Matteo Cantiello edited Conclusions.tex  about 10 years ago

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Nevertheless it is interesting to consider what angular momentum transport mechanisms could be responsible for the strong coupling implied by the asteroseismic observations. One candidate are gravity waves excited by the convective envelope during the RGB, as these can potentially lead to some transport of angular momentum. While a similar process has been discussed in the context of the Sun's rotational profile \citep{Zahn:1997,Charbonnel:2005}, more work needs to be done to understand the details of the excitation and propagation of gravity waves \citep[See e.g.][]{Lecoanet:2013,Rogers:2013}  Another possibility is that some large scale magnetic field is present in and above the stellar core at the end of the main sequence, providing some coupling between core and envelope. This magnetic field could be either of fossil origin (similar to what has been discussed in the context of explaining the internal rotation profile of the Sun) or be generated by a convective dynamo in the H-burning core during the main sequence. Dynamo action is favorable as, given the typical rotational velocities of 1.5-3.0$\mso$ stars during the main sequence, Rossby numbers are usually smaller than 1, implying an $\alpha\Omega$-dynamo could be at work in the core. The equipartition magnetic field is $B_{\rm{eq}} = \varv_c\,\sqrt{4\pi\rho}$, \varv_c\,\sqrt{4\pi\rho}$;  assuming $B_{\phi}\sim B_r\sim B_{\rm{eq}}$ the resulting magnetic stress is $S= \frac{B_r B_{\phi}}{4\pi}$ and the associated diffusivity is $\nu \sim \frac{S}{\rho q \Omega}$, where $q=-\frac{\partial \log \Omega}{\partial \log r}$ is the shear. Typical convective velocities in the core of main sequence, low-mass stars are on the order $0.01\kms$ resulting in $B_{\rm{eq}}\sim 10^4-10^5 G$. Some of the magnetic flux will diffuse in the radiative layers above the convective core, but this is expected to affect only a small fraction of the star as the Ohmic diffusion timescale is much longer than the main sequence timescale.   % Given the typical values of the density and shear in the region between contracting core and the expanding envelope, %we find that to match resulting magnetic diffusivity is of the order $. Compared to the artificial diffusivity %required to explain the early RGB asteroseismic observations... \textbf{TBD: Matteo complete this piece of the %discussion putting in numbers. Depending on how promising the results are we might wanna keep this out of the paper.}  Overall assessing whether such a mechanism can explain the observed rotation rates requires following the coupled evolution of shear and magnetic fields.