Observational constraints on the likelihood of $^{26}$Al in planet-forming environments. (arXiv:2011.09971v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Reiter_M/0/1/0/all/0/1">Megan Reiter</a>
Recent work suggests that $^{26}$Al may determine the water budget in
terrestrial exoplanets as its radioactive decay dehydrates planetesimals
leading to rockier compositions. Here I consider the observed distribution of
$^{26}$Al in the Galaxy and typical star-forming environments to estimate the
likelihood of $^{26}$Al enrichment during planet formation. I do not assume
Solar-System-specific constraints as I am interested in enrichment for
exoplanets generally. Observations indicate that high-mass stars dominate the
production of $^{26}$Al with nearly equal contributions from their winds and
supernovae. $^{26}$Al abundances are comparable to those in the early Solar
System in the high-mass star-forming regions where most stars (and thereby most
planets) form. These high abundances appear to be maintained for a few Myr,
much longer than the 0.7 Myr half-life. Observed bulk $^{26}$Al velocities are
an order of magnitude slower than expected from winds and supernovae. These
observations are at odds with typical model assumptions that $^{26}$Al is
provided instantaneously by high velocity mass loss from supernovae and winds.
Regular replenishment of $^{26}$Al especially when coupled with the small age
differences that are common in high-mass star-forming complexes, may
significantly increase the number of star/planet-forming systems exposed to
$^{26}$Al. Exposure does not imply enrichment, but the order of magnitude
slower velocity of $^{26}$Al may alter the fraction that is incorporated into
planet-forming material. Together, this suggests that the conditions for rocky
planet formation are not rare, nor are they ubiquitous, as small regions like
Taurus that lack high-mass stars to produce $^{26}$Al may be less likely to
form rocky planets. I conclude with suggested directions for future studies.
Recent work suggests that $^{26}$Al may determine the water budget in
terrestrial exoplanets as its radioactive decay dehydrates planetesimals
leading to rockier compositions. Here I consider the observed distribution of
$^{26}$Al in the Galaxy and typical star-forming environments to estimate the
likelihood of $^{26}$Al enrichment during planet formation. I do not assume
Solar-System-specific constraints as I am interested in enrichment for
exoplanets generally. Observations indicate that high-mass stars dominate the
production of $^{26}$Al with nearly equal contributions from their winds and
supernovae. $^{26}$Al abundances are comparable to those in the early Solar
System in the high-mass star-forming regions where most stars (and thereby most
planets) form. These high abundances appear to be maintained for a few Myr,
much longer than the 0.7 Myr half-life. Observed bulk $^{26}$Al velocities are
an order of magnitude slower than expected from winds and supernovae. These
observations are at odds with typical model assumptions that $^{26}$Al is
provided instantaneously by high velocity mass loss from supernovae and winds.
Regular replenishment of $^{26}$Al especially when coupled with the small age
differences that are common in high-mass star-forming complexes, may
significantly increase the number of star/planet-forming systems exposed to
$^{26}$Al. Exposure does not imply enrichment, but the order of magnitude
slower velocity of $^{26}$Al may alter the fraction that is incorporated into
planet-forming material. Together, this suggests that the conditions for rocky
planet formation are not rare, nor are they ubiquitous, as small regions like
Taurus that lack high-mass stars to produce $^{26}$Al may be less likely to
form rocky planets. I conclude with suggested directions for future studies.
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