141 research outputs found
Avalanches on a conical bead pile: scaling with tuning parameters
Uniform spherical beads were used to explore the behavior of a granular
system near its critical angle of repose on a conical bead pile. We found two
tuning parameters that could take the system to a critical point where a simple
power-law described the avalanche size distribution as predicted by
self-organized criticality, which proposed that complex dynamical systems
self-organize to a critical point without need for tuning. Our distributions
were well described by a simple power-law with the power {\tau} = 1.5 when
dropping beads slowly onto the apex of a bead pile from a small height.
However, we could also move the system from the critical point using either of
two tuning parameters: the height from which the beads fell onto the top of the
pile or the region over which the beads struck the pile. As the drop height
increased, the system did not reach the critical point yet the resulting
distributions were independent of the bead mass, coefficient of friction, or
coefficient of restitution. All our apex-dropping distributions for any type of
bead (glass, stainless steel, zirconium) showed universality by scaling onto a
common curve with {\tau} = 1.5 and {\sigma} = 1.0, where 1/{\sigma} is the
power of the tuning parameter. From independent calculations using the moments
of the distribution, we find values for {\tau} = 1.6 \pm 0.1 and {\sigma} =
0.91 \pm 0.15. When beads were dropped across the surface of the pile instead
of solely on the apex, then the system also moved from the critical point and
again the avalanche size distributions fell on a common curve when scaled
similarly using the same values of {\tau} and {\sigma}. We also observed that
an hcp structure on the base of the pile caused an emergent structure in the
pile that had six faces with some fcc or hcp structure.Comment: 8 pages, 6 figures; submitted to Granular Matter; Reformatted into
LaTeX from Word; Fixed typo in uncertainty of tau; Rearranged two paragraphs
to improve flo
TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844
Data from the newly-commissioned \textit{Transiting Exoplanet Survey
Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf
located 15 pc away. The planet has a radius of and
orbits the star every 11 hours. Although the existence of an atmosphere around
such a strongly irradiated planet is questionable, the star is bright enough
(, ) for this possibility to be investigated with transit and
occultation spectroscopy. The star's brightness and the planet's short period
will also facilitate the measurement of the planet's mass through Doppler
spectroscopy.Comment: 10 pages, 4 figures. Submitted to ApJ Letters. This letter makes use
of the TESS Alert data, which is currently in a beta test phase, using data
from the pipelines at the TESS Science Office and at the TESS Science
Processing Operations Cente
TOI 540 b: A Planet Smaller than Earth Orbiting a Nearby Rapidly Rotating Low-mass Star
We present the discovery of TOI 540 b, a hot planet slightly smaller than
Earth orbiting the low-mass star 2MASS J05051443-4756154. The planet has an
orbital period of days ( 170 ms) and a radius of , and is likely terrestrial based on the observed
mass-radius distribution of small exoplanets at similar insolations. The star
is 14.008 pc away and we estimate its mass and radius to be and , respectively. The
star is distinctive in its very short rotational period of hours and correspondingly small Rossby number of 0.007 as
well as its high X-ray-to-bolometric luminosity ratio of based on a serendipitous XMM-Newton detection during a slew operation.
