930 research outputs found
Combining Spatial and Temporal Logics: Expressiveness vs. Complexity
In this paper, we construct and investigate a hierarchy of spatio-temporal
formalisms that result from various combinations of propositional spatial and
temporal logics such as the propositional temporal logic PTL, the spatial
logics RCC-8, BRCC-8, S4u and their fragments. The obtained results give a
clear picture of the trade-off between expressiveness and computational
realisability within the hierarchy. We demonstrate how different combining
principles as well as spatial and temporal primitives can produce NP-, PSPACE-,
EXPSPACE-, 2EXPSPACE-complete, and even undecidable spatio-temporal logics out
of components that are at most NP- or PSPACE-complete
The Chemical Composition of Cernis 52 (BD+31 640)
We present an abundance analysis of the star Cernis 52 in whose spectrum we
recently reported the napthalene cation in absorption at 6707.4 {\AA}. This
star is on a line of sight to the Perseus molecular complex. The analysis of
high-resolution spectra using a chi^2-minimization procedure and a grid of
synthetic spectra provides the stellar parameters and the abundances of O, Mg,
Si, S, Ca, and Fe. The stellar parameters of this star are found to be T_{eff}
= 8350 +- 200 K, logg= 4.2 +- 0.4 dex. We derived a metallicity of [Fe/H] =
-0.01 +- 0.15. These stellar parameters are consistent with a star of
\Msun in a pre-main-sequence evolutionary stage. The stellar spectrum is
significantly veiled in the spectral range 5150-6730 {\AA} up to almost 55 per
cent of the total flux at 5150 {\AA} and decreasing towards longer wavelengths.
Using Johnson-Cousins and 2MASS photometric data, we determine a distance to
Cernis 52 of 231 pc considering the error bars of the stellar
parameters. This determination places the star at a similar distance to the
young cluster IC 348. This together with its radial velocity, v_r=13.7+-1 km/s,
its proper motion and probable young age support Cernis 52 as a likely member
of IC 348. We determine a rotational velocity of v\sin i=65 +- 5 km/s for this
star. We confirm that the stellar resonance line of \ion{Li}{1} at 6707.8 {\AA}
is unable to fit the broad feature at 6707.4 {\AA}. This feature should have a
interstellar origin and could possibly form in the dark cloud L1470 surrounding
all the cluster IC 348 at about the same distance.Comment: Accepted for publication in The Astrophysical Journa
Automated spectroscopic abundances of A and F-type stars using echelle spectrographs I. Reduction of ELODIE spectra and method of abundance determination
This paper presents an automated method to determine detailed abundances for
A and F-type stars. This method is applied on spectra taken with the ELODIE
spectrograph. Since the standard reduction procedure of ELODIE is optimized to
obtain accurate radial velocities but not abundances, we present a more
appropriate reduction procedure based on IRAF. We describe an improvement of
the method of Hill & Landstreet (1993) for obtaining Vsini, microturbulence and
abundances by fitting a synthetic spectrum to the observed one. In particular,
the method of minimization is presented and tested with Vega and the Sun. We
show that it is possible, in the case of the Sun, to recover the abundances of
27 elements well within 0.1 dex of the commonly accepted values.Comment: 12 pages, 10 figures, accepted for publication in A&
Hot DQ White Dwarfs: Something Different
We present a detailed analysis of all the known Hot DQ white dwarfs in the
Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to
have carbon dominated atmospheres. Our spectroscopic and photometric analysis
reveals that these objects all have effective temperatures between ~18,000 and
24,000 K. The surface composition is found to be completely dominated by
carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or
determination of trace amount in a few cases). We find that the surface gravity
of all objects but one seems to be ''normal'' and around log g = 8.0 while one
is likely near log g = 9.0. The presence of a weak magnetic field is directly
detected by spectropolarimetry in one object and is suspected in two others. We
propose that these strange stars could be cooled down versions of the weird
PG1159 star H1504+65 and form a new family of hydrogen and helium deficient
objects following the post-AGB phase. Finally, we present the results of full
nonadiabatic calculations dedicated specifically to each of the Hot DQ that
show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity
variations. This result is in excellent agreement with recent observations by
Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among
6 Hot DQ white dwarfs surveyed in February 2008.Comment: 33 pages, 7 figures, accepted for publication in Ap
An Analysis of the Shapes of Interstellar Extinction Curves. VI. The Near-IR Extinction Law
We combine new HST/ACS observations and existing data to investigate the
wavelength dependence of NIR extinction. Previous studies suggest a power-law
form, with a "universal" value of the exponent, although some recent
observations indicate that significant sight line-to-sight line variability may
exist. We show that a power-law model provides an excellent fit to most NIR
