68 research outputs found
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
The VMC survey - XV : The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations
Date of Acceptance: 19/03/2015We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t/yr) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t/yr) ≤ 9.4 - while two excesses of clusters are seen at log(t/yr) ~9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ~7.3, but it also hosts some of intermediate ages, log(t/yr) ~9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t/yr) < 9.0, which could have been induced by interactions with the LMC.Peer reviewedFinal Accepted Versio
Surface albedo changes with time on Titan’s possible cryovolcanic sites: Cassini/VIMS processing and geophysical implications
We present a study on Titan’s possibly cryovolcanic and varying regions as suggested from previous studies [e.g. 1;2;7]. These regions, which are potentially subject to change over time in brightness and are located close to the equator, are Tui Regio, Hotei Regio, and Sotra Patera. We apply two methods on Cassini/VIMS data in order to retrieve their surface properties and monitor any temporal variations. First, we apply a statistical method, the Principal Component Analysis (PCA) [3;4] where we manage to isolate regions of distinct and diverse chemical composition called ‘Region of interest – RoI’. Then, we focus on retrieving the spectral differences (with respect to the Huygens landing site albedo) among the RoIs by applying a radiative transfer code (RT) [5;3]. Hence, we are able to view the dynamical range and evaluate the differences in surface albedo within the RoIs of the three regions. In addition, using this double procedure, we study the temporal surface variations of the three regions witnessing albedo changes with time for Tui Regio from 2005-2009 (darkening) and Sotra Patera from 2005-2006 (brightening) at all wavelengths [3]. The surface albedo variations and the presence of volcanic-like features within the regions in addition to a recent study [6] that calculates Titan's tidal response are significant indications for the connection of the interior with the cryovolcanic candidate features with implications for the satellite’s astrobiological potential
The evolution of the atmosphere and surface of Titan from Cassini infrared observations
Saturn’s Earth-like satellite Titan has a thick and dense atmosphere consisting of nitrogen (98.4%), methane (1.6%) and trace gases such as hydrocarbons and nitriles [1]. The condensed organics are deposited on the surface and the atmosphere-surface-interior interactions shape the ground. In particular, Titan’s methane cycle, similarly to the Earth’s hydrologic cycle, plays an important role in these exchanges by transporting methane at all layers. By applying our radiative transfer code (ARTT) to Cassini/CIRS data taken during Titan flybys from 2004-2010 and to the 1980 Voyager 1 flyby values inferred from the reanalysis of the Infrared Radiometer Spectrometer (IRIS) spectra, as well as to the intervening ground- and space- based observations (such as with ISO), we study the stratospheric evolution over a Titanian year (V1 encounter Ls=9° was reached in mid-2010)
On the Properties of the HI shells in the Small Magellanic CLoud
There are 509 expanding neutral hydrogen shells catalogued in the Small
Magellanic Cloud (SMC), all apparently very young, with dynamical ages of a few
Myr. To examine their relationship with young stellar objects we
cross-correlated the shell catalogue with various catalogues of OB
associations, super giants, Cepheids, WR stars, supernova remnants, and star
clusters. The incidence of chance line-ups was estimated via Monte-Carlo
simulations, and found to be high. However, it is important that there are 1.5
times more shells that are {\em not} spatially correlated to an OB association,
than shells that are. Moreover, 59 of the 509 shells lie mainly in low stellar
density fields and have no young stellar objects associated with them, and
therefore no obvious energy source. It is shown that, on the whole, the
properties of these "empty" shells are very similar to the properties of the
rest of the shells, once selection biases are taken into account. Theoretical
implications of the results are discussed.Comment: 15 pages, 8 figures accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Image relaxation by use of the Potts model with a fast deterministic method
We demonstrate a close relationship between classical models from statistical physics and Markov randomfield models for image labeling purposes. A picture is taken of a real image or map, defined by a measure of intensity over a set of pixels. Possible class (or gray-level) values are assigned to spin values in Potts theory. We present a continuous analysis of image relaxation by mean-field theory and apply it by using standard and extended Potts models. Accurate relaxation results were obtained with a specific deterministic method called mean-field fast annealing. © 1997 Optical Society of America
Solar image segmentation by use of mean field fast annealing
We present a "continuous” analysis of a solar image
in order to address the problem of image segmentation. Our approach
is based on combinatorial optimization methods and in particular on
Mean Field Fast Annealing (MFFA).
Mean-field theory gives a deterministic nature to our algorithm
while its efficiency is improved by a fast cooling schedule.
We show how this method can be used to separate efficiently the
regions of different solar activity giving a tool for a future
automated recognition and classification of sunspots
A spatial regularization method preserving local photometry for Richardson-Lucy restoration
In this article we give a simple method of spatial regularization
deriving from the Richardson-Lucy (RL) algorithm in order to overcome the problem
of noise amplification during the image reconstruction process.
It is very important in astronomy
to regularize images while controlling their photometric behavior.
We propose a new reconstruction method preserving
both the global and local image photometric properties.
A mathematical presentation
is described here. This method was applied to
an image of Titan -a satellite of Saturn -acquired in near infrared
with the adaptive optics system ADONIS installed at the ESO 3.6 m telescope
in La Silla (Chile). The local photometric results here are compared using
contours normalized at the same intensity levels
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