A&A 378, 936-945 (2001)
DOI: 10.1051/0004-6361:20011255
C. S. Jeffery - R. Aznar Cuadrado
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
Received 21 June 2001 / Accepted 4 September 2001
Abstract
BI Lyn has previously been misclassified as an evolved binary
system containing either a hot subdwarf or white dwarf and a thick
accretion disk. New intermediate dispersion spectra are presented
which demonstrate clearly that the hot component is a luminous
low-mass helium star and the cool companion is a rapidly rotating
G-type giant. Techniques of spectrum synthesis have been used to
establish the dimensions of both components. Although the orbital
period of the system remains unknown, other phenomena are entirely
consistent with these observed dimensions. A 0.34 d periodic
photometric variation could be explained by pulsations in the B-type
primary and, by analogy with other H-deficient binaries, it is
suggested that the variable absorption in H
may be due to a
gas stream flowing through the inner Lagrangian point.
Key words: stars: fundamental parameters - stars: binaries - stars: chemically peculiar - stars: individual: BI Lyn
BI Lyn (=PG0900+400) has been identified
as a composite binary containing a hot subdwarf primary with effective
temperature
and a cool
main-sequence star of spectral type
K3 (Ferguson et al. 1984).
As such, it was proposed to be the potential progenitor
of a cataclysmic variable (CV) system. The subsequent passage from being a
suspected CV progenitor to becoming a "nova-like'' CV
(Downes et al. 1997; Ritter & Kolb 1998) appears to have come
from the juxtaposition of a small-amplitude light variation (Lipunova & Shugarov 1991)
with a suggested thick accretion disk (Ferguson et al. 1984) by compilers of the
variable star lists (Kazarovets et al. 1993).
There is, however, no evidence for any long-term variability, although
there may be a 0.338 day photometric variation (Lipunova & Shugarov 1990; Lipunova & Shugarov 1991;
Kuczawska et al. 1993).
It was only
following a more detailed spectroscopic study that firm evidence of
variability,
manifest in the behaviour of the blue-shifted H
absorption component of a broader emission line, was obtained (Wade & Potter 1995).
Deconvolution of the composite spectra confirmed the spectral types of the
hot and cool components (Orosz et al. 1997).
BI Lyn stands out amongst the composite stars
of Ferguson et al. (1984) for having a distinctly red flux distribution,
where all other composites have blue or flat spectra. Orosz et al. (1997)
refer to PG0900+400 specifically for several reasons. It is the only
member of their own sample to show H
emission. It appears to have
an unusually large rotation velocity (
).
In contrast to other stars in their sample,
which show the He I D3 absorption near 5876 Å with
equivalent widths between 0.5 and 1.0 Å, the D3 line in
PG0900+400 has an equivalent width of 2.0 Å or more. Low-resolution
spectroscopy (Kuczawska et al. 1993; Liu & Hu 2000) also points to numerous prominent
He I lines which are stronger than neighbouring Balmer lines.
Taken overall, the literature on BI Lyn suggests a hot helium-rich
subdwarf with a K-type main-sequence companion or other cool source.
Broad absorption lines, variable H
emission and a 0.34 d
light variation suggest that the latter could include a thick accretion disk.
In evolutionary terms it would lie between
a common-envelope phase (red giant plus main sequence star) and a
cataclysmic variable (white dwarf plus main sequence star). Being helium rich,
it could represent a very important phase of binary
evolution.
The star caught our attention during a new intermediate-dispersion survey
of composite spectrum subdwarfs. Fortunately, we had overlooked the SIMBAD
designation "V
BI Lyn - Nova-like Star'' and had retained
the star in our sample. In our ignorance of the literature discussed above,
an immediate examination of the new spectra suggested a quite different
interpretation. This has been fully confirmed by subsequent observations and
more detailed analysis.
