A&A 368, 932-938 (2001)
DOI: 10.1051/0004-6361:20010047
S. Zoa1,2 -
W. Og
oza2,3
1 - Astronomical Observatory of the Jagiellonian University,
ul. Orla 171, 30-244 Cracow, Poland
2 -
Mt. Suhora Observatory of the Pedagogical University,
ul. Podchorazych 2, 30-084 Cracow, Poland
3 -
N. Copernicus Astronomical Center, Polish Academy of Sciences,
ul. Bartycka 18, 00-716 Warsaw, Poland
Received 4 April 2000/ Accepted 2 January 2001
Abstract
New photoelectric observations of the interacting binary V367 Cyg
were made during two consecutive seasons, 1996 and 1997,
using the two-channel photometer at Mt. Suhora Observatory.
The BVRI light curves are analyzed and system parameters
are derived for two alternative models: with and without an accretion disk.
A contact configuration is obtained for the no-disk
model. The semidetached model, with a disk around
the invisible component gives a better fit and, in addition, explains
most of the observed features of V367 Cyg.
The disk in V367 Cyg has a radius of about
,
almost
completely filling the secondary component's Roche lobe.
Mass is transferred from the less massive (
)
to the more massive
(
)
star at a high rate of 5-7 10
/yr.
Key words: binaries: eclipsing, accretion disks, individual: V367 Cyg
V367 Cyg (BD+38
4235,
,
,
,
Sp = A7Iapevar,
RA = 20
47
,
(J2000)),
is an eclipsing system that belongs
to the Serpentids group. Several photometric light curves
have been published, mostly obtained in the UBV system (Heiser 1961;
Fresa 1966; Kalw & Pustylnik 1975; Akan 1987). The authors
observed variations in depth of both minima as well as intrinsic
fluctuations in the light curves. In addition, an asymmetry of both minima
was noticed.
V367 Cyg is a multiple system. Two optical companions were reported: C, which is faint (13.7 mag) and about 2 arcsec distant from V367 Cyg and D (HD 198288), supposed to be only 0.2 arcsec away but of comparable brightness. The D component was at first resolved and then undetected by the speckle technique (McAlister & Hartkopf 1988) and some authors questioned its existence. Most of the recent work on V367 Cyg does not include star D in the model of the system.
Only the primary component is observed spectroscopically. The
underlying stellar spectrum is dominated by strong, sharp
shell lines mostly of singly-ionized metals.
Emission lines of highly-ionized elements such as NV, CIV and SiIV were also
detected (Heiser 1962; Aydin et al. 1978). MgII 4481 is the only
clearly visible photospheric line. Other lines belonging to the primary
component were noticed as blends
or asymmetries of the shell lines. Comparing the photospheric wings of the
Balmer lines with models computed by Kurucz, Aydin et al. (1978) derived the
primary star's temperature
K and surface gravity
.
Although Glazunova & Menchenkova (1989) announced the detection of the MgII
4481 line originating in the secondary star, this interpretation has
been ruled out by Schneider et al. (1993). Based on new
CCD data, they attributed the proposed secondary's line to the FeI
4482.2 shell line. Unfortunately, this result leaves the mass ratio of the
system still not determined from the radial velocity curves.
The light curve of V367 Cyg was
modeled by Li & Leung (1987), who found a contact configuration
for this system. However, in a more recent paper, Pavlovski et al. (1992) have shown
that the light curve can also be described quite well by a
completely different geometry, namely the semidetached configuration.
From the light curve synthesis, the authors derived a solution
with a mass ratio q=0.56 (primary/secondary) and inclination
.
In this model, the
enigmatic secondary component is an accretion disk hiding the mass gaining
star (
K). Within the model proposed by Pavlovski et al. (1992),
the disk is represented by a slab of vertical thickness
A, radius
A and uniform temperature distribution.
Low resolution spectra made by IUE were analyzed by Hack et al. (1984).
