Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays. (arXiv:2007.12707v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gaches_B/0/1/0/all/0/1">Brandt A. L. Gaches</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walch_S/0/1/0/all/0/1">Stefanie Walch</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walch_S/0/1/0/all/0/1">Stella S. R. Walch</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Munker_C/0/1/0/all/0/1">Carsten Münker</a>
The radioactive decay of aluminum-26 ($^{26}$Al) is an important heating
source in early planet formation. Since its discovery, there have been several
mechanisms proposed to introduce $^{26}$Al into protoplanetary disks, primarily
through contamination by external sources. We propose a local mechanism to
enrich protostellar disks with $^{26}$Al through irradiation of the
protostellar disk surface by cosmic rays accelerated in the protostellar
accretion shock. We calculate the $^{26}$Al enrichment, [$^{26}$Al/$^{27}$Al],
at the surface of the protostellar disk in the inner AU throughout the
evolution of low-mass stars, from M-dwarfs to proto-Suns. Assuming constant
mass accretion rates, $dot{m}$, we find that irradiation by MeV cosmic rays
can provide significant enrichment on the disk surface if the cosmic rays are
not completely coupled to the gas in the accretion flow. Importantly, we find
that low accretion rates, $dot{m} < 10^{-7}$ M$_{odot}$ yr$^{-1}$, are able
to produce canonical amounts of $^{26}$Al, $[^{26}{rm Al}/^{27}{rm Al}]
approx 5times10^{-5}$. These accretion rates are experienced at the
transition from Class I- to Class II-type protostars, when it is assumed that
calcium-aluminum-rich inclusions condense in the inner disk. We conclude that
irradiation of the inner disk surface by cosmic ray protons accelerated in
accretion shocks at the protostellar surface may be an important mechanism to
produce $^{26}$Al. Our models show protostellar cosmic rays may be a viable
model to explain the enrichment of $^{26}$Al found in the Solar System.
The radioactive decay of aluminum-26 ($^{26}$Al) is an important heating
source in early planet formation. Since its discovery, there have been several
mechanisms proposed to introduce $^{26}$Al into protoplanetary disks, primarily
through contamination by external sources. We propose a local mechanism to
enrich protostellar disks with $^{26}$Al through irradiation of the
protostellar disk surface by cosmic rays accelerated in the protostellar
accretion shock. We calculate the $^{26}$Al enrichment, [$^{26}$Al/$^{27}$Al],
at the surface of the protostellar disk in the inner AU throughout the
evolution of low-mass stars, from M-dwarfs to proto-Suns. Assuming constant
mass accretion rates, $dot{m}$, we find that irradiation by MeV cosmic rays
can provide significant enrichment on the disk surface if the cosmic rays are
not completely coupled to the gas in the accretion flow. Importantly, we find
that low accretion rates, $dot{m} < 10^{-7}$ M$_{odot}$ yr$^{-1}$, are able
to produce canonical amounts of $^{26}$Al, $[^{26}{rm Al}/^{27}{rm Al}]
approx 5times10^{-5}$. These accretion rates are experienced at the
transition from Class I- to Class II-type protostars, when it is assumed that
calcium-aluminum-rich inclusions condense in the inner disk. We conclude that
irradiation of the inner disk surface by cosmic ray protons accelerated in
accretion shocks at the protostellar surface may be an important mechanism to
produce $^{26}$Al. Our models show protostellar cosmic rays may be a viable
model to explain the enrichment of $^{26}$Al found in the Solar System.
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