Image | Date: JD | Wavelength | Res. | exp |
-2 450 000 | Å | s | ||
INT+IDS | ||||
155679 | 1264.427 | 3820-4686 | 2500 | 300 |
155725 | 1264.547 | 6070-6930 | 3900 | 300 |
155728 | 1264.553 | 7975-8813 | 5100 | 300 |
155928 | 1265.603 | 3821-4687 | 2500 | 300 |
WHT+ISIS | ||||
337312 | 1676.371 | 4246-4653 | 5600 | 100 |
337313 | 1676.374 | 4246-4653 | 5600 | 200 |
337318 | 1676.383 | 3898-4304 | 5100 | 200 |
337319 | 1676.385 | 3898-4304 | 5100 | 200 |
337414 | 1677.358 | 6364-6769 | 8300 | 61 |
337415 | 1677.359 | 6364-6769 | 8300 | 200 |
337416 | 1677.360 | 4597-5004 | 6000 | 200 |
337417 | 1677.363 | 6364-6769 | 8300 | 200 |
337418 | 1677.363 | 4597-5004 | 6000 | 200 |
337419 | 1677.367 | 4246-4653 | 5600 | 200 |
337420 | 1677.367 | 8408-8803 | 11 100 | 300 |
Image | Date: JD | B star | G star | |||
-2 450 000 | v | ![]() |
v | ![]() |
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|
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||||||
155679 | 1264.427 | 36.8 | 3.1 | 24.9 | 3.4 | 12.0 |
155928 | 1265.603 | 37.8 | 4.1 | 21.4 | 2.1 | 16.4 |
337312 | 1676.371 | 28.1 | 1.6 | 39.1 | 3.1 | -11.0 |
337313 | 1676.374 | 17.7 | 3.4 | 39.0 | 3.1 | -21.3 |
337318 | 1676.383 | 40.5 | 3.1 | 13.6 | 0.8 | 26.9 |
337319 | 1676.385 | 40.3 | 2.5 | 15.4 | 1.4 | 24.9 |
337416 | 1677.360 | 10.5 | 3.0 | 34.6 | 2.6 | -24.1 |
337418 | 1677.363 | 19.6 | 3.4 | 34.4 | 2.1 | -14.8 |
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||||||
155728 | 1264.553 | - | - | 19.3 | 0.2 | - |
337420 | 1677.367 | - | - | 27.3 | 0.5 | - |
Mean | 28.9 | 26.9 | 1.1 | |||
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11.7 | 9.5 | 21.1 |
![]() |
Figure 1:
INT intermediate-resolution
spectra of BI Lyn, showing sharp He I lines,
weaker Balmer lines, a veil of metal lines from a
cool component (top: INT images 155679+155928), a P Cygni profile
in H![]() |
Open with DEXTER |
Intermediate dispersion spectroscopy of a sample of candidate subdwarf
B stars with cool companions was carried out with the Isaac Newton telescope
on 1999 March 26 and 27, using the 235 mm camera of the
Intermediate Dispersion Spectrograph,
R1200R and R1200B diffraction gratings and the TEK5 (11241124) CCD.
Wavelength regions were chosen to determine the atmospheric parameters
of the sdB stars (blue) and their cool companions (infrared calcium triplet).
Red spectra (around H
)
were obtained in an effort to obtain
spectrophotometry of the cool companions (cf. Jeffery & Pollacco 1998).
All observations and data reduction followed standard procedures,
including bias-subtraction, flat-fielding and wavelength calibration,
and are described in full by Aznar Cuadrado (Aznar Cuadrado 2001). Details of the
observations of BI Lyn are given in Table 1 and the
spectra are shown in Fig. 1.
Our first reaction upon seeing the quick-look extraction of the
blue spectrum of PG0900+400 was that we were looking at an extreme
helium star. This was in ignorance of the history described in the
introduction and on the expectation that we should see a typical
subdwarf B spectrum with strong and broad Balmer lines superposed by
a weak cool-star spectrum. In fact, the neutral helium spectrum stands
out clearly while the Balmer lines are weak and narrow. Other features
include C II 4267 Å, Mg II
4481 Å and Ca H+K
(Fig. 1). Meanwhile
the rest of the spectrum is very rich in absorption lines. The authors
were immediately reminded of the spectra of cool helium stars such as
PV Tel and
Sgr (Jeffery et al. 1987). Closer inspection
disavowed this conclusion and raised several difficulties.
First of all we consider the evidence of the helium lines in the
blue. There is no evidence for the Pickering series of He II,
so the Balmer lines are due to hydrogen and the effective temperature
cannot exceed 35 000 K.
The fact that He I 4388 Å is
considerably stronger than H
implies that the star is
hydrogen deficient.
