ASTEROSEISMIC CLASSIFICATION OF STELLAR POPULATIONS AMONG 13,000 RED GIANTS OBSERVED BY KEPLER

Abstract

Of the more than 150 000 targets followed by the Kepler Mission, about 10% were selected as red giants. Due to their high scientific value, in particular for Galaxy population studies and stellar structure and evolution, their Kepler light curves were made public in late 2011. More than 13 000 (over 85%) of these stars show intrinsic flux variability caused by solar-like oscillations making them ideal for large scale asteroseismic investigations. We automatically extracted individual frequencies and measured the period spacings of the dipole modes in nearly every red giant. These measurements naturally classify the stars into various populations, such as the red giant branch, the low-mass (M/M ⊙ 1.8) helium-core-burning red clump, and the higher-mass (M/M ⊙ 1.8) secondary clump. The period spacings also reveal that a large fraction of the stars show rotationally induced frequency splittings. This sample of stars will undoubtedly provide an extremely valuable source for studying the stellar population in the direction of the Kepler field, in particular when combined with complementary spectroscopic surveys.

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.…”

Triennial Report (2012-2015): The Legacy Issue

“…Recent result show strong indications this effect is due to strong magnetic fields inside the stars -a result that could seed a new line of research in the next few years. The shear numbers of red giants with detected oscillations has risen by orders of magnitude in a period of only 2-3 years, to roughly 15-20 thousand stars , Kallinger et al 2010, Hekker et al 2011, Miglio et al 2012, Stello et al 2013. With these large numbers of stars we can expect to resolve new questions and opened up completely new ways of exploring red giants using asteroseismology in the decade to come.…”

“…With data from the Kepler spacecraft the revolution for solar-like variables continued with detections of oscillations in hundreds of dwarfs and subgiants (Chaplin et al 2011) as well as thousands of red-giant stars (Hekker et al 2011;Stello et al 2013). For the giant stars mixed-modes have been detected (Beck et al 2011;Bedding et al 2011) which make it possible to study the inner parts of these stars in a more direct manner.…”

“…From the rotational splitting the inclination angle of the spin axis of the star can also be derived. For planet-host stars with eclipsing planets this has led to the detection of spinorbit (mis)alignments (Chaplin et al 2013;Huber et al 2013). These (mis)alignments provide vital information regarding the formation and evolution of planetary systems.…”