1997MNRAS.289...10W
Mon. Not. R. Astron. Soc. 289,10-20 (1997)
Radio and infrared structure of the colliding-wind Wolf-Rayet
system WR 147
P. M. Williams, 1 S. M. Dougherty,2,3 R. J. Davis,4 K. A. van der Rueht,5 M. F. Bode6 and
D. Y. A. Setia Gunawan5
1Royal
Observatory, Blackford Hill, Edinburgh EH93HJ
& Astronomy, University of Calgary, 2500 University Drive Nw. Calgary, Alberta T2N IN4, Canada
3 Dominion Radio Astrophysical Observatory, PO Box 248, White Lake Road, Penticton, British Columbia V2A 6K3, Canada
4 University of Manchester Nuffield Radio Astronomy Laboratories, Jodrell Bank, Macclesfield, Cheshire SKI I 9DL
5 Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
6Astrophysics Group, School of Electrical Engineering, Electronics and Physics, Liverpool John Moores University, Byrom St, Liverpool L3 3AF
2 Physics
Accepted 1997 February 12. Received 1997 February 12; in original form 1996 December 30
ABSTRACT
New, high-resolution infrared and radio images of the X-ray-Iuminous Wolf-Rayet system
WR 147 (AS 431) are presented. The 5-GHz radio image resolves both components of this
double source. The emission from one component, that to the south and associated with the
wind of the WN8 star, has an extent of -170 x 253 mas 2 , indicating that the stellar wind is not
spherically symmetric. The second, non-thermal component -0.6 arc sec north of the WN8
star is extended east-west to -267 mas. The infrared image reveals the presence of a
companion to the WN8 star, close to the non-thermal radio source but slightly (-60 mas)
more distant from the WN8 star. The companion is t::.K - 3 mag fainter than the WN8 star and
has a luminosity of a BO.5V star, just sufficiently luminous to possess a stellar wind capable of
colliding with that of the WN8 star. The presence of the non-thermal emission between the two
stars and much closer to that with the weaker wind is direct evidence for a colliding-wind
origin for the emission. A significant portion of the X -ray emission can also be accounted for
by the release of energy in the wind collision. Comparison of the non-thermal flux with those
of the three systems incorporating WC-type stars (WR 125, WR 140 and WR 146) shows a
correlation with velocity of the WR wind.
Keywords: stars: individual: WR 147 - stars: Wolf-Rayet - radio continuum: stars - X-rays:
stars.
1
INTRODUCTION
Wolf-Rayet (WR) stars emit free-free continuum radiation from
their extended ionized stellar wind envelopes. This emission is
readily observable in the infrared and, particularly, radio wavelength regions where the continuum spectrum has the form Sv ex pC/.,
with spectral index O! = 0.7-0.8, depending on the conditions in
the wind. In addition to this emission, a number ofWR stars exhibit
strong, non-thermal emission which is characterized by zero or
negative spectral index, variable flux density, and typically high
brightness temperature (TB - 106 _108 K). This emission is
thought to be synchrotron radiation arising from electrons accelerated to relativistic velocities by first-order Fermi acceleration in
shocks, arising from either wind instabilities in single stars (e.g.
Chen & White 1994), or the region where the winds of two
components of a massive binary collide (e.g. Eichler & Usov
1993). A third mechanism, acceleration of the electrons to relativistic velocities by magnetic field compression from the
colliding winds, has been proposed by Jardine, Allen & Pollock
(1996).
Four WR stars have been studied as non-thermal emitters:
WR 125, WR 140 (lID 193793), WR 146 and WR 147 (Williams
1996). Of these, WR 140 (WC7+04-5V) has been demonstrated to
be a binary system (Williams et al. 1990a), while spectroscopic
evidence for O-type companions to the WC stars in WR 125 and
146 was found by Williams et al. (1 994b) and Dougherty et al.
(1996), suggesting that the non-thermal emission in all three
systems arises from the interaction of the WR winds with their
companions.
In the case of WR 140, the non-thermal emission was shown to
vary in phase with the system's 7.94-yr period by Williams et al.