The ratio of He I
4471 Å to Mg II
4481 Å requires
that the temperature of the helium star must be higher than
that of PV Tel (15 000 K: Walker & Schönberner 1981).
The calcium infrared triplet is strong (Fig. 1), proof positive of a star with spectral type G or later, even given the peculiar properties of stellar spectra in the absence of hydrogen.
Having examined the literature and discovered that we were not the
first to encounter these ambiguities,
we concluded that we must be looking at a composite spectrum binary
containing a helium star and, given previous work, a G or K star. The
question that arises is whether we are looking at two giants (or
supergiants), or at two dwarfs. The possibility that both components
are giants is intriguing. H
emission
(Fig. 1) has
in fact been seen elsewhere in hydrogen-deficient giants such as
Sgr (Seydel 1929).
A preliminary analysis based on these data (Aznar Cuadrado et al. 12th EADN Predoctoral School. Selected topics on binary stars. Observation and physical processes. La Laguna, Spain. September 6-17 1999, unpublished) failed to resolve the surface gravities of both cool and hot components in a manner consistent with the relative angular radii determined from spectrophotometry and with sensible values of the masses for both stars. A likely reason was that the cool-star spectrum contaminates the hot-star spectrum so severely that the lines are simply too heavily blended at this spectral resolution. Consequently, spectral fitting by least-squares minimisation was poorly constrained. It was therefore necessary to obtain higher resolution spectra, and to combine these with previous ultraviolet and visual spectrophotometry.
Further intermediate dispersion spectroscopy of BI Lyn
was carried out with the William Herschel telescope
on 2000 May 11 and 12, using ISIS,
R1200R and R1200B diffraction gratings and the TEK2 (red arm) and TEK4
(blue arm) (11241124) CCDs. With approximately double the
spectral resolution, similar wavelength regions were chosen
as before. Details are given in Table 1.
All observations and data reduction followed standard procedures,
including bias-subtraction, flat-fielding and wavelength calibration,
are again described in full by Aznar Cuadrado (Aznar Cuadrado 2001).
Where possible, spectra were obtained in pairs to avoid the risk of
a cosmic ray hit on an important line. These multiple spectra were
coadded. In addition, data from three contiguous wavelength regions
in the blue were combined to form a single spectrum (
Å)
for use in the subsequent analysis. Each region was normalized by
fitting a low-order polynomial to regions of pseudo-continuum. Overlapping
wavelength regions were inspected to ensure that the overlap was good
before merging the individual sections. If necessary, the pseudo-continuum
was adjusted to ensure this condition was fulfilled. The result was
three spectra for analysis with wavelength ranges
3900-5000 Å,
6360-6760 Å and
8400-8800 Å. Unfortunately the
8400-8800 Å was too noisy and contaminated by flat-field fringes to be useful
for spectral analysis, although it could be used for radial velocity
measurements.
BI Lyn was observed within two IUE programmes in 1995. The low-resolution image numbers are LWP30587, SWP54558, SWP54559, SWP54560, SWP56235, of which SWP54559 and SWP54560 were made with the small aperture and the remainder with the large aperture. These images have been recovered from the IUE Final Archive as Newly Extracted Spectra. The short-wavelength IUE spectrum is typical of a hot subdwarf, showing very strong He II, C III, IV, N V, and Si IV absorption lines.
Broad-band colours have been reported by Ferguson et al. (1984) and by Lipunova & Shugarov (1991), from which we have adopted V=12.85, B-V=0.2, U-B=-0.96, V-R=0.3 and V-I=0.58.
By combining these data, a single description of the flux distribution between 1100 and 9000 Å has been derived (Fig. 2). For the time being, we have chosen to discount the possibility of variability in the overall flux distribution. If present, it does not exceed 0.2 mag (Lipunova & Shugarov 1990) in V.
Radial velocities were measured by cross-correlation. The cross-correlation
templates were taken to be the best-fit model spectra for each stellar component
as described in the next section. Since a velocity needs to be
known a priori to perform this analysis, a two-step iteration for velocity
was effected. Strong lines common to both stars (e.g.
Balmer lines) were excluded from the cross-correlation.
