title: Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes.
abstract: When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star's radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected 'mixed modes'. By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.
authors.affiliation: Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, 3001 Leuven, Belgium. paul.beck@ster.kuleuven.be
authors.forename: Paul G
authors.initials: PG
authors.lastname: Beck
authors.forename: Josefina
authors.initials: J
authors.lastname: Montalban
authors.forename: Thomas
authors.initials: T
authors.lastname: Kallinger
authors.forename: Joris
authors.initials: J
authors.lastname: De Ridder
authors.forename: Conny
authors.initials: C
authors.lastname: Aerts
authors.forename: Rafael A
authors.initials: RA
authors.lastname: García
authors.forename: Saskia
authors.initials: S
authors.lastname: Hekker
authors.forename: Marc-Antoine
authors.initials: MA
authors.lastname: Dupret
authors.forename: Benoit
authors.initials: B
authors.lastname: Mosser
authors.forename: Patrick
authors.initials: P
authors.lastname: Eggenberger
authors.forename: Dennis
authors.initials: D
authors.lastname: Stello
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Elsworth
authors.forename: Søren
authors.initials: S
authors.lastname: Frandsen
authors.forename: Fabien
authors.initials: F
authors.lastname: Carrier
authors.forename: Michel
authors.initials: M
authors.lastname: Hillen
authors.forename: Michael
authors.initials: M
authors.lastname: Gruberbauer
authors.forename: Jørgen
authors.initials: J
authors.lastname: Christensen-Dalsgaard
authors.forename: Andrea
authors.initials: A
authors.lastname: Miglio
authors.forename: Marica
authors.initials: M
authors.lastname: Valentini
authors.forename: Timothy R
authors.initials: TR
authors.lastname: Bedding
authors.forename: Hans
authors.initials: H
authors.lastname: Kjeldsen
authors.forename: Forrest R
authors.initials: FR
authors.lastname: Girouard
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Hall
authors.forename: Khadeejah A
authors.initials: KA
authors.lastname: Ibrahim