51 research outputs found
The quest for companions to post-common envelope binaries: III. A reexamination of HW Virginis
We report new mid-eclipse times of the short-period sdB/dM binary HW Vir,
which differ substantially from the times predicted by a previous model. The
proposed orbits of the two planets in that model are found to be unstable. We
present a new secularly stable solution, which involves two companions orbiting
HW VIr with periods of 12.7 yr and 55 +/-15 yr. For orbits coplanar with the
binary, the inner companion is a giant planet with mass M_3 sin i_3 = 14 M_Jup
and the outer one a brown dwarf or low-mass star with a mass of M_4 sin i_4 =
30-120 M_Jup. Using the mercury6 code, we find that such a system would be
stable over more than 10^7 yr, in spite of the sizeable interaction. Our model
fits the observed eclipse-time variations by the light-travel time effect
alone, without invoking any additional process, thereby providing support for
the planetary hypothesis of the eclipse-time variations in close binaries. The
signature of non-Keplerian orbits may be visible in the data.Comment: accepted by A&
The most plausible explanation of the cyclical period changes in close binaries: the case of the RS CVn-type binary WW Dra
We searched the orbital period changes in 182 EA-type (including the 101
Algol systems used by \cite{hal89}), 43 EB-type and 53 EW-type binaries with
known both the mass ratio and the spectral type of their secondary components.
We reproduced and improved the same diagram as Hall's (1989) according to the
new collected data. Our plots do not support the conclusion derived by
\cite{hal89} that all cases of cyclical period changes are restricted to
binaries having the secondary component with spectral types later than F5. The
presence of period changes also among stars with secondary component of early
type indicates that the magnetic activity is one cause, but not the only one,
for the period variation. It is discovered that cyclic period changes, likely
due to the presence of a third body are more frequent in EW-type binaries among
close binaries. Therefore, the most plausible explanation of the cyclical
period changes is the LTTE via the presence of a third body. By using the
century-long historical record of the times of light minimum, we analyzed the
cyclical period change in the Algol binary WW Dra. It is found that the orbital
period of the binary shows a cyclic variation
with an amplitude of . The cyclic oscillation
can be attributed to the LTTE via a third body with a mass no less than . However, no spectral lines of the third body were discovered
indicating that it may be a candidate black hole. The third body is orbiting
the binary at a distance shorter than 14.4 AU and it may play an important role
in the evolution of this system.Comment: 9 pages, 5 figures, published by MNRA
A model of AW UMa
The contact binary AW UMa has an extreme mass ratio, with the more massive
component (the current primary) close to the main sequence, while the low mass
star at q ~ 0.1 (the current secondary) has a much larger radius than a main
sequence star of a comparable mass. We propose that the current secondary has
almost exhausted hydrogen in its center and is much more advanced in its
evolution, as suggested by Stepien. Presumably the current secondary lost most
of its mass during its evolution with part of it transferred to the current
primary. After losing a large fraction of its angular momentum, the binary may
evolve into a system of FK Com type.Comment: 5 pages, 6 figures, Accepted to MNRAS, content change
Carbon deficiencies in the primaries of some classical Algols
The equivalent widths of C II 4267 \AA line were measured for the
mass-gaining primary stars of the 18 Algol-type binary systems. The comparison
of the EWs of the gainers with those of the single standard stars having the
same effective temperature and luminosity class clearly indicates that they are
systematically smaller than those of the standard stars. The primary components
of the classical Algols, located in the main-sequence band of the HR diagram,
appear to be C poor stars. We estimate relative to the Sun
as -1.91 for GT Cep, -1.88 for AU Mon and -1.41 for TU Mon, indicating poorer C
abundance. An average differential carbon abundance has been estimated to be
-0.82 dex relative to the Sun and -0.54 dex relative to the main-sequence
standard stars. This result is taken to be an indication of the transferring
material from the evolved less-massive secondary components to the gainers such
that the CNO cycle processed material changed the original abundance of the
gainers. There appear to be relationships between the EWs of C II
4267 \AA line and the rates orbital period increase and mass transfer in some
Algols. As the mass transfer rate increases the EW of the C II line decreases,
which indicates that accreted material has not been completely mixed yet in the
surface layers of the gainers. This result supports the idea of mixing as an
efficient process to remove the abundance anomaly built up by accretion.
Chemical evolution of the classical Algol-type systems may lead to constrains
on the initial masses of the less massive, evolved, mass-losing stars.Comment: 10 pages, 4 figures, accepted in MNRA
The eclipsing LMC star OGLE05155332--6925581: a clue for Double Periodic Variables
We investigate the nature of OGLE05155332-6925581, one of the brightest
members of the enigmatic group of Double Periodic Variables (DPVs) recently
found in the Magellanic Clouds. The modeling of archival orbital light curves
(LCs), along with the analysis of the radial velocities suggest that this
object is a semi--detached binary with the less massive star transferring
matter to the more massive and less evolved star, in an Algol--like
configuration. We find evidence for additional orbital variability and
H emission, likely caused by an accretion disc around the primary star.