The heliocentric velocities measured from each observation
are given in Table 2. Errors on individual
velocities are formal errors from fitting a Gaussian to the ccf peak. The
actual errors are probably much larger, but difficult to determine
quantitatively; the typical ccf width is 200-300
.
The standard
deviation about the mean (
)
may be a better indication.
Wavelength calibration and other systematic errors may be responsible for
anomalous measurements from spectra around H
and one blue spectrum
(WHT image 337419). These are not shown in Table 2.
The average velocities for both components, 29 and 27
,
respectively, are
consistent with previous velocity measurements (Orosz et al. 1997).
![]() |
Figure 2:
Ultraviolet and visual spectrophotometry of BI Lyn
(histogram) together with the best fitting theoretical flux distribution
(polyline and horizontal bars).
The latter represents the sum of two model atmospheres (dashed
lines) with effective temperature and angular radii
![]() ![]() ![]() ![]() |
Open with DEXTER |
The available spectra of BI Lyn suggest that it comprises a hot helium-rich source and a cooler source since we see absorption lines due to plasma in two quite different ionization states. In constructing a model to fit these spectra, a number of assumptions - or approximations - influence our conclusions. Many of these are necessary because both the number of free parameters and the limited number of constraints preclude an exhaustive search of solution space.
The fundamental assumption is that both cool and
hot absorption sources are primarily stellar. This seems to
hold well for the hot source. This remains a good working hypothesis
even though high rotational broadening in the cool
source spectrum and the presence of H
emission makes a purely
stellar identification less secure.
For two stellar sources, the principal free parameters which govern the measured spectrum are as follows (subscripts refer to the hot and cool source respectively):
Thus the second group of assumptions are as follows.
The abundances of all elements other than hydrogen and helium are
in proportion to their cosmic abundances, with
.
The helium abundance of the
cool star is normal:
.
The adopted microturbulent velocities are typical for early-type stars
and main-sequence late-type stars
.
The latter assumption is very important as it affects both the
metallicity
(see above) and the derived
radius ratio R2/R1. We have adopted
in order
that the latter quantity as derived from spectral fitting be as consistent
as possible with
derived from spectrophotometry
where we have also used cool star models computed with
.
Secondary effects on
and
are
not significant here.
The third level of assumptions concerns the physics.
The stars are assumed to be spherically symmetric
and not to vary significantly over time. The approximations of
plane-parallel geometry, local thermodynamic, radiative and
hydrostatic equilibrium have been assumed valid for modelling
their atmospheres (rapid rotation and H
emission may
compromise these).
However, with these assumptions and approximations, it becomes possible, in principle, to solve for the remaining 12 (!) free parameters by modelling the overall flux distribution and the intermediate-resolution spectra of BI Lyn.
The methods for fitting flux distributions and high-resolution spectra by least-squares minimization within grids of models have been described in detail elsewhere (Jeffery et al. 2001). The primary codes used are BINFIT and SFIT.
BINFIT is an extension of the
single star code TFIT used to model the flux distribution of a
binary system containing a hot and a cool star. Fitting of each
component is done within a one-dimensional grid, normally
,
convolved with an extinction curve
and angular
radius
.
The flux distributions used in the model grids are of
low resolution and taken directly from the output of LTE model atmosphere
codes STERNE (Jeffery et al. 2001) or ATLAS (Kurucz 1979).
SFIT is used to model intermediate and high-resolution stellar spectra
by interpolation in three-dimensional model grids, normally
,
and an abundance parameter, e.g.
or
.
Versions
exist for both single and binary stars, and appropriate allowance
is made for velocity shifts, rotational broadening, and instrumental
broadening. Given the relative radii of stars in a binary, the
spectra are added correctly at the absolute flux level and then
normalized to the true total continuum. Provision is made to renormalize
the observed spectrum to the true continuum in order to optimize the
fit. SFIT may be used to measure any or all of
,
,
abundance,
,
and R2/R1. In practise,
only two or three variables should be solved for simultaneously,
although R2/R1 should always be a free parameter. Consequently,
the use of SFIT is iterative, particularly for binaries.
The radial velocity shifts v may be found by cross-correlating the observed spectrum with individual components of a synthetic spectrum which is a sufficiently good approximation to the final solution. Providing the hot and cool star spectra are sufficiently different, cross-correlation will automatically identify the individual components in the observed spectrum, although it is important to exclude strong features present in both spectra (e.g. Balmer lines).
