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W. Schmutz Institute of Astronomy, ETH Zentrum, CH-8092 Zürich, Switzerland
W. D. Vacca, L. Close, and J. Rayner Institute for Astronomy, Honolulu, Hl 96822
T. R. Geballe Joint Astronomy Centre, Hilo, Hl 96720, USA
H. Schild and R. Walder Institute of Astronomy, Zürich, Switzerland
The HST images have only recently been acquired and their analysis is currently underway. Here we report the possible detection of emission at a distance of about 03. This result is preliminary and needs confirmation by NIC-1 observations with its better sampling of the stellar profile.
binaries: close - stars: individual: Cyg X-3 - stars: Wolf-Rayet - infrared: stars - circumstellar matter
Cyg X-3 is one of the most luminous X-ray sources in the sky. On the
basis of its periodic X-ray variability, it has been interpreted as a
binary system in which substantial mass transfer occurs from a
companion onto a collapsed object (Bonnet-Bidaud & Chardin 1988).
Unfortunately, the enormous
reddening toward Cyg X-3 (EB-V>5) effectively prevents any optical
studies of this system (V > 24,
,
).
However, recent infrared observations of
Cyg X-3 have provided important information regarding the nature of
this object. The first K-band spectrum of the system, obtained by van
Kerkwijk et al. (1992), revealed strong and broad emission lines of He
I and He II and it was suggested that the mass donor is a
Wolf-Rayet (WR) star. Schmutz et al. (1996) found that the K-band emission
lines exhibited periodic wavelength shifts, presumably due to the
orbital motion of the WR star around the compact companion. The mass
function for the system was found to be 2.3 and reasonable estimates of
the mass of the WR star yielded a mass of the compact object of
M
;
thus, the compact object is an excellent black
hole candidate. These results strengthen the suggestion that Cyg X-3 is
representative of the endpoint of massive binary evolution.
Despite theoretical predictions for the formation of systems
composed of a WR star and a compact companion
(van den Heuvel & de Loore 1973), observational searches
for such objects had yielded negative results
(Willis et al. 1989; Moffat 1992; St-Louis et al. 1993) until the
observation by van Kerkwijk et al. (1992). The evolutionary steps for the
formation of such a system are O+O
RSG+O
WR+O
SN+O
c+O
c+RSG
c+WR, where c denotes a compact object. Numerical
calculations (Iben & Livio 1993)
indicate that the common envelope phase (c+RSG) is the
critical phase in the evolution, and few c+WR systems are expected to
exist as a result of the complete disruption of the secondary during
this phase. The fact that only one c+WR system has been found to date
indicates that it is indeed rare for a system to survive the common
envelope phase. Thus Cyg X-3 provides an excellent and completely unique
laboratory for testing the models of massive binary evolution.
It has been estimated that on the order of a few tens of solar masses of
material was lost during the common envelope phase of Cyg
X-3 and various models predict that the material should be concentrated
in the orbital plane (Livio & Soker 1988; Terman et al. 1994).
The current fast wind from the WR
star should interact with this slowly expanding circumbinary
material. This situation should lead to a ``ring'' of condensed matter
around the system, probably broken up into individual knots due to
Rayleigh-Taylor and other instabilities
(Garcia-Segura & Mac Low 1995), similar to the ring
observed around SN1987A. This is demonstrated in Fig. 1, which presents
predictions for the density contours resulting from
a 2-D numerical simulation in which a fast stellar wind collides
with a slowly expanding shell
(Folini & Walder 1998, personal communication;
see also Walder & Folini 1998).
