We analyse the angular momentum evolution from the red giant branch (RGB) to the horizontal branch (HB) and along the HB. Using rotation velocities for stars in the globular cluster M13, we find that the required angular momentum for the fast rotators is up to 1±3 orders of magnitude (depending on some assumptions) larger than that of the Sun. Planets of masses up to 5 times Jupiter's mass and up to an initial orbital separation of ,2 au are sufficient to spin-up the RGB progenitors of most of these fast rotators. Other stars have been spun-up by brown dwarfs or low-mass main-sequence stars. Our results show that the fast rotating HB stars have been probably spun-up by planets, brown dwarfs or low-mass main-sequence stars while they evolved on the RGB. We argue that the angular momentum considerations presented in this paper further support the`planet second parameter' model. In this model, the`second parameter' process, which determines the distribution of stars on the HB, is interaction with low-mass companions, in most cases with gas-giant planets, and in a minority of cases with brown dwarfs or low-mass main-sequence stars. The masses and initial orbital separations of the planets (or brown dwarfs or low-mass main-sequence stars) form a rich spectrum of different physical parameters, which manifests itself in the rich varieties of HB morphologies observed in the different globular clusters.

We examine the envelope properties of asymptotic giant branch (AGB) stars as they evolve on the upper AGB and during the early post‐AGB phase. Because of the high mass‐loss rate, the envelope mass decreases by more than an order of magnitude. This makes the density profile below the photosphere much shallower, and the entropy profile much steeper. We discuss the possible role of these changes in the profiles in the onset of the high mass‐loss rate (superwind) and the large deviation from spherical mass loss at the termination of the AGB. We concentrate on the idea that the shallower density profile and steeper entropy profile allow the formation of cool magnetic spots, above which dust forms much more easily.

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