Magnetic field study in single late giants

Konstantinova-Antova, R.; Aurière, M.; Alécian, E.; Petit, P.; Donati, J.; Wade, G. A.; Boeva, S.; Iliev, L.; Kostov, A.

doi:10.1063/1.2993684

Cited by 3 publications

(1 citation statement)

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“…Moreover, wind-reversed chromospheric lines, such as Mg II, show persistent blueshifts signifying mass outflow with velocities increasing with height and reaching terminal velocity of 30-70 km/s (which is greater than the stellar escape velocity) within 1 -2R (Carpenter et al 1995;Robinson et al 1998). Recent observations reveal surface magnetic field at the level of a few Gauss in non-coronal giants to about 60 G in coronal giants and supergiants (Auriere et al 2010;Konstantinova-Antova et al 2008;2009;Tsvetkova et al 2013). For example, the coronal giant β Cet shows an average bipolar photospheric field, fB =20 G (f is the filling factor), while coronal observations of Fe XXI lines imply magnetic confinement with coronal field of about 300 G suggesting that f is less than a few percent.…”

Section: Observational Constrains On Stellar Chromospheric Heatingmentioning

confidence: 99%

Toward A Self Consistent MHD Model of Chromospheres and Winds From Late Type Evolved Stars

Airapetian,

Leake,

Carpenter

2014

Preprint

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We present the first magnetohydrodynamic model of the stellar chromospheric heating and acceleration of the outer atmospheres of cool evolved stars, using α Tau as a case study. We used a 1.5D MHD code with a generalized Ohm's law that accounts for the effects of partial ionization in the stellar atmosphere to study Alfvén wave dissipation and wave reflection. We have demonstrated that due to inclusion of the effects of ion-neutral collisions in magnetized weakly ionized chromospheric plasma on resistivity and the appropriate grid resolution, the numerical resistivity becomes 1-2 orders of magnitude smaller than the physical resistivity. The motions introduced by non-linear transverse Alfvén waves can explain non-thermally broadened and non-Gaussian profiles of optically thin UV lines forming in the stellar chromosphere of α Tau and other late-type giant and supergiant stars. The calculated heating rates in the stellar chromosphere due to resistive (Joule) dissipation of electric currents, induced by upward propagating non-linear Alfvén waves, are consistent with observational constraints on the net radiative losses in UV lines and the continuum from α Tau. At the top of the chromosphere, Alfvén waves experience significant reflection, producing downward propagating transverse waves that interact with upward propagating waves and produce velocity shear in the chromosphere. Our simulations also suggest that momentum deposition by non-linear Alfvén waves becomes significant in the outer chromosphere at 1 stellar radius from the photosphere. The calculated terminal velocity and the mass loss rate are consistent with the observationally derived wind properties in α Tau.

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Section: Observational Constrains On Stellar Chromospheric Heatingmentioning

confidence: 99%

Toward A Self Consistent MHD Model of Chromospheres and Winds From Late Type Evolved Stars

Airapetian,

Leake,

Carpenter

2014

Preprint

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Direct detection of a magnetic field in the photosphere of the single M giant EK Bootis

Konstantinova-Antova

Aurière

Charbonnel

et al. 2010

A&A

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Aims. We study the fast rotating M 5 giant EK Boo by means of spectropolarimetry to obtain direct and simultaneous measurements of both the magnetic field and activity indicators, in order to infer the origin of the activity in this fairly evolved giant. Methods. We used the new spectropolarimeter NARVAL at the Bernard Lyot Telescope (Observatoire du Pic du Midi, France) to obtain a series of Stokes I and Stokes V profiles for EK Boo. Using the least square deconvolution (LSD) technique we were able to detect the Zeeman signature of the magnetic field. We measured its longitudinal component by means of the averaged Stokes V and Stokes I profiles. The spectra also permitted us to monitor the Ca ii K&H chromospheric emission lines, which are well known as indicators of stellar magnetic activity. Results. From ten observations obtained between April 2008 and March 2009, we deduce that EK Boo has a magnetic field, which varied in the range of −0.1 to −8 G. On March 13, 2009, a complex structure of Stokes V was observed, which might indicate a dynamo. We also determined the initial mass and evolutionary stage of EK Boo, based on up-to-date stellar evolution tracks. The initial mass is in the range of 2.0−3.6 M , and EK Boo is either on the asymptotic giant branch (AGB), at the onset of the thermal pulse phase, or at the tip of the first (or red) giant branch (RGB). The fast rotation and activity of EK Boo might be explained by angular momentum dredge-up from the interior, or by the merging of a binary. In addition, we observed eight other M giants, which are known as X-ray emitters, or to be rotating fast for their class. For one of these, β And, presumably also an AGB star, we have a marginal detection of magnetic field, and a longitudinal component B l of about 1 G was measured. More observations like this will answer the question whether EK Boo is a special case, or whether magnetic activity is, rather, more common among M giants than expected.

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Magnetic field structure in single late-type giants: The weakG-band giant 37 Comae from 2008 to 2011

Tsvetkova

Petit

Konstantinova-Antova

et al. 2017

A&A

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Aims. This work studies the magnetic activity of the late-type giant 37 Com. This star belongs to the group of weak Gband stars that present very strong carbon deficiency in their photospheres. The paper is a part of a global investigation into the properties and origin of magnetic fields in cool giants. Methods. We use spectropolarimetric data, which allows the simultaneous measurement of the longitudinal magnetic field B l , line activity indicators (Hα, Ca ii IRT, S-index) and radial velocity of the star, and consequently perform a direct comparison of their time variability. Mean Stokes V profiles are extracted using the least squares deconvolution (LSD) method. One map of the surface magnetic field of the star is reconstructed via the Zeeman Doppler imaging (ZDI) inversion technique. Results. A periodogram analysis is performed on our dataset and it reveals a rotation period of 111 days. We interpret this period to be the rotation period of 37 Com. The reconstructed magnetic map reveals that the structure of the surface magnetic field is complex and features a significant toroidal component. The time variability of the line activity indicators, radial velocity and magnetic field B l indicates a possible evolution of the surface magnetic structures in the period from 2008 to 2011. For completeness of our study, we use customized stellar evolutionary models suited to a weak G-band star. Synthetic spectra are also calculated to confirm the peculiar abundance of 37 Com. Conclusions. We deduce that 37 Com is a 6.5 M⊙ weak G-band star located in the Hertzsprung gap, whose magnetic activity is probably due to dynamo action.

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Magnetic field study in single late giants

Cited by 3 publications

References 0 publications

Toward A Self Consistent MHD Model of Chromospheres and Winds From Late Type Evolved Stars

Toward A Self Consistent MHD Model of Chromospheres and Winds From Late Type Evolved Stars

Direct detection of a magnetic field in the photosphere of the single M giant EK Bootis

Magnetic field structure in single late-type giants: The weakG-band giant 37 Comae from 2008 to 2011

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