HG-WELS Bigger Picture and Goals

Giant stars go through all sorts of interesting structural changes as they evolve from the MS onto the giant branch. These structural changes result in some interesting observable changes – as the convective zone deepens, it changes the angular momentum and chemical composition of the surface layers (what we can most easily observe). Some giants are observed to be rapidly rotating. Some of them have high lithium abundances. The Li, in particular, is hard to explain. Li is so weakly held together that the star doesn’t have to try very hard to destroy it – it can be destroyed at temperatures lower than what it takes to make He. Some people think that the giant star is actually making Li near the base of the convection zone, and then the convection brings it up to the surface where we can see it. Some think that those stars that have Li might have just recently ingested a planet or BD companion (e.g., Siess & Livio 1999 MNRAS 308, 1133 or Carlberg et al. 2012, ApJ, 757, 109), and the addition of that material mixed into the outer layers of the star by convection is what changes the abundances (adding Li). This process would also affect the rotation rate (angular momentum) of the outer layers. Some people (e.g., de la Reza et al. 1997, ApJL, 482, 77) noticed that the high-Li stars also seemed to have IRAS IR excesses, which they postulated came from a shell ejected from the star. This is controversial. IRAS had low spatial resolution and was not terribly sensitive. WISE now provides better spatial resolution and sensitivity but does not go out as far in wavelength as IRAS did. We are looking at samples of these Li K giants to see if we (a) still see IR excesses where de la Reza et al. did with IRAS (with better spatial resolution, these might go away); and (b) for a better-defined sample of K giants (from Carlberg et al 2012), do we see a correlation between IR excess and Li like the de la Reza sample did?