The model atmospheres and flux distributions used to analyse the hot star are computed with the plane-parallel LTE code STERNE, which is adapted for dealing with hydrogen-deficient stellar atmospheres. The high-resolution spectra are calculated with the LTE code SPECTRUM. Both codes are described more fully by Jeffery et al. (2001).
Model atmospheres used in this investigation were calculated on a
three-dimensional rectangular grid defined
by
,
,
and composition
,
[0.01,0.99,0.0],
[0.05,0.95,0.0]and
[0.1,0.9,-1.0]. The larger value of
or
is reduced to
compensate for the trace elements. Coarse model grids for
,
0.5, 0.7, 0.9, 0.95, 0.99 and 1.0, with corresponding
and
,
are also available, as are fine grids with
and
for selected areas of (
)
space.
Synthetic spectra were calculated on wavelength intervals
3900-5000 Å (blue),
6360-6770 Å (red/H)
and
8400-8800 Å (CaT). Linelists were
taken from the list of assessed data for hot stars LTE/_LINES
(Jeffery 1991). Microturbulent velocity
and solar abundances for
all elements other than hydrogen and helium were assumed (see above).
Model atmospheres and flux distributions used to analyse the cool star
were taken from the Kurucz' standard grid of ATLAS models (Kurucz 1993),
for
,
,
and
.
High resolution spectra were calculated in the same spectral regions as
for the hot star using Kurucz' code SYNTHE
(Kurucz 1991; Jeffery et al. 1996). Grids with microturbulent velocities
and 10.0
were computed.
Star | 1 | ![]() |
2 | ![]() |
|
Spectrophotometry | |||||
![]() |
0.00 | 0.02 | |||
![]() |
28.6 | 1.0 | 5.84 | 0.96 | kK |
![]() |
0.55 | 0.01 | 4.09 | 0.10 | 10-11 rad |
![]() |
5 | a | 2 | a |
![]() |
R/R1 | 1 | 7.44 | 0.03 | ||
Spectroscopy | |||||
![]() |
30.1 | 0.01 | kK)b | ||
![]() |
28.6 | a | 5.84 | a | kK |
![]() |
3.6 | 0.1 | 3.2 | 0.3 | (cgs) |
![]() |
0.95 | 0.01 | 0.1 | a | |
![]() |
0.0 | a | 0.0 | a | |
![]() |
0 | a | 120 | 20 |
![]() |
![]() |
5 | a | 2 | a |
![]() |
R/R1 | 1 | 4.9 | 0.5 | ||
a Assumed value. | |||||
b Free solution not used. |
Assumed values | ||||||
![]() |
![]() |
![]() |
||||
R2/R1 | 4.9a | 0.5 | 7.4b | 0.03 | 4.9a | 0.5 |
![]() |
0.5 | 0.05 | 0.5 | 0.05 | 1.0 | 0.1 |
Derived values | ||||||
![]() |
1.85 | 0.34 | 1.85 | 0.34 | 2.62 | 0.48 |
![]() |
3.32 | 0.10 | 3.32 | 0.10 | 3.62 | 0.10 |
![]() |
9.09 | 1.89 | 13.80 | 2.51 | 12.85 | 2.68 |
![]() |
1.93 | 0.11 | 2.29 | 0.10 | 2.23 | 0.11 |
d/kpc | 5.01 | 1.05 | 7.61 | 1.40 | 7.09 | 1.49 |
![]() |
4.8 | 1.8 | 11.0 | 3.9 | 9.6 | 3.5 |
![]() |
1.0 | 0.4 | 2.2 | 0.8 | 1.9 | 0.7 |
a R2/R1 from spectroscopy. | ||||||
b R2/R1 from spectrophotometry. | ||||||
c Trial values. |
The method for measuring T
With
Much more information is available in the line spectrum than can be obtained
from photometry alone. The object of spectrum synthesis is to find, given
assumptions that have been introduced already, a model for the overall spectrum
that best matches the observation by, for example, minimizing the square
of residuals between model and data. Techniques for doing this with
single stars have been described already (Jeffery et al. 2001). The extension to
binary stars has been developed by Aznar Cuadrado & Jeffery (in preparation)
and described by Aznar Cuadrado (Aznar Cuadrado 2001).