They found that the system is heavily reddened, with E(B-V) between
0.50 and 0.55. By comparing the dereddened flux in the ultraviolet continuum
with Kurucz (1979) models, it turned out that one star is not sufficient
to explain the observed distribution. The best fit was obtained assuming two
stars, with temperatures 8000 K (
)
and 10000 K.
Contrary to expectations based on BV photometry, the secondary
component is hotter than the primary one.
Hack et al. proposed that the eclipsing body is
a cool disk around the secondary star with
K.
Polarimetric observations of V367 Cyg have been published by Elias (1993). He constructed a two-component model for the circumbinary matter. The variable linear polarization component arises from an electron envelope around the stars. Constant linear component as well as variable circular polarization are produced by an extensive envelope of material consisting of neutral hydrogen and dust surrounding the whole system. Recently Berdyugin & Tarasov (1997) found, contrary to the results by Elias, that the circular polarization (within the observational errors) was zero. They also argue that the intrinsically variable polarization probably results from light scattering by a gas stream that flows from the primary to the secondary through the inner Lagrangian point. The analysis of the polarization variability gave the orbital inclination to be roughly 82 degrees.
The most recent model of the interacting binary V367 Cyg was proposed
by Tarasov & Berdyugin (1998) (TB98) based on new, high-dispersion CCD
spectroscopic observations. TB98 have determined radial velocities of the
primary component from the MgII 4481 line at several phases.
Combining them with previously published radial velocities, TB98
confirmed long-period variations in systemic velocity
and derived
a new value for the primary star semi-amplitude
kms-1 (leading to a new value for the mass function
and
).
The authors claim detection of a faint emission component of the
HeI
6678 line. They argue that the HeI
6678 emission is
formed in an accretion disk surrounding the invisible component and
have used it to determine
kms-1, and, consequently
,
.
An inspection of their Fig. 1 shows, however, that there is no distinct
feature in the complex line profile that could be reliably identified as a
"component''. TB98 concluded that V367 Cyg is in an initial phase of rapid mass transfer
(as previously suggested by Paczynski 1971),
with matter flowing from the more massive to the less massive component.
In this paper we present the analysis of new photometric light curves of V367 Cyg obtained in the BVRI filters. We describe the new observations in the next section. Description of modeling of the new photometric light curves is presented in Sect. 3, while the results are discussed in the last section.
We decided to re-observe this star in order to obtain a complete light curve in the shortest possible time, with the hope of reducing intrinsic variations of the light curve. In addition, we wanted to extend photometry to the Rand I bands.
V367 Cyg was observed with the 60 cm telescope and two-channel
photometer at the Mt. Suhora Observatory. A description of the equipment
was published by Kreiner et al. (1993).
We used BD+38
4242 as the comparison
star and BD+38
4239 as the check star.
Data were collected during two consecutive
seasons in 1996 and 1997. During this time, we succeeded in obtaining complete
light curves in the BVRI filters. All data were reduced in the usual
way. First we cross-calibrated both channels for different sensitivity; next,
the correction for differential extinction was applied. The data, magnitude
differences between the variable and comparison stars, was left in the
instrumental system. The phases were calculated using the ephemeris
published by Pavlovski et al. (1992).
Large scatter (which for such a bright star cannot be explained by observational errors) is visible also in our data throughout all phases. However, its amplitude slightly decreases towards longer wavelengths. An asymmetry of the secondary minimum is also present in the new observations.
parameter | with ![]() |
without ![]() |
configuration | contact | near-contact |
phase shift |
![]() |
![]() |
i |
![]() |
![]() |
![]() |
*8000 K | *8000 K |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
**3.008 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
**2.190 | **2.193 |
![]() |
**2.412 | **2.473 |
![]() |
**2.563 | **2.651 |
![]() |
**2.870 | **2.834 |
![]() |
![]() |
*0.000 |
![]() |
![]() |
*0.000 |
![]() |
![]() |
*0.000 |
![]() |
![]() |
*0.000 |
![]() |
0.762 | 0.820 |
![]() |
0.741 | 0.796 |
![]() |
0.726 | 0.780 |
![]() |
0.705 | 0.763 |
![]() |
0.238 | 0.180 |
![]() |
0.259 | 0.204 |
![]() |
0.274 | 0.220 |
![]() |
0.295 | 0.237 |
![]() |
4.373 | 4.399 |
* - not adjusted, ** - computed,
: W-D program input values.
