1,037 research outputs found
Reduced dynamics of Ward solitons
The moduli space of static finite energy solutions to Ward's integrable
chiral model is the space of based rational maps from \CP^1 to itself
with degree . The Lagrangian of Ward's model gives rise to a K\"ahler metric
and a magnetic vector potential on this space. However, the magnetic field
strength vanishes, and the approximate non--relativistic solutions to Ward's
model correspond to a geodesic motion on . These solutions can be compared
with exact solutions which describe non--scattering or scattering solitons.Comment: Final version, to appear in Nonlinearit
Laser-only adaptive optics achieves significant image quality gains compared to seeing-limited observations over the entire sky
Adaptive optics laser guide star systems perform atmospheric correction of
stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order
laser-correction. The requirement of a sufficiently bright guide star in the
field-of-view to correct tip-tilt limits sky coverage. Here we show an
improvement to effective seeing without the need for nearby bright stars,
enabling full sky coverage by performing only laser-assisted wavefront
correction. We used Robo-AO, the first robotic AO system, to comprehensively
demonstrate this laser-only correction. We analyze observations from four years
of efficient robotic operation covering 15,000 targets and 42,000 observations,
each realizing different seeing conditions. Using an autoguider (or a
post-processing software equivalent) and the laser to improve effective seeing
independent of the brightness of a target, Robo-AO observations show a 39+/-19%
improvement to effective FWHM, without any tip-tilt correction. We also
demonstrate that 50% encircled-energy performance without tip-tilt correction
remains comparable to diffraction-limited, standard Robo-AO performance.
Faint-target science programs primarily limited by 50% encircled-energy (e.g.
those employing integral field spectrographs placed behind the AO system) may
see significant benefits to sky coverage from employing laser-only AO.Comment: Accepted for publication in The Astronomical Journal. 7 pages, 6
figure
Building the Evryscope: Hardware Design and Performance
The Evryscope is a telescope array designed to open a new parameter space in
optical astronomy, detecting short timescale events across extremely large sky
areas simultaneously. The system consists of a 780 MPix 22-camera array with an
8150 sq. deg. field of view, 13" per pixel sampling, and the ability to detect
objects down to Mg=16 in each 2 minute dark-sky exposure. The Evryscope,
covering 18,400 sq.deg. with hours of high-cadence exposure time each night, is
designed to find the rare events that require all-sky monitoring, including
transiting exoplanets around exotic stars like white dwarfs and hot subdwarfs,
stellar activity of all types within our galaxy, nearby supernovae, and other
transient events such as gamma ray bursts and gravitational-wave
electromagnetic counterparts. The system averages 5000 images per night with
~300,000 sources per image, and to date has taken over 3.0M images, totaling
250TB of raw data. The resulting light curve database has light curves for 9.3M
targets, averaging 32,600 epochs per target through 2018. This paper summarizes
the hardware and performance of the Evryscope, including the lessons learned
during telescope design, electronics design, a procedure for the precision
polar alignment of mounts for Evryscope-like systems, robotic control and
operations, and safety and performance-optimization systems. We measure the
on-sky performance of the Evryscope, discuss its data-analysis pipelines, and
present some example variable star and eclipsing binary discoveries from the
telescope. We also discuss new discoveries of very rare objects including 2 hot
subdwarf eclipsing binaries with late M-dwarf secondaries (HW Vir systems), 2
white dwarf / hot subdwarf short-period binaries, and 4 hot subdwarf reflection
binaries. We conclude with the status of our transit surveys, M-dwarf flare
survey, and transient detection.Comment: 24 pages, 24 figures, accepted PAS
EvryFlare II: Rotation Periods of the Cool Flare Stars in TESS Across Half the Southern Sky
We measure rotation periods and sinusoidal amplitudes in Evryscope light
curves for 122 two-minute K5-M4 TESS targets selected for strong flaring. The
Evryscope array of telescopes has observed all bright nearby stars in the
South, producing two-minute cadence light curves since 2016. Long-term,
high-cadence observations of rotating flare stars probe the complex
relationship between stellar rotation, starspots, and superflares. We detect
periods from 0.3487 to 104 d, and observe amplitudes from 0.008 to 0.216 g'
mag. We find the Evryscope amplitudes are larger than those in TESS with the
effect correlated to stellar mass (p-value=0.01). We compute the Rossby number
(Ro), and find our sample selected for flaring has twice as many intermediate
rotators (0.040.44) rotators; this may
be astrophysical or a result of period-detection sensitivity. We discover 30
fast, 59 intermediate, and 33 slow rotators. We measure a median starspot
coverage of 13% of the stellar hemisphere and constrain the minimum magnetic
field strength consistent with our flare energies and spot coverage to be 500
G, with later-type stars exhibiting lower values than earlier-types. We observe
a possible change in superflare rates at intermediate periods. However, we do
not conclusively confirm the increased activity of intermediate rotators seen
in previous studies. We split all rotators at Ro~0.2 into Prot10
d bins to confirm short-period rotators exhibit higher superflare rates, larger
flare energies, and higher starspot coverage than do long-period rotators, at
p-values of 3.2 X 10^-5, 1.0 X 10^-5, and 0.01, respectively.Comment: 16 pages, 8 figures, 3 tables. Ancillary machine-readable files
included. Accepted for publication in ApJ (proofs submitted). Includes
