542 research outputs found
Cross-Document Pattern Matching
We study a new variant of the string matching problem called cross-document
string matching, which is the problem of indexing a collection of documents to
support an efficient search for a pattern in a selected document, where the
pattern itself is a substring of another document. Several variants of this
problem are considered, and efficient linear-space solutions are proposed with
query time bounds that either do not depend at all on the pattern size or
depend on it in a very limited way (doubly logarithmic). As a side result, we
propose an improved solution to the weighted level ancestor problem
Efficient Seeds Computation Revisited
The notion of the cover is a generalization of a period of a string, and
there are linear time algorithms for finding the shortest cover. The seed is a
more complicated generalization of periodicity, it is a cover of a superstring
of a given string, and the shortest seed problem is of much higher algorithmic
difficulty. The problem is not well understood, no linear time algorithm is
known. In the paper we give linear time algorithms for some of its versions ---
computing shortest left-seed array, longest left-seed array and checking for
seeds of a given length. The algorithm for the last problem is used to compute
the seed array of a string (i.e., the shortest seeds for all the prefixes of
the string) in time. We describe also a simpler alternative algorithm
computing efficiently the shortest seeds. As a by-product we obtain an
time algorithm checking if the shortest seed has length at
least and finding the corresponding seed. We also correct some important
details missing in the previously known shortest-seed algorithm (Iliopoulos et
al., 1996).Comment: 14 pages, accepted to CPM 201
The spectroscopic evolution of the recurrent nova T Pyxidis during its 2011 outburst I. The optically thick phase and the origin of moving lines in novae
The nova T Pyx was observed with high resolution spectroscopy (R ~ 65000)
spectroscopy, beginning 1 day after discovery of the outburst and continuing
through the last visibility of the star at the end of May 2011. The
interstellar absorption lines of Na I, Ca II, CH, CH, and archival H I 21
cm emission line observations have been used to determine a kinematic distance.
Interstellar diffuse absorption features have been used to determine the
extinction independent of previous assumptions. Sample Fe-peak line profiles
show the optical depth and radial velocity evolution of the discrete
components. We propose a distance to T Pyx 4.5kpc, with a strict lower
limit of 3.5 kpc (the previously accepted distance). We derive an extinction,
E(B-V)0.1, that is higher than previous estimates. The first
observation, Apr. 15, displayed He I, He II, C III, and N III emission lines
and a maximum velocity on P Cyg profiles of the Balmer and He I lines of
2500 km s characteristic of the fireball stage. These ions were
undetectable in the second spectrum, Apr. 23, and we use the recombination time
to estimate the mass of the ejecta, M for a filling factor
. Numerous absorption line systems were detected on the Balmer, Fe-peak, Ca
II, and Na I lines, mirrored in broader emission line components, that showed
an "accelerated" displacement in velocity. We also show that the time sequence
of these absorptions, which are common to all lines and arise only in the
ejecta, can be described by recombination front moving outward in the expanding
gas without either a stellar wind or circumstellar collisions.Comment: Accepted for publication in Astronomy & Astrophysics Letters
(17/8/11
Cooler and bigger than thought? Planetary host stellar parameters from the InfraRed Flux Method
Effective temperatures and radii for 92 planet-hosting stars as determined
from the InfraRed Flux Method (IRFM) are presented and compared with those
given by other authors using different approaches. The IRFM temperatures we
have derived are systematically lower than those determined from the
spectroscopic condition of excitation equilibrium, the mean difference being as
large as 110 K. They are, however, consistent with previous IRFM studies and
with the colors derived from Kurucz and MARCS model atmospheres. Comparison
with direct measurements of stellar diameters for 7 dwarf stars, which
approximately cover the range of temperatures of the planet-hosting stars,
suggest that the IRFM radii and temperatures are reliable in an absolute scale.
A better understanding of the fundamental properties of the stars with planets
will be achieved once this discrepancy between the IRFM and the spectroscopic
temperature scales is resolved.Comment: 15 pages, 4 figures. Accepted for publication in Ap
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
Supernova type Ia luminosities, their dependence on second parameters, and the value of H_0
A sample of 35 SNe Ia with good to excellent photometry in B and V, minimum
internal absorption, and 1200 < v < \approx 30000 km/s is compiled from the
literature. As far as their spectra are known they are all Branch-normal. For
29 of the SNe Ia also peak magnitudes in I are known. The SNe Ia have uniform
colors at maximum, i.e. =-0.012 mag (sigma=0.051) and =-0.276 mag
(sigma=0.078). In the Hubble diagram they define a Hubble line with a scatter
of =0.21-0.16 mag, decreasing with wavelength. The scatter is further
reduced if the SNe Ia are corrected for differences in decline rate Delta_m_15
or color (B-V). A combined correction reduces the scatter to sigma<=0.13 mag.
After the correction no significant dependence remains on Hubble type or
galactocentric distance. The Hubble line suggests some curvature which can be
differently interpreted. A consistent solution is obtained for a cosmological
model with Omega_M=0.3, Omega_Lambda=0.7, which is indicated also by much more
distant SNe Ia. Absolute magnitudes are available for eight equally blue
(Branch-normal) SNe Ia in spirals, whose Cepheid distances are known. If their
well defined mean values of M_B, M_V, and M_I are used to fit the Hubble line
to the above sample of SNe Ia one obtains H_0=58.3 km/s/Mpc, or, after
adjusting all SNe Ia to the average values of Delta_m_15 and (B-V), H_0=60.9
km/s/Mpc. Various systematic errors are discussed whose elimination tends to
decrease H_0. The finally adopted value at the 90-percent level, including
random and systematic errors, is H_0=58.5 +/- 6.3 km/s/Mpc. Several higher
values of H_0 from SNe Ia, as suggested in the literature, are found to depend
on large corrections for variations of the light curve parameter and/or on an
unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.Comment: 42 pages, including 9 figures; submitted to Ap
Five-Year Optical and Near Infrared Observations of the Extremely Slow Nova V1280 Scorpii
We present optical (, , , and ) and near
infrared (, and ) photometric and spectroscopic observations
of a classical nova V1280 Scorpii for five years from 2007 to 2011. Our
photometric observations show a declining event in optical bands shortly after
the maximum light which continues 250 days. The event is most probably
caused by a dust formation. The event is accompanied by a short ( 30
days) re-brightening episode ( 2.5 mag in ), which suggests a
re-ignition of the surface nuclear burning. After 2008, the band
observations show a very long plateau at around = 10.5 for more than 1000
days until April 2011 ( 1500 days after the maximum light). The nova had
taken a very long time ( 50 months) before entering the nebular phase
(clear detection of both [\ion{O}{iii}] 4959 and 5007) and is still continuing
to generate the wind caused by H-burning. The finding suggests that V1280 Sco
is going through the historically slowest evolution. The interval from the
maximum light (2007 February 16) to the beginning of the nebular phase is
longer than any previously known slow novae: V723 Cas (18 months), RR Pic (10
months), or HR Del (8 months). It suggests that the mass of a white dwarf in
the V1280 Sco system might be 0.6 M_\mathrm{\sun} or smaller. The distance,
based on our measurements of the expansion velocity combined with the directly
measured size of the dust shell, is estimated to be 1.1 0.5 kpc.Comment: 17 pages, 14 figures, accepted for publication in A&
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
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