318 research outputs found
Optimal Computation of Overabundant Words
The observed frequency of the longest proper prefix, the longest proper suffix, and the longest infix of a word w in a given sequence x can be used for classifying w as avoided or overabundant. The definitions used for the expectation and deviation of w in this statistical model were described and biologically justified by Brendel et al. (J Biomol Struct Dyn 1986). We have very recently introduced a time-optimal algorithm for computing all avoided words of a given sequence over an integer alphabet (Algorithms Mol Biol 2017). In this article, we extend this study by presenting an O(n)-time and O(n)-space algorithm for computing all overabundant words in a sequence x of length n over an integer alphabet. Our main result is based on a new non-trivial combinatorial property of the suffix tree T of x: the number of distinct factors of x whose longest infix is the label of an explicit node of T is no more than 3n-4. We further show that the presented algorithm is time-optimal by proving that O(n) is a tight upper bound for the number of overabundant words. Finally, we present experimental results, using both synthetic and real data, which justify the effectiveness and efficiency of our approach in practical terms
Determining the spectroscopic mass ratio in interacting binaries: Application to X-Ray Nova Sco 1994
We present a model for determining the mass ratio in interacting binaries by
directly fitting the observed spectrum with synthetic spectra. We make direct
use of NextGen model atmospheres intensities which are the most comprehensive
and detailed models available for cool stars. We fully take into account the
varying temperature and gravity across the secondary star's photosphere, by
incorporating the synthetic spectra into the secondary star's Roche geometry.
As a result, we determine the exact rotationally broadened spectrum of the
secondary star and so eliminate the need for a limb-darkening law, and the
uncertainties associated with it. As an example we determine the mass ratio for
the well studied soft X-ray transient Nova Sco 1994. In order to obtain a more
accurate determination of the mass ratio, which does not depend on assumptions
about the rotation profile and limb-darkening coefficients, we use our model to
compute the exact rotationally broadened model spectrum, which we compare
directly with the observed intermediate resolution spectrum of Nova Sco 1994.
We determine the mass ratio of Nova Sco 1994 to be 0.419+/-0.028 (90 percent
confidence), which is the most accurate determination of the binary mass ratio
in an X-ray binary. This result combined with the binary mass function and
inclination angle gives a refined black hole mass of 5.99+\-0.42 Mo (90 percent
confidence). We also perform simulations which show that, for an F-type
secondary star, the standard rotation profile with zero and continuum value for
the line limb-darkening coefficient gives a value for q that brackets the value
found using the full geometrical treatment.Comment: 11 pages including 5 figures, accepted by MNRA
Optimal Computation of Avoided Words
The deviation of the observed frequency of a word from its expected
frequency in a given sequence is used to determine whether or not the word
is avoided. This concept is particularly useful in DNA linguistic analysis. The
value of the standard deviation of , denoted by , effectively
characterises the extent of a word by its edge contrast in the context in which
it occurs. A word of length is a -avoided word in if
, for a given threshold . Notice that such a word
may be completely absent from . Hence computing all such words na\"{\i}vely
can be a very time-consuming procedure, in particular for large . In this
article, we propose an -time and -space algorithm to compute all
-avoided words of length in a given sequence of length over a
fixed-sized alphabet. We also present a time-optimal -time and
-space algorithm to compute all -avoided words (of any
length) in a sequence of length over an alphabet of size .
