Neutrons for $s$-processing at $A \gtrsim 85$ are mainly provided by \ctanb in AGB stars, requiring some proton penetration below the envelope so far assumed to be of small mass ($\lesssim 10^{-3}$ \ms). However, models with rotation suggested that there excessive $^{14}$N would hamper $s$-processing. On the other hand, $s$-element abundances in Galaxies require \ct-rich layers more extended in mass. We present new calculations for clarifying the above issues, aiming at understanding if the solar composition can help in fixing the extension of the \ctb ``pocket''. We show: i) that mixing ``from bottom to top'' (like in magnetic buoyancy or other forced mechanisms) might in principle form a \ctb reservoir much larger than assumed so far; ii) that stellar models at a suitable metallicity, using a similarly extended pocket would reproduce the main $s$-component as accurately as before; iii) that with the extended pocket the previously envisaged contributions from an unknown nucleosynthesis process ($LEPP$) would no longer be required. The new scheme also fulfils the requirements of C-star luminosities. Consisting of a few large neutron exposures, it would imply a large production of nuclei below $A = 90$; in particular, $^{86,\;87}$Sr would be fully synthesized by AGB stars, while $^{88}$Sr, $^{89}$Y and $^{94}$Zr would be contributed more efficiently by the new model. We suggest some tests, which would probably say a final word on the real extension of the \ctb pocket.
s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Sourve in the 13C-pocket
TRIPPELLA, OSCAR;BUSSO, Maurizio Maria;MAIORCA, ENRICO;PALMERINI, SARA
2014
Abstract
Neutrons for $s$-processing at $A \gtrsim 85$ are mainly provided by \ctanb in AGB stars, requiring some proton penetration below the envelope so far assumed to be of small mass ($\lesssim 10^{-3}$ \ms). However, models with rotation suggested that there excessive $^{14}$N would hamper $s$-processing. On the other hand, $s$-element abundances in Galaxies require \ct-rich layers more extended in mass. We present new calculations for clarifying the above issues, aiming at understanding if the solar composition can help in fixing the extension of the \ctb ``pocket''. We show: i) that mixing ``from bottom to top'' (like in magnetic buoyancy or other forced mechanisms) might in principle form a \ctb reservoir much larger than assumed so far; ii) that stellar models at a suitable metallicity, using a similarly extended pocket would reproduce the main $s$-component as accurately as before; iii) that with the extended pocket the previously envisaged contributions from an unknown nucleosynthesis process ($LEPP$) would no longer be required. The new scheme also fulfils the requirements of C-star luminosities. Consisting of a few large neutron exposures, it would imply a large production of nuclei below $A = 90$; in particular, $^{86,\;87}$Sr would be fully synthesized by AGB stars, while $^{88}$Sr, $^{89}$Y and $^{94}$Zr would be contributed more efficiently by the new model. We suggest some tests, which would probably say a final word on the real extension of the \ctb pocket.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
