Matteo Cantiello edited Red Clump.tex  about 9 years ago

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\subsection{Red Clump}  After reaching the tip of the red giant branch branch,  stars ignite helium in their cores. In stars with mass $M \le 2\mso$ the off-center ignition of helium leads to a runaway nuclear fusion removing the degeneracy of the core. This process, lasting about $2$ Myrs, is called core He-flash and has a specific asteroseismic signature \cite{Bildsten_2011}. The core He-flash, due to its large luminosity release, is characterized by episodes of turbulent convection that propagate through the whole He core. Stars with $M > 2\mso$ do not experience the He-flash and gently transit from H-shell burning to He core burning. For all stars the burning of $\sim 0.5\mso$ of Helium occurs in a convective core, due to the steep temperature dependency of the triple-alpha nuclear reactions. %Stars in this phase, lasting $\sim$ 100 Myrs, are called red clump stars. Similarly to the case of early RGB stars, the tunneling integral is such that $\ell=1$ modes are expected to have substantial inertia in the g-mode cavity. As a matter of fact mixed modes are observed on the clump, allowing for the determination of core rotation rates. Therefore stars with suppressed dipoles on the RGB are expected to show suppressed dipoles during He-core burning \texbf{if} their internal magnetic fields are not radically different.   The apparent absence of suppressed $\ell=1$ modes in clump stars means that these stars must have lost their large amplitude magnetic fields during or shortly after the He-flash. During the He-flash the size of the core increases by a factor of 3 due to the degeneracy removal. While the magnetic field would slightly decrease in amplitude due to conservation of magnetic flux, this can not explain the fact that clump stars show no suppression at all. A possibility is that the stable configuration of the fossil field is unable to survive the various phases of turbulent convection associated with the He-flash. The vigorous convection might destroy the field, or re-arrange it in an unstable configuration, which then decays on the Alfven timescale (for $B\approx10^5$ G and $0.3\rso$ this is of order months, many orders of magnitude shorter than the burning timescale on the clump).