Furthermore, we provide a tight asymptotic upper bound for the number of
-avoided words and the expected length of the longest one. We make
available an open-source implementation of our algorithm. Experimental results,
using both real and synthetic data, show the efficiency of our implementation
A Thousand and One Nova Outbursts
Multicycle nova evolution models have been calculated over the past twenty
years, the number being limited by numerical constraints. Here we present a
long-term evolution code that enables a continuous calculation through an
unlimited number of nova cycles for an unlimited evolution time, even up to (or
exceeding) a Hubble time. Starting with two sets of the three independent nova
parameters -- the white dwarf mass, the temperature of its isothermal core, and
the rate of mass transfer on to it -- we have followed the evolution of two
models, with initial masses of 1 and 0.65 solar masses, accretion rates
(constant throughout each calculation) of 1e-11 and 1e-9 solar-masses/yr, and
relatively high initial temperatures (as they are likely to be at the onset of
the outburst phase), through over 1000 and over 3000 cycles, respectively. The
results show that although on the short-term consecutive outbursts are almost
identical, on the long-term scale the characteristics change. This is mainly
due to the changing core temperature, which decreases very similarly to that of
a cooling white dwarf for a time, but at a slower rate thereafter. As the white
dwarf's mass continually decreases, since both models lose more mass than they
accrete, the central pressure decreases accordingly. The outbursts on the
massive white dwarf change gradually from fast to moderately fast, and the
other characteristics (velocity, abundance ratios, isotopic ratios) change,
too. Very slowly, a steady state is reached, where all characteristics, both in
quiescence and in outburst, remain almost constant. For the less massive white
dwarf accreting at a high rate, outbursts are similar throughout the evolution.Comment: To be published in MNRA
Chemical stratification in the atmosphere of Ap star HD 133792. Regularized solution of the vertical inversion problem
High spectral resolution studies of cool Ap stars reveal conspicuous
anomalies of the shape and strength of many absorption lines. This is a
signature of large atmospheric chemical gradients produced by the selective
radiative levitation and gravitational settling of chemical species. Here we
present a new approach to mapping the vertical chemical structures in stellar
atmospheres. We have developed a regularized chemical inversion procedure that
uses all information available in high-resolution stellar spectra. The new
technique for the first time allowed us to recover chemical profiles without
making a priori assumptions about the shape of chemical distributions. We have
derived average abundances and applied the vertical inversion procedure to the
high-resolution VLT UVES spectra of the weakly magnetic, cool Ap star HD
133792. Our analysis yielded improved estimates of the atmospheric parameters
of HD 133792. We show that this star has negligible vsini and the mean magnetic
field modulus =1.1+/-0.1 kG. We have derived average abundances for 43 ions
and obtained vertical distributions of Ca, Si, Mg, Fe, Cr, and Sr. All these
elements except Mg show high overabundance in the deep layers and solar or
sub-solar composition in the upper atmosphere of HD 133792. In contrast, the Mg
abundance increases with height. We find that transition from the
metal-enhanced to metal-depleted zones typically occurs in a rather narrow
range of depths in the atmosphere of HD 133792. Based on the derived
photospheric abundances, we conclude that HD 133792 belongs to the rare group
of evolved cool Ap stars, which possesses very large Fe-peak enhancement, but
lacks a prominent overabundance of the rare-earth elements.Comment: Accepted by A&A; 12 pages, 9 figure
Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
Aims. We aim to provide observational constraints on diffusion models that
predict peculiar chemical abundances in the atmospheres of Am stars. We also
intend to check if chemical peculiarities and slow rotation can be explained by
the presence of a weak magnetic field.
Methods. We have obtained high resolution, high signal-to-noise ratio spectra
of eight previously-classified Am stars, two normal A-type stars and one Blue
Straggler, considered to be members of the Praesepe cluster. For all of these
stars we have determined fundamental parameters and photospheric abundances for
a large number of chemical elements, with a higher precision than was ever
obtained before for this cluster. For seven of these stars we also obtained
spectra in circular polarization and applied the LSD technique to constrain the
longitudinal magnetic field.
Results. No magnetic field was detected in any of the analysed stars. HD
73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to
have the abundances of a normal A-type star. Am classification is not confirmed
for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry
that is in good agreement with the observations. There is a generally good
agreement between abundance predictions of diffusion models and values that we
have obtained for the remaining Am stars. However, the observed Na and S
abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively.
Li appears to be overabundant in three stars of our sample.Comment: Accepted for publication on A&
Constraining the solutions of an inverse method of stellar population synthesis
In three previous papers (Pelat 1997, 1998 and Moultaka & Pelat 2000), we set
out an inverse stellar population synthesis method which uses a database of
stellar spectra. Unlike other methods, this one provides a full knowledge of
all possible solutions as well as a good estimation of their stability;
moreover, it provides the unique approximate solution, when the problem is
overdetermined, using a rigorous minimization procedure. In Boisson et al.
(2000), this method has been applied to 10 active and 2 normal galaxies. In
this paper we analyse the results of the method after constraining the
solutions. Adding {\it a priori} physical conditions on the solutions
constitutes a good way to regularize the synthesis problem. As an illustration
we introduce physical constraints on the relative number of stars taking into
account our present knowledge of the initial mass function in galaxies. In
order to avoid biases on the solutions due to such constraints, we use
constraints involving only inequalities between the number of stars, after
dividing the H-R diagram into various groups of stellar masses. We discuss the
results for a well-known globular cluster of the galaxy M31 and discuss some of
the galaxies studied in Boisson et al. (2000). We find that, given the spectral
resolution and the spectral domain, the method is very stable according to such
constraints (i.e. the constrained solutions are almost the same as the
unconstrained one). However, an additional information can be derived about the
evolutionary stage of the last burst of star formation, but the precise age of
this particular burst seems to be questionable.Comment: Accepted in A&A. 15 pages, 5 figures and 6 table
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