Chromium Nucleosynthesis and Silicon-Carbon Shell Mergers in Massive Stars. (arXiv:1906.07218v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Cote_B/0/1/0/all/0/1">Benoit C&#xf4;t&#xe9;</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jones_S/0/1/0/all/0/1">Samuel Jones</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Herwig_F/0/1/0/all/0/1">Falk Herwig</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pignatari_M/0/1/0/all/0/1">Marco Pignatari</a>

We analyze the production of the element Cr in galactic chemical evolution
(GCE) models using the NuGrid nucleosynthesis yields set. We show that the
unusually large [Cr/Fe] abundance at [Fe/H] $approx 0$ reported by previous
studies using those yields and predicted by our Milky Way model originates from
the merging of convective Si-burning and C-burning shells in a 20 $M_odot$
model at metallicity $Z=0.01$, about an hour before the star explodes. This
merger mixes the incomplete burning material in the Si shell, including
$^{51}$V and $^{52}$Cr, out to the edge of the carbon/oxygen (CO) core. The
adopted supernova model ejects the outer 2 $M_odot$ of the CO core, which
includes a significant fraction of the Cr-rich material. When including this 20
$M_odot$ model at $Z=0.01$ in the yields interpolation scheme of our GCE model
for stars in between 15 and 25 $M_odot$, we overestimate [Cr/Fe] by an order
of magnitude at [Fe/H] $approx$ 0 relative to observations in the Galactic
disk. This raises a number of questions regarding the occurrence of Si-C shell
mergers in nature, the accuracy of different simulation approaches, and the
impact of such mergers on the pre-supernova structure and explosion dynamics.
According to the conditions in this 1D stellar model, the substantial
penetration of C-shell material into the Si-shell could launch a
convective-reactive global oscillation, if a merger does take place. In any
case, GCE provides stringent constraints on the outcome of this stellar
evolution phase.

We analyze the production of the element Cr in galactic chemical evolution
(GCE) models using the NuGrid nucleosynthesis yields set. We show that the
unusually large [Cr/Fe] abundance at [Fe/H] $approx 0$ reported by previous
studies using those yields and predicted by our Milky Way model originates from
the merging of convective Si-burning and C-burning shells in a 20 $M_odot$
model at metallicity $Z=0.01$, about an hour before the star explodes. This
merger mixes the incomplete burning material in the Si shell, including
$^{51}$V and $^{52}$Cr, out to the edge of the carbon/oxygen (CO) core. The
adopted supernova model ejects the outer 2 $M_odot$ of the CO core, which
includes a significant fraction of the Cr-rich material. When including this 20
$M_odot$ model at $Z=0.01$ in the yields interpolation scheme of our GCE model
for stars in between 15 and 25 $M_odot$, we overestimate [Cr/Fe] by an order
of magnitude at [Fe/H] $approx$ 0 relative to observations in the Galactic
disk. This raises a number of questions regarding the occurrence of Si-C shell
mergers in nature, the accuracy of different simulation approaches, and the
impact of such mergers on the pre-supernova structure and explosion dynamics.
According to the conditions in this 1D stellar model, the substantial
penetration of C-shell material into the Si-shell could launch a
convective-reactive global oscillation, if a merger does take place. In any
case, GCE provides stringent constraints on the outcome of this stellar
evolution phase.

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