extinction curves, but that the value of the power, beta, varies significantly
from sight line-to-sight line. Therefore, it seems that a "universal NIR
extinction law" is not possible. Instead, we find that as beta decreases, R(V)
[=A(V)/E(B-V)] tends to increase, suggesting that NIR extinction curves which
have been considered "peculiar" may, in fact, be typical for different R(V)
values. We show that the power law parameters can depend on the wavelength
interval used to derive them, with the beta increasing as longer wavelengths
are included. This result implies that extrapolating power law fits to
determine R(V) is unreliable. To avoid this problem, we adopt a different
functional form for NIR extinction. This new form mimics a power law whose
exponent increases with wavelength, has only 2 free parameters, can fit all of
our curves over a longer wavelength baseline and to higher precision, and
produces R(V) values which are consistent with independent estimates and
commonly used methods for estimating R(V). Furthermore, unlike the power law
model, it gives R(V)'s that are independent of the wavelength interval used to
derive them. It also suggests that the relation R(V) = -1.36 E(K-V)/E(B-V) -
0.79 can estimate R(V) to +/-0.12. Finally, we use model extinction curves to
show that our extinction curves are in accord with theoretical expectations.Comment: To appear in the Astrophysical Journa
Screened thermonuclear reactions and predictive stellar evolution of detached double-lined eclipsing binaries
The low energy fusion cross sections of charged-particle nuclear reactions
(and the respective reaction rates) in stellar plasmas are enhanced due to
plasma screening effects. We study the impact of those effects on predictive
stellar evolution simulations for detached double-lined eclipsing binaries. We
follow the evolution of binary systems (pre-main sequence or main sequence
stars) with precisely determined radii and masses from 1.1Mo to 23Mo (from
their birth until their present state). The results indicate that all the
discrepancies between the screened and unscreened models (in terms of
luminosity, stellar radius, and effective temperature) are within the
observational uncertainties. Moreover, no nucleosynthetic or compositional
variation was found due to screening corrections. Therefore all thermonuclear
screening effects on the charged-particle nuclear reactions that occur in the
binary stars considered in this work (from their birth until their present
state) can be totally disregarded. In other words, all relevant
charged-particle nuclear reactions can be safely assumed to take place in a
vacuum, thus simplifying and accelerating the simulation processes.Comment: 5 RevTex pages,no figures. Accepted for publication in Phys.Rev.
Binaries with total eclipses in the LMC: potential targets for spectroscopy
35 Eclipsing binaries presenting unambiguous total eclipses were selected
from a subsample of the list of Wyrzykowski et al. (2003). The photometric
elements are given for the I curve in DiA photometry, as well as approximate
Teff and masses of the components. The interest of these systems is stressed in
view of future spectroscopic observations.Comment: 4 pages, 1 figure; poster presented at the conference "Close binaries
in the 21st Century: new opportunities and challenges", Syros, 27-30 June
200
A detailed spectropolarimetric analysis of the planet hosting star WASP-12
The knowledge of accurate stellar parameters is paramount in several fields
of stellar astrophysics, particularly in the study of extrasolar planets, where
often the star is the only visible component and therefore used to infer the
planet's fundamental parameters. Another important aspect of the analysis of
planetary systems is the stellar activity and the possible star-planet
interaction. Here we present a self-consistent abundance analysis of the planet
hosting star WASP-12 and a high-precision search for a structured stellar
magnetic field on the basis of spectropolarimetric observations obtained with
the ESPaDOnS spectropolarimeter. Our results show that the star does not have a
structured magnetic field, and that the obtained fundamental parameters are in
good agreement with what was previously published. In addition we derive
improved constraints on the stellar age (1.0-2.65 Gyr), mass (1.23-1.49 M/M0),
and distance (295-465 pc). WASP-12 is an ideal object to look for pollution
signatures in the stellar atmosphere. We analyse the WASP-12 abundances as a
function of the condensation temperature and compare them with those published
by several other authors on planet hosting and non-planet hosting stars. We
find hints of atmospheric pollution in WASP-12's photosphere, but are unable to
reach firm conclusions with our present data. We conclude that a differential
analysis based on WASP-12 twins will probably clarify if an atmospheric
pollution is present, the nature of this pollution and its implications in the
planet formation and evolution. We attempt also the direct detection of the
circumstellar disk through infrared excess, but without success.Comment: 49 pages, 11 figures, Accepted for publication on Ap
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