6,433 research outputs found
Phase transitions between dilute and dense axion stars
We study the nature of phase transitions between dilute and dense axion stars
interpreted as self-gravitating Bose-Einstein condensates. We develop a
Newtonian model based on the Gross-Pitaevskii-Poisson equations for a complex
scalar field with a self-interaction potential involving an
attractive term and a repulsive term. Using a Gaussian
ansatz for the wave function, we analytically obtain the mass-radius relation
of dilute and dense axion stars for arbitrary values of the self-interaction
parameter . We show the existence of a critical point
above which a first order phase transition takes
place. We qualitatively estimate general relativistic corrections on the
mass-radius relation of axion stars. For weak self-interactions
, a system of self-gravitating axions forms a stable
dilute axion star below a general relativistic maximum mass and collapses into a black hole above that
mass. For strong self-interactions , a system of
self-gravitating axions forms a stable dilute axion star below a Newtonian
maximum mass , collapses
into a dense axion star above that mass, and collapses into a black hole above
a general relativistic maximum mass . Dense axion stars explode below a Newtonian minimum
mass and form dilute axion
stars of large size or disperse away. We determine the phase diagram of
self-gravitating axions and show the existence of a triple point
separating dilute axion stars, dense axion stars,
and black holes. We make numerical applications for QCD axions and ultralight
axions
Collisions of Dark Matter Axion Stars with Astrophysical Sources
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, 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 () in the axion star, and a ratio of mass to
radius cubed characterizing the ordinary star (). 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.Comment: 19 pages, 5 figures. v2: References added, typos correcte
Formation of Relativistic Axion Stars
Axions and axion-like particles are compelling candidates for the missing
dark matter of the universe. As they undergo gravitational collapse, they can
form compact objects such as axion stars or even black holes. In this paper, we
study the formation and distribution of such objects. First, we simulate the
formation of compact axion stars using numerical relativity with aspherical
initial conditions that could represent the final stages of axion dark matter
structure formation. We show that the final states of such collapse closely
follow the known relationship of initial mass and axion decay constant .
Second, we demonstrate with a toy model how this information can be used to
scan a model density field to predict the number densities and masses of such
compact objects. In addition to being detectable by the LIGO/VIRGO
gravitational wave interferometer network for axion mass of eV, we show using peak statistics that for , there
exists a "mass gap" between the masses of axion stars and black holes formed
from collapse
Astrophysical Axion Bounds
Axion emission by hot and dense plasmas is a new energy-loss channel for
stars. Observational consequences include a modification of the solar
sound-speed profile, an increase of the solar neutrino flux, a reduction of the
helium-burning lifetime of globular-cluster stars, accelerated white-dwarf
cooling, and a reduction of the supernova SN 1987A neutrino burst duration. We
review and update these arguments and summarize the resulting axion
constraints.Comment: Contribution to Axion volume of Lecture Notes in Physics, 20 pages, 3
figure
Probing Axions with Radiation from Magnetic Stars
Recent experiments suggest that polarized photons may couple significantly to
pseudoscalar particles such as axions. We study the possible observational
signatures of axion-photon coupling for radiation from magnetic stars, with
particular focus on neutron stars. We present general methods for calculating
the axion-photon conversion probability during propagation through a varying
magnetized vacuum as well as across an inhomogeneous atmosphere. Partial
axion-photon conversion may take place in the vacuum region outside the neutron
star. Strong axion-photon mixing occurs due to a resonance in the atmosphere,
and depending on the axion coupling strength and other parameters, significant
axion-photon conversion can take place at the resonance. Such conversions may
produce observable effects on the radiation spectra and polarization signals
from the star. We also apply our results to axion-photon propagation in the Sun
and in magnetic white dwarfs. We find that there is no appreciable conversion
of solar axions to photons during the propagation.Comment: 12 pages, 11 figures. Minor changes. PRD accepte
QCD Axion Star Collapse with the Chiral Potential
In a previous work, we analyzed collapsing axion stars using the low-energy
instanton potential, showing that the total energy is always bounded and that
collapsing axion stars do not form black holes. In this paper, we provide a
proof that the conclusions are unchanged when using instead the more general
chiral potential for QCD axions.Comment: 11 page
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