Journal Article

Non-radial oscillation modes with long lifetimes in giant stars

Samadi, Réza, Kallinger, Thomas, Baudin, Frédéric, Carrier, Fabien, Weiss, Werner W., Auvergne, Michel, Deleuil, Magali, Barge, Pierre, Barban, Caroline, Hatzes, Artie P., Hekker, Saskia, De Ridder, Joris, Baglin, Annie

2009

Overview

Taking the pulse of red giants When main-sequence stars like the Sun near the end of their life, they expand to become oscillating red giants. Such evolved stars could in principle provide stringent tests of stellar theory via analysis of radial and non-radial stellar oscillations. Until now it has been unclear whether non-radial modes are observable at all in red giants. De Ridder et al . now report the presence of both radial and non-radial oscillations in over 300 giant stars. For some red giants, mode lifetimes are of the order of a month. Current stellar evolution theory cannot account for these observations. Towards the end of their lives, stars like the Sun expand greatly to become red giant stars that oscillate. Such evolved stars could provide stringent tests of stellar theory through the analysis of radial and non-radial stellar oscillations. Here, the presence of such oscillations in more than 300 giant stars is reported, with mode lifetimes of some of the giants in the order of a month. Towards the end of their lives, stars like the Sun greatly expand to become red giant stars. Such evolved stars could provide stringent tests of stellar theory, as many uncertainties of the internal stellar structure accumulate with age. Important examples are convective overshooting and rotational mixing during the central hydrogen-burning phase, which determine the mass of the helium core, but which are not well understood 1 . In principle, analysis of radial and non-radial stellar oscillations can be used to constrain the mass of the helium core. Although all giants are expected to oscillate 2 , it has hitherto been unclear whether non-radial modes are observable at all in red giants, or whether the oscillation modes have a short or a long mode lifetime 3 , 4 , 5 , 6 , 7 , which determines the observational precision of the frequencies. Here we report the presence of radial and non-radial oscillations in more than 300 giant stars. For at least some of the giants, the mode lifetimes are of the order of a month. We observe giant stars with equally spaced frequency peaks in the Fourier spectrum of the time series, as well as giants for which the spectrum seems to be more complex. No satisfactory theoretical explanation currently exists for our observations.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Astronomy / Astrophysics / Earth, ocean, space / Exact sciences and technology / Giant and subgiant stars / Giant stars / Helium / Humanities and Social Sciences / letter / Lifetime / multidisciplinary / Noise / Normal stars (by class): general or individual / Physics / Properties / Pulsations, oscillations, and stellar seismology / Red giants / Science / Science (multidisciplinary) / Spectrum allocation / Stars / Stellar characteristics and properties / Stellar oscillations / Studies