16,597 research outputs found
Anapole Dark Matter
We consider dark matter (DM) that interacts with ordinary matter exclusively
through an electromagnetic anapole, which is the only allowed electromagnetic
form factor for Majorana fermions. We show that unlike DM particles with an
electric or magnetic dipole moment, anapole dark matter particles annihilate
exclusively into fermions via purely p-wave interactions, while tree-level
annihilations into photons are forbidden. We calculate the anapole moment
needed to produce a thermal relic abundance in agreement with cosmological
observations, and show that it is consistent with current XENON100 detection
limits on the DM-nucleus cross-section for all masses, while lying just below
the detection threshold for a mass ~ 30-40 GeV.Comment: 7 pages, 5 figures, v3: version to appear in PL
Secular Evolution of Hierarchical Planetary Systems
(Abridged) We investigate the dynamical evolution of coplanar hierarchical
two-planet systems where the ratio of the orbital semimajor axes alpha=a_1/a_2
is small. The orbital parameters obtained from a multiple Kepler fit to the
radial velocity variations of a star are best interpreted as Jacobi coordinates
and Jacobi coordinates should be used in any analyses of hierarchical planetary
systems. An approximate theory that can be applied to coplanar hierarchical
two-planet systems with a wide range of masses m_j and orbital eccentricities
e_j is the octupole-level secular perturbation theory (OSPT). The OSPT shows
that if the ratio of the maximum orbital angular momenta, lambda \approx
(m_1/m_2) alpha^{1/2}, for given a_j is approximately equal to a critical value
lambda_{crit}, then libration of the difference in the longitudes of periapse,
w_1-w_2, about either 0 or 180 deg. is almost certain, with possibly large
amplitude variations of both e_j. We establish that the OSPT is highly accurate
for systems with alpha<0.1 and reasonably accurate even for systems with alpha
as large as 1/3, provided that alpha is not too close to a significant
mean-motion commensurability or above the stability boundary. The HD 168443
system is not in a secular resonance and its w_1-w_2 circulates. The HD 12661
system is the first extrasolar planetary system found to have w_1-w_2 librating
about 180 deg. The libration of w_1-w_2 and the large-amplitude variations of
both e_j in the HD 12661 system are consistent with the analytic results on
systems with lambda \approx lambda_{crit}. The HD 12661 system with the best-
fit orbital parameters and sin i = 1 is affected by the close proximity to the
11:2 commensurability, but small changes in the outer orbital period can result
in configurations that are not affected by mean-motion commensurabilities.Comment: 32 pages, including 8 figures; uses AASTeX v5.0; accepted for
publication in Ap
A Primordial Origin of the Laplace Relation Among the Galilean Satellites
Understanding the origin of the orbital resonances of the Galilean satellites
of Jupiter will constrain the longevity of the extensive volcanism on Io, may
explain a liquid ocean on Europa, and may guide studies of the dissipative
properties of stars and Jupiter-like planets. The differential migration of the
newly formed Galilean satellites due to interactions with a circumjovian disk
can lead to the primordial formation of the Laplace relation n_1 - 3 n_2 + 2
n_3 = 0, where the n_i are the mean orbital angular velocities of Io, Europa,
and Ganymede, respectively. This contrasts with the formation of the resonances
by differential expansion of the orbits from tidal torques from Jupiter.Comment: 13 pages, including 4 figures; uses scicite.st
Mode Variational LSTM Robust to Unseen Modes of Variation: Application to Facial Expression Recognition
Spatio-temporal feature encoding is essential for encoding the dynamics in
video sequences. Recurrent neural networks, particularly long short-term memory
(LSTM) units, have been popular as an efficient tool for encoding
spatio-temporal features in sequences. In this work, we investigate the effect
of mode variations on the encoded spatio-temporal features using LSTMs. We show
that the LSTM retains information related to the mode variation in the
sequence, which is irrelevant to the task at hand (e.g. classification facial
expressions). Actually, the LSTM forget mechanism is not robust enough to mode
variations and preserves information that could negatively affect the encoded
spatio-temporal features. We propose the mode variational LSTM to encode
spatio-temporal features robust to unseen modes of variation. The mode
variational LSTM modifies the original LSTM structure by adding an additional
cell state that focuses on encoding the mode variation in the input sequence.
To efficiently regulate what features should be stored in the additional cell
state, additional gating functionality is also introduced. The effectiveness of
the proposed mode variational LSTM is verified using the facial expression
recognition task. Comparative experiments on publicly available datasets
verified that the proposed mode variational LSTM outperforms existing methods.
Moreover, a new dynamic facial expression dataset with different modes of
variation, including various modes like pose and illumination variations, was
collected to comprehensively evaluate the proposed mode variational LSTM.
Experimental results verified that the proposed mode variational LSTM encodes
spatio-temporal features robust to unseen modes of variation.Comment: Accepted in AAAI-1
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