3,781 research outputs found
The centre and periphery of conscious thought
This paper is about whether shifts in attention can alter what it is like to think. I begin by taking up the hypothesis that attention structures consciousness into a centre and a periphery, following Watzl's (2014; 2017) understanding of the distinction between the centre and periphery of the field of consciousness. Then I show that introspection leads to divided results about whether attention structures conscious thought into a centre and a periphery -- remarks by Martin (1997) and Phillips (2012) suggest a negative answer, whereas remarks by Maher (1923) and Chudnoff (2013) suggest a positive answer. Lastly, I argue that there is behavioural evidence that lends weight to the 'yes' side of the introspective dispute. My argument makes use of Garavan's (1998) study of forming and maintaining two mental counts at once
Universal hinges and the bounds of sense
According to DaniĆØle Moyal-Sharrock, Wittgensteinās On Certainty presents a theory of hinges, and hinges have a role to play in a foundationalist epistemology (2013). Michael Williams (2005) and Annalisa Coliva (2013 ) have claimed that the hinges are not suitable to play such a role as they are not shared universally. Moyal-Sharrock has replied that a subset of the hinges is suitable to play such a role: the āuniversalā hinges. I argue that for Moyal-Sharrockās reply to be sustained, she must construe the set of universal hinges much more narrowly than she does currently. For instance, Moyal-Sharrock claims that āI have a brainā is a universal hinge, which consigns people who know nothing about brains to stand outside the bounds of sense. I also provide a novel way of thinking about the universal hinges, which I argue is better textually motivated than Moyal-Sharrockās own way, and which provides a set of hinges more suitable to play a role in foundationalist epistemology
Thermo-Resistive Instability of Hot Planetary Atmospheres
The atmospheres of hot Jupiters and other strongly-forced exoplanets are
susceptible to a thermal instability in the presence of ohmic dissipation, weak
magnetic drag and strong winds. The instability occurs in radiatively-dominated
atmospheric regions when the ohmic dissipation rate increases with temperature
faster than the radiative (cooling) rate. The instability domain covers a
specific range of atmospheric pressures and temperatures, typically P ~ 3-300
mbar and T ~ 1500-2500K for hot Jupiters, which makes it a candidate mechanism
to explain the dayside thermal "inversions" inferred for a number of such
exoplanets. The instability is suppressed by high levels of non-thermal
photoionization, in possible agreement with a recently established
observational trend. We highlight several shortcomings of the instability
treatment presented here. Understanding the emergence and outcome of the
instability, which should result in locally hotter atmospheres with stronger
levels of drag, will require global non-linear atmospheric models with adequate
MHD prescriptions.Comment: 13 pages, 3 figures, accepted for publication in ApJ
Exoplanet albedo spectra and colors as a function of planet phase, separation, and metallicity
First generation optical coronagraphic telescopes will obtain images of cool
gas and ice giant exoplanets around nearby stars. The albedo spectra of
exoplanets at planet-star separations larger than about 1 AU are dominated by
reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption
features. We consider how exoplanet albedo spectra and colors vary as a
function of planet-star separation, metallicity, mass, and observed phase for
Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs
with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with
10x and 30x. Our model planets orbit a solar analog parent star at separations
of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to
compute temperature-pressure profiles. The giant exoplanets are cloud-free at
0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and
10 AU. For each model planet we compute moderate resolution spectra as a
function of phase. The presence and structure of clouds strongly influence the
spectra. Since the planet images will be unresolved, their phase may not be
obvious, and multiple observations will be needed to discriminate between the
effects of planet-star separation, metallicity, and phase. We consider the
range of these combined effects on spectra and colors. For example, we find
that the spectral influence of clouds depends more on planet-star separation
and hence temperature than metallicity, and it is easier to discriminate
between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In
addition to alkalis and methane, our Jupiter models show H2O absorption
features near 0.94 {\mu}m. We also predict that giant exoplanets receiving
