180 research outputs found
Constraints on the Progenitor of SN 2016gkg From Its Shock-Cooling Light Curve
SN 2016gkg is a nearby Type IIb supernova discovered shortly after explosion.
Like several other Type IIb events with early-time data, SN 2016gkg displays a
double-peaked light curve, with the first peak associated with the cooling of a
low-mass extended progenitor envelope. We present unprecedented
intranight-cadence multi-band photometric coverage of the first light-curve
peak of SN 2016gkg obtained from the Las Cumbres Observatory Global Telescope
network, the Asteroid Terrestrial-impact Last Alert System, the Swift satellite
and various amateur-operated telescopes. Fitting these data to analytical
shock-cooling models gives a progenitor radius of ~25-140 solar radii with
~2-30 x 10^-2 solar masses of material in the extended envelope (depending on
the model and the assumed host-galaxy extinction). Our radius estimates are
broadly consistent with values derived independently (in other works) from HST
imaging of the progenitor star. However, the shock-cooling model radii are on
the lower end of the values indicated by pre-explosion imaging. Hydrodynamical
simulations could refine the progenitor parameters deduced from the
shock-cooling emission and test the analytical models.Comment: Accepted by ApJ
Brightness variation distributions among main belt asteroids from sparse light curve sampling with Pan-STARRS 1
The rotational state of asteroids is controlled by various physical
mechanisms including collisions, internal damping and the
Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. We have analysed the
changes in magnitude between consecutive detections of approximately 60,000
asteroids measured by the PanSTARRS 1 survey during its first 18 months of
operations. We have attempted to explain the derived brightness changes
physically and through the application of a simple model. We have found a
tendency toward smaller magnitude variations with decreasing diameter for
objects of 1 < D < 8 km. Assuming the shape distribution of objects in this
size range to be independent of size and composition our model suggests a
population with average axial ratios 1 : 0.85 \pm 0.13 : 0.71 \pm 0.13, with
larger objects more likely to have spin axes perpendicular to the orbital
plane.Comment: 10 pages, 10 figures, accepted by MNRA
ATLAS: A High-Cadence All-Sky Survey System
Technology has advanced to the point that it is possible to image the entire
sky every night and process the data in real time. The sky is hardly static:
many interesting phenomena occur, including variable stationary objects such as
stars or QSOs, transient stationary objects such as supernovae or M dwarf
flares, and moving objects such as asteroids and the stars themselves. Funded
by NASA, we have designed and built a sky survey system for the purpose of
finding dangerous near-Earth asteroids (NEAs). This system, the "Asteroid
Terrestrial-impact Last Alert System" (ATLAS), has been optimized to produce
the best survey capability per unit cost, and therefore is an efficient and
competitive system for finding potentially hazardous asteroids (PHAs) but also
for tracking variables and finding transients. While carrying out its NASA
mission, ATLAS now discovers more bright () supernovae candidates than
any ground based survey, frequently detecting very young explosions due to its
2 day cadence. ATLAS discovered the afterglow of a gamma-ray burst independent
of the high energy trigger and has released a variable star catalogue of
5 sources. This, the first of a series of articles describing
ATLAS, is devoted to the design and performance of the ATLAS system. Subsequent
articles will describe in more detail the software, the survey strategy,
ATLAS-derived NEA population statistics, transient detections, and the first
data release of variable stars and transient lightcurves.Comment: 26 pages, 12 figures, submitted to PAS
Observational Constraints on the Catastrophic Disruption Rate of Small Main Belt Asteroids
We have calculated 90% confidence limits on the steady-state rate of
catastrophic disruptions of main belt asteroids in terms of the absolute
magnitude at which one catastrophic disruption occurs per year (HCL) as a
function of the post-disruption increase in brightness (delta m) and subsequent
brightness decay rate (tau). The confidence limits were calculated using the
brightest unknown main belt asteroid (V = 18.5) detected with the Pan-STARRS1
(Pan-STARRS1) telescope. We measured the Pan-STARRS1's catastrophic disruption
detection efficiency over a 453-day interval using the Pan-STARRS moving object
