24,091 research outputs found
Feedback computability on Cantor space
We introduce the notion of feedback computable functions from to
, extending feedback Turing computation in analogy with the standard
notion of computability for functions from to . We then
show that the feedback computable functions are precisely the effectively Borel
functions. With this as motivation we define the notion of a feedback
computable function on a structure, independent of any coding of the structure
as a real. We show that this notion is absolute, and as an example characterize
those functions that are computable from a Gandy ordinal with some finite
subset distinguished
Waves as the source of apparent twisting motions in sunspot penumbrae
The motion of dark striations across bright filaments in a sunspot penumbra
has become an important new diagnostic of convective gas flows in penumbral
filaments. The nature of these striations has, however, remained unclear. Here
we present an analysis of small scale motions in penumbral filaments in both
simulations and observations. The simulations, when viewed from above, show
fine structure with dark lanes running outwards from the dark core of the
penumbral filaments. The dark lanes either occur preferentially on one side or
alternate between both sides of the filament. We identify this fine structure
with transverse (kink) oscillations of the filament, corresponding to a
sideways swaying of the filament. These oscillations have periods in the range
of 5-7 min and propagate outward and downward along the filament. Similar
features are found in observed G-band intensity time series of penumbral
filaments in a sunspot located near disk center obtained by the Broadband
Filter Imager (BFI) on board {\it Hinode}. We also find that some filaments
show dark striations moving to both sides of the filaments. Based on the
agreement between simulations and observations we conclude that the motions of
these striations are caused by transverse oscillations of the underlying bright
filaments.Comment: Accepted for publication in Astrophysical Journal on 8th April 201
WASP-1: A lithium- and metal-rich star with an oversized planet
In this paper we present our results of a comprehensive spectroscopicanalysis
of WASP-1, the host star to the exoplanet WASP-1b. We derive T_eff = 6110 +/-
45 K, log g = 4.28 +/- 0.15, and [M/H] = 0.23 +/- 0.08, and also a high
abundance of lithium, log n(Li) = 2.91 +/- 0.05. These parameters suggests an
age for the system of 1-3 Gyr and a stellar mass of 1.25-1.35 M_sun. This means
that WASP-1 has properties very similar to those of HD 149026, the host star
for the highest density planet yet detected. Moreover, their planets orbit at
comparable distances and receive comparable irradiating fluxes from their host
stars. However, despite the similarity of WASP-1 with HD 149026, their planets
have strongly different densities. This suggests that gas-giant planet density
is not a simple function of host-star metallicity or of radiation environment
at ages of ~2 Gyr.Comment: Accepted for publication in MNRAS. 6 pages, 4 figure
Magnetic Flux Transport at the Solar Surface
After emerging to the solar surface, the Sun's magnetic field displays a
complex and intricate evolution. The evolution of the surface field is
important for several reasons. One is that the surface field, and its dynamics,
sets the boundary condition for the coronal and heliospheric magnetic fields.
Another is that the surface evolution gives us insight into the dynamo process.
In particular, it plays an essential role in the Babcock-Leighton model of the
solar dynamo. Describing this evolution is the aim of the surface flux
transport model. The model starts from the emergence of magnetic bipoles.
Thereafter, the model is based on the induction equation and the fact that
after emergence the magnetic field is observed to evolve as if it were purely
radial. The induction equation then describes how the surface flows --
differential rotation, meridional circulation, granular, supergranular flows,
and active region inflows -- determine the evolution of the field (now taken to
be purely radial). In this paper, we review the modeling of the various
processes that determine the evolution of the surface field. We restrict our
attention to their role in the surface flux transport model. We also discuss
the success of the model and some of the results that have been obtained using
this model.Comment: 39 pages, 15 figures, accepted for publication in Space Sci. Re
Data users note: Apollo 17 lunar photography
The availability of Apollo 17 pictorial data is announced as an aid to the selection of the photographs for study. Brief descriptions are presented of the Apollo 17 flight, and the photographic equipment used during the flight. The following descriptions are also included: service module photography, command module photography, and lunar surface photography
UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star
We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P=
0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR
Hubble Space Telescope with the time and spectral resolutions of 27 s and 15
km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV
doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K).