This is consistent with the X-ray emission being observed at a maximum value of
as predicted for the most rapidly rotating M
dwarfs. TOI 540 b may be an alluring target to study atmospheric erosion due to
the strong stellar X-ray emission. It is also among the most accessible targets
for transmission and emission spectroscopy and eclipse photometry with JWST,
and may permit Doppler tomography with high-resolution spectroscopy during
transit. This discovery is based on precise photometric data from TESS and
ground-based follow-up observations by the MEarth team.Comment: 18 pages, 7 figures. Accepted for publication in The Astronomical
Journa
LSST: from Science Drivers to Reference Design and Anticipated Data Products
(Abridged) We describe here the most ambitious survey currently planned in
the optical, the Large Synoptic Survey Telescope (LSST). A vast array of
science will be enabled by a single wide-deep-fast sky survey, and LSST will
have unique survey capability in the faint time domain. The LSST design is
driven by four main science themes: probing dark energy and dark matter, taking
an inventory of the Solar System, exploring the transient optical sky, and
mapping the Milky Way. LSST will be a wide-field ground-based system sited at
Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m
effective) primary mirror, a 9.6 deg field of view, and a 3.2 Gigapixel
camera. The standard observing sequence will consist of pairs of 15-second
exposures in a given field, with two such visits in each pointing in a given
night. With these repeats, the LSST system is capable of imaging about 10,000
square degrees of sky in a single filter in three nights. The typical 5
point-source depth in a single visit in will be (AB). The
project is in the construction phase and will begin regular survey operations
by 2022. The survey area will be contained within 30,000 deg with
, and will be imaged multiple times in six bands, ,
covering the wavelength range 320--1050 nm. About 90\% of the observing time
will be devoted to a deep-wide-fast survey mode which will uniformly observe a
18,000 deg region about 800 times (summed over all six bands) during the
anticipated 10 years of operations, and yield a coadded map to . The
remaining 10\% of the observing time will be allocated to projects such as a
Very Deep and Fast time domain survey. The goal is to make LSST data products,
including a relational database of about 32 trillion observations of 40 billion
objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures
available from https://www.lsst.org/overvie
A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients
We present a search for extragalactic fast blue optical transients (FBOTs)
during Phase I of the Zwicky Transient Facility (ZTF). We identify 38
candidates with durations above half-maximum light 1 d < t1/2 < 12 d, of which
28 have blue (g-r<-0.2 mag) colors at peak light. Of the 38 transients (28
FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae
(SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type
IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published
previously) had high-S/N predominantly featureless spectra and luminous radio
emission: AT2018lug and AT2020xnd. Seven (five) did not have a definitive
classification: AT 2020bdh showed tentative broad H in emission, and AT
2020bot showed unidentified broad features and was 10 kpc offset from the
center of an early-type galaxy. Ten (six) have no spectroscopic observations or
redshift measurements. We present multiwavelength (radio, millimeter, and/or
X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type
IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits
on the presence of radio emission for 22 of the transients. All X-ray and radio
observations resulted in non-detections; we rule out AT2018cow-like X-ray and
radio behavior for five FBOTs and more luminous emission (such as that seen in
the Camel) for four additional FBOTs. We conclude that exotic transients
similar to AT2018cow, the Koala, and the Camel represent a rare subset of
FBOTs, and use ZTF's SN classification experiments to measure the rate to be at
most 0.1% of the local core-collapse SN rate.Comment: Replaced following peer-review process. 46 pages, 20 figures.
Accepted for publication in Ap
Flea Diversity as an Element for Persistence of Plague Bacteria in an East African Plague Focus
Plague is a flea-borne rodent-associated zoonotic disease that is caused by Yersinia pestis and characterized by long quiescent periods punctuated by rapidly spreading epidemics and epizootics. How plague bacteria persist during inter-epizootic periods is poorly understood, yet is important for predicting when and where epizootics are likely to occur and for designing interventions aimed at local elimination of the pathogen. Existing hypotheses of how Y. pestis is maintained within plague foci typically center on host abundance or diversity, but little attention has been paid to the importance of flea diversity in enzootic maintenance. Our study compares host and flea abundance and diversity along an elevation gradient that spans from low elevation sites outside of a plague focus in the West Nile region of Uganda (∼725–1160 m) to higher elevation sites within the focus (∼1380–1630 m). Based on a year of sampling, we showed that host abundance and diversity, as well as total flea abundance on hosts was similar between sites inside compared with outside the plague focus. By contrast, flea diversity was significantly higher inside the focus than outside. Our study highlights the importance of considering flea diversity in models of Y. pestis persistence
A Phosphatidic Acid Binding/Nuclear Localization Motif Determines Lipin1 Function in Lipid Metabolism and Adipogenesis
A polybasic motif in the metabolic regulator lipin1 is both a membrane anchor and a nuclear localization sequence required for lipin1 function in phospholipid metabolism and adipogenesis
- …