2,537 research outputs found
Extremal limits and black hole entropy
Taking the extremal limit of a non-extremal Reissner-Nordstr\"om black hole
(by externally varying the mass or charge), the region between the inner and
outer event horizons experiences an interesting fate -- while this region is
absent in the extremal case, it does not disappear in the extremal limit but
rather approaches a patch of . In other words, the approach to
extremality is not continuous, as the non-extremal Reissner-Nordstr\"om
solution splits into two spacetimes at extremality: an extremal black hole and
a disconnected space. We suggest that the unusual nature of this limit
may help in understanding the entropy of extremal black holes.Comment: 10 pages, 3 figures. Minor corrections and added reference
Pay Attention to How You Drive: Safe and Adaptive Model-Based Reinforcement Learning for Off-Road Driving
Autonomous off-road driving is challenging as risky actions taken by the
robot may lead to catastrophic damage. As such, developing controllers in
simulation is often desirable as it provides a safer and more economical
alternative. However, accurately modeling robot dynamics is difficult due to
the complex robot dynamics and terrain interactions in unstructured
environments. Domain randomization addresses this problem by randomizing
simulation dynamics parameters, however this approach sacrifices performance
for robustness leading to policies that are sub-optimal for any target
dynamics. We introduce a novel model-based reinforcement learning approach that
aims to balance robustness with adaptability. Our approach trains a System
Identification Transformer (SIT) and an Adaptive Dynamics Model (ADM) under a
variety of simulated dynamics. The SIT uses attention mechanisms to distill
state-transition observations from the target system into a context vector,
which provides an abstraction for its target dynamics. Conditioned on this, the
ADM probabilistically models the system's dynamics. Online, we use a Risk-Aware
Model Predictive Path Integral controller (MPPI) to safely control the robot
under its current understanding of the dynamics. We demonstrate in simulation
as well as in multiple real-world environments that this approach enables safer
behaviors upon initialization and becomes less conservative (i.e. faster) as
its understanding of the target system dynamics improves with more
observations. In particular, our approach results in an approximately 41%
improvement in lap-time over the non-adaptive baseline while remaining safe
across different environments
Racial profiling data collection: is it worth the costs?
Discusses the cost/benefits of collecting data on racial profiling
Characterizing Circumgalactic Gas around Massive Ellipticals at z~0.4 - II. Physical Properties and Elemental Abundances
We present a systematic investigation of the circumgalactic medium (CGM)
within projected distances d<160 kpc of luminous red galaxies (LRGs). The
sample comprises 16 intermediate-redshift (z=0.21-0.55) LRGs of stellar mass
M_star>1e11 M_sun. Combining far-ultraviolet Cosmic Origin Spectrograph spectra
from the Hubble Space Telescope and optical echelle spectra from the ground
enables a detailed ionization analysis based on resolved component structures
of a suite of absorption transitions, including the full HI Lyman series and
various ionic metal transitions. By comparing the relative abundances of
different ions in individually-matched components, we show that cool gas (T~1e4
K) density and metallicity can vary by more than a factor of ten in in an LRG
halo. Specifically, metal-poor absorbing components with <1/10 solar
metallicity are seen in 50% of the LRG halos, while gas with solar and
super-solar metallicity is also common. These results indicate a complex
multiphase structure and poor chemical mixing in these quiescent halos. We
calculate the total surface mass density of cool gas, \Sigma_cool, by applying
the estimated ionization fraction corrections to the observed HI column
densities. The radial profile of \Sigma_cool is best-described by a projected
Einasto profile of slope \alpha=1 and scale radius r_s=48 kpc. We find that
typical LRGs at z~0.4 contain cool gas mass of M_cool= (1-2) x1e10 M_sun at
d<160 kpc (or as much as 4x1e10 M_sun at d<500 kpc), comparable to the cool CGM
mass of star-forming galaxies. Furthermore, we show that high-ionization OVI
and low-ionization absorption species exhibit distinct velocity profiles,
highlighting their different physical origins. We discuss the implications of
our findings for the origin and fate of cool gas in LRG halos.Comment: Accepted for publication in MNRAS after a minor revision. 23 pages,
14 figures, and a 29-page Appendix with 27 additional figure
Structure of Frequency-Interacting RNA Helicase from \u3ci\u3eNeurospora crassa\u3c/i\u3e Reveals High Flexibility in a Domain Critical for Circadian Rhythm and RNA Surveillance
The FRH (frequency-interacting RNA helicase) protein is the Neurospora crassa homolog of yeast Mtr4, an essential RNA helicase that plays a central role in RNA metabolism as an activator of the nuclear RNA exosome. FRH is also a required component of the circadian clock, mediating protein interactions that result in the rhythmic repression of gene expression. Here we show that FRH unwinds RNA substrates in vitro with a kinetic profile similar to Mtr4, indicating that while FRH has acquired additional functionality, its core helicase function remains intact. In contrast with the earlier FRH structures, a new crystal form of FRH results in an ATP binding site that is undisturbed by crystal contacts and adopts a conformation consistent with nucleotide binding and hydrolysis. Strikingly, this new FRH structure adopts an arch domain conformation that is dramatically altered from previous structures. Comparison of the existing FRH structures reveals conserved hinge points that appear to facilitate arch motion. Regions in the arch have been previously shown to mediate a variety of protein-protein interactions critical for RNA surveillance and circadian clock functions. The conformational changes highlighted in the FRH structures provide a platform for investigating the relationship between arch dynamics and Mtr4/FRH function
Puma predation on radiocollared and uncollared bighorn sheep
BackgroundWe used Global Positioning System (GPS) data from radiocollared pumas (Puma concolor) to identify kill sites of pumas preying upon an endangered population of bighorn sheep (Ovis canadensis) in southern California. Our aims were to test whether or not pumas selected radiocollared versus uncollared bighorn sheep, and to identify patterns of movement before, during, and after kills.FindingsThree pumas killed 23 bighorn sheep over the course of the study, but they did not preferentially prey on marked (radiocollared) versus unmarked bighorn sheep. Predation occurred primarily during crepuscular and nighttime hours, and 22 kill sites were identified by the occurrence of 2 or more consecutive puma GPS locations (a cluster) within 200 m of each other at 1900, 0000, and 0600 h.ConclusionWe tested the "conspicuous individual hypothesis" and found that there was no difference in puma predation upon radiocollared and uncollared bighorn sheep. Pumas tended to move long distances before and after kills, but their movement patterns immediately post-kill were much more restricted. Researchers can exploit this behaviour to identify puma kill sites and investigate prey selection by designing studies that detect puma locations that are spatially clustered between dusk and dawn
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