Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars

Abstract

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell 1,2 . Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion 3 . Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes 4 . Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies 5 . Here we report observations of gravity-mode period spacings in red giants 6 that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly 50 seconds) and those that are also burning helium (period spacing 100 to 300 seconds).Oscillations in red giants, like those in the Sun, are thought to be excited by near-surface convection. The observed oscillation spectra are indeed remarkably Sun-like, with a broad range of radial and nonradial modes in a characteristic comb pattern [7][8][9][10][11]

Adiabatic Solar-Like Oscillations in Red Giant Stars

“…Models with a large density contrast show a frequency difference between consecutive modes that is much smaller than for models at the bottom of the RGB or in the He-B phase. These properties have been measured by Bedding et al (2011) in the spectra of red giants observed by Kepler and also in those of CoRoT red giants (Mosser et al, 2011a). At a given ∆ ν (that of the red clump) the difference of period (∆ P) between consecutive modes gathers the stars in two groups: one characterized by targets with ∆ P > 100 s and one with ∆ P < 60 s. The comparison with theoretical computations allows us to identify these two groups with stars that are burning He at the center, for the first group, and with stars that are still burning H in a shell during the ascending RGB for the second one, and then to lift the degeneracy between RGB and He-B models with the same ∆ ν and ν max .…”

Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes

“…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' 3,4 . By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface.…”

Front Matter: Volume 8152

“…One of the most exciting recent results is the ability of Kepler to distinguish between giant stars that are still burning hydrogen in a shell surrounding the accumulating helium core, and those giants that have initiated helium burning in the core. 43 Finally, the Kepler data are also being used to study a wide variety of oscillating and pulsating stars, including RR Lyr stars, which can more than double their brightness every 12 hours, as well as cataclysmic variable stars.…”