315 research outputs found
Hot Jupiters from Secular Planet--Planet Interactions
About 25 per cent of `hot Jupiters' (extrasolar Jovian-mass planets with
close-in orbits) are actually orbiting counter to the spin direction of the
star. Perturbations from a distant binary star companion can produce high
inclinations, but cannot explain orbits that are retrograde with respect to the
total angular momentum of the system. Such orbits in a stellar context can be
produced through secular (that is, long term) perturbations in hierarchical
triple-star systems. Here we report a similar analysis of planetary bodies,
including both octupole-order effects and tidal friction, and find that we can
produce hot Jupiters in orbits that are retrograde with respect to the total
angular momentum. With distant stellar mass perturbers, such an outcome is not
possible. With planetary perturbers, the inner orbit's angular momentum
component parallel to the total angular momentum need not be constant. In fact,
as we show here, it can even change sign, leading to a retrograde orbit. A
brief excursion to very high eccentricity during the chaotic evolution of the
inner orbit allows planet-star tidal interactions to rapidly circularize that
orbit, decoupling the planets and forming a retrograde hot Jupiter.Comment: accepted for publication by Nature, 3 figures (version after proof -
some typos corrected
Characterization of exoplanets from their formation I: Models of combined planet formation and evolution
A first characterization of many exoplanets has recently been achieved by the
observational determination of their radius. For some planets, a measurement of
the luminosity has also been possible, with many more directly imaged planets
expected in the future. The statistical characterization of exoplanets through
their mass-radius and mass-luminosity diagram is thus becoming possible. This
is for planet formation and evolution theory of similar importance as the
mass-distance diagram. Our aim in this and a companion paper is to extend our
formation model into a coupled formation and evolution model. We want to
calculate in a self-consistent way all basic characteristics (M,a,R,L) of a
planet and use the model for population synthesis calculations. Here we show
how we solve the structure equations describing the gaseous envelope not only
during the early formation phase, but also during gas runaway accretion, and
during the evolutionary phase at constant mass on Gyr timescales. We then study
the in situ formation and evolution of Jupiter, the mass-radius relationship of
giants, the influence of the core mass on the radius and the luminosity both in
the "hot start" and the "cold start" scenario. We put special emphasis on the
comparison with other models. We find that our results agree very well with
those of more complex models, despite a number of simplifications. The upgraded
model yields the most important characteristics of a planet from its beginning
as a seed embryo to a Gyr old planet. This is the case for all planets in a
synthetic planetary population. Therefore, we can now use self-consistently the
statistical constraints coming from all major observational techniques. This is
important in a time where different techniques yield constraints on very
diverse sub-populations of planets, and where its is challenging to put all
these constraints together in one coherent picture.Comment: Accepted to A&A. Identical as v1 except for additional online data
reference and corrected typos. 23 pages, 11 figure
From Select Agent to an Established Pathogen: The Response to \u3ci\u3ePhakopsora pachyrhizi\u3c/i\u3e (Soybean Rust) in North America
The pathogen causing soybean rust, Phakopsora pachyrhizi, was first described in Japan in 1902. The disease was important in the Eastern Hemisphere for many decades before the fungus was reported in Hawaii in 1994, which was followed by reports from countries in Africa and South America. In 2004, P. pachyrhizi was confirmed in Louisiana, making it the first report in the continental United States. Based on yield losses from countries in Asia, Africa, and South America, it was clear that this pathogen could have a major economic impact on the yield of 30 million ha of soybean in the United States. The response by agencies within the United States Department of Agriculture, industry, soybean check-off boards, and universities was immediate and complex. The impacts of some of these activities are detailed in this review. The net result has been that the once dreaded disease, which caused substantial losses in other parts of the world, is now better understood and effectively managed in the United States. The disease continues to be monitored yearly for changes in spatial and temporal distribution so that soybean growers can continue to benefit by knowing where soybean rust is occurring during the growing season
Extrasolar planet population synthesis IV. Correlations with disk metallicity, mass and lifetime
Context. This is the fourth paper in a series showing the results of planet
population synthesis calculations.
Aims. Our goal in this paper is to systematically study the effects of
important disk properties, namely disk metallicity, mass and lifetime on
fundamental planetary properties.
Methods. For a large number of protoplanetary disks we calculate a population
of planets with our core accretion formation model including planet migration
and disk evolution.