Since V367 Cyg is a multiple system, we initially limited the solutions to these which include a third light in the list of free parameters.
![]() |
Figure 1:
Obtained configuration of V367 Cyg in the no-disk model. The lines
show potentials ![]() ![]() ![]() ![]() |
Open with DEXTER |
Assuming that the system consists of only stellar components, our first attempt
was to find the best solution using the Wilson-Devinney code (Wilson & Devinney
1971; Wilson 1993).
The Monte Carlo method (Price 1976; Barone et al. 1988; Zoa et al. 1997) was employed as the search procedure
instead of the original differential corrections (DC).
The Monte Carlo method can locate a global minimum
within the set ranges of parameters.
Normal points were calculated from the original observations: 78 in the I filter and 85 in the B, V and R filters. We applied the following weighting scheme for our data. The V and R light curves were considered more accurate and assigned weights twice larger than those of the B and I data. Additionally, in each light curve points in the deeper minimum have larger weights than those outside.
The temperature of the primary component (star No. 1 in the W-D model),
which is observed spectroscopically, was fixed at 8000 K.
The limb darkening coefficients were adopted as functions of the temperature
and wavelength from Díaz-Cordovés et al. (1995) and Claret et al. (1995).
Additionally, we set albedos and gravity darkening
coefficients at their theoretical values.
The following parameters were adjusted: the phase shift, inclination,
temperature of the secondary component, potentials of the stars, the mass ratio,
luminosity of the primary and a third light.
We used a multidimensional search array consisting of 2000 elements. Each
element consists of all parameters needed for computing a synthetic light curve and an
additional parameter describing the quality of the fit.
Initially, the search array was filled with randomly chosen parameters from
assumed ranges. In subsequent computations, each successive trial produced a new set of
parameters which was compared with the worst one stored in the search array. If its
quality was better than that of the worst element, the
latter was replaced in the search array.
This procedure was continued until the difference between
the best and the worst elements of the search array were smaller than 1 percent.
The computations were repeated in the same way as described above but assuming
.
Final results are presented in Table 1.
The errors listed in Table 1 are standard
's computed from
all values of each adjusted parameter stored the search array.
The results are also shown graphically in Fig. 1.
In this figure, the cross-sections of the search arrays in the -qplane are shown for both cases considered. The two clouds of points
shown in Fig. 1 represent the best solutions stored in the search
arrays at the end of search.
As one can see, the contact configuration was obtained for the model
which included a third light and near-contact if
.
Next, we considered a semidetached model with an accretion disk.
Such a solution would explain some observed properties
of V367 Cyg not understood within the model described in the previous section,
i.e. the presence of highly-ionized, metal lines or invisibility of the
spectrum of the hotter star.
The model is based on the W-D code, modified to include an optically thick disk
(Zoa 1991, 1992).
It requires four parameters
attributed to the disk: its radius (
), luminosity (
),
outer temperature (
)
(in this paper, temperature distribution is
assumed as for the stationary-accretion case),
and thickness (
) - i.e. the
angle between the disk surface and the orbital plane.
The disk thickness is assumed to increase linearly with radial distance,
which can be a first order approximation for moderately thick disks.
In our model, the accretion disk surrounds the secondary star. The primary star is assumed to fill its Roche lobe. The deeper minimum in V367 Cyg would then be caused by the eclipse of the
star whose spectrum is observed by an accretion disk.
