ASTROPHYSICAL IMPACT OF THE UPDATED⁹Be(p,α)⁶Li AND¹⁰B(p,α)⁷Be REACTION RATES AS DEDUCED BY THM

Abstract

The complete understanding of the stellar abundances of lithium, beryllium, and boron represents one of the most interesting open problems in astrophysics. These elements are largely used to probe stellar structure and mixing phenomena in different astrophysical scenarios, such as pre-main-sequence or main-sequence stars. Their different fragility against (p,α) burning reactions allows one to investigate different depths of the stellar interior. Such fusion mechanisms are triggered at temperatures between T ≈ (2-5) × 10 6 K, thus defining a corresponding Gamow energy between ≈ 3-10 keV, where S(E)-factor measurements need to be performed to get reliable reaction rate evaluations. The Trojan Horse Method is a well defined procedure to measure cross sections at Gamow energies overcoming the uncertainties due to low-energy S(E)-factor extrapolation as well as electron screening effects. Taking advantage of the THM measure of the 9 Be(p,α) 6 Li and 10 B(p,α) 7 Be cross sections, the corresponding reaction rates have been calculated and compared with the evaluations by the NACRE collaboration, widely used in the literature. The impact on surface abundances of the updated 9 Be and 10 B (p,α) burning rates is discussed for pre-MS stars.

The Trojan Horse Method as a tool for investigating astrophysically relevant fusion reactions

“…In the last work of [18], the impact of the THM measurements for the (p,α) cross sections involving 9 Be and 10 B has been evaluated. In particular, the 9 Be(p,α) 6 Li THM measurement has been firstly investigated in [19] while an improved measurement is given in [20].…”

“…In particular, the 9 Be(p,α) 6 Li THM measurement has been firstly investigated in [19] while an improved measurement is given in [20]. The latter one has been used to firstly extract the bare-nucleus S b (E)-factor and then for calculating the corresponding reaction rate, by means of standard formulas (see [18] for details). The THM S b (E)-factor measurements are shown in the left panel of Fig.1, where the experimental points are shown as black-circles with their errors.…”

“…The full line represents the corresponding fit leading to a value of S(0)=21.8±0.8 (MeV b) while the dashed line gives its enhancing due to an electron screening potential of U e =676±86 eV [20]. The fit of the THM S(E)-factor has been then used in [18] for calculating the corresponding reaction rate through the standard formula…”

The Trojan Horse Method application on the 10B(p,α0)7Be reaction cross section measurements

“…Since depletion of light elements such as Li, Be and B occurs at different depths, this reaction can be used as probe for internal stellar structure, so, analysis of the resulting atmospheric abundances of these elements can help in understanding and confining the mixing processes [3][4][5].…”

New direct measurement of the10B(p,α)7Be reaction with the activation technique

“…As a consequence, when material is mixed from the surface down to the inner layers of a star, e-mail: rdepalo@pd.infn.it each element survives to different depths [2]. Moreover, the relative abundances of the stable boron isotopes 10 B and 11 B in pre-MS stars provides additional information on early stellar evolution: the N( 11 B)/N( 10 B) ratio for different stellar masses and metallicities shows an increase with time for stellar masses in the range 0.1 -0.3 solar masses [3]. Therefore, by measuring the amount of each isotope on the surface of a star it is possible to shed light on stellar mixing in the early phases of stellar evolution, provided that the cross sections of all the reactions which may affect the elemental abundances are known.…”