processing system (MOPS) and a simple model for the catastrophic disruption
event's photometric behavior in a small aperture centered on the catastrophic
disruption event. Our simplistic catastrophic disruption model suggests that
delta m = 20 mag and 0.01 mag d-1 < tau < 0.1 mag d-1 which would imply that H0
= 28 -- strongly inconsistent with H0,B2005 = 23.26 +/- 0.02 predicted by
Bottke et al. (2005) using purely collisional models. We postulate that the
solution to the discrepancy is that > 99% of main belt catastrophic disruptions
in the size range to which this study was sensitive (100 m) are not
impact-generated, but are instead due to fainter rotational breakups, of which
the recent discoveries of disrupted asteroids P/2013 P5 and P/2013 R3 are
probable examples. We estimate that current and upcoming asteroid surveys may
discover up to 10 catastrophic disruptions/year brighter than V = 18.5.Comment: 61 Pages, 10 Figures, 3 Table
A First Catalog of Variable Stars Measured by the Asteroid Terrestrial-impact Last Alert System (ATLAS)
The Asteroid Terrestrial-impact Last Alert System (ATLAS) carries out its
primary planetary defense mission by surveying about 13000 deg^2 at least four
times per night. The resulting data set is useful for the discovery of variable
stars to a magnitude limit fainter than r~18, with amplitudes down to 0.01 mag
for bright objects. Here we present a Data Release One catalog of variable
stars based on analyzing 142 million stars measured at least 100 times in the
first two years of ATLAS operations. Using a Lomb-Scargle periodogram and other
variability metrics, we identify 4.7 million candidate variables which we
analyze in detail. Through Space Telescope Science Institute, we publicly
release lightcurves for all of them, together with a vector of 169
classification features for each star. We do this at the level of unconfirmed
candidate variables in order to provide the community with a large set of
homogeneously analyzed photometry and avoid pre-judging which types of objects
others may find most interesting. We use machine learning to classify the
candidates into fifteen different broad categories based on lightcurve
morphology. About 10% (430,000 stars) pass extensive tests designed to screen
out spurious variability detections: we label these as `probable' variables. Of
these, 230,000 receive specific classifications as eclipsing binaries,
pulsating, Mira-type, or sinusoidal variables: these are the `classified'
variables. New discoveries among the probable variables number more than
300,000, while 150,000 of the classified variables are new, including about
10,000 pulsating variables, 2,000 Mira stars, and 70,000 eclipsing binaries.Comment: Accepted by AJ; gives instructions for querying ATLAS variable star
database; this new version has nicer lightcurve figure
Asteroid models reconstructed from ATLAS photometry
The Asteroid Terrestrial-impact Last Alert System (ATLAS) is an all-sky
survey primarily aimed at detecting potentially hazardous near-Earth asteroids.
Apart from the astrometry of asteroids, it also produces their photometric
measurements that contain information about asteroid rotation and their shape.
To increase the current number of asteroids with a known shape and spin state,
we reconstructed asteroid models from ATLAS photometry that was available for
approximately 180,000 asteroids observed between 2015 and 2018. We made use of
the light-curve inversion method implemented in the Asteroid@home project to
process ATLAS photometry for roughly 100,000 asteroids with more than a hundred
individual brightness measurements. By scanning the period and pole parameter
space, we selected those best-fit models that were, according to our setup, a
unique solution for the inverse problem. We derived ~2750 unique models, 950 of
them were already reconstructed from other data and published. The remaining
1800 models are new. About half of them are only partial models, with an
unconstrained pole ecliptic longitude. Together with the shape and spin, we
also determined for each modeled asteroid its color index from the cyan and
orange filter used by the ATLAS survey. We also show the correlations between
the color index, albedo, and slope of the phase-angle function. The current
analysis is the first inversion of ATLAS asteroid photometry, and it is the
first step in exploiting the huge scientific potential that ATLAS photometry
has. ATLAS continues to observe, and in the future, this data, together with
other independent photometric measurements, can be inverted to produce more
refined asteroid models
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