386 research outputs found
Document Retrieval on Repetitive Collections
Document retrieval aims at finding the most important documents where a
pattern appears in a collection of strings. Traditional pattern-matching
techniques yield brute-force document retrieval solutions, which has motivated
the research on tailored indexes that offer near-optimal performance. However,
an experimental study establishing which alternatives are actually better than
brute force, and which perform best depending on the collection
characteristics, has not been carried out. In this paper we address this
shortcoming by exploring the relationship between the nature of the underlying
collection and the performance of current methods. Via extensive experiments we
show that established solutions are often beaten in practice by brute-force
alternatives. We also design new methods that offer superior time/space
trade-offs, particularly on repetitive collections.Comment: Accepted to ESA 2014. Implementation and experiments at
http://www.cs.helsinki.fi/group/suds/rlcsa
Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy
With the HDS (High Dispersion Spectrograph) on the Subaru telescope, we
obtained high resolution optical region spectra of three red giant stars (cos
4, cos 82, and cos 347) in the Ursa Minor dwarf spheriodal galaxy. Chemical
abundances in these stars have been analysed for 26 elements including alpha-,
iron-peak, and neutron capture elements. All three stars show low abundances of
alpha-elements (Mg, Si, and Ca) and two stars (cos 82 and cos 347) show high
abundance of Mn compared to Galactic halo stars of similar metallicity. One
star (cos 4) has been confirmed to be very metal deficient ([Fe/H]=-2.7) and
found to show anomalously low abundances of Mn, Cu, and Ba. In another star cos
82 ([Fe/H]=-1.5), we have found large excess of heavy neutron-capture elements
with the general abundance pattern similar to the scaled solar system r-process
abundance curve. These observational results are rather puzzling: low
abundances of alpha-elements and high abundance of Mn seem to sugggest a
significant contribution of SNe Ia at low metallicity, while there is no hint
of s-process (i.e., AGB stars) contribution even at [Fe/H]=-1.5, suggesting a
peculiar nucleosynthetic history of the UMi dSph galaxy.Comment: 26 pages with 12 figures, accepted to PAS
Chemical Abundances in the Secondary Star of the Black Hole Binary V4641 Sagittarii (SAX J1819.3-2525)
We report on detailed spectroscopic studies performed for the secondary star
in the black hole binary (micro-quasar) V4641 Sgr in order to examine its
surface chemical composition and to see if its surface shows any signature of
pollution by ejecta from a supernova explosion. High-resolution spectra of
V4641 Sgr observed in the quiescent state in the blue-visual region are
compared with those of the two bright well-studied B9 stars (14 Cyg and
Cap) observed with the same instrument. The effective temperature of V4641 Sgr
(10500 200 K) is estimated from the strengths of He~{\sc i} lines, while
its rotational velocity, sin (95 10 km s), is
estimated from the profile of the Mg~{\sc ii} line at 4481 \AA. We obtain
abundances of 10 elements and find definite over-abundances of N (by 0.8 dex or
more) and Na (by 0.8 dex) in V4641 Sgr. From line-by-line comparisons of eight
other elements (C, O, Mg, Al, Si, Ti, Cr, and Fe) between V4641 Sgr and the two
reference stars, we conclude that there is no apparent difference in the
abundances of these elements between V4641 Sgr and the two normal late B-type
stars, which have been reported to have solar abundances. An evolutionary model
of a massive close binary system has been constructed to explain the abundances
observed in V4641 Sgr. The model suggests that the progenitor of the black hole
forming supernova was as massive as ~ 35 Msun on the main-sequence and, after
becoming a ~ 10 Msun He star, underwent "dark" explosion which ejected only N
and Na-rich outer layer of the He star without radioactive Ni.Comment: 13 pages, 14 figures. Accepted for publication in the PASJ:
Publications of the Astronomical Society of Japa
Lanthanides and other spectral oddities in a Centauri
Context: There is considerable interest in the helium variable a Cen as a
bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce
III and other possible lanthanides in the spectrum the of hottest chemically
peculiar (CP) star in which these elements have been found. A {Kr II line
appears within a broad absorption which we suggest may be due to a high-level
transition in C II. Methods: Wavelengths and equivalent widths are measured on
high-resolution UVES spectra, analyzed, and their phase-variations
investigated. Results: New, robust identifications of Ce III and Kr II are
demonstrated. Nd III is likely present. A broad absorption near 4619[A] is
present at all phases of a Cen, and in some other early B stars. Conclusions:
The presence of lanthanides in a Cen strengthens the view that this star is a
significant link between the cooler CP stars and the hotter helium-peculiar
stars. Broad absorptions in a Cen are not well explained.Comment: Research Note accepted by Astronomy and Astrophysics; 4 pages, 4
Figs. 2 Table
Implementation of the Backlund transformations for the Ablowitz-Ladik hierarchy
The derivation of the Backlund transformations (BTs) is a standard problem of
the theory of the integrable systems. Here, I discuss the equations describing
the BTs for the Ablowitz-Ladik hierarchy (ALH), which have been already
obtained by several authors. The main aim of this work is to solve these
equations. This can be done in the framework of the so-called functional
representation of the ALH, when an infinite number of the evolutionary
equations are replaced, using the Miwa's shifts, with a few equations linking
tau-functions with different arguments. It is shown that starting from these
equations it is possible to obtain explicit solutions of the BT equations. In
other words, the main result of this work is a presentation of the discrete BTs
as a superposition of an infinite number of evolutionary flows of the
hierarchy. These results are used to derive the superposition formulae for the
BTs as well as pure soliton solutions.Comment: 20 page
An Improved Algorithm for Fast K-Word Proximity Search Based on Multi-Component Key Indexes
A search query consists of several words. In a proximity full-text search, we
want to find documents that contain these words near each other. This task
requires much time when the query consists of high-frequently occurring words.
