People involved at OAB: Bragaglia, Possenti
During a large term project dedicated to finding and understanding millisecond pulsars (MSP) in globular clusters, a particularly interesting system was discovered in NGC 6397 and optically identified (Ferraro et al. 2001)
This MSP binary system contains a tidally deformed star.
This is the first case of a MSP in a binary system with such
an exotic companion and it could represent the first detection
of a new-born MSP. The discovery of this system, thanks to the unusual
brightness of the companion (V=16.6), allowed us
unprecedented detailed spectroscopic observations,
opening a new unexplored window to the study of MSPs in clusters.
We have performed
phase resolved spectroscopy at high resolution (with UVES@VLT)
which allowed us to determine the radial
velocity curve and many orbital parameters of this binary system
(Ferraro et al. 2003). These observations have shown
a surprising detection
of strong HeI absorption lines, completely unexpected
in a low-temperature star as the MSP companion. This
feature implies the existence of atmospheric regions at
K, significantly warmer that the rest of the star. The
intensity of the lines correlates with the orbital phase, suggesting
the presence of a small spot onto the companion surface, heated by the
millisecond pulsar flux.
We have also performed the analysis of the complex structure of
the H
emission line, which has allowed us to map the structure
of the gas released by the donor star (Sabbi et al. 2003a).
Finally, abundance analysis has been performed, even with some difficulties
due to the high rotational velocity (about 50 km s
). We have found
that elemental abundances are fully compatible with the ones of ``normal''
stars in NGC 6397, with the exception of Li, Ca, and C.
The almost complete absence of C suggests that the star has been heavily
peeled, and favours the scenario where the companion is a MSTO star which
has lost most of its mass (Sabbi et al. 2003b).
This research is in collaboration with D'Amico (INAF-OA Cagliari),
Ferraro and Sabbi (Dip. Astr. BO).