![]() |
The size of the expected emission region should depend on the velocity
of expansion of the shell of material expelled during the common
envelope phase and on the time since the common envelope phase. The
expansion velocity in turn, is given by the accumulated momentum from
the wind that accelerated the shell's mass. (The initial expansion
from the shell can probably be neglected.) For a typical duration of the
WR phase,
yr, and an assumed average expansion
of
kms-1, we estimate an extension of 0.3pc. At
10kpc, the commonly adopted distance of Cyg X-3 (Dickey 1983), this
corresponds to an angular separation of 6. This estimate is
highly uncertain, and a search for nebular emission at both larger and
smaller separations is also necessary. For example, typical sizes of WR ring
nebulae are on the order of a few pc (Tutukov 1982), i.e. on the order
of 20 at a distance of Cyg X-3.
The morphology of the emission nebula can provide observational
constraints for the predictions of the common envelope mass loss and
the nebular emission can be used to place constraints on the initial
mass of the system. Contamination by swept up interstellar material
can be excluded as the source of any possible nebular emission
detected relatively close to Cyg X-3 (i.e. anywhere within the field of
view of the NIC-2 camera), because the predicted SN explosion prior to
the common envelope phase should have cleared out this region.
Detected nebular Pa
emission would therefore allow us to
calculate the circumstellar mass and obtain an estimate of the lower
limit for the initial mass of the WR star. This assumes that most of
the ionized material is density bounded. Alternatively, if the nebula
is ionization bounded, then we can probe the far UV radiation field,
especially if we detect the nebula in the light of He I,
[Fe II], and [S III]. In either case, an unambiguous
detection of nebular emission close to Cyg X-3 would confirm the
predicted common envelope phase with its dramatic mass loss.
![]() |
There is observational evidence for
extended nebular emission associated with the Cyg X-3 system.
Spectroscopic observations of Cyg X-3, obtained with
seeing and slit sizes, reveal the presence of narrow emission lines
superimposed on the broad WR emission features. An example of this
emission can be seen in Figs. 3 and 4 of Schmutz et al. (1996).
These narrow lines are
much stronger than any variable emission features often seen on top of
the WR emission lines in the spectra of WR binary systems and attributed to the wind-wind
collision zones (Moffat et al. 1996).
Therefore, the narrow emissions in Cyg X-3 are
probably not related to the WR wind but originate at larger distances
from the system.
We have recently obtained ground-based near-infrared images of Cyg X-3, using adaptive optics (AO) techniques, which also seem to suggest that extended nebular emission may be present in this system. In Fig. 2 (from Vacca et al. 1998) we present our H-band image, obtained with the University of Hawaii Adaptive Optics system on the CFHT. The total integration time for this image was 600sec. It was constructed by co-adding 20 individual frames, each with an exposure time of 30sec. The FWHM of this image is about 05. Because the only suitable reference star that can be used to make the AO wavefront corrections is 30 away from Cyg X-3 and has a brightness of only V=15 (both values are at the limit of what can be used in current AO systems), our exposure resulted in a PSF which is slightly elongated to the SE, in the direction of this reference star. Nevertheless, this image indicated that we may have detected extended emission to the west of Cyg X-3 in the region of object #2. Unfortunately the detection is marginal because of problems with the flat-fielding and background sky subtraction. But it is sufficiently far from the point sources that we thought it might be real until we saw the HST NICMOS images. In fact, it is an artifact resulting from the flat-fielding.
Recently, Ogley et al. (1997) have reported that Cyg X-3 appears to be slightly extended in their deep K-band images and they suggested that a second point source, a factor of 11 fainter than the primary object, was located within 056 of Cyg X-3. (Dilution by a second stellar object might also explain why the broad WR emission lines seen in the K-band spectra have unusually small equivalent widths for a WR star (van Kerkwijk et al. 1992; van Kerkwijk 1993; Schmutz et al. 1996). However, our AO images (Fig. 2; Vacca et al. 1998), do not confirm this suggestion. Despite the slight elongation of our PSF, a second point source with the claimed separation and brightness would be easily recognizable on our image. (We also do not find a second source in the K-band image). NICMOS images, with their much smaller PSF cores, easily settle this question and place much stricter limits on the brightness and separation of any possible additional source.
Cyg X-3 was observed with HST and NICMOS over 3 orbits on 1998 March 13.