Under perfect conditions (e.g. noise-free data),
a residual-minimization procedure could solve for many free
parameters simultaneously. In practice, it is necessary to hold most parameters fixed
while solving for two or three at a time, and iterating around several parameters
until an optimum solution is obtained. With the assumptions introduced
already, the final solution for BI Lyn is
given in Table 3. The errors cited in fitted quantities are
formal errors from the
A free spectroscopic solution for effective temperature gave
The surface gravity and helium abundance of the hot star were
well determined spectroscopically; there is negligible rotational
broadening (at the instrumental resolution) in the line profiles.
It is clear that the hot component is a hydrogen-deficient giant
with
The cool star is more difficult to analyse, mainly because of substantial
rotation broadening
With the assumptions given and parameters deduced, a free solution gives
The best-fit model spectra for both hot and cool stars, their sum according
to the given relative radii and a comparison with the observed spectrum
are given for all spectra regions in Figs. 3-6.
Given the measured dimensions of BI Lyn, a number of other properties
including masses, luminosities and distance may be estimated.
However these depend on which value for R2/R1 is adopted, an estimate
for the hot star mass M1 and the quality of the
These solutions must be reconciled with the position
of BI Lyn which, with
A better solution might be achieved if we have overestimated g2 by, for
example, placing the continuum around the infrared calcium triplet
too low or underestimating the hot star flux at these wavelengths.
With
In any event, the components must be highly evolved. A solution
comprising a 0.5
The surface gravity for the hot star places it on an evolutionary
track for a post-AGB star of
The referee has rightly pointed out that the detailed results of
our analysis are subject to the LTE assumption used in the analysis of
the early-type star. Being a giant, departures from LTE can be
significant and will affect, in particular,
the predicted equivalent widths of the He I lines. Other
spectral features may also be affected. Non-LTE calculations for
hydrogen-poor hydrogen-helium atmospheres have been available for some
years, but fully line-blanketed NLTE models for hydrogen-poor atmospheres
with
BI Lyn has been found to be variable in light with an amplitude
of A period of
Given the resolution and inhomogeneity of the spectra,
the short time base over which observations were obtained
and the rotational broadening of the cool star spectrum, no
significant changes in the radial velocity of either component could be identified
(Table 2). The mean difference in velocity of the two components is
negligible. High-resolution studies of a specific spectral region over an extended
interval will be necessary to establish the orbital period and velocity amplitudes
in this system. These will be extremely important observations and should be undertaken
as a matter of urgency since they will give the mass ratio directly and
independently from the spectroscopic analysis.
Variations in H
Although the data are sparse, the qualitative behaviour of H
We have obtained intermediate dispersion spectra of the supposed hot subdwarf
binary BI Lyn. These have been analysed, together with the overall
flux distribution, to establish the dimensions of both hot and cool components.
The hot component is clearly demonstrated to be a low-mass hydrogen-deficient
star with low surface gravity. The cool component is most likely a
giant of approximately one to a few solar masses. The hydrogen-deficiency
of the hot star is probably the result of a common-envelope phase during which
the outer envelope was entirely removed or transferred to the cool companion.
The hot star luminosity suggests that it lies on a post-AGB evolution track.
Previous reports of variability in light and H
Note added in proof: A striking similarity between BI Lyn and HD 128220 has been drawn to our attention. The latter is a binary containing an O subdwarf and a rapidly rotating G-type primary, probably spun up by mass exchange (Howard I. D., & Heber U. 1990, PASP, 102, 912). Thus both systems consist of a hot post-AGB star and a more-massive rapidly-rotating cool companion.
and angular radii
for both components in
composite systems containing a hot subdwarf and a cool companion using IUE
spectrophotometry and optical-IR broad-band photometry has been described
elsewhere (Aznar Cuadrado & Jeffery 2001). In fact PG0900+400 appeared in the sample analyzed,
yielding
,
,
with
R2/R1=8.0.
However it had been assumed that
and
.
For this paper, the data were re-analyzed iteratively with the optical data
(see below). Consequently, quite different model atmosphere grids were
adopted in the final analysis, with
and
.