If we cannot avoid this task by excluding high-frequently occurring words from
consideration by declaring them as stop words, then we can optimize our
solution by introducing additional indexes for faster execution. In a previous
work, we discussed how to decrease the search time with multi-component key
indexes. We had shown that additional indexes can be used to improve the
average query execution time up to 130 times if queries consisted of
high-frequently occurring words. In this paper, we present another search
algorithm that overcomes some limitations of our previous algorithm and
provides even more performance gain.
This is a pre-print of a contribution published in Arai K., Kapoor S., Bhatia
R. (eds) Intelligent Systems and Applications. IntelliSys 2020. Advances in
Intelligent Systems and Computing, vol 1251, published by Springer, Cham. The
final authenticated version is available online at:
https://doi.org/10.1007/978-3-030-55187-2_3
Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy
Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the
Sextans dwarf spheroidal galaxy are determined based on high resolution
spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph.
(1) The Fe abundances derived from the high resolution spectra are in good
agreement with the metallicity estimated from the Ca triplet lines in low
resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by
previous estimates from the Ca triplet, is confirmed by our measurements,
although we note that high resolution spectroscopy for a larger sample of stars
will be necessary to estimate the true fraction of stars with such low
metallicity. (2) While one object shows an overabundance of Mg (similar to
Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar
to the solar value. This is the first time that low Mg/Fe ratios at such low
metallicities have been found in a dwarf spheroidal galaxy. No evidence for
over-abundances of Ca and Ti are found in these five stars, though the
measurements for these elements are less certain. Possible mechanisms to
produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are
discussed. (3) Ba is under-abundant in four objects, while the remaining two
stars exhibit large and moderate excesses of this element. The abundance
distribution of Ba in this galaxy is similar to that in the Galactic halo,
indicating that the enrichment of heavy elements, probably by the r-process,
started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.Comment: 15 pages, 6 figures, 6 tables, A&A, in pres
A Keck/HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. I. Testing Giant Planet Formation and Migration Scenarios
We describe a high-precision Doppler search for giant planets orbiting a
well-defined sample of metal-poor dwarfs in the field. This experiment
constitutes a fundamental test of theoretical predictions which will help
discriminate between proposed giant planet formation and migration models. We
present here details on the survey as well as an overall assessment of the
quality of our measurements, making use of the results for the stars that show
no significant velocity variation.Comment: 25 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Abundance Analysis of Planetary Host Stars I. Differential Iron Abundances
We present atmospheric parameters and iron abundances derived from
high-resolution spectra for three samples of dwarf stars: stars which are known
to host close-in giant planets (CGP), stars for which radial velocity data
exclude the presence of a close-in giant planetary companion (no-CGP), as well
as a random sample of dwarfs with a spectral type and magnitude distribution
similar to that of the planetary host stars (control). All stars have been
observed with the same instrument and have been analyzed using the same model
atmospheres, atomic data and equivalent width modeling program. Abundances have
been derived differentially to the Sun, using a solar spectrum obtained with
Callisto as the reflector with the same instrumentation. We find that the iron
abundances of CGP dwarfs are on average by 0.22 dex greater than that of no-CGP
dwarfs. The iron abundance distributions of both the CGP and no-CGP dwarfs are
different than that of the control dwarfs, while the combined iron abundances
have a distribution which is very similar to that of the control dwarfs. All
four samples (CGP, no-CGP, combined, control) have different effective
temperature distributions. We show that metal enrichment occurs only for CGP
dwarfs with temperatures just below solar and approximately 300 K higher than
solar, whereas the abundance difference is insignificant at Teff around 6000 K.Comment: 52 pages (aastex 11pt, preprint style), including 17 figures and 13
tables; accepted for publication in AJ (scheduled for the October 2003 issue
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