80 research outputs found
Avalanches and the Distribution of Reconnection Events in Magnetized Circumstellar Disks
Cosmic rays produced by young stellar objects can potentially alter the
ionization structure, heating budget, chemical composition, and accretion
activity in circumstellar disks. The inner edges of these disks are truncated
by strong magnetic fields, which can reconnect and produce flaring activity
that accelerates cosmic radiation. The resulting cosmic rays can provide a
source of ionization and produce spallation reactions that alter the
composition of planetesimals. This reconnection and particle acceleration are
analogous to the physical processes that produce flaring in and heating of
stellar coronae. Flaring events on the surface of the Sun exhibit a power-law
distribution of energy, reminiscent of those measured for Earthquakes and
avalanches. Numerical lattice-reconnection models are capable of reproducing
the observed power-law behavior of solar flares under the paradigm of
self-organized criticality. One interpretation of these experiments is that the
solar corona maintains a nonlinear attractor -- or ``critical'' -- state by
balancing energy input via braided magnetic fields and output via reconnection
events. Motivated by these results, we generalize the lattice-reconnection
formalism for applications in the truncation region of magnetized disks. Our
numerical experiments demonstrate that these nonlinear dynamical systems are
capable of both attaining and maintaining criticality in the presence of
Keplerian shear and other complications. The resulting power-law spectrum of
flare energies in the equilibrium attractor state is found to be nearly
universal in magnetized disks. This finding indicates that magnetic
reconnection and flaring in the inner regions of circumstellar disks occur in a
manner similar to activity on stellar surfaces
Differences in signal contrast and camouflage among different colour variations of a stomatopod crustacean, Neogonodactylus oerstedii
Animal colouration is often a trade-off between background matching for camouflage from predators, and conspicuousness for communication with con- or heterospecifics. Stomatopods are marine crustaceans known to use colour signals during courtship and contests, while their overall body colouration may provide camouflage. However, we have little understanding of how stomatopods perceive these signals in their environment or whether overall body coloration does provide camouflage from predators. Neogonodactylus oerstedii assess meral spot colour during contests, and meral spot colour varies depending on local habitat. By calculating quantum catch for N. oerstedii’s 12 photoreceptors associated with chromatic vision, we found that variation in meral spot total reflectance does not function to increase signal contrast in the local habitat. Neogonodactylus oerstedii also show between-habitat variation in dorsal body colouration. We used visual models to predict a trichromatic fish predator’s perception of these colour variations. Our results suggest that sandy and green stomatopods are camouflaged from a typical fish predator in rubble fields and seagrass beds, respectively. To our knowledge, this is the first study to investigate signal contrast and camouflage in a stomatopod. These results provide new insight into the function and evolution of colouration in a species with a complex visual system
eleanor: An open-source tool for extracting light curves from the TESS Full-Frame Images
During its two year prime mission the Transiting Exoplanet Survey Satellite
(TESS) will perform a time-series photometric survey covering over 80% of the
sky. This survey comprises observations of 26 24 x 96 degree sectors that are
each monitored continuously for approximately 27 days. The main goal of TESS is
to find transiting planets around 200,000 pre-selected stars for which fixed
aperture photometry is recorded every two minutes. However, TESS is also
recording and delivering Full-Frame Images (FFIs) of each detector at a 30
minute cadence. We have created an open-source tool, eleanor, to produce light
curves for objects in the TESS FFIs. Here, we describe the methods used in
eleanor to produce light curves that are optimized for planet searches. The
tool performs background subtraction, aperture and PSF photometry,
decorrelation of instrument systematics, and cotrending using principal
component analysis. We recover known transiting exoplanets in the FFIs to
validate the pipeline and perform a limited search for new planet candidates in
Sector 1. Our tests indicate that eleanor produces light curves with
significantly less scatter than other tools that have been used in the
literature. Cadence-stacked images, and raw and detrended eleanor light curves
for each analyzed star will be hosted on MAST, with planet candidates on
ExoFOP-TESS as Community TESS Objects of Interest (CTOIs). This work confirms
the promise that the TESS FFIs will enable the detection of thousands of new
exoplanets and a broad range of time domain astrophysics.Comment: 21 pages, 13 figures, 2 tables, Accepted to PAS
Potential Melting of Extrasolar Planets by Tidal Dissipation
Tidal heating on Io due to its finite eccentricity was predicted to drive
surface volcanic activity, which was subsequently confirmed by the
spacecrafts. Although the volcanic activity in Io is more
complex, in theory volcanism can be driven by runaway melting in which the
tidal heating increases as the mantle thickness decreases. We show that this
runaway melting mechanism is generic for a composite planetary body with liquid
core and solid mantle, provided that (i) the mantle rigidity, , is
comparable to the central pressure, i.e.
for a body with density , surface gravitational acceleration , and
radius , (ii) the surface is not molten, (iii) tides deposit
sufficient energy, and (iv) the planet has nonzero eccentricity. We calculate
the approximate liquid core radius as a function of ,
and find that more than of the core will melt due to this runaway for
. From all currently confirmed exoplanets, we
find that the terrestrial planets in the L98-59 system are the most promising
candidates for sustaining active volcanism. However, uncertainties regarding
the quality factors and the details of tidal heating and cooling mechanisms
prohibit definitive claims of volcanism on any of these planets. We generate
synthetic transmission spectra of these planets assuming Venus-like atmospheric
compositions with an additional 5, 50, and SO component, which is a
tracer of volcanic activity. We find a preference for a
model with SO with 5-10 transits with for L98-59bcd.Comment: 16 pages, 8 Figures, accepted for publication in Ap
HFF-DeepSpace photometric catalogs of the 12 Hubble frontier fields, clusters, and parallels : photometry, photometric redshifts, and stellar masses
We present Hubble multi-wavelength photometric catalogs, including (up to) 17 filters with the Advanced Camera for Surveys and Wide Field Camera 3 from the ultra-violet to near-infrared for the Hubble Frontier Fields and associated parallels. We have constructed homogeneous photometric catalogs for all six clusters and their parallels. To further expand these data catalogs, we have added ultra-deep KS-band imaging at 2.2. mu m from the Very Large Telescope HAWK-I and Keck-I MOSFIRE instruments. We also add post-cryogenic Spitzer imaging at 3.6 and 4.5. mu m with the Infrared Array Camera (IRAC), as well as archival IRAC 5.8 and 8.0. mu m imaging when available. We introduce the public release of the multi-wavelength (0.2-8 mu m) photometric catalogs, and we describe the unique steps applied for the construction of these catalogs. Particular emphasis is given to the source detection band, the contamination of light from the bright cluster galaxies (bCGs), and intra-cluster light (ICL). In addition to the photometric catalogs, we provide catalogs of photometric redshifts and stellar population properties. Furthermore, this includes all the images used in the construction of the catalogs, including the combined models of bCGs and ICL, the residual images, segmentation maps, and more. These catalogs are a robust data set of the Hubble Frontier Fields and will be an important aid in designing future surveys, as well as planning follow-up programs with current and future observatories to answer key questions remaining about first light, reionization, the assembly of galaxies, and many more topics, most notably by identifying high-redshift sources to target
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