53 research outputs found
The United Nations Basic Space Science Initiative: The TRIPOD concept
Since 1990, the United Nations is annually holding a workshop on basic space
science for the benefit of the worldwide development of astronomy. Additional
to the scientific benefits of the workshops and the strengthening of
international cooperation, the workshops lead to the establishment of
astronomical telescope facilities through the Official Development Assistance
(ODA) of Japan. Teaching material, hands-on astrophysics material, and variable
star observing programmes had been developed for the operation of such
astronomical telescope facilities in an university environment. This approach
to astronomical telescope facility, observing programme, and teaching astronomy
has become known as the basic space science TRIPOD concept. Currently, a
similar TRIPOD concept is being developed for the International Heliophysical
Year 2007, consisting of an instrument array, data taking and analysis, and
teaching space science.Comment: 8 pages, LaTe
Decoding of the light changes in eclipsing Wolf-Rayet binaries I. A non-classical approach to the solution of light curves
We present a technique to determine the orbital and physical parameters of
eclipsing eccentric Wolf-Rayet + O-star binaries, where one eclipse is produced
by the absorption of the O-star light by the stellar wind of the W-R star. Our
method is based on the use of the empirical moments of the light curve that are
integral transforms evaluated from the observed light curves. The optical depth
along the line of sight and the limb darkening of the W-R star are modelled by
simple mathematical functions, and we derive analytical expressions for the
moments of the light curve as a function of the orbital parameters and the key
parameters of the transparency and limb-darkening functions. These analytical
expressions are then inverted in order to derive the values of the orbital
inclination, the stellar radii, the fractional luminosities, and the parameters
of the wind transparency and limb-darkening laws. The method is applied to the
SMC W-R eclipsing binary HD 5980, a remarkable object that underwent an
LBV-like event in August 1994. The analysis refers to the pre-outburst
observational data. A synthetic light curve based on the elements derived for
the system allows a quality assessment of the results obtained.Comment: Accepted for publication in Astronomy & Astrophysic
Eclipsed X-ray flares in binary stars: geometrical constraints on the flare's location and size
The observation of eclipses during X-rays flares taking place in active cool
stars binaries allows us to calculate the position and size of the flares. This
information cannot be derived by analyzing the decay of the flares, a
frequently used approach in the literature that requires the assumption of a
physical model. We make use of the eclipsing light curve to constrain the set
of possible solutions, from the geometrical point of view, in two flares of
Algol, and one flare in VW Cep. We make use of a technique developed with the
system SV Cam (i~90 deg) and generalize it to binary systems with arbitrary
inclination. The method simulates all possible geometrical situations that can
produce the times of the four contacts of the eclipse. As an approximation we
assume that the emitting region has a spherical shape that remains unchanged
during the eclipse. We however show that this is a good approximation for the
problem. The solutions observed indicate that in two of the three cases the
flare cannot be polar (lat<55 deg) and in a third one the flare can be placed
either near the pole or at other latitudes. The emitting regions must have a
small size (0.002-0.5 R_*), but if interpreted as the apex of coronal loops,
their length could actually be up to 3.1 R_* for one of the Algol flares. These
measurements imply a lower limit to the electron density in the emitting region
between log n_e(cm^-3) 10.4 and 14.0, and a magnetic field between 70 and 3500
G. Similar results are found if the emitting region is assumed to be
loop-shaped.Comment: 8 pages (17 in online version), 9 figures (18 in online version).
Figure 12 is a set of animated GIF, available also at
http://laeff.esa.es/users/jsanz/Papers/Eclipses/Videosweb . Accepted for
publication in A&
Rotational Broadening and Doppler Tomography of the Quiescent X-Ray Nova Centaurus X-4
We present high and intermediate resolution spectroscopy of the X-ray nova
Centaurus X-4 during its quiescent phase. Our analysis of the absorption
features supports a K3-K5V spectral classification for the companion star,
which contributes approximately 75 % of the total flux at Halpha. Using the
high resolution spectra we have measured the secondary star's rotational
broadening to be V_rot*sin(i) = 43 +/- 6 km/s and determined a binary mass
ratio of q=0.17 +/- 0.06. Combining our results for K_2 and q with the
published limits for the binary inclination, we constrain the mass of the
compact object and the secondary star to the ranges 0.49 < M_1 < 2.49 Msun and
0.04 < M_2 < 0.58 Msun. A Doppler image of the Halpha line shows emission
coming from the secondary star, but no hotspot is present. We discuss the
possible origins of this emission.Comment: 10 pages, 4 figures, accepted by MNRA
Genetic-Algorithm-based Light Curve Optimization Applied to Observations of the W UMa star BH Cas
I have developed a procedure utilizing a Genetic-Algorithm-based optimization
scheme to fit the observed light curves of an eclipsing binary star with a
model produced by the Wilson-Devinney code. The principal advantages of this
approach are the global search capability and the objectivity of the final
result. Although this method can be more efficient than some other comparably
global search techniques, the computational requirements of the code are still
considerable. I have applied this fitting procedure to my observations of the W
UMa type eclipsing binary BH Cassiopeiae. An analysis of V-band CCD data
obtained in 1994/95 from Steward Observatory and U- and B-band photoelectric
data obtained in 1996 from McDonald Observatory provided three complete light
curves to constrain the fit. In addition, radial velocity curves obtained in
1997 from McDonald Observatory provided a direct measurement of the system mass
ratio to restrict the search. The results of the GA-based fit are in excellent
agreement with the final orbital solution obtained with the standard
differential corrections procedure in the Wilson-Devinney code.Comment: 9 pages, 2 figures, 2 tables, uses emulateapj.st
The ``Outside-In'' Outburst of HT Cassiopeiae
We present results from photometric observations of the dwarf nova system HT
Cas during the eruption of November 1995. The data include the first
two--colour observations of an eclipse on the rise to outburst. They show that
during the rise to outburst the disc deviates significantly from steady state
models, but the inclusion of an inner-disc truncation radius of about 4
and a ``flared'' disc of semi-opening angle of produces
acceptable fits. The disc is found to have expanded at the start of the
outburst to about , as compared to quiescent measurements. The
accretion disc then gradually decreases in radius reaching during
the last stages of the eruption. Quiescent eclipses were also observed prior to
and after the eruption and a revised ephemeris is calculated.Comment: 9 pages, 11 figures, to appear in MNRA
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
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