significant new material, including starspot color that depends on stellar
mass, more rotation periods, potential changes in activity during spin-down,
and examples of binary rotator
Robo-AO Kepler Survey IV: the effect of nearby stars on 3857 planetary candidate systems
We present the overall statistical results from the Robo-AO Kepler planetary
candidate survey, comprising of 3857 high-angular resolution observations of
planetary candidate systems with Robo-AO, an automated laser adaptive optics
system. These observations reveal previously unknown nearby stars blended with
the planetary candidate host star which alter the derived planetary radii or
may be the source of an astrophysical false positive transit signal. In the
first three papers in the survey, we detected 440 nearby stars around 3313
planetary candidate host stars. In this paper, we present observations of 532
planetary candidate host stars, detecting 94 companions around 88 stars; 84 of
these companions have not previously been observed in high-resolution. We also
report 50 more-widely-separated companions near 715 targets previously observed
by Robo-AO. We derive corrected planetary radius estimates for the 814
planetary candidates in systems with a detected nearby star. If planetary
candidates are equally likely to orbit the primary or secondary star, the
radius estimates for planetary candidates in systems with likely bound nearby
stars increase by a factor of 1.54, on average. We find that 35
previously-believed rocky planet candidates are likely not rocky due to the
presence of nearby stars. From the combined data sets from the complete Robo-AO
KOI survey, we find that 14.5\pm0.5% of planetary candidate hosts have a nearby
star with 4", while 1.2% have two nearby stars and 0.08% have three. We find
that 16% of Earth-sized, 13% of Neptune-sized, 14% of Saturn-sized, and 19% of
Jupiter-sized planet candidates have detected nearby stars.Comment: Accepted to the Astronomical Journa
Variables in the Southern Polar Region Evryscope 2016 Dataset
The regions around the celestial poles offer the ability to find and
characterize long-term variables from ground-based observatories. We used
multi-year Evryscope data to search for high-amplitude (~5% or greater)
variable objects among 160,000 bright stars (Mv < 14.5) near the South
Celestial Pole. We developed a machine learning based spectral classifier to
identify eclipse and transit candidates with M-dwarf or K-dwarf host stars -
and potential low-mass secondary stars or gas giant planets. The large
amplitude transit signals from low-mass companions of smaller dwarf host stars
lessens the photometric precision and systematics removal requirements
necessary for detection, and increases the discoveries from long-term
observations with modest light curve precision. The Evryscope is a robotic
telescope array that observes the Southern sky continuously at 2-minute
cadence, searching for stellar variability, transients, transits around exotic
stars and other observationally challenging astrophysical variables. In this
study, covering all stars 9 < Mv < 14.5, in declinations -75 to -90 deg, we
recover 346 known variables and discover 303 new variables, including 168
eclipsing binaries. We characterize the discoveries and provide the amplitudes,
periods, and variability type. A 1.7 Jupiter radius planet candidate with a
late K-dwarf primary was found and the transit signal was verified with the
PROMPT telescope network. Further followup revealed this object to be a likely
grazing eclipsing binary system with nearly identical primary and secondary K5
stars. Radial velocity measurements from the Goodman Spectrograph on the 4.1
meter SOAR telescope of the likely-lowest-mass targets reveal that six of the
eclipsing binary discoveries are low-mass (.06 - .37 solar mass) secondaries
with K-dwarf primaries, strong candidates for precision mass-radius
measurements.Comment: 32 pages, 17 figures, accepted to PAS
Robo-AO Kepler Survey V: The effect of physically associated stellar companions on planetary systems
The Kepler light curves used to detect thousands of planetary candidates are
susceptible to dilution due to blending with previously unknown nearby stars.
With the automated laser adaptive optics instrument, Robo-AO, we have observed
620 nearby stars around 3857 planetary candidates host stars. Many of the
nearby stars, however, are not bound to the KOI. In this paper, we quantify the
association probability between each KOI and detected nearby stars through
several methods. Galactic stellar models and the observed stellar density are
used to estimate the number and properties of unbound stars. We estimate the
spectral type and distance to 145 KOIs with nearby stars using multi-band
observations from Robo-AO and Keck-AO. We find most nearby stars within 1" of a
Kepler planetary candidate are likely bound, in agreement with past studies. We
use likely bound stars as well as the precise stellar parameters from the
California Kepler Survey to search for correlations between stellar binarity
and planetary properties. No significant difference between the binarity
fraction of single and multiple planet systems is found, and planet hosting
stars follow similar binarity trends as field stars, many of which likely host
their own non-aligned planets. We find that hot Jupiters are ~4x more likely
than other planets to reside in a binary star system. We correct the radius
estimates of the planet candidates in characterized systems and find that for
likely bound systems, the estimated planetary candidate radii will increase on
average by a factor of 1.77, if either star is equally likely to host the
planet. We find that the planetary radius gap is robust to the impact of
dilution, and find an intriguing 95%-confidence discrepancy between the radius
distribution of small planets in single and binary systems.Comment: 19 pages, 12 figures, submitted to AAS Journal
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