Speaker
Description
Globular clusters (GC) constitute a rather unique probe of galactic and stellar evolution theory, but their formation and peculiar chemical patterns are still not completely understood.
The most striking feature is the presence of multiple stellar populations, showing strong star-to-star variations in light element abundances. The distribution of heavier elements is instead constant across different stellar generations, making a contribution to their formation from supernovae-processed material unlikely.
One of the strongest GC formation models suggests that new stars are produced from matter processed by nuclear burning cycles and slowly ejected during the AGB phase, while supernova explosions violently sweep nebulae out of the GC, hindering further star formation.
This model explains many GC observables, but fails in predicting the observed O depletion and Na enrichement, the so-called O-Na anti-correlation.
A possible nuclear solution to this long-standing issue could be a $\mathrm{^{23}Na(p, \alpha)^{20}Ne}$ reaction rate a factor 2 to 5 lower than currently assumed.
This reaction is responsible for Na destruction and is dominated by a narrow resonance at $E_{CM}$=133 keV, never observed before.
Tentative upper limits for its strength have been placed depending on the unknown proton momentum transfer value $I_p$, carrying five orders of magnitude of uncertainty. After a first unsuccessful study, no attempts at direct measurement have been made, since the expected event rate is too low to be detectable on Earth's surface.
At LUNA (\textit{Laboratory for Underground Nuclear Astrophysics}), exploiting the combination of its known background reduction and the high proton beam intensities achievable, it will be possible to measure the strength of this resonance over a few months.
As part of the ELDAR project (UKRI ERC StG), a new array of silicon detectors has been specifically designed to detect charged particles at LUNA and its commissioning will begin in the next months. We are also exploring new production procedures for sodium targets, in collaboration with \textit{Laboratori Nazionali di Legnaro} (Italy), to reduce their sensitivity to humidity and improve their stability under beam bombardment.
The extensive experimental campaign will allow us to perform a deep study of the $\mathrm{^{23}Na(p, \alpha)^{20}Ne}$, with a particular focus on the resonance that could possibly solve the GC puzzle.
