant Stars ◽  
First Results ◽  
Red Giant Stars ◽  
Red Giant ◽  
Exoplanet Systems

AbstractWe present and highlight the first results of the three main exoplanet surveys we are currently conducting at Universidad de Chile: CHEPS, Red Giant Exoplanets (radial velocity), and TraMoS (transit lightcurves). We have several interesting candidates at the Calan-Hertfordshire Extrasolar Planet Search (CHEPS) project, which is aimed at searching for the currently missing southern bright transiting planets at a few m/s radial velocity precision. Using the same technique, we are also characterizing the planetary population in a constrained sample of Red Giant stars. The Transit Monitoring from the South (TraMoS) project is aimed both at improving transit parameters and at detecting any kind of lightcurve variability from several known southern exoplanet systems.

scholarly journals Asteroseismology of the Hyades red giant and planet host ϵ Tauri

2019 ◽  
Vol 622 ◽  
pp. A190 ◽  
Author(s):  
T. Arentoft ◽  
F. Grundahl ◽  
T. R. White ◽  
D. Slumstrup ◽  
R. Handberg ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Seismic Analysis ◽  
Frequency Separation ◽  
Series Data ◽  
Physical Parameters ◽  
Evolutionary Status ◽  
Evolutionary Stage ◽  
Red Giants ◽  
Model Calculations ◽  
Red Giant

Context. Asteroseismic analysis of solar-like stars allows us to determine physical parameters such as stellar mass, with a higher precision compared to most other methods. Even in a well-studied cluster such as the Hyades, the masses of the red giant stars are not well known, and previous mass estimates are based on model calculations (isochrones). The four known red giants in the Hyades are assumed to be clump (core-helium-burning) stars based on their positions in colour-magnitude diagrams, however asteroseismology offers an opportunity to test this assumption. Aims. Using asteroseismic techniques combined with other methods, we aim to derive physical parameters and the evolutionary stage for the planet hosting star ϵ Tau, which is one of the four red giants located in the Hyades. Methods. We analysed time-series data from both ground and space to perform the asteroseismic analysis. By combining high signal-to-noise radial-velocity data from the ground-based SONG network with continuous space-based data from the revised Kepler mission K2, we derive and characterize 27 individual oscillation modes for ϵ Tau, along with global oscillation parameters such as the large frequency separation Δν and the ratio between the amplitude of the oscillations measured in radial velocity and intensity as a function of frequency. The latter has been measured previously for only two stars, the Sun and Procyon. Combining the seismic analysis with interferometric and spectroscopic measurements, we derive physical parameters for ϵ Tau, and discuss its evolutionary status. Results. Along with other physical parameters, we derive an asteroseismic mass for ϵ Tau of M = 2.458 ± 0.073 M⊙, which is slightly lower than previous estimates, and which leads to a revised minimum mass of the planetary companion. Noting that the SONG and K2 data are non-simultaneous, we estimate the amplitude ratio between intensity and radial velocity to be 42.2 ± 2.3 ppm m−1 s, which is higher than expected from scaling relations.

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