Horizontal Branch (HB) stars are characterized by core-helium burning and shell-hydrogen burning. The star location in effective temperature along the Zero Age Horizontal Branch (ZAHB) depends on almost all stellar parameters (composition, age, rotation, etc.). The wide colour distribution of the HB, called the HB morphology, is the result of large differences in the envelope mass of stars having the same core mass, at the same evolutionary stage. The HB phase behaves as an amplifier, displaying the record of both initial conditions and of any variations and perturbations in the evolution of the star from its birth up to the HB stage. Therefore, reading properly the HB morphologies can yield a better understanding of Population II stellar evolution in general, and of the specific stellar systems, clusters or galaxies, in particular.
However, it appears that our comprehension of the HB phase and its precursors is incomplete.
Canonical stellar theory cannot adequately explain the wide variety of HB morphologies observed in Galactic globular clusters, ranging from short red HBs to long extended "blue tails’’.
Stellar rotation and chemical anomalies of HB stars
Several important questions are still challenging our understanding of angular momentum redistribution in stars along their evolutionary path. One of these is rotation of horizontal branch (stars, which are additionaly affected by our incomplete comprehension of the HB phase and its precursors.
The progenitors of HB stars are expected to have shed most of their angular momentum via
magnetically-coupled winds during their early evolution on the main sequence and, as a consequence, to have had modest rotation rates, of about of 2 km/s, like the Sun. Moreover, we would expect a further slowing down of those stars, due to the radius increase and mass loss during the red giant branch ascent. Surprisingly, however, rotation values as high as 30-40 km/s have been observed among horizontal branch stars, both in globular clusters (GC) (Behr 2003a, Recio-Blanco et al., 2002, 2004) and the field (Behr 2003b).
In addition, fast rotating stars are found only among stars cooler than 11, 500 K. This temperature value plays an important role in the HB picture: hotter stars (and at the same time cooler than about 20,000 K) have anomalously bright Str¨omgren u magnitudes (Grundahl et al. 1998, 1999, G99) and gravities that are systematically lower than predicted by evolutionary models (Moehler et al. 1995, 2003). This can be attributed (G99; Hui-Bon-Hoa et al. 2000) to the onset of strong effects of levitation, due to microscopic diffusion, and radiation pressure, whose combined effects can be seen in stars lacking significant sub-atmospheric convection zones (Sweigart 2002). This is the case of hot HB stars, whose atmospheres are expected to be strongly depleted in He (Michaud et al. 1983), and to exhibit large over-abundances of elements like Fe, Ti, P (Richer et al. 2000). All these features are actually observed (Behr 2003a; Fabbian, Recio-Blanco et al. 2005). As mentioned before, all the HB stars with Teff hotter than 11,500 K analyzed so far, show vsini values (the projected rotational velocity) 12 km/s, indicating that the bulk of these stars must be intrinsically slow rotators.
Indeed, at Teff 11,500 K there is an abrupt change in the rotational velocity distribution.
Among the cooler stars (Teff < 11,500 K), there is a group rotating at 15 km/s or less, and a fast rotating group reaching 30 km/s. Such discontinuity seems to suggest a link between HB rotation and the onset of radiative levitation processes on the stellar atmosphere(Michaud et al. 2008) . Molecular weight barriers preventing the transfer of angular momentum from the core and HB mass loss have been proposed to explain this phenomenom (Sills & Pinsonneault, 2000; Vink & Cassisi, 2002).
Our group, is involved in serveral projects related to the stellar rotation and chemical anomalies of HB stars in collaboration with people at the Padova University (Italy) and the GRAAL (Universite de Montpellier).