9,671 research outputs found
3D Temperature Mapping of Solar Photospheric Fine Structure Using Ca II H Filtergrams
Context. The wings of the Ca II H and K lines provide excellent photospheric
temperature diagnostics. At the Swedish 1-meter Solar Telescope the blue wing
of Ca II H is scanned with a narrowband interference filter mounted on a
rotation stage. This provides up to 0"10 spatial resolution filtergrams at high
cadence that are concurrent with other diagnostics at longer wavelengths. Aims.
The aim is to develop observational techniques that provide the photospheric
temperature stratification at the highest spatial resolution possible and use
those to compare simulations and observations at different heights. Methods. We
use filtergrams in the Ca II H blue wing obtained with a tiltable interference
filter at the SST. Synthetic observations are produced from 3D HD and 3D MHD
numerical simulations and degraded to match the observations. The temperature
structure obtained from applying the method to the synthetic data is compared
with the known structure in the simulated atmospheres and with observations of
an active region. Cross-correlation techniques using restored non-simultaneous
continuum images are used to reduce high-altitude, small-scale seeing signal
introduced from the non-simultaneity of the frames when differentiating data.
Results. Temperature extraction using high resolution filtergrams in the Ca II
H blue wing works reasonably well when tested with simulated 3D atmospheres.
The cross-correlation technique successfully compensates the problem of
small-scale seeing differences and provides a measure of the spurious signal
from this source in differentiated data. Synthesized data from the simulated
atmospheres (including pores) match well the observations morphologically at
different observed heights and in vertical temperature gradients.Comment: Accepted the 10/10/2012 for publication in Astronomy & Astrophysics
in Section 9, The Sun. Published the 03/12/2012 v1 to v2: changed submission
metadata v2 to v3: small changes to match published versio
The role of active galactic nuclei in galaxy formation
We use Monte-Carlo Markov chain techniques to constrain acceptable parameter
regions for the Munich L-Galaxies semi-analytic galaxy formation model.
Feedback from active galactic nuclei (AGN) is required to limit star-formation
in the most massive galaxies. However, we show that the introduction of tidal
stripping of dwarf galaxies as they fall into and merge with their host systems
can lead to a reduction in the required degree of AGN feedback. In addition,
the new model correctly reproduces both the metallicity of large galaxies and
the fraction of intracluster light.Comment: Monster's Fiery Breath Conference Proceedings, 4 page
Exact Solutions for Boson-Fermion Stars in (2+1) dimensions
We solve Einstein equations coupled to a complex scalar field with infinitely
large self-interaction, degenerate fermions, and a negative cosmological
constant in dimensions. Exact solutions for static boson-fermion stars
are found when circular symmetry is assumed. We find that the minimum binding
energy of boson-fermion star takes a negative value if the value of the
cosmological constant is sufficiently small.Comment: 19 pages, 5 figures, RevTeX 3.0, second revised versio
Metallic slabs: Perturbative treatments based on jellium
We examine first-order perturbative results based on jellium for the surface
energy of slabs of simple metals, using various local pseudopotentials
(Ashcroft, Heine-Abarenkov and evanescent core). The difference between the
pseudopotential and the jellium potential is averaged along the plane parallel
to the surface. We compare these perturbative results with those of the
stabilized jellium model (a modification of the regular jellium model in which
the perturbation appears in the energy functional right from the outset) and
with the output of other perturbative and non-perturbative calculations.Comment: 15 pages, 2 figures, to appear in Prog. Surf. Sc
Quasi-complete intersection homomorphisms
Extending a notion defined for surjective maps by Blanco, Majadas, and
Rodicio, we introduce and study a class of homomorphisms of commutative
noetherian rings, which strictly contains the class of locally complete
intersection homomorphisms, while sharing many of its remarkable properties.Comment: Final version, to appear in the special issue of Pure and Applied
Mathematics Quarterly dedicated to Andrey Todorov. The material in the first
four sections has been reorganized and slightly expande
Cluster decomposition, T-duality, and gerby CFT's
In this paper we study CFT's associated to gerbes. These theories suffer from
a lack of cluster decomposition, but this problem can be resolved: the CFT's
are the same as CFT's for disconnected targets. Such theories also lack cluster
decomposition, but in that form, the lack is manifestly not very problematic.
In particular, we shall see that this matching of CFT's, this duality between
noneffective gaugings and sigma models on disconnected targets, is a worldsheet
duality related to T-duality. We perform a wide variety of tests of this claim,
ranging from checking partition functions at arbitrary genus to D-branes to
mirror symmetry. We also discuss a number of applications of these results,
including predictions for quantum cohomology and Gromov-Witten theory and
additional physical understanding of the geometric Langlands program.Comment: 61 pages, LaTeX; v2,3: typos fixed; v4: writing improved in several
sections; v5: typos fixe
Monte Carlo Markov Chain parameter estimation in semi-analytic models of galaxy formation
We present a statistical exploration of the parameter space of the De Lucia and Blaizot version of the Munich semi-analytic (SA) model built upon the Millennium dark matter simulation. This is achieved by applying a Monte Carlo Markov Chain method to constrain the six free parameters that define the stellar and black hole mass functions at redshift zero. The model is tested against three different observational data sets, including the galaxy K-band luminosity function, B - V colours and the black hole-bulge mass relation, separately and combined, to obtain mean values, confidence limits and likelihood contours for the best-fitting model. Using each observational data set independently, we discuss how the SA model parameters affect each galaxy property and find that there are strong correlations between them. We analyse to what extent these are simply reflections of the observational constraints, or whether they can lead to improved understandings of the physics of galaxy formation.
When all the observations are combined, we find reasonable agreement between the majority of the previously published parameter values and our confidence limits. However, the need to suppress dwarf galaxy formation requires the strength of the supernova feedback to be significantly higher in our best-fitting solution than in previous work.
To balance this, we require the feedback to become ineffective in haloes of lower mass than before, so as to permit the formation of sufficient high-luminosity galaxies: unfortunately, this leads to an excess of galaxies around L*. Although the best fit is formally consistent with the data, there is no region of parameter space that reproduces the shape of galaxy luminosity function across the whole magnitude range.
For our best fit, we present the model predictions for the bJ-band luminosity and stellar mass functions. We find a systematic disagreement between the observed mass function and the predictions from the K-band constraint, which we explain in light of recent works that suggest uncertainties of up to 0.3 dex in the mass determination from stellar population synthesis models.
We discuss modifications to the SA model that might simultaneously improve the fit to the observed mass function and reduce the reliance on excessive supernova feedback in small haloes
Chromospheric Inversions of a Micro-flaring Region
We use spectropolarimetric observations of the Ca II 8542~\AA\ line, taken
from the Swedish 1-m Solar Telescope (SST), in an attempt to recover dynamic
activity in a micro-flaring region near a sunspot via inversions. These
inversions show localized mean temperature enhancements of 1000~K in the
chromosphere and upper photosphere, along with co-spatial bi-directional
Doppler shifting of 5 - 10 km s. This heating also extends along a
nearby chromospheric fibril, co-spatial to 10 - 15 km s down-flows.
Strong magnetic flux cancellation is also apparent in one of the footpoints,
concentrated in the chromosphere. This event more closely resembles that of an
Ellerman Bomb (EB), though placed slightly higher in the atmosphere than is
typically observed.Comment: 9 pages, 9 figures, accepted in ApJ. Movies are stored here:
https://star.pst.qub.ac.uk/webdav/public/areid/Microflare
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