The mean quiescent CIV flux state was close to the saturated value and 100
times the solar one. The line profile (after removing the rotational and
instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM =
70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two
separate broadening mechanisms and velocity fields at the coronal base. It is
possible that TR transient events (random multiple velocities), with a large
surface coverage, give rise to the broadening of the narrow component,while
true microflaring is responsible for the broad one.
The transition region was observed to flare frequently on different time
scales and magnitudes. The largest impulsive flare seen in the CIV 1549
emission reached in less than one minute the peak differential emission measure
(10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower
quiescent level.The 3 min average line profile of the flare was blue-shifted
(-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have
been due to a chromospheric heating and subsequent evaporation by an electron
beam, accelerated (by reconnection) at the apex of a coronal loop.Comment: to be published in AJ (April 98), 3 tables and 7 figures as separate
PS-files, print Table 2 as a landscap
Constructing and Characterising Solar Structure Models for Computational Helioseismology
In this paper, we construct background solar models that are stable against
convection, by modifying the vertical pressure gradient of Model S
(Christensen-Dalsgaard et al., 1996, Science, 272, 1286) relinquishing
hydrostatic equilibrium. However, the stabilisation affects the eigenmodes that
we wish to remain as close to Model S as possible. In a bid to recover the
Model S eigenmodes, we choose to make additional corrections to the sound speed
of Model S before stabilisation. No stabilised model can be perfectly
solar-like, so we present three stabilised models with slightly different
eigenmodes. The models are appropriate to study the f and p1 to p4 modes with
spherical harmonic degrees in the range from 400 to 900. Background model CSM
has a modified pressure gradient for stabilisation and has eigenfrequencies
within 2% of Model S. Model CSM_A has an additional 10% increase in sound speed
in the top 1 Mm resulting in eigenfrequencies within 2% of Model S and
eigenfunctions that are, in comparison with CSM, closest to those of Model S.
Model CSM_B has a 3% decrease in sound speed in the top 5 Mm resulting in
eigenfrequencies within 1% of Model S and eigenfunctions that are only
marginally adversely affected. These models are useful to study the interaction
of solar waves with embedded three-dimensional heterogeneities, such as
convective flows and model sunspots. We have also calculated the response of
the stabilised models to excitation by random near-surface sources, using
simulations of the propagation of linear waves. We find that the simulated
power spectra of wave motion are in good agreement with an observed SOHO/MDI
power spectrum. Overall, our convectively stabilised background models provide
a good basis for quantitative numerical local helioseismology. The models are
available for download from http://www.mps.mpg.de/projects/seismo/NA4/.Comment: 35 pages, 23 figures Changed title Updated Figure 1
Model selection and prediction of outcomes in recent onset schizophrenia patients who undergo cognitive training.
Predicting treatment outcomes in psychiatric populations remains a challenge, but is increasingly important in the pursuit of personalized medicine. Patients with schizophrenia have deficits in cognition, and targeted cognitive training (TCT) of auditory processing and working memory has been shown to improve some of these impairments; but little is known about the baseline patient characteristics predictive of cognitive improvement. Here we use a model selection and regression approach called least absolute shrinkage and selection operator (LASSO) to examine predictors of cognitive improvement in response to TCT for patients with recent onset schizophrenia. Forty-three individuals with recent onset schizophrenia randomized to undergo TCT were assessed at baseline on measures of cognition, symptoms, functioning, illness duration, and demographic variables. We carried out 10-fold cross-validation of LASSO for model selection and regression. We followed up on these results using linear models for statistical inference. No individual variable was found to correlate with improvement in global cognition using a Pearson correlation approach, and a linear model including all variables was also found not to be significant. However, the LASSO model identified baseline global cognition, education, and gender in a model predictive of improvement on global cognition following TCT. These findings offer guidelines for personalized approaches to cognitive training for patients with schizophrenia
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