Title: Collisions of Dark Matter Axion Stars with Astrophysical Sources,
Abstract: If QCD axions form a large fraction of the total mass of dark matter, then
axion stars could be very abundant in galaxies. As a result, collisions with
each other, and with other astrophysical bodies, can occur. We calculate the
rate and analyze the consequences of three classes of collisions, those
occurring between a dilute axion star and: another dilute axion star, an
ordinary star, or a neutron star. In all cases we attempt to quantify the most
important astrophysical uncertainties; we also pay particular attention to
scenarios in which collisions lead to collapse of otherwise stable axion stars,
and possible subsequent decay through number changing interactions. Collisions
between two axion stars can occur with a high total rate, but the low relative
velocity required for collapse to occur leads to a very low total rate of
collapses. On the other hand, collisions between an axion star and an ordinary
star have a large rate, $\Gamma_\odot \sim 3000$ collisions/year/galaxy, and
for sufficiently heavy axion stars, it is plausible that most or all such
collisions lead to collapse. We identify in this case a parameter space which
has a stable region and a region in which collision triggers collapse, which
depend on the axion number ($N$) in the axion star, and a ratio of mass to
radius cubed characterizing the ordinary star ($M_s/R_s^3$). Finally, we
revisit the calculation of collision rates between axion stars and neutron
stars, improving on previous estimates by taking cylindrical symmetry of the
neutron star distribution into account. Collapse and subsequent decay through
collision processes, if occurring with a significant rate, can affect dark
matter phenomenology and the axion star mass distribution.