We used the NIC-2 camera
in combination with the narrow band filters F187N, centered on Pa,
and F190N, for the adjacent continuum region.
For both filters we obtained 4 dither positions
with 2 step size. The total exposure times were 68 min and
34 min for the filters F187, and F190N, respectively.
For the analysis presented here we used the mosaiced images as delivered
by the standard STScI pipeline. The standard processing leaves
a clearly visible offset in the bias levels of the four
detector quadrants but otherwise, particularly in terms of image sharpness,
the processing resulted in excellent images.
In Fig. 3 we show the resulting image of the Cyg X-3
field.
![]() |
With the HST NICMOS observations we were searching for 4 types of emission:
![]() |
We can also address item #2 relatively easily. Using the intensities of the
stars D and Z we scale the image F190N with a factor of 0.97
to that of F187N and we produce a difference image F187N-F190N.
An investigation of this difference image does not reveal any extended
emission. The only signatures left above the background
are strong Pa
emission at the location
of Cyg X-3 and imperfect cancelations of the stellar images of
star D and Z that leave paired positive and negative intensities.
(The negative-positive residuals from stars D and Z integrate to zero
intensity, of course, because we have used them to scale the
continuum observation to that of the Pa
.) The strength of the
Pa
excess emission of Cyg X-3 is 40% of the continuum. This
value agrees very well with the emission strength estimated from ground-based
spectroscopy of Cyg X-3 (see Fig. 4 of Schmutz et al. 1996).
In fact, the emission is not from Pa
at all, but rather the broad
He II 6-8 stellar wind line.
With the difference image described above we have also addressed, in part,
item #3. However, if there is a nebular structure like a ring
nebulae around Wolf-Rayet stars then this would be outside the field
of the NIC-2 camera. We also have Pa
images of NIC-1 and NIC-3
that are obtained in parallel with the NIC-2 exposures. With the
parallel exposures we probe regions about 30 and 80 to the west of Cyg X-3. Although these images are not in focus they
would still allow to detect extended nebular emission. In the difference
images of NIC-1 and NIC-3 there is also no obvious nebular emission.
Of course, with this parallel exposures we are only covering only a small
fraction
of the total area out to, say, 100. Thus, item #3 cannot
be addressed conclusively with our HST NICMOS observations.
The only remaining issue is item # 1, nebular emission close
(i.e. )
to Cyg X-3. The detection of such emission requires
a deconvolution of the stellar image of Cyg X-3. As can be seen on Fig. 3
the point-spread function (PSF) of NICMOS is quite complicated. But the good news
is that it is supposed to be very reproducible, apart for some ``breathing''
effects which change the focus during an orbit.
![]() |
The extended nebular emission can also be detected in the original
image. In Fig. 6 we plot a west-east cut
![]() |
In view of the importance of Cyg X-3 to the understanding of massive binary evolution -- as the only representative of the c+WR class -- we have searched for nebular emission that would testify to the events during its previous evolutionary phase. We suspect that the previous phase was a common envelope phase which implies that the non-degenerate star has lost a substantial fraction of its mass. This mass is expected to be still around the system, probably in a ring. To date, there have been no clear detections of emission around Cyg X-3. However, on the basis of the first part of our HST observations, we report here the possible detection of matter close to Cyg X-3. Our result is preliminary and at this stage of the analysis it is still not clear whether the features seen are artifacts produced by subtle imperfections in our PSF. Clearly, we need a better sampling of the profiles of the Cyg X-3 image and of the PSF. We hope to achieve this in an upcoming HST observation with NIC-1 camera. If the extended nebular emission is confirmed, we will estimate the mass lost from the donor star during the common envelope phase. This result could have a profound impact on models of binary evolution.
WS is grateful to R. Hook and S. Stolovy for enlightening discussions on the properties of NICMOS PSF. WDV is grateful to Alan Stockton for partial support. Support for this work was also provided by NASA through grant number G)-07838.01-96A from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.