Following Hack et al. (1984), we fixed the temperature of the primary (the mass loser) at 8000 K. As in the no-disk model, we included a third light in the list of free parameters but also checked for a possible solution without third light. The linear limb darkening coefficients were taken from Díaz-Cordovés et al. (1995) and Claret et al. (1995), according the temperatures and filter wavelengths, and again we set albedos and gravity darkening coefficients equal to their theoretical values.
The normal mode of the W-D code operation is when the control parameter
IPB is set to 0. For such a case, the relative luminosity of the secondary star
()
is not a free parameter but is computed
from geometrical parameters, the luminosity
of the primary component, temperatures and a
radiation law (either black body or Carbon-Gingerich atmosphere models).
The IPB parameter can be set to 1. Then the coupling between (
)
and
(
)
is severed and both luminosities can be adjusted.
If there is an optically thick disk in a binary system, then for
high inclination, the disk can completely obscure the mass gaining component.
Indeed after some trial runs we encountered just such a situation with
V367 Cyg, and set the control parameter IPB=1.
The following parameters were fitted: inclination, the mass ratio,
luminosities of both stars, the secondary star potential, the disk
parameters and the third light. The temperature of the secondary star was
fixed at 10000 K, after Hack et al. (1984). Computations were done
until the difference between the best and the worst elements in the search
array was less than 1 percent.
Next, we switched to the gradient search algorithm, setting the initial values
of the parameters to those for which the best fit was derived.
The results are presented in Table 2. The errors listed
in Table 2 are standard
's computed for each free
parameter from all elements of the search array.
We derived a configuration
where the disk completely obscures the mass gainer, for which there
was no convergence either for the secondary star's potential or its
relative luminosity (
)
and there are no entries for these parameters
in Table 2. Comparison between the synthetic and observed light curves is shown
in Figs. 2 and 3 (the disk model with third light).
One can see views of the disk model at phase 0.0 in Fig. 4.
parameter | with ![]() |
without ![]() |
configuration | semidetached | semidetached |
phase shift | *0.0000 | *0.0000 |
i |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
*8000 K | *8000 K |
![]() |
||
![]() |
**2.765 | **3.462 |
![]() |
![]() |
![]() |
![]() |
||
![]() |
||
![]() |
||
![]() |
||
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
*0.000 |
![]() |
![]() |
*0.000 |
![]() |
![]() |
*0.000 |
![]() |
![]() |
*0.000 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
4.245 | 4.581 |
* - not adjusted, ** - computed,
: W-D program input values.
![]() |
Figure 2: Comparison between the disk model (lines) and observations (dots) - B and V filters |
Open with DEXTER |
![]() |
Figure 3: Comparison between the disk model (lines) and observations (dots) - R and I filters |
Open with DEXTER |
![]() |
Figure 4:
View of the model of V367 Cyg at phase 0.0
(disk model with and without ![]() |
Open with DEXTER |
Finally, we checked the model proposed by TB98. The computations were performed with the gradient search method only and with the IPB parameter set to 1. The mass ratio parameter was fixed at q=2.08 (primary/secondary), the value derived by TB98. We also fixed the temperatures at 10000 K and 8000 K (for the secondary and primary, respectively) and inclination at 82 degrees. It turned out that a good agreement with the new data can be obtained only for very thick disks exceeding the Roche lobe of the secondary star.
Combining the results from this paper (no-disk and disk models)
with the mass function
and
(TB98), we can derive
the absolute parameters of the components, presented in Table 3.
Since the secondary star is completely obscured by the accretion disk we have
no information about its radius if the disk model (IPB=1) solution is
considered. However, if we assume that
this component is a Main Sequence star, for its mass
of
its radius should be
,
while the temperature
K (Harmanec 1988). The solution
without
would result in the following parameters of the
secondary component:
,
and
K.
The accretion disk has a radius of about 23
and outer vertical thickness
of about 6
.
