VITESSES
RADIALES, ÉLÉMENTS ORBITAUX ET
MODÈLES
POUR 25 SYSTÈMES
DOUBLES

MAURICE IMBERT

Laboratoire
d’Astrophysique de Marseille

Observatoire
Astronomique de Marseille Provence

2
Place Le Verrier,
F-13248 Marseille Cedex 04, France

E-mail:
maurice.imbert@oamp.fr

*Abstract.* Spectroscopic
orbits of 25 binary stars, including 12 double-lined
systems, are obtained, mainly, from 1402 *Coravel*
radial velocity measurements.

For the four systems, HD20507, HD28033, HD175635
and HD201193, already studied by other authors, the new spectroscopic
orbits
are either better or

of quality comparable to that previously published. The
geometrical and physical parameters of these systems were estimated
with the
photometric available

data and parallaxes from Hipparcos. These data enabled us
to find a model compatible with the whole of the observations. Four
systems,
HD19926, HD57267,

HD128255 and HD175635 are composed of a cool giant and a hot
main sequence star. Most of the observations were performed at

Observatoire de
Haute-Provence (CNRS). The paper is based, in part, on observations
collected
at the European Southern Observatory, La Silla,

*Key
words:* Stars: binaries: spectroscopic −
Stars: individual: HD4134, HD5129,
HD16287, HD18328, HD19926, HD20507, HD21168, HD21312, HD28007,

HD28033, HD57267,
HD66553, HD90162, HD120510, HD128255, HD135100, HD142653, HD156432,
HD175635,
HD201193, HD217492, HD226099,

HD238932, BD+43°164.

LIGHT-CURVE
VARIABILITY
OF RZ CEPHEI.

ALEXANDRU POP, RODICA ROMAN

Astronomical
Institute of the

Astronomical
Observatory

Str.
Cireşilor 19, 400487

E-mail:
apop@academie.cj.edu.ro,
rroman@math.ubbcluj.ro

*Abstract*. Six data sets on
the RRc star RZ Cephei, published between 1959
and 1997, are analyzed via Fourier-decomposition

technique, and their
Fourier-type structural parameters are computed. The shape and the
amplitude of
the light curve seem to be

variable, while one of the comparison stars used (BD
+ 64°1699) is
suspected to be variable.

*Key
words:* variable stars – RR Lyrae stars –
light-curve variability.

LIGHT-CURVE VARIABILITY
OF RZ CEPHEI.

CLUJ-NAPOCA

ALEXANDRU POP, RODICA ROMAN

Astronomical
Institute of the

Astronomical
Observatory Cluj-Napoca

Str.
Cireşilor
19, 400487 Cluj-Napoca, Romania

E-mail:
apop@academie.cj.edu.ro,
rroman@math.ubbcluj.ro

*Abstract*. Fourteen data
sets consisting of 1694 observations on the RRc star
RZ Cephei, obtained between 1972 and 1975, are analysed

through Fourier
decomposition technique and their Fourier type structural parameters
were
computed. In spite of the quite high noise level,

the amplitude of the light
curve is confirmed to be variable. The hypothesis concerning the
variability of
the comparison star used (BD +64^{°}1699)

has to be further taken into consideration.

*Key
words*: variable stars – RR Lyrae stars –
light curve variability.

ANALYSIS OF THE FIRST GROUND-BASED OBSERVATIONS OF THE ECLIPSING BINARY HIP 12039 (V376 AND)

ALEXANDRU
DUMITRESCU^{1},
MARIAN DORU ŞURAN^{1}, HELEN
ROVITHIS-LIVANIOU^{2}, LUBOMIR ILIEV^{3}

^{1 }Astronomical
Institute of the

Str.
Cuţitul de
Argint 5, 040557 Bucharest, Romania

E-mail:
[alex,
suran]@aira.astro.ro

^{2 }University
of

Section of Astrophysics, Astronomy and Mechanics

Panepistimiopolis,
Zografos 15784, Athens, Greece

E-mail:
elivan@phys.uoa.gr

^{3 }Institute
of Astronomy of the

72
Tsarigradsko Shosse, BG-1784, Sofia, Bulgaria

E-mail:
liliev@astro.bas.bg

*Abstract*. We derive a
photometric solution for V376 And, which was
discovered by HIPPARCOS/TYCHO (HIP 12039), as a new

This was
achieved using the W-D code to analyze the first ground-based
photoelectric
observations of the system. The latter were carried out at

Rozhen observatories between 2000 and 2004. Moreover, the differences
between
our and Hipparcos light curves, suggest that the system is quite
active, as it

exhibits a pronounced O’Connell effect, too. So, a spot model was
applied to
explain the light curve behavior of V376 And.