(1990a), who proposed a model in which a non-thermal source in
the wind-collision region was taken to be much closer to the 04-5
star than to the WC star on account of the higher mass-loss rate of
the latter. The non-thermal source therefore moves with the 04-5
star inside the stellar wind of the WC star during the orbit. As a
© 1997RAS
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
The colliding-wind Wolf-Rayet system WR 147
result, the free-free opacity in the WR wind along lines of sight to
the collision region changes throughout the orbit as the geometry
changes. Our ability to observe the non-thermal emission requires
the absence of appreciable free-free opacity along our line of sight
to the emission region which, in the case ofWR 140, occurs for only
a fraction of each period. This model has been developed quantitatively by Eichler & Usov (1993). Multi-frequency light curves
were observed and used by Williams, van der Hucht & Spoelstra
(1994a) and White & Becker (1995) to study the circumstellar
extinction and intrinsic non-thermal emission. In particular, White
& Becker argued from the variation of the intrinsic non-thermal flux
that the undisturbed wind of the WR component was not spherical,
but flattened somewhat into a disc.
The non-thermal radio emission from WR 125 faded between
1984 and 1988, perhaps due to an increase in circumstellar extinction as the system approached periastron passage shortly before its
dust-formation episode (Williams et al. 1992). The non-thermal
emission had not returned by 1995 (Contreras et al. 1996), indicating that, if the variation is periodic like that of WR 140, the
period is in excess of 13 yr. This is consistent with the minimum 15yr interval between dust-formation episodes inferred by Williams
(1996) from the infrared light curves.
Dougherty et al. (1996) have recently resolved the radio emission
from WR 146 into two components and suggested that the nonthermal component is associated with a wind-collision region
between the WC6 star and a companion star. From optical spectroscopy the companion has been identified as late O-type, expected to
have a much lower mass-loss rate than the WC6 star. Consequently,
the wind-collision region and non-thermal source are expected to be
much closer to the 0 star than to the WC star. The wide separation
(D cos i -140 au) results in a low circumstellar extinction along our
sight-line to the non-thermal source through the WR wind.
Strong radio, infrared and X-ray emission from WR 147 (AS
431) was first reported by Caillault et al. (1985), who also identified
it from a red spectrum as a heavily reddened WN8-type star. The
spectral type was confirmed by Conti & Massey (1989). From the
radio flux density Caillault et al. derived an unusually high massloss rate (-3.9 x 10-4 Mo yr- I ), assuming a distance of2 kpc and
a wind terminal velocity v= = 2000 km S-I. At the same time, they
recognized the uncertainty of the nature of the radio emission: its
partial resolution suggested a thermal source while its negative
spectral index suggested that it was non-thermal.
This apparent contradiction was removed by high-resolution,
5-GHz MERLIN observations of WR 147 which demonstrated that
the emission arose in two components. One component was shown
to be coincident with the optical image of the WN8 star and
identified as its thermal wind emission; and the other, 0.6 arcsec
to the north, was identified with the (possible) source of nonthermal emission (Moran et al. 1989). From multi-epoch, multifrequency VLA observations, Churchwell et al. (1992) confirmed
that the northern component was a non-thermal emitter with a
2-6cm spectral index of - - 0.5, compared with a spectral index
of - + 0.6 for the southern component, consistent with free-free
emission from the wind of the WN8 star. The two components were
observed to be joined by a bridge of emission at 15 GHz, indicating
that the sources were spatially related rather than being a chance
alignment of the WR star with an unrelated non-thermal source.
Churchwell et al. (1992) used a combination of optical and
infrared photometry to re-determine the reddening and distance to
WR 147. Their new distance of 630 pc made WR 147 the second
closest WR star to the Sun. From considering the radio flux from
the southern, stellar wind component, Churchwell et al. derived a
11
mass-loss rate of 4.2 x 10- 5 Mo yr- I, similar to those of other WR
stars. Contreras et al. (1996) found a similar value from a 43-GHz
observation, taken to be uncontaminated by non-thermal emission.
In his survey of X-ray emission from WR stars, Pollock (1987)
determined a 0.2-4 keY X-ray luminosity of (47 ± 6) x 1032 erg
s-I from Einstein hard-band observations of WR 147, assuming a
distance of 1.9 kpc. Re-scaled to a distance of 630 pc, this becomes
(5.2 ± 0.7) x 1032 erg S-I, or 0.13 ± 0.02 Lo. The extinction
derived by Churchwell et al. implies a hydrogen column density
NH - 2.3 X 1022 cm- 2 , 20 per cent higher than that adopted by
Pollock, so that derived luminosity will be higher also. The high
extinction may also account for the low ROSATO.2-2.4 keY X-ray
luminosity of (0.58 ± 0.31) x 1032 erg S-I reported by Pollock,
Haberl & Corcoran (1995), although variability cannot be ruled out.
