259 research outputs found
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
A New Multiple Stellar System in the Solar Neighborhood
Adaptive optics corrected images obtained with the CIAO instrument at the
Subaru 8.2-meter telescope show the presence of two subarsecond companions to
the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a
single member of the IC 2391 supercluster. The two companions share the same
proper motion as the primary and are redder. Their projected separations from
the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated
masses for the two new companions depend strongly on the age of the system. For
the range of ages found in the literature for IC 2391 supercluster members
(from 35 Myr to 200 Myr), the expected masses range from 0.2 M to 0.4
M for the B component, and from 0.09 M to 0.22 M for
the C component. The determination of the dynamical mass of the faintest
component of GJ 900 will yield the age of the system using theoretical
evolutionary tracks. The apparent separations of the GJ 900 system components
meet the observational criterion for an unstable Trapezium-type system, but
this could be a projection effect. Further observations are needed to establish
the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003
Analysis of roles and groups in blogosphere
In the paper different roles of users in social media, taking into
consideration their strength of influence and different degrees of
cooperativeness, are introduced. Such identified roles are used for the
analysis of characteristics of groups of strongly connected entities. The
different classes of groups, considering the distribution of roles of users
belonging to them, are presented and discussed.Comment: 8th International Conference on Computer Recognition Systems, CORES
201
Solar Ellerman Bombs in 1D Radiative Hydrodynamics
Recent observations from the Interface Region Imaging Spectrograph (IRIS)
appear to show impulsive brightenings in high temperature lines, which when
combined with simultaneous ground based observations in H, appear
co-spatial to Ellerman Bombs (EBs). We use the RADYN 1-dimensional radiative
transfer code in an attempt to try and reproduce the observed line profiles and
simulate the atmospheric conditions of these events. Combined with the MULTI/RH
line synthesis codes, we compute the H, Ca II 8542~\AA, and Mg II h \&
k lines for these simulated events and compare them to previous observations.
Our findings hint that the presence of superheated regions in the photosphere
(10,000 K) is not a plausible explanation for the production of EB
signatures. While we are able to recreate EB-like line profiles in H,
Ca II 8542~\AA, and Mg II h \& k, we cannot achieve agreement with all of these
simultaneously.Comment: Accepted into ApJL. 4 Figures, 1 Tabl
Intensity enhancement of O VI ultraviolet emission lines in solar spectra due to opacity
Opacity is a property of many plasmas, and it is normally expected that if an
emission line in a plasma becomes optically thick, its intensity ratio to that
of another transition that remains optically thin should decrease. However,
radiative transfer calculations undertaken both by ourselves and others predict
that under certain conditions the intensity ratio of an optically thick to thin
line can show an increase over the optically thin value, indicating an
enhancement in the former. These conditions include the geometry of the
emitting plasma and its orientation to the observer. A similar effect can take
place between lines of differing optical depth. Previous observational studies
have focused on stellar point sources, and here we investigate the
spatially-resolved solar atmosphere using measurements of the I(1032 A)/I(1038
A) intensity ratio of O VI in several regions obtained with the Solar
Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the
Solar and Heliospheric Observatory (SoHO) satellite. We find several I(1032
A)/I(1038 A) ratios observed on the disk to be significantly larger than the
optically thin value of 2.0, providing the first detection (to our knowledge)
of intensity enhancement in the ratio arising from opacity effects in the solar
atmosphere. Agreement between observation and theory is excellent, and confirms
that the O VI emission originates from a slab-like geometry in the solar
atmosphere, rather than from cylindrical structures.Comment: 17 pages, 4 figures, ApJ Letters, in pres
Methinks: Enabling Sophisticated Comment Management in the Social Web
User reviews, comments and votes on the Social Web form the modern version of word-of-mouth
communication, which has a huge impact on people’s shopping habits, businesses and the overall market.
Despite that, systems have so far limited practical success in helping consumers and businesses analysing,
managing and understanding Social Web content. In this paper, we present a new tool that leverages a
combination of techniques from Semantic Web, Computational Argumentation and Crowdsourcing to support
this activity, through an intuitive and functional user interface
Intermittent prophylactic antibiotics for bronchiectasis
This is a protocol for a Cochrane Review (Intervention). The objectives are as follows: To evaluate the safety and efficacy of intermittent prophylactic antibiotics in the treatment of adults and children with bronchiectasis
Are superflares on solar analogues caused by extra-solar planets?
