Seismic probing of the first dredge-up event through the eccentric red-giant and red-giant spectroscopic binary KIC 9163796

Abstract

Context. Binaries in double-lined spectroscopic systems (SB2) provide a homogeneous set of stars. Differences of parameters, such as age or initial conditions, which otherwise would have strong impact on the stellar evolution, can be neglected. The observed differences are determined by the difference in stellar mass between the two components. The mass ratio can be determined with much higher accuracy than the actual stellar mass. Aims. In this work, we aim to study the eccentric binary system KIC 9163796, whose two components are very close in mass and both are low-luminosity red-giant stars. Methods. We analyse four years of Kepler space photometry and we obtained high-resolution spectroscopy with the Hermes instrument. The orbital elements and the spectra of both components were determined using spectral disentangling methods. The effective temperatures, and metallicities were extracted from disentangled spectra of the two stars. Mass and radius of the primary were determined through asteroseismology. The surface rotation period of the primary is determined from the Kepler light curve. From representative theoretical models of the star, we derived the internal rotational gradient, while for a grid of models, the measured lithium abundance is compared with theoretical predictions. Results. From seismology the primary of KIC 9163796 is a star of 1.39±0.06 M ⊙ , while the spectroscopic mass ratio between both components can be determined with much higher precision by spectral disentangling to be 1.015±0.005. With such mass and a difference in effective temperature of 600 K from spectroscopy, the secondary and primary are, respectively, in the early and advanced stage of the first dredge-up event on the red-giant branch. The period of the primary's surface rotation resembles the orbital period within 10 days.The radial rotational gradient between the surface and core in KIC 9163796 is found to be 6.9 +2.0 −1.0 . This is a low value but not exceptional if compared to the sample of typical single field stars. The seismic average of the envelope's rotation agrees with the surface rotation rate. The lithium abundance is in agreement with quasi rigidly-rotating models. Conclusions. The agreement between the surface rotation with the seismic result indicates that the full convective envelope is rotating quasi-rigidly. The models of the lithium abundance are compatible with a rigid rotation in the radiative zone during the main sequence. Because of the many constraints offered by oscillating stars in binary systems, such objects are important test beds of stellar evolution.

Testing tidal theory for evolved stars by using red giant binaries observed by Kepler

“…Such a two-layer model is commonly used in the representative forward-modelling approach in asteroseismology. Recent theoretical and observational results by Di Mauro et al (2016) and Beck et al (2018), respectively, have confirmed that this simplification can be used.…”

Asteroseismic sensitivity to internal rotation along the red-giant branch

“…When the asteroseismic surface rotation rate is roughly equal to the independent measurement of the surface rotation, the star rotates as a solid body in the convection zone. Beck et al (2018) applied the above approach to the primary component of the binary KIC 9163796 and showed that the amount of differential rotation in the convection zone is small, meaning that the convection zone is nearly rotating as a solid body. This could provide important hints for underlying angular momentum transport mechanisms.…”

“…Internal rotation rates as a function of depth have been estimated for a number of red giants using a forward modelling approach (Beck et al 2014(Beck et al , 2018 or by means of rotational inversions (Deheuvels et al 2012(Deheuvels et al , 2014Di Mauro et al 2016Triana et al 2017). However, the exact shape of the internal rotation profile remains unknown, as only core and surface rotation rates can currently be measured.…”

Solar-Like Oscillators in the Kepler Era: A Review

“…One obvious example is testing tidal interaction theories [e.g., (Remus et al, 2012)]. Beck et al (Beck et al, 2018a) used a sample of binaries with at least one red-giant star to show that the equilibrium tide is sufficient to explain the distribution of the measured orbital eccentricities and periods. As another important example, a prediction of stellar evolution theory is that stars climbing the red-giant branch will "dredge-up" nuclear-processed material as the surface convection zone deepens, potentially altering the surface composition.…”