Churchwell et al. examined several possible mechanisms for the
X-ray emission. They found that only the capture of a slow, dense,
equatorially enhanced wind from the WN8 star by a neutron star
could produce the observed flux, since a colliding-wind massivebinary model appeared to be ruled out by the speckle observations
of Lortet et al. (1987), who found no optical companion with
Ilm < 3 at A = 7000 Awithin 1.5 arcsec of the WN8 star. However,
Churchwell et al. could not rule out the possibility that the X-rays
were produced in a collision between the wind of the WN star and
that of an unseen companion.
We re-observed WR 147 in the radio to examine the structure
evident in the maps by Moran et al. and Churchwell et al., exploiting
the higher resolution possible with MERLIN following the commissioning of the Cambridge antenna. A preliminary reduction of
these data was presented by Davis et al. (1995). In addition, we
extended the search for a companion to the infrared, given the
severe interstellar extinction suffered by WR 147. We also observed
the system at mid-infrared and millimetre wavelengths to form a
spectral energy distribution extending from the optical to the radio
wavelength regime.
Our new 2-fLm map shows the presence of an infrared companion
- 3 mag fainter than the WN8 star and a little beyond the nonthermal radio component. We take this as evidence that WR 147 is a
double star and discuss the system as a colliding-wind binary. We
compare the wind properties of WR 147 with those of other WR
systems showing non-thermal radio emission.
2
RADIO OBSERVATIONS
Radio observations were obtained using the MERLIN array on 1992
June 24 (C band, 5 GHz) and 1993 April 11 (L band, 1.6 GHz) with
nominal resolutions of 50 and 150 mas respectively. The total onsource integration times were 14.5 and 11 h at 1.6 and 5 GHz
respectively. To establish the complex antenna gains, the on-source
observations were interleaved with frequent observations of the
nearby radio-bright quasar 2005 + 403. The absolute flux scale was
established by observation of 3C 286 and the unresolved sources
OQ 208 (1.6 GHz) and 0552+398 (5 GHz). The fluxes adopted for
3C 286 and determined for the other sources, including the phasereference source 2005 + 403, are given in Table 1.
The data were initially amplitude-calibrated using MERLIN
software, and then transferred to the NRAO AlPS package for
phase calibration and imaging. The 'dirty' images were deconvolved using the CLEAN algorithm (see Cornwell & Braun 1989, and
references therein), giving the final synthesized images shown in
Fig. 1. The images presented here have a resolution approximately
three times that of the data of Moran et al., which had a synthesized
beam FWHM of 150 x 130 mas2 • This is largely due to the fact that
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
12
P. M. Williams et al.
15 GHz. Our observation confirms the spatial connection between
WR l47N and S and rules out the possibility that N is a background
radio source in a chance alignment with S. As yet, the nature of the
bridge is not known.
Fluxes for the two components were derived by fitting twodimensional-Gaussian functions to the emission features (AlPS
routine JMF1T) in Fig 2. At the full 5-GHz resolution of MERLIN
(Fig. 1) this method gives a poor estimate of flux, largely
because neither the radio emission from N nor that from S is
well approximated by a Gaussian function. In addition, it is
apparent from the many small patches of low-surface-brightness
emission in the 5-GHz image that some flux is resolved out by
MERLIN. For comparison, we have determined the fluxes for the
features in Fig. 1 by integrating the flux over the area occupied
by each source (AlPS routine TVSTAT). In addition, we also
convolved the data with lower resolution beams to estimate the
total flux from WR 147 as detected by MERLIN. The fluxes at
1.6 GHz were determined in a similar fashion. These fluxes are
given in Table 2.
In Table 3 we give the positions derived from Fig. 2. The error in
the radio positions is dominated by the error in the position of the
radio phase-reference source relative to the optical reference frame.
For the MERLIN calibrator used in these observations this is
approximately ± 10 mas. The separation of the two radio components is 575 ± 15 mas. This compares very well with the separations of 600 mas derived by Moran et al. and 580 mas by
Churchwell et al.
Table 1. Fluxes of the radio calibrators.
Source
3C286
0552+398
2005+403
OQ208
5GHz
(Jy)
1.6 GHz
(Jy)
7.31 a
6.65
2.83
13.64a
2.36
1.08
aFrom Baars et al. (1977).
the present observations were made after the Cambridge antenna
was added to MERLIN, increasing both baseline coverage and the
sensitivity on the longest baselines.