Results. We find a large number of correlations: Regarding the planetary
initial mass function, metallicity, disk mass and disk lifetime have different
roles: For high [Fe/H], giant planets are more frequent. For high disk masses,
giant planets are more massive. For long disk lifetimes, giant planets are both
more frequent and massive. At low metallicities, very massive giant planets
cannot form, but otherwise giant planet mass and metallicity are uncorrelated.
In contrast, planet masses and disk gas masses are correlated. The sweet spot
for giant planet formation is at 5 AU. In- and outside this distance, higher
planetesimals surface densities are necessary. Low metallicities can be
compensated by high disk masses, and vice versa, but not ad infinitum. At low
metallicities, giant planets only form outside the ice line, while at high
metallicities, giant planet formation occurs throughout the disk. The extent of
migration increases with disk mass and lifetime and usually decreases with
metallicity. No clear correlation of metallicity and the semimajor axis of
giant planets exists because in low [Fe/H] disks, planets start further out,
but migrate more, whereas for high [Fe/H] they start further in, but migrate
less. Close-in low mass planets have a lower mean metallicity than Hot
Jupiters.
Conclusions. The properties of protoplanetary disks are decisive for the
properties of planets, and leave many imprints.Comment: 23 pages, 16 figures. Accepted for A&
Accreting Protoplanets in the LkCa 15 Transition Disk
Exoplanet detections have revolutionized astronomy, offering new insights
into solar system architecture and planet demographics. While nearly 1900
exoplanets have now been discovered and confirmed, none are still in the
process of formation. Transition discs, protoplanetary disks with inner
clearings best explained by the influence of accreting planets, are natural
laboratories for the study of planet formation. Some transition discs show
evidence for the presence of young planets in the form of disc asymmetries or
infrared sources detected within their clearings, as in the case of LkCa 15.
Attempts to observe directly signatures of accretion onto protoplanets have
hitherto proven unsuccessful. Here we report adaptive optics observations of
LkCa 15 that probe within the disc clearing. With accurate source positions
over multiple epochs spanning 2009 - 2015, we infer the presence of multiple
companions on Keplerian orbits. We directly detect H{\alpha} emission from the
innermost companion, LkCa 15 b, evincing hot (~10,000 K) gas falling deep into
the potential well of an accreting protoplanet.Comment: 35 pages, 3 figures, 1 table, 9 extended data item
Discovery of the Coldest Imaged Companion of a Sun-Like Star
We present the discovery of a brown dwarf or possible planet at a projected
separation of 1.9" = 29 AU around the star GJ 758, placing it between the
separations at which substellar companions are expected to form by core
accretion (~5 AU) or direct gravitational collapse (typically >100 AU). The
object was detected by direct imaging of its thermal glow with Subaru/HiCIAO.
At 10-40 times the mass of Jupiter and a temperature of 550-640 K, GJ 758 B
constitutes one of the few known T-type companions, and the coldest ever to be
imaged in thermal light around a Sun-like star. Its orbit is likely eccentric
and of a size comparable to Pluto's orbit, possibly as a result of
gravitational scattering or outward migration. A candidate second companion is
detected at 1.2" at one epoch.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter
Collisional Evolution of Irregular Satellite Swarms: Detectable Dust around Solar System and Extrasolar Planets
Since the 1980's it has been becoming increasingly clear that the Solar
System's irregular satellites are collisionally evolved. We derive a general
model for the collisional evolution of an irregular satellite swarm and apply
it to the Solar System and extrasolar planets. Our model reproduces the Solar
System's complement of observed irregulars well, and suggests that the
competition between grain-grain collisions and Poynting-Robertson (PR) drag
helps set the fate of the dust. Because swarm collision rates decrease over
time the main dust sink can change with time, and may help unravel the
accretion history of synchronously rotating regular satellites that show
brightness asymmetries. Some level of dust must be present on AU scales around
the Solar System's giant planets, which we predict may be at detectable levels.