Interstellar
extinction was still found to be negligible, but the different distribution of
opacity in the hydrogen-deficient atmosphere of the hot star resulted in a higher T
being obtained for both components.
,
the earlier measurement of
(Ferguson et al. 1984) was approximately recovered.
However, with
,
the cool star appears to be somewhat hotter than the K3 spectral type
indicated
before. The broader spectral range covered by our data should lead to a
more robust result than the flux ratio method (Wade 1980). It is
noted that the errors are formal errors and do not allow for systematic errors as
may be introduced, for example, by an inappropriate choice for
unconstrained model atmosphere parameters.
The relative radii of the two stars, given by their angular diameters, is
.
Other parameters of the fit are given in Table 3.
The best-fit model flux distribution is shown together with the data in
Fig. 2.
Figure 3:
Normalized blue spectrum of BI Lyn (bottom: d)) together
with a best fit composite model spectrum c) formed by adding models with
a)
,
,
(top) and
b)
,
,
assuming that the relative radii
R2/R1=4.9. The model spectra have been
velocity shifted and degraded to match the observed spectral resolution
(1 Å) (WHT images: 337312 + 337313 + 337318 + 337319 + 337416 + 337418 + 337419).
A detailed comparison between the observed spectrum and best-fit model
is shown in Fig. 4.
Open with DEXTER
Figure 4:
Normalized blue spectrum of BI Lyn (histogram)
together with the best fit composite model spectrum (polyline: see Fig. 3c)
showing the fit in detail.
Open with DEXTER
Figure 5:
As Fig. 3 in the region of the
infared calcium triplet (INT image: 155728).
Open with DEXTER
Figure 6:
As Fig. 3 in the region of H
(WHT images: 337414 + 337415 + 337417).
Open with DEXTER
3.4 Spectrum synthesis
minimization. Other errors have been
propagated from these. Systematic errors
(e.g. choice of
,
)
can significantly affect these.
kK, but could not provide
because
of a lack of temperature-sensitive diagnostics.
To maintain consistency in the radius ratio, the spectrophotometric
solutions for
and
were retained.
with an atmosphere containing some 5 per cent
hydrogen and 95 per cent helium (by number).
.
Metallicity, microturbulent
velocity, helium abundance and gravity all affect the strength of
the metal line spectrum, which is then heavily smeared by the rotation.
The infrared calcium triplet could provide a good gravity indicator
if
were known. Assuming
we find
.
in both blue and infared spectral regions. This is
determined solely by the strength of the cool star absorption spectrum,
so is clearly assumption dependent. Given the nature of these, it is
satisfactory that R2/R1 is within 40% of the value obtained from
photometry.
3.5 Stellar dimensions
measurement.
Possible values, given choices for each of these parameters, are shown in
Table 4. The given distance is derived from the measured
angular diameter and the derived radius of the cool star (
).
A similar result is obtained from the apparent visual magnitude and derived
luminosities.
,
is substantially (
3 kpc) out of the Galactic plane in the
anticentre direction. It therefore seems preferable to seek the lowest
possible mass for the cool star in which case
a solution with
and
is indicated. The latter is comparable with the
companion star mass estimated for another hydrogen-deficient binary
-
Sgr (Schönberner & Drilling 1983; Dudley & Jeffery 1990).
and
or 2.5, a cool star
mass
or 1
would be easier to reconcile with the Galactic
position.
B-type helium star and a 1-5
G-type
giant (luminosity class II - III) is consistent with the
observational data presented in this paper. It is suggested that
BI Lyn is a post common-envelope binary in which the primary
(helium star) has almost completely shed its outer hydrogen-rich
envelope together with substantial angular momentum. A large fraction
of this has been transferred to the cooler star. The envelope of the
cool star may not yet be in thermal equilibrium - there may still be
a main-sequence star underneath giving it the semblance of a giant.
This would account for the apparently advanced evolutionary state of
the cool star at the same time as the hot star is in a relatively
short-lived phase of evolution.
(Schönberner 1983).
have not yet been successfully
computed (Rauch 1996). Their eventual arrival will affect the
detailed results presented here but not the overall conclusions
concerning the dimensions of BI Lyn.
Figure 8:
Residuals of four spectra with respect to the mean
WHT spectrum in the region of H
,
labelled by image number.