At 5 GHz, WR 147 is resolved into two components, Nand S, as
previously observed by Moran et al. (1989) and Churchwell et al.
(1992). At 1.6 GHz, component N is easily observed but, at the
position of S, the radio emission is only detected at around the 3a
level. Moran et al. showed that the position of WR 147S was
coincident with the optical position of the WR star.
We have convolved the 5-GHz data with a beam similar to that of
Moran et al. to enhance low-surface-brightness emission (Fig. 2). It
can be seen that there is low-surface-brightness emission extending
to the north of N, and between N and S. This latter emission is the
'bridge' of emission that Churchwell et al. (1992) observed at
(a)
40 10 38.6
(b)
401038.6
-'
0
38.4
S
It)
0
38.0
en
§.
Z
0
0
@
z
0
;::
0
c(
38.0
S
It)
en
37.8
o
c(
Z
Z
w
38.2
§.
37.8
:::;
0
0
~
;::
c
0
G
38.2
38.4
37.6
37.4
37.2
37.0
36.8
0
0
Ij
0
0
:::;
0
w
37.6
c
37.4
+
(j'J
37.2
37.0
36.8
•
Figure 1. The final deconvolved MERLIN images at (a) 5 GHz with a synthesized beam FWHM of 58 x 57 mas 2 and (b) 1.6 GHz with a beam of 178 x 172 mas 2 .
The beams are shown in the lower right-hand corners of the images. Both images are at the full resolution of MERLIN (i.e. with no tapering) and are presented at
identical scales. The la TIllS noise levels in the images are 89 and 13511Jy beam- J and the contour levels represent -3,3,5,7,9,110', and
3,6,9,12,15,18,20,220' respectively. The negative contours are denoted by dashed lines. The crosses in (b) indicate the positions of Nand S observed in
the 5-GHz image.
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
The colliding-wind Wolf-Rayet system WR 147
40 10 38.6
Table 2. Fluxes of the radio emission from WR 147 from MERLIN
maps.
:J
.J
o
38.4
'.
p
Sv(N)
S.(S)
(GHz)
(mJy)
(mJy)
5
1.6
38.0
"'~
e.z
0
~Z
::::;
I.)
w
37.6
Method
9.2:!: 1.8
4.5:!: l.0
11.4 :!: 0.3
5.9:!: 0.2
6.8 :!: O.la
1l.8 :!: OJ a
11.4:!: 0.6
16.3 :!: 0.2b
0.058 x 0.057
0.15 x 0.13
0.3 x 0.3
0.178xO.172
2x2
TVSTAT
JMFIT
JMFIT
JMFIT
JMFIT
a,bThe total flux detected with MERLIN at (a) 5 GHz, and (b)
1.6 GHz.
37.8
Q
Beam
,
38.2
6'
13
0
Table 3. Positions (equinox B 1950, epoch 1992.42) of the 5GHz components relative to the extragalactic radio frame, and
source sizes derived from model fits to the visibility data.
37.4
37.2
N
S
.!l
37.0
RA
(:!:0.".01)
20h34m
Dec
(:!:0.".0l)
40° 10'
53 s .853
53 s .860
0.".070
38".00
37".43
0.".57
36.8
aFWHM;
Figure 2. The 5-GHz data in Fig. 1 convolved with a 150 x 130 mas 2 beam,
similar to that of Moran et al. (1989) (see lower right-hand comer). This
increases the sensitivity to extended emission. A bridge of emission between
WR 147S and WR 147N is clearly observed. The 10' rms of the image is
82 fLJy beam- 1 and the contour levels are -3,3,5,7,11,15,19,23,27,31,
350'. Negative contours are denoted by dashed lines.
Both N and S are clearly resolved in our observations, N being
elongated with an apparent east-west orientation, and S clearly not
a symmetric, uniform emission region, as often assumed a priori in
calculations, for example, of mass-loss rates in hot star winds.
Indeed, WR 147S appears to have a complex intensity distribution.
However, since possibly as much as half of the emission is resolved
out by MERLIN (see below), any detailed analysis of the intensity
distribution of S requires the addition of contemporaneous lower
spatial frequency visibility data than observed with MERLIN.