*Key
words:*
photoelectric photometry – eclipsing variable stars – V376 And.

HOMOGENEOUS STELLAR MODEL IN POST-NEWTONIAN APPROXIMATION

VASILE URECHE

“Babeş-Bolyai” University, Faculty of Mathematics and Computer Science

Str.
M.
Kogălniceanu 1, Cluj-Napoca, Romania

E-mail:
vureche@math.ubbcluj.ro

*Abstract*. Expanding the
equation of Tolman-Oppenheimer-Volkoff for the
hydrostatic equilibrium of a relativistic star to first order in 1/*c*^{2},

the solution for the homogeneous model is obtained in analytical form.
At the
nuclear density kg m^{-3}, the
mass of the star will be 1.31 *M*_{☼}

and the radius reaches 14.6 km. These are the typical values for a
neutron
star.

*Key
words:* astrophysics − stellar models –
post-Newtonian approximation.

LINEAR STELLAR MODEL IN POST-NEWTONIAN APPROXIMATION

VASILE
URECHE^{ 1}, RODICA ROMAN^{ 2}

^{1 }“Babeş-Bolyai”
University, Faculty of Mathematics and Computer Science

Str.
M.
Kogălniceanu 1, Cluj-Napoca, Romania

E-mail:
vureche@math.ubbcluj.ro

^{2 }Astronomical
Institute of the

Astronomical
Observatory Cluj-Napoca

Str.
Cireşilor
19, 400487 Cluj-Napoca, Romania

E-mail:
rroman@math.ubb.cluj.ro

*Abstract*. Expanding the
equation of hydrostatic equilibrium for a
relativistic star to first order in 1/*c*^{2}, the linear
stellar
model is analyzed and

the solution is numerically obtained. Fig. 1 gives the
variation of the pressure for three values of *p*_{c}, while Figs. 2 and 3 give the
variation of

mass and radius with the central density ρ_{c}. For *p*_{c} *=* 0.1 and ρ_{c}
= 10^{18} kg m^{−3},
the mass of the star will be 1.6 *M _{☼}* and the radius
will reach

14.5 km. These results are in good agreement with the observational data.

*Key
words:* astrophysics – stellar models –
post-Newtonian approximation.

ON
THE ACCURACY OF
STÖRMER/VERLET METHOD AS
NUMERICAL INTEGRATOR
OF
THE *N*-BODY PROBLEM

CĂLIN-IOAN GHEORGHIU, ADRIAN CRISTIAN MUREŞAN

“Tiberiu
Popoviciu” Institute of Numerical Analysis

CP
68, 400110
Cluj-Napoca, Romania

E-mail:
[ghcalin,
acmuresan]@ictp.acad.ro

*Abstract*. The
Newton-Störmer/Verlet-leapfrog method (S/V) is a symplectic
and symmetric one of order two, which, when applied to separable

Hamiltonian
dynamical systems, becomes explicit and conserves quadratic first
integrals,
e.g., the angular momentum in the *N*-body
problem. As for

high accuracy long-time computations required by dynamical
systems coming from astronomy, the order two is too low, we consider
composed
S/V

methods up to order 10. Beyond all these remarkable qualities, our
numerical experiments on the outer as well as inner Solar system (*N*-body problem

for *N* larger than 5) show that, in practice, the
conservation of the
Hamiltonian of the flow, up to a prescribed order, still remains an
open task.
Specifically,

the order at which the Hamiltonian is conserved seems to be quasi
independent of the order of the composed method in spite of the fact
that the
compensated

summation, as a specific technique to reduce the round-off errors,
was used. In order to clear up this aspect we perform a round-off error
analysis of S/V

method as a one-step method applied to an arbitrary vector
field. Despite of the fact that we succeeded in improving the classical
round-off error estimates,

numerical experiments lead to the opinion that the
accumulation of round-off errors bears the main responsibility for the
lack of
accuracy in long time energy

conservation.

*Key
words:* *N*-body problem –
composed
Störmer/Verlet method.