Mixed modes in red giants: a window on stellar evolution

Abstract

Context. The detection of oscillations with a mixed character in subgiants and red giants allows us to probe the physical conditions in their cores. Aims. With these mixed modes, we aim at determining seismic markers of stellar evolution. Methods. Kepler asteroseismic data were selected to map various evolutionary stages and stellar masses. Seismic evolutionary tracks were then drawn with the combination of the frequency and period spacings. Results. We measured the asymptotic period spacing for 1178 stars at various evolutionary stages. This allows us to monitor stellar evolution from the main sequence to the asymptotic giant branch and draw seismic evolutionary tracks. We present clear quantified asteroseismic definitions that characterize the change in the evolutionary stages, in particular the transition from the subgiant stage to the early red giant branch, and the end of the horizontal branch. Conclusions. The seismic information is so precise that clear conclusions can be drawn independently of evolution models. The quantitative seismic information can now be used for stellar modeling, especially for studying the energy transport in the heliumburning core or for specifying the inner properties of stars entering the red or asymptotic giant branches. Modeling will also allow us to study stars that are identified to be in the helium-subflash stage, high-mass stars either arriving or quitting the secondary clump, or stars that could be in the blue-loop stage.

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

“…Indeed, asteroseismic studies of clump stars (Montalbán et al 2013;Stello et al 2013;Mosser et al 2014;Bossini et al 2015;Constantino et al 2015) and subdwarf B stars (Van Grootel et al 2010a, 2010bCharpinet et al 2011;Schindler et al 2015) indicate that the convective core is somewhat larger than predicted by stellar evolution codes, even using optimistic overshooting prescriptions. MS convective cores may also be somewhat larger and show evidence for enhanced mixing (Moravveji et al 2015).…”

“…This technique utilizes observations of mixed modes, oscillations that behave as pressure waves (p-modes) near the stellar surface and gravity waves (g-modes) in the stellar core (Scuflaire 1974;Osaki 1975;Aizenman et al 1977;Dziembowski et al 2001;Christensen-Dalsgaard 2004;Dupret et al 2009). Mixed modes have been used successfully to determine the evolutionary status of red giant stars Stello et al 2013;Mosser et al 2014), and to measure their internal rotation rate (Beck et al 2012;Mosser et al 2012;Deheuvels et al 2014Deheuvels et al , 2015. Turbulent convection in the red giant envelope excites these modes, with part of the wave energy leaking through the evanescent region into the gmode cavity.…”

Helium signature in red giant oscillation patterns observed byKepler

“…Original light curves were processed and corrected from phenomenological effects such as outliers, jump and drifts according to the method of García et al (2011). We used the set of 1142 stars for which Mosser et al (2014) deduced the evolutionary status from the identification of the mixed-mode pattern. This allowed us to select oscillation spectra with a high signal-to-noise ratio as measured by an autocorrelation amplitude above 50 (Mosser & Appourchaux 2009) …”

“…We use stars with an evolutionary state already determined by Mosser et al (2014) in order to investigate the behaviour of the glitches as a function of the evolutionary status. In Sect.…”

Determining stellar parameters of asteroseismic targets: going beyond the use of scaling relations

“…Although such corrections are expected to have little impact on the stellar radii (and hence on the distances), they are expected to reduce the errors in the derived stellar masses, hence on the derived ages for giants. Second, there are other asteroseismic parameters as well -like for instance the period spacing of mixed modes, ∆P (Beck et al 2011;Mosser et al 2014) -that can be used to estimate stellar parameters, although not via so easy-to-use scaling relations as those above-mentioned.…”