People involved at OAB: Beccari, Bellazzini, Carretta, Fusi Pecci, Lanzoni
The actual nature of the Blue Straggler Stars (BSS) has been a puzzle for many years, and their formation mechanism is still not completely understood. BSS are more massive than the normal MS stars, either because of mass transfer between binary companions (MT-BSS), possibly up to the coalescence of the binary system, or merger of two single or binary stars driven by stellar collisions (COL-BSS).
To finally unveil their nature and their formation mechanisms, we are using
several different and complementary approaches, including high-resolution and
multi-wavelength photometric observations, deep high-resolution spectroscopy,
and Monte-Carlo dynamical simulations.
i) Radial distributions. We performed high-resolution and wide-field photometry in ultraviolet and optical bands to study the radial distribution of BSS within their host globular clusters (GCs). While normal cluster populations (such as red giant and horizontal branch stars) do not show any spatial segregation, the radial distribution of BSS was found to be bimodal (i.e., highly peaked in the centre, decreasing at intermediate radii, and rising again outward) in several GCs (such as M3, 47 Tucanae, NGC 6752, M5, M55). Suitable dynamical simulations have been used to show that such a bimodality can be explained only if a sizable fraction (-40%) of the cluster BSS population is made of MT-BSS, responsible for the external rising branch of the distribution, with the balance being COL-BSS, mainly contributing to the central peak. This suggests that both formation channels are simultaneously at work in GCs. Once a larger sample of GCs will be studied with such an approach, the detailed comparison between the BSS population properties, and the cluster structural and dynamical characteristics will allow us to shed light on the complex interplay between stellar evolution and dynamical processes in dense stellar systems (Lanzoni et al. 2007a,b,c; Mapelli et al. 2006).
This work is in collaboration with Ferraro, Dalessandro, Sollima
(Univ. Bologna), Rood, Schiavon (Univ. of Virginia, USA), Mapelli (Zurich
Univ., Switzerland), Sigurdsson (Pennsylvania State Univ., USA), Sanna
(Univ. Roma Tor Vergata), Manicini (Univ. Firenze).
ii) The BSS surface abundances. We used the FLAMES facility at the VLT to obtain high-resolution spectra of 43 BSSs in 47 Tuc. We analysed their surface abundance pattern and discovered that a subpopulation of BSSs shows a significant depletion of carbon and oxygen with respect to the dominant population. This suggests the presence of CNO burning products on the BSS surface, coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster (Ferraro et al. 2006).
This work is in collaboration with Ferraro, Compagni (Univ. of Bologna),
Sabbi (STScI), Gratton, Lucatello (INAF-Padova Obs.), Piotto (Univ. of
Padova), Rood (Univ. of Virginia, USA), Sills (McMaster Univ., Canada),
iii) Anomalous Population Fractions. We used archived HST-ACS images to probe the evolved populations of the GC 47 Tucanae. We found an excess of asymptotic giant branch stars in the cluster core. We interpret this feature as the signature of an extra-population likely made of massive stars (possibly BSS) originated by the evolution of binary systems, and currently experiencing the first ascending red giant branch (Beccari et al. 2006).
This work is in collaboration with F.R. Ferraro (Univ. of Bologna).