Only the INT spectrum shows a significant change in the absorption strength.
Open with DEXTER
4 Variability
4.1 Photometry
0.1 mag and a period of 0.33818 d (Lipunova & Shugarov 1990, 1991).
Reports of other periodicities (280 s: Lipunova & Shugarov 1991, 1117 s: Kuczawska et al. 1993)
have not been confirmed. These find possible explanations in a CV model
as the orbital period and oscillations of the white dwarf respectively.
An orbital period of 0.34 d was not detected spectroscopically (Orosz et al. 1997).
The 0.34 d period cannot be associated with the cool star rotation;
a star with
,
and
has a
minimum rotation period of 2.6 d.
0.3 d can be explained by pulsations (radial or
non-radial) in a luminous hot helium star. A corollary would be
V2076 Oph (Lynas-Gray et al. 1987), a luminous helium star with
,
and non-radial pulsation periods
0.7 and 1.1 days. The observed periods of these strange-mode
pulsations (Saio 1995) depend critically on the stellar mass,
luminosity and which modes are most excited. V2076 Oph
is thought to have a mass
(Lynas-Gray et al. 1987).
4.2 Radial velocity
4.3 H
are more difficult to interpret reliably,
particularly since the promised report of 1995 March observations
(Wade & Potter 1995; Orosz et al. 1997) has still to appear. The available data
describe a broad emission with a variable and narrower absorption
component. We observed a change in the absorption
strength of H
between 1998 and 1999,
but not during the short interval of the 1999 observations (Fig. 7).
A rapid increase in absorption strength has been reported elsewhere (Wade & Potter 1995).
Such phenomena have previously been seen in hydrogen-deficient
supergiant binaries (e.g.
Sgr) on a timescale
comparable with their orbital periods.
Sgr is a single-lined spectroscopic binary with an
orbital period of 138 days (Wilson 1914). The primary is an early-type
supergiant with
(Dudley & Jeffery 1993) and an
extremely low hydrogen surface abundance (Schönberner & Drilling 1983). Being of the
6th magnitude, it has been scrutinized for over a century (Campbell 1899).
It is interesting to review some of the reports concerning H
:
More recent data (Frame et al. 1995) confirm
the strongly variable nature of H
a very broad, weak emission band, roughly 8 Å in width,
can be seen'' (Greenstein 1943);
appeared...'' (Greenstein 1950);
occur in the envelope
of the system'' (Greenstein 1950);
absorption
line appears during an interval of about 40 days centered about the phase...
at which the primary is farthest from us (e.g. Bidelman 1949; Hack 1960)''
(Nariai 1967);
absorption and emission.
Absorption episodes are roughly correlated with orbital phase although they
may switch off for several cycles at a time.
The preferred model is that of a supersonic jet (Nariai 1967) generated by
material passing through the inner Lagrangian point on to the secondary
and which eclipses an extended envelope around the primary responsible for the
broad emission. Similar phenomena have been seen in the three other known
hydrogen-deficient binaries KS Per (Nariai 1972), V426 Car = CPD
(Frame et al. 1995) and HDE320156 = LSS4300 (Frame et al. 1995).
in BI Lyn and
Sgr may be similar.
Consequently a similar interpretation involving a
circumstellar envelope and a gas-stream flowing through the inner Lagrangian point
may be appropriate. The rapid increase in absorption reported
in H
(Wade & Potter 1995)
would correspond to eclipse of
the circumstellar material by the gas stream. Clearly, more
detailed work including determination of the orbital period and the
phase-dependency of the H
profile will be required to confirm this
hypothesis.
5 Conclusion
emission in BI Lyn
are consistent with the behaviour of other H-deficient giants, being caused
by pulsations and mass transfer respectively. Further observations are required
to determine the orbital period and mass ratio, to verify the pulsation
hypothesis and to correlate the H
behaviour with orbital phase.
This research has made extensive use of software
provided through UK PPARC Collaborative Computational Project No. 7
for "the Analysis of Astronomical Spectra'', software and facilities provided
through the UK PPARC Starlink project and the SIMBAD database,
operated at CDS, Strasbourg, France. We acknowledge financial support
from the Northern Ireland Department of Culture, Arts and Leisure.
References
Copyright ESO 2001