Meanwhile, we can estimate the size and aspect ratio of the 5GHz emission from WR 147N and S from a simple analysis of the
visibility data presented here. After locating the peak of S at the
phase-tracking centre of the image, we removed the flux contributed
by N from the visibility data using the AlPS routine UVSUB. The
remaining visibility data were then binned into a 20 x 20 grid to
improve the signal-to-noise ratio. A two-dimensional Gaussian fit
to the gridded data then gives an estimate of the source size, flux
density and position angle. A similar procedure was followed for N.
The derived sizes are given in Table 3. From these results, the aspect
ratios for N and S are 3.4 ::!:: 0.5 and 1.5 ::!:: 0.15 respectively.
Also, in a long-term monitoring programme (van der Hucht et al.
1995), three more 5-GHz observations were taken with the WSRT
in the first half of 1993 with a 3.7-arcsec synthesized beam and
reduced using Westerbork's NEWSTAR software. The flux densities,
on the Baars et al. scale, are consistent within the uncertainties and
are averaged for a '1993.3' flux density in Table 4.
Our MERLIN flux densities (Table 2) are significantly lower than
our 1993 WSRT data and those derived by Churchwell et al. and in
(Ja
0';
(mas)
(mas)
x
267:!: 27 79:!: 10
170:!: 14 253:!: 18
PAb
n
93 :!: 3
65:!: 7
bposition angle, moving east from north.
other studies. These have yielded total 5-GHz fluxes -36 mJy
(Table 4), usually from the VLA in the compact C, C-D or D
configurations. In these configurations, WR 147 is largely unresolved and these fluxes closely represent the total flux, from both
compact and extended components ofWR 147. The higher resolution A-configuration observations of Churchwell et a1. gave slightly
lower flux densities, but the differences are barely significant.
Churchwell et al. also reported Westerbork observations with a
3.5-arcsec beam. We averaged the 1989 and 1990 6-cm data to give
flux densities for epochs '1989.7' and '1990.4'.
Comparison with the total5-GHz flux of 18.6 ::!:: 0.3 mJy that we
derive suggests two possibilities: either MERLIN detected only half
of the total emission of the source; or the source may have been
significantly fainter at the time of our observations (1992) than at
other times. Given that the stellar wind component, S, is the
principal contributor to the 5-GHz flux from WR 147 (Fig. 3,
later), the variability option seems unlikely. A -50 per cent fall
in the emission by a stellar wind implies a -40 per cent fall in the
star's mass-loss rate over a period of years followed by a recovery in
-180 d, little more than the -130-d dynamical time-scale of the
wind. This cannot be ruled out, but such a phenomenon has not been
observed from previous radio and extensive infrared observations of
WR winds.
The alternative to variability is that a significant fraction of the
5-GHz flux from WR 147 is from a more extended component than
observed by MERLIN, which is only capable of detecting emission
on size-scales up to about 700 mas at 5 GHz (on account of its
shortest antenna spacing being 6.4 kro). Support for this view comes
from the fact that the flux detected by Moran et al. is rather lower
than that cited in their paper because they used too high a flux for the
calibrator, 2005+403. The corrected flux is given in Table 4 and is
also lower than those determined with the VLA and WSRT, which
are mutually consistent and show no evidence for significant
variability. This is consistent with the small dispersion of the
near-infrared magnitudes observed by Churchwell et al. in 1983
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
14
P. M. Williams et ai.
Table 4. Comparison of previous 5-GHz fluxes from WR 147.
N (mJy)
Date
1984.3
1984.5
1984.9
1985.1
1988.1
1989.7
1990.4
1992.5
1993.3
1995.3
S (mJy)
11.9 ± 0.4a
13.3 ± O.4a
8.3 ± 0.3
21.4 ± 0.4
19.4 ± 0.4
16.7 ± 0.3
6.8 ± 0.1
11.8 ± 0.1
TotaI (mJy)
Telescope
Ref.
35.3
38
33.3
32.7
25.0
36.3
33.8
18.6
37.2
38.36
VLA(C)
VLA(C-D)
VLA(A)
VLA(A)
MERLIN
WSRT
WSRT
MERLIN
WSRT
VLA(D)
Abbott et aI.
Caillault et aI.
Churchwell et aI.
Churchwell et aI.
Moran et aI. b
Churchwell et aI.
Churchwell et aI.
this paper
this paper
Contreras et aI.