We noticed that convergence of the outer disk temperature parameter was poorer
than for others. Therefore, the disk temperature (shown in Table 4)
has been computed by comparing the vertical thickness
of the disk resulting from modeling and computed
from the
-disk, z-structure models (Smak 1984, 1992).
We have estimated the mass transfer rate in two ways: first,
from the outer disk temperature, assuming steady state accretion,
and second, from an aproximate formula given by Smak (1989).
This was done for V367 Cyg and also for similar, previously analyzed systems: UU Cnc and W Cru.
Such calculations gave the results presented in Table 4.
Note that the mass transfer rate for W Cru differs from that published
by Zoa (1996) due to an error discovered in the code used for calculation
of the maximum disk temperature for the stationary accretion case.
These two methods agree well for V367 Cyg but for very large disks
(W Cru and UU Cnc) discrepancies are significant.
The mass transfer rate of order of
/yr
in V367 Cyg should produce the following period change:
,
which is of the same order as that
for
Lyr. Unfortunately, the O-C diagram is not so well covered and
the scatter in the O-C values is so large (Kreiner 1999, private communication)
that no decisive conclusions can be drawn yet. Future, high precision
measurements of times of minima are needed to resolve this matter.
no-disk | disk | disk | |
parameter | no ![]() |
with ![]() |
no ![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
*2.3 ![]() |
*2.9 ![]() |
A |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
|
![]() |
![]() |
![]() |
|
![]() |
![]() |
![]() |
* - if a Main Sequence star.
B, V, R and I light curves of V367 Cyg, collected during two consecutive observing seasons, were analyzed with the Wilson-Devinney code (no-disk model) and its modification that also accounts for the presence of an accretion disk (disk model).
Within the no-disk model we derived a contact configuration. The temperature
of the primary component was fixed at 8000 K. The secondary's temperature
should be about 6500-6700 K. This solution would imply that V367 Cyg is
a massive (total mass about
)
contact system.
A significant amount of third light is required,
ranging from 18 percent in the I filter to almost 23 percent in B.
It is too low, however, to be due to star D, and much too high
if the star C is the additional contribution (see comments below
on the existence of star D). As suggested by the referee, the third light
may be just a numerical artifact. It is possible that a spurious solution was
obtained due to the use of the wrong model (not accounting for disk effects)
for a system with an accretion disk.
Due to large disagreement in the third light contribution the no-disk solution with third
light was finally discarded.
In our disk model, matter is transferred from the lower mass component
to the more massive star which is
completely obscured by the accretion disk. The latter result was derived
due to our very crude model. We assumed the disk to be optically thick
while the real disk structure should have an atmosphere (most likely extended).
Our model does not account for the effect of some of the radiation from the
mass accreting star passing through the disk
atmosphere. Such an effect could be a possible explanation
for the observed 10000 K continuum.
We obtained different mass ratios: q=0.44 if we included third light and
q=0.83 for a solution without .
Since there is evidence
for a third star in V367 Cyg, the light curve solution should include
.
However, this turned out to be a risky procedure, especially in the disk model.
As one can see in Fig. 4, the disk is not completely obscured
by the primary star and the light from these parts would have the same effect
on the light curve as a third light.
The amount of third light we obtained is about ten times larger
than star C should contribute.
stationary | |||
object | data | accretion | Smak (1989) |
V367 Cyg | ZO |
![]() |
![]() |
UU Cnc | APT |
![]() |
![]() |
UU Cnc | KRK |
![]() |
![]() |
W Cru | Pazzi |
![]() |
![]() |
W Cru | Marino |
![]() |
![]() |
ZO - this paper, disk solution without ![]() APT - APT data (Eaton et al. 1991); KRK - Cracow data (Winiarski & Zo ![]() Pazzi - Pazzi (1993); Marino - Marino et al. (1988). |
Additional check for consistency of our models can be done with simple
calculations of predicted absolute magnitudes of the system which
could be compared with that derived from the apparent magnitude, known reddening
and distance to V367 Cyg.
