Bibcode
García, R. A.; Pérez-Hernández, F.; Benomar, O.; Silva Aguirre, V.; Ballot, J.; Davies, G. R.; Doğan, G.; Stello, D.; Christensen-Dalsgaard, J.; Houdek, G.; Lignières, F.; Mathur, S.; Takata, M.; Ceillier, T.; Chaplin, W. J.; Mathis, S.; Mosser, B.; Ouazzani, R. M.; Pinsonneault, M. H.; Reese, D. R.; Régulo, C.; Salabert, D.; Thompson, M. J.; van Saders, J. L.; Neiner, C.; De Ridder, J.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 563, id.A84, 17 pp.
Advertised on:
3
2014
Revista
Número de citas
49
Número de citas referidas
41
Descripción
Context. The continuous high-precision photometric observations provided
by the CoRoT and Kepler space missions have allowed us to understand the
structure and dynamics of red giants better using asteroseismic
techniques. A small fraction of these stars show dipole modes with
unexpectedly low amplitudes. The reduction in amplitude is more
pronounced for stars with a higher frequency of maximum power,
νmax. Aims: In this work we want to characterise
KIC 8561221 in order to confirm that it is currently the least evolved
star among this peculiar subset and to discuss several hypotheses that
could help explain the reduction of the dipole mode amplitudes.
Methods: We used Kepler short- and long-cadence data combined with
spectroscopic observations to infer the stellar structure and dynamics
of KIC 8561221. We then discussed different scenarios that could
contribute to reducing the dipole amplitudes, such as a fast-rotating
interior or the effect of a magnetic field on the properties of the
modes. We also performed a detailed study of the inertia and damping of
the modes. Results: We have been able to characterise 36
oscillations modes, in particular, a few dipole modes above
νmax that exhibit nearly normal amplitudes. The
frequencies of all the measured modes were used to determine the overall
properties and the internal structure of the star. We have inferred a
surface rotation period of ~91 days and uncovered a variation in the
surface magnetic activity during the last 4 years. The analysis of the
convective background did not reveal any difference compared to "normal"
red giants. As expected, the internal regions of the star probed by the
ℓ = 2 and 3 modes spin 4 to 8 times faster than the surface.
Conclusions: With our grid of standard models we are able to properly
fit the observed frequencies. Our model calculation of mode inertia and
damping give no explanation for the depressed dipole modes. A
fast-rotating core is also ruled out as a possible explanation. Finally,
we do not have any observational evidence of a strong deep magnetic
field inside the star.
Table 3 and Appendix A are available in electronic form at http://www.aanda.org
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