±
±
±
±
±
±
±
±
±
±
0.13
1
0.6
0.6
0.4
0.4
1.2
0.2
0.2
0.09
These uncertainties are estimates taking the background rms in the synthesized images and
multiplying by estimates of the size of the source relative to the beam size. b Using a flux
density of 3.4 Jy for 2005+403 instead of 4.4 Jy used by Moran et aI.
a
Table 5. Mid-infrared photometry ofWR 147.
Band
M
[8.75]
[9.7]
[11.6]
[12.5]
Q
mag.
2.85
2.47
2.77
2.15
1.75
1.31
that of the WR 147S-N system, it is unlikely that N lies in the
higher density equatorial plane of S unless both systems are inclined
at angles that combine to bring this about. More probably,
WR 147N lies at an intermediate latitude ofWR 147S. Accordingly,
in our calculations involving the wind density ofWR 147S, we will
use the 'spherical' wind mass-loss rate derived conventionally, but
will note the possible effects of departures from this.
Table 6. Submillimetre photometry of WR 147.
Date
450fLm
(Jy)
800fLm
(mJy)
1.1mm
(mJy)
1.3mm
(mJy)
2.0mm
(mJy)
1991
1992
1.2±0.2 357±70
368±40
292±15
263±30
280±30
287±30
275±30
230±50
and 1990, and agreement with those from Caillault et al., showing
the flux level from the WR 147 stellar wind to be constant to within
about 5 per cent. We therefore conclude that MERLIN resolves out
approximately half the flux, presumably from an extended component. It is not clear why the fluxes from the two MERLIN studies
differ but, as Moran et al. did not give details of how their fluxes
were calculated, we cannot take this further at present. Therefore,
we will rely on our MERLIN observations for discussion of
structure but use fluxes from Churchwell et aI. for the luminosities.
The shape of the non-thermal component, N, will be discussed in
Section 5 but, as to that of S, we note that this is the first time that an
asymmetric geometry in a WR star wind has been observed directly.
Previously, the wind of "{2 Velorum had been resolved at 5 GHz by
Hogg (1985), who found that the flux density observed at a given
VLA antenna separation was independent of the position angle of
the baseline, implying a spherical wind. Polarimetric observations
suggest that a few WR stars have flattened winds (Schulte-Ladbeck
1995) and, as noted above, White & Becker (1995) inferred from
the radio light curve of WR 140 that its wind is flattened. Theoretical studies of the winds of rotating WR stars (Cassinelli, Ignace &
Bjorkman 1995) show that high-density wind-compressed zones
(WCZs) can form in the stars' equatorial planes. The density
contrasts between the equatorial and higher latitudes are significantly smaller than those of the wind-compressed discs (WCDs)
proposed for Be stars, but can still reach -18 depending on the wind
acceleration and stellar rotational velocity (Ignace, Cassinelli &
Bjorkman 1996). As the position angle of WR 147S differs from
3 SPECTRAL ENERGY DISTRIBUTION OF
WR147
Churchwell et al. used optical and near-infrared photometry to
re-determine the reddening and distance to WR 147. We also
obtained mid-infrared photometry in 1983 and use these data,
together with data from lRAS and submillimetre and millimetre
observations, to extend the near-infrared spectral energy distribution (SED) of WR 147 to longer wavelengths for comparison with
the radio fluxes of the two components.
The mid-infrared observations were made using the United
Kingdom Infrared Telescope (UKIRT) on 1983 July 14 (the night
after the first set of near-infrared data reported by Churchwell et al.)
with the common-user bolometer photometer, UKTI. Both components ofWR 147 were included in the 5-arcsec focal-plane aperture,
and we used the intermediate band-pass (.lA - 1 fLm) filters in the
8-13 fLm region, together with the broad-band 4.8-fLm M and
20-fLm Q filters, to help define the SED. The magnitudes are
given in Table 5. The relative faintness of the 9.7-fLm magnitude
compared with the other magnitudes in the 8-13 fLm region is
attributable to extinction by the 'silicate' feature, consistent with
the high reddening (Av = 11.5 mag, Churchwell et al.) towards
WR147.
The mid-infrared data were strengthened and extended using
lRAS fluxes (also obtained in 1983) from Cohen (1995), which were
corrected for the spectral shape evident from the ground-based data
following the prescription in the lRAS Explanatory Supplement
(Beichman et al. 1988).
Our submillimetre and millimetre observations were made with
the common-user 3He-cooled bolometer UKT14 (Duncan et al.
