The s-process main component is mainly produced in low-mass AGB stars by the C-13(alpha, n)O-16 reaction, requiring proton injection from the envelope. The C-13 pocket was typically assumed to involve a small mass (<= 10(-3) M-circle dot) (Gallino et al. 1998), but models with rotation suggest that N-14 hampers s-processing in such tiny layers. Recent spectroscopy of young open clusters, showing enhancements of s-element abundances with respect to the Sun, have indicated a more effective s-process production requiring C-13-rich layers extended in mass (>= 4 x 10(-3) M-circle dot). We speculated that mixing driven by magnetic buoyancy (as in Maiorca et al. 2012; Trippella et al. 2014, or other forced mechanisms "from bottom to top") can form a C-13 reservoir larger than assumed so far, covering most of the He-rich layers. We present new calculations (Trippella et al. 2014) aimed at understanding if the solar composition helps to constrain the C-13-pocket extension. Stellar models at a fixed metallicity, based on a large C-13 reservoir, reproduce the main s-component as accurately as before and don't require any nuclear contribution from an unknown nucleosynthesis processes (LEPP). These models also avoid problems of mixing at the envelope border and meet requirements from C-star luminosities. A large production of nuclei below A = 90 is expected, so that (86)'Sr-87 may be fully synthesized by AGB stars, while Sr-88, Y-89 and Zr-94 are contributed more efficiently than before.
Can the Main Component of the s-Process in AGB Stars Constrain the C-13-Pocket Formation?
TRIPPELLA, OSCAR;BUSSO, Maurizio Maria;MAIORCA, ENRICO;PALMERINI, SARA
2015
Abstract
The s-process main component is mainly produced in low-mass AGB stars by the C-13(alpha, n)O-16 reaction, requiring proton injection from the envelope. The C-13 pocket was typically assumed to involve a small mass (<= 10(-3) M-circle dot) (Gallino et al. 1998), but models with rotation suggest that N-14 hampers s-processing in such tiny layers. Recent spectroscopy of young open clusters, showing enhancements of s-element abundances with respect to the Sun, have indicated a more effective s-process production requiring C-13-rich layers extended in mass (>= 4 x 10(-3) M-circle dot). We speculated that mixing driven by magnetic buoyancy (as in Maiorca et al. 2012; Trippella et al. 2014, or other forced mechanisms "from bottom to top") can form a C-13 reservoir larger than assumed so far, covering most of the He-rich layers. We present new calculations (Trippella et al. 2014) aimed at understanding if the solar composition helps to constrain the C-13-pocket extension. Stellar models at a fixed metallicity, based on a large C-13 reservoir, reproduce the main s-component as accurately as before and don't require any nuclear contribution from an unknown nucleosynthesis processes (LEPP). These models also avoid problems of mixing at the envelope border and meet requirements from C-star luminosities. A large production of nuclei below A = 90 is expected, so that (86)'Sr-87 may be fully synthesized by AGB stars, while Sr-88, Y-89 and Zr-94 are contributed more efficiently than before.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