We also predict whether dust produced by extrasolar circumplanetary swarms can
be detected. The coronagraphic instruments on JWST will have the ability to
detect the dust generated by these swarms, which are most detectable around
planets that orbit at tens of AU from the youngest stars. Because the
collisional decay of swarms is relatively insensitive to planet mass, swarms
can be much brighter than their host planets and allow discovery of
Neptune-mass planets that would otherwise remain invisible. This dust may have
already been detected. The observations of the planet Fomalhaut b can be
explained as scattered light from dust produced by the collisional decay of an
irregular satellite swarm around a 10 Earth-mass planet. Such a swarm comprises
about 5 Lunar masses worth of irregular satellites. Finally, we consider what
happens if Fomalhaut b passes through Fomalhaut's main debris ring, which
allows the circumplanetary swarm to be replenished through collisions with ring
planetesimals. (abridged)Comment: accepted to MNRA
The Palomar/Keck Adaptive Optics Survey of Young Solar Analogs: Evidence for a Universal Companion Mass Function
We present results from an adaptive optics survey for substellar and stellar
companions to Sun-like stars. The survey targeted 266 F5-K5 stars in the 3Myr
to 3Gyr age range with distances of 10-190pc. Results from the survey include
the discovery of two brown dwarf companions (HD49197B and HD203030B), 24 new
stellar binaries, and a triple system. We infer that the frequency of
0.012-0.072Msun brown dwarfs in 28-1590AU orbits around young solar analogs is
3.2% (+3.1%,-2.7%; 2sigma limits). The result demonstrates that the deficiency
of substellar companions at wide orbital separations from Sun-like stars is
less pronounced than in the radial velocity "brown dwarf desert." We infer that
the mass distribution of companions in 28-1590AU orbits around solar-mass stars
follows a continuous dN/dM_2 ~ M_2^(-0.4) relation over the 0.01-1.0Msun
secondary mass range. While this functional form is similar to that for
<0.1Msun isolated objects, over the entire 0.01-1.0Msun range the mass
functions of companions and of isolated objects differ significantly. Based on
this conclusion and on similar results from other direct imaging and radial
velocity companion surveys in the literature, we argue that the companion mass
function follows the same universal form over the entire range between 0-1590AU
in orbital semi-major axis and 0.01-20Msun in companion mass. In this context,
the relative dearth of substellar versus stellar secondaries at all orbital
separations arises naturally from the inferred form of the companion mass
function.Comment: Final version accepted by ApJ Supplements. 50 pages, including 12
tables + 16 figures. Version with full tables available at
http://www.astro.sunysb.edu/metchev/PUBLICATIONS/cmf.pd
Higher media multi-tasking activity is associated with smaller gray-matter density in the anterior cingulate cortex
Media multitasking, or the concurrent consumption of multiple media forms, is increasingly prevalent in today’s society and has been associated with negative psychosocial and cognitive impacts. Individuals who engage in heavier media-multitasking are found to perform worse on cognitive control tasks and exhibit more socio-emotional difficulties. However, the neural processes associated with media multi-tasking remain unexplored. The present study investigated relationships between media multitasking activity and brain structure. Research has demonstrated that brain structure can be altered upon prolonged exposure to novel environments and experience. Thus, we expected differential engagements in media multitasking to correlate with brain structure variability. This was confirmed via Voxel-Based Morphometry (VBM) analyses: Individuals with higher Media Multitasking Index (MMI) scores had smaller gray matter density in the anterior cingulate cortex (ACC). Functional connectivity between this ACC region and the precuneus was negatively associated with MMI. Our findings suggest a possible structural correlate for the observed decreased cognitive control performance and socio-emotional regulation in heavy media-multitaskers. While the cross-sectional nature of our study does not allow us to specify the direction of causality, our results brought to light novel associations between individual media multitasking behaviors and ACC structure differences
Competing Neural Responses for Auditory and Visual Decisions
Why is it hard to divide attention between dissimilar activities, such as reading and listening to a conversation? We used functional magnetic resonance imaging (fMRI) to study interference between simple auditory and visual decisions, independently of motor competition. Overlapping activity for auditory and visual tasks performed in isolation was found in lateral prefrontal regions, middle temporal cortex and parietal cortex. When the visual stimulus occurred during the processing of the tone, its activation in prefrontal and middle temporal cortex was suppressed. Additionally, reduced activity was seen in modality-specific visual cortex. These results paralleled impaired awareness of the visual event. Even without competing motor responses, a simple auditory decision interferes with visual processing on different neural levels, including prefrontal cortex, middle temporal cortex and visual regions
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