The Hipparcos parallax is very small ( mas
0.63 mas) indicating
a distance of about 1.1 kpc (although with large error: 0.66 kpc to
4 kpc). The distance derived from the 0.22
m feature in the ultraviolet
spectrum is about 0.7 kpc (Elias 1993).
The distance of 0.7 kpc will require
,
while
1.1 kpc
and 4 kpc
.
Taking effective temperatures and radii from Table 3 we have:
for the no-disk, no
solution,
for the disk model without third light and
for the disk model with third light.
The disk model with l
gives an absolute magnitude of V367 Cyg
defintely too low so we conclude that the disk solution with third
light could also be a numerical artifact due to the large number of free
parameters. A determination of the star C magnitudes in different filters
independent from light curve modeling
(i.e. by CCD observations) would help to resolve
the problem of third light and allow more reliable determination of the
mass ratio of the system on the basis of light curve modeling.
If the distance to V367 Cyg is close to 0.7 kpc the closest match is obtained
for the disk model without third light. However if the distance is about 1 kpc,
the no-disk solution gives almost perfect agreement. Unfortunately, on
the basis of the above calculations we cannot distinguish between the
no-disk and disk solutions.
For the disk model, the primary component parameters are: mass about
and radius about
.
The disk around the invisible component is large, its radius being about
23
,
almost completely filling the mass gainer's Roche lobe.
The disk luminosity increases from the B to I wavelengths, its contribution
reaching about 21% of the total light in the I filter.
The disk and no-disk models gave a comparable fit to the observed light curves.
The best fit was obtained for the disk model with third light.
The fits within the no-disk model also gave a good description of
the observations, which is not surprising as the light curves of a high inclination system
with an accretion disk resemble those of contact systems. As was
shown by Zoa (1995), the best fit will likely result in a contact
configuration when the Wilson-Devinney model is used to solve light
curves of a high-inclination binary system with one component hidden in
an optically thick accretion disk.
The latest model for V367 Cyg proposed by TB98
has been verified with our new photometric data. The quality of the fit is
worse than that of our disk model. In order to reproduce the observed light
curves with the mass ratio claimed by Berdyugin & Tarasov (1997),
the disk must either exceed the Roche lobe or be very thick. If the disk
radius is limited to the side radius of the secondary star Roche lobe, the
parameter needs to be close to 35 degrees. This is not a moderately
thick disk and it should be hotter than the primary (visible) component.
Finally, we looked for star D in the photometric data. For this purpose,
we made a grid of solutions (within the disk model),
with the third light parameter fixed at chosen values.
The gradient search method was employed and only the V light curve
was used. We set as the starting parameters those derived earlier.
The third light parameter was considered in the range from 0 to
0.6, with step 0.05. It turned out that the quality of the fit deteriorates
quickly for increasing third light, being unacceptable already for
.
The conclusion is that there cannot be such a bright third star in V367 Cyg.
The inspection of the Hipparcos catalogue confirms this inference - no such
star is listed in the Hipparcos catalogue of multiple stars.
Neither the Roche nor disk models can explain the observed asymmetry of the shallower minimum and slightly different heights of maxima. Either a spot should be added to the Roche model or a non circular disk assumed to improve the fits.
Acknowledgements
The computations were performed at ACK "Cyfronet'' in Cracow under grant No. KBN/UJ/015/95 which we gratefully acknowledge. The authors thank M. Drozdz, D. Marchev, J. Krzesinski and G. Pajdosz for obtaining some observations. We would like to thank Prof. J. Smak for stimulating discussions and critical reading of the manuscript and Prof. J. Mikoajewska for discovering a bug in the code used to compute the mass transfer rate for W Cru. We acknowlege comments of an anonymous referee which allowed us to improve presentation of this work.