greater insolation than Jupiter will exhibit higher equator to pole temperature
gradients than are found on Jupiter and thus may have differing atmospheric
dynamics.Comment: 62 pages, 19 figures, 6 tables Accepted for publication in Ap
Effects of Helium Phase Separation on the Evolution of Giant Planets
We present the first models of Saturn and Jupiter to couple their evolution
to both a radiative-atmosphere grid and to high-pressure phase diagrams of
hydrogen with helium. The purpose of these models is to quantify the
evolutionary effects of helium phase separation in Saturn's deep interior. We
find that prior calculated phase diagrams in which Saturn's interior reaches a
region of predicted helium immiscibility do not allow enough energy release to
prolong Saturn's cooling to its known age and effective temperature. We explore
modifications to published phase diagrams that would lead to greater energy
release, and find a modified H-He phase diagram that is physically reasonable,
leads to the correct extension of Saturn's cooling, and predicts an atmospheric
helium mass fraction Y_atmos in agreement with recent estimates. We then expand
our inhomogeneous evolutionary models to show that hypothetical extrasolar
giant planets in the 0.15 to 3.0 Jupiter mass range may have T_effs 10-15 K
greater than one would predict with models that do not incorporate helium phase
separation.Comment: 4 pages. Contribution to 'The Search for Other Worlds', Oct 2003,
University of Marylan
Bayesian Analysis of Hot Jupiter Radius Anomalies: Evidence for Ohmic Dissipation?
The cause of hot Jupiter radius inflation, where giant planets with K are significantly larger than expected, is an open question and
the subject of many proposed explanations. Rather than examine these models
individually, this work seeks to characterize the anomalous heating as a
function of incident flux, , needed to inflate the population of
planets to their observed sizes. We then compare that result to theoretical
predictions for various models. We examine the population of about 300 giant
planets with well-determined masses and radii and apply thermal evolution and
Bayesian statistical models to infer the anomalous power as a function of
incident flux that best reproduces the observed radii. First, we observe that
the inflation of planets below about M=0.5 \;\rm{M}_\rm{J} appears very
different than their higher mass counterparts, perhaps as the result of mass
loss or an inefficient heating mechanism. As such, we exclude planets below
this threshold. Next, we show with strong significance that
increases with towards a maximum of at K, and then decreases as temperatures increase further, falling
to at T_\rm{eff}= 2500 K. This high-flux decrease in inflation
efficiency was predicted by the Ohmic dissipation model of giant planet
inflation but not other models. We also explicitly check the thermal tides
model and find that it predicts far more variance in radii than is observed.
Thus, our results provide evidence for the Ohmic dissipation model and a
functional form for that any future theories of hot Jupiter radii
can be tested against.Comment: 14 pages, 14 figures, accepted to The Astronomical Journal. This
revision revises the description of statistical methods for clarity, but the
conclusions remain the sam
Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
Transiting planet surveys like Kepler have provided a wealth of information
on the distribution of planetary radii, particularly for the new populations of
super-Earth and sub-Neptune sized planets. In order to aid in the physical
interpretation of these radii, we compute model radii for low-mass rocky
planets with hydrogen-helium envelopes. We provide model radii for planets 1-20
Earth masses, with envelope fractions from 0.01-20%, levels of irradiation
0.1-1000x Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide
simple analytic fits that summarize how radius depends on each of these
parameters. Most importantly, we show that at fixed composition, radii show
little dependence on mass for planets with more than ~1% of their mass in their
envelope. Consequently, planetary radius is to first order a proxy for
planetary composition for Neptune and sub-Neptune sized planets. We recast the
observed mass-radius relationship as a mass-composition relationship and
discuss it in light of traditional core accretion theory. We discuss the
transition from rocky super-Earths to sub-Neptune planets with large volatile
envelopes. We suggest 1.75 Earth radii as a physically motivated dividing line
between these two populations of planets. Finally, we discuss these results in
light of the observed radius occurrence distribution found by Kepler.Comment: 17 pages, 9 figures, 7 tables, submitted to Ap
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