Stellar flares with times more energy than the largest solar
flare have been detected from 9 normal F and G main sequence stars (Schaefer,
King & Deliyannis 1999). These superflares have durations of hours to days and
are visible from at least x-ray to optical frequencies. The absence of
world-spanning aurorae in historical records and of anomalous extinctions in
the geological record indicate that our Sun likely does not suffer superflares.
In seeking to explain this new phenomenon, we are struck by its similarity to
large stellar flares on RS Canum Venaticorum binary systems, which are caused
by magnetic reconnection events associated with the tangling of magnetic fields
between the two stars. The superflare stars are certainly not of this class,
although we propose a similar flare mechanism. That is, superflares are caused
by magnetic reconnection between fields of the primary star and a close-in
Jovian planet. Thus, by only invoking known planetary properties and
reconnection scenarios, we can explain the energies, durations, and spectra of
superflares, as well as explain why our Sun does not have such events.Comment: 13 pages, Accepted for publication in Ap
The Active Corona of HD 35850 (F8 V)
We present Extreme Ultraviolet Explorer spectroscopy and photometry of the
nearby F8 V star HD 35850 (HR 1817). The EUVE spectra reveal 28 emission lines
from Fe IX and Fe XV to Fe XXIV. The Fe XXI 102, 129 A ratio yields an upper
limit for the coronal electron density, log n < 11.6 per cc. The EUVE SW
spectrum shows a small but clearly detectable continuum. The line-to-continuum
ratio indicates approximately solar Fe abundances, 0.8 < Z < 1.6. The resulting
emission-measure distribution is characterized by two temperature components at
log T of 6.8 and 7.4. The EUVE spectra have been compared with non-simultaneous
ASCA SIS spectra of HD 35850. The SIS spectrum shows the same temperature
distribution as the EUVE DEM analysis. However, the SIS spectral firs suggest
sub-solar abundances, 0.34 < Z < 0.81. Although some of the discrepancy may be
the result of incomplete X-ray line lists, we cannot explain the disagreement
between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance.
Given its youth (t ~ 100 Myr), its rapid rotation (v sin i ~ 50 km/s), and its
high X-ray activity (Lx ~ 1.5E+30 ergs/s), HD 35850 may represent an activity
extremum for single, main-sequence F-type stars. The variability and EM
distribution can be reconstructed using the continuous flaring model of Guedel
provided that the flare distribution has a power-law index of 1.8. Similar
results obtained for other young solar analogs suggest that continuous flaring
is a viable coronal heating mechanism on rapidly rotating, late-type,
main-sequence stars.Comment: 32 pages incl. 14 figures and 3 tables. To appear in the 1999 April
10 issue of The Astrophysical Journa
The AU Microscopii Debris Disk: Multiwavelength Imaging and Modeling
(abridged) Debris disks around main sequence stars are produced by the
erosion and evaporation of unseen parent bodies. AU Microscopii (GJ 803) is a
compelling object to study in the context of disk evolution across different
spectral types, as it is an M dwarf whose near edge-on disk may be directly
compared to that of its A5V sibling beta Pic. We resolve the disk from 8-60 AU
in the near-IR JHK' bands at high resolution with the Keck II telescope and
adaptive optics, and develop a novel data reduction technique for the removal
of the stellar point spread function. The point source detection sensitivity in
the disk midplane is more than a magnitude less sensitive than regions away
from the disk for some radii. We measure a blue color across the near-IR bands,
and confirm the presence of substructure in the inner disk. Some of the
structural features exhibit wavelength-dependent positions. The disk
architecture and characteristics of grain composition are inferred through
modeling. We approach the modeling of the dust distribution in a manner that
complements previous work. Using a Monte Carlo radiative transfer code, we
compare a relatively simple model of the distribution of porous grains to a
broad data set, simultaneously fitting to midplane surface brightness profiles
and the spectral energy distribution. Our model confirms that the large-scale
architecture of the disk is consistent with detailed models of steady-state
grain dynamics. Here, a belt of parent bodies from 35-40 AU is responsible for
producing dust that is then swept outward by the stellar wind and radiation
pressures. We infer the presence of very small grains in the outer region, down
to sizes of ~0.05 micron. These sizes are consistent with stellar mass-loss
rates Mdot_* << 10^2 Mdot_sun.Comment: ApJ accepted, 56 pages, preprint style. Version in emulateapj with
high-resolution figures available at http://tinyurl.com/y6ent
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