As in the case of the circumprimary disc seems to be more luminous
than the primary, but we do not detect orbital period changes. We find that the
LC follows a loop in the color--magnitude diagram during the long cycle; the
system is redder when brighter and the rising phase is bluer than during
decline. Infrared excess is also present. The source of the long--term
periodicity is not eclipsed, indicating its circumbinary origin. Strong
asymmetries, discrete absorption components (DACs) and a shift are new
and essential observational properties in the infrared H I lines. The DACs
strength and RV follow a saw--teeth pattern during the orbital cycle. We
suggest that the system experiences supercycles of mass outflow feeding a
circumbinary disc. Mass exchange and mass loss could produce comparable but
opposite effects in the orbital period on a long time scale, resulting in a
quasi--constancy of this parameter.Comment: submitted to MNRA
The solar-type eclipsing binary system LL Aquarii
The eclipsing binary LL Aqr consists of two late-type stars in an eccentric
orbit with a period of 20.17 d. We use an extensive light curve from the
SuperWASP survey augmented by published radial velocities and UBV light curves
to measure the physical properties of the system. The primary star has a mass
of 1.167 +/- 0.009 Msun and a radius of 1.305 +/- 0.007 Rsun. The secondary
star is an analogue of the Sun, with a mass and radius of 1.014 +/- 0.006 Msun
and 0.990 +/- 0.008 Rsun respectively. The system shows no signs of stellar
activity: the upper limit on spot-induced rotational modulation is 3 mmag, it
is slowly rotating, has not been detected at X-ray wavelengths, and the calcium
H and K lines exhibit no emission. Theoretical stellar models provide a good
match to its properties for a sub-solar metal abundance of Z = 0.008 and an age
of 2.5 Gyr. Most low-mass eclipsing binary systems are found to have radii
larger than expected from theoretical predictions, blamed on tidally-enhanced
magnetic fields in these short-period systems. The properties of LL Aqr support
this scenario: it exhibits negligible tidal effects, shows no signs of magnetic
activity, and matches theoretical models well.Comment: Accepted for publication in A&A. 8 pages, 5 figures (3 reduced in
size), 7 tables. A full-size preprint, the data, and a catalogue of
well-studied detached eclipsing binary star systems can found at
http://www.astro.keele.ac.uk/jkt
Long-term luminosity variations and period changes in CG Cygni
We report new photometry of CG Cyg from 1998–2002.
We also analyze published photometry collected since 1965. An O–C analysis
reveals evidence of a third body in the system having a 51 year orbital
period. After removing the effects of the third body, we note a remaining 22.5 year cyclic variation in the O–C curve that is similar to the period of the
luminosity variations. Modeling the spot activity for 28Â VÂ band light curves
obtained between 1965 and 2002 confirms the presence of two active longitude
belts (ALBs) at approximately and . Most of the spots occur at mid
latitudes with a few at low latitudes. We note no spots at latitudes greater
thanÂ
H
We present the results of spectroscopic observations of
the active eclipsing binary ER Vul in the 5860–6700 Å spectral range
carried out at the Catania Astrophysical Observatory.
Accurate measurements of radial velocities by using the cross-correlation
technique have been made and a new orbital solution is given. All
spectra display chromospheric emission which fills in the Hα absorption
lines of both components.
The equivalent width (EW) of the Hα emission has been measured by subtraction
of a synthetic spectrum built up with spectra of inactive
standard stars. The EW of total Hα emission, arisen from
both components, shows a phase-dependent variation.
It reaches its maximum value just before the primary eclipse.
We were able to separate the contributions to the Hα emission from
the individual components at phases far from the eclipses.
We have found that the secondary, cooler component is the most active and its
Hα residual emission shows the same trend as
the total Hα emission, along orbital phase.
This Hα equivalent width variation
can be due to plage-like structures on the chromosphere of secondary star.
In addition, a decrease of the Hα EW around the ingress phase of the
primary eclipse is apparent
High-speed photometry of the pre-cataclysmic binary HWÂ Virginis and its orbital period change
The broad band B and V light curves of the pre-cataclysmic eclipsing binary HW Vir were
obtained. All the available eclipse timings, including the new ones, spanning 19Â years were
analyzed under the third-body hypothesis. The residuals between the observed and calculated times of
mid-eclipse show a long-term sinusoidal variation. The analysis yields the parameters of the third-body
orbit, as well as limiting mass for the tertiary object. The result of this analysis gives
a light-time semi-amplitude of 112 s, an orbital period of 18.8 yr and an eccentricity of 0.12.
The mass of the third star is below the theoretical threshold of 0.07 for a hydrogen burning star.
Its minimum mass is about 0.022Â and for a wide range of inclinations of third-body orbit, i.e.
, the mass is smaller than 0.07Â . Therefore, we suggest that the third star
may be a brown dwarf candidate. Combining the semi-amplitudes of the radial velocities
and the photometric light curve solution has allowed us to model the short-period detached binary HWÂ Vir.
The luminosity and radius of the dM companion are slightly larger than that given by low-mass models
The brightness variations and orbital period changes of RT Lacertae
The light curves of the chromospherically active eclipsing binary RT Lacertae obtained from 1993 to 1999 are analyzed here. The variation of the brightness at mid-eclipses and at maxima is carefully re-examined. The largest variation was obtained at mid-primary, where the more massive, hotter component occults the less massive cooler secondary star. Therefore, we suggest that the variation of the system's brightness mainly arises from the more massive star. The mean brightness of the system indicates a cyclic change. It showed at least two jumps during the last 22 years. The first occurred in 1984 and the second in 1994. Therefore, the length of the magnetic cycle appears to be about ten years. All the timings of the mid-eclipses obtained so far were collected and analyzed under the assumption of the third body hypothesis. A
period of 94 yr was found for the third body orbit. The variation of the systemic velocity of the eclipsing pair seems to confirm this suggestion. The time delay and advance due to the orbit of the eclipsing pair around the third component were computed and subtracted from the original residuals obtained with the linear light elements. The remaining residuals also show a quasi-periodic change. The period of this change was calculated to be about 18 yr. This second O-C change may be related to the magnetic activity of the more massive component
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