1990) on the James Clerk Maxwell Telescope (JCMT) on 1991
March 21-23 and 1992 May 5-6. The beam size is dependent on
wavelength but was always sufficiently large to include both
components of WR 147. Flux calibration was by observation of
Uranus and secondary standards (Sandell 1994). For the relative
© 1997 RAS, MNRAS 289, 10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
The colliding-wind Wolf-Rayet system WR 147
The spectrum of WR 147N is very different. Plotted in Fig. 3 is
the 5-15 GHz spectrum observed by Churchwell et al. in 1984,
which has a spectral index ex - -0.4. They did not resolve the
components at 1.45 GHz but, from subtracting the extrapolated
wind spectrum from the total flux, estimated a 1.45-GHz flux for N
which suggested a slightly flatter spectrum longward of 5 GHz.
From the relatively low upper limit to the 22-GHz flux found by
Churchwell et al. in 1985, there appears to be a sharp highfrequency cut-off which needs confirmation and examination. The
integrated spectrum ofN gives a non-thermal luminosity of7 x 1028
erg S-I. Also plotted are our 5- and 1.65-GHz fluxes of N. As
described above, the differences between our fluxes and those of
Churchwell et al. are most probably attributable to MERLIN's
inability to detect extended emission. Consequently, the agreement
between the spectral index (-0.45) indicated by our 1.6- and 5-GHz
observations and that by Churchwell et al. is fortuitous.
g
o
A
A
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.
~
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c:
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,
til, ,
)(
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G:
,
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,
d
q
4
,,
'0,
Av = 11.5, a(S) = 0.66
o
10
15
100
1000
Wavelength v.m)
Figure 3. Spectral energy distribution (SED) ofWR 147 from de-reddened
optical (L':,), ground-based infrared (e) and lRAS (0) data. Our JCMT data
are marked *, the 250-GHz flux given by Altenhoff, Thurn & Wendker
(1994) is marked 0 and the 43-GHz flux from Contreras et al. (1996) is
marked 0 - all including both components of WR 147. Radio data for the
separate components from Churchwell et al. are marked EB and from this
paper (N only) *. The shift between the fluxes of N from Churchwell et al.
and the present study (see text) is evident. The dashed line has the form
S. DC pO.66, appropriate for a stellar wind spectrum.
extinctions at different wavelengths, we followed Stevens &
Robson (1994). The fluxes are given in Table 6.
These optical-infrared data were de-reddened by Av = 11.5
mag and are plotted in Fig. 3. It is evident that the de-reddened
mid-infrared-millimetre fluxes and the radio fluxes ofWR 147S fit
a power-law SED, S. IX pOI., characteristic of a stellar wind. This
index of this spectrum (ex = 0.66 ::!:: 0.02 by least squares) is
slightly greater than that (0.6) predicted (e.g. Wright & Barlow
1975) for an isothermal, spherically symmetric, steady-state wind,
but less than those (ex = 0.78) observed for the WC systems
WR 140 (Williams 1996) and WR 146 (Dougherty et al. 1996).
The higher spectral indices are due to the ion density falling off
more rapidly than r2. This could be caused by a number of factors:
the failure of the winds to have reached their terminal velocities at
the radio continuum forming regions, the ionization fraction
changing with radius (cf.l Vel: Williams et al. 1990b; Leitherer
& Robert 1991) or a geometry that is changing with radius. The
450-fJ,m flux lies about 30' above the power-law wind spectrum, but
the observed flux came from one night only and the high, and
variable, atmospheric extinction at this wavelength makes calibration difficult.
THE INFRARED IMAGE AND COMPANION
Infrared observations of WR 147 were obtained in the Service
Observing Programme of the United Kingdom Infrared Telescope
(UKIRT) on 1996 May 28 using the IRCAM3 camera and ALICE
array control system (Aspin et al. 1997). The camera was configured to give an image scale of 0.057 arcsec pixel- 1 and operated in
shift-and-add mode. Each observation is broken up into a large
number of very short integrations. Provided that the field contains
one, dominant point source, the centroid of this source on the array
can be located for each integration and the images shifted in real
time to bring them into alignment. This system is very well suited to
searching for faint companions to bright point sources - such as the
WN8 star in WR 147. Two such observations through the K filter,
each comprising 10 000 integrations of 36-ms duration, were added
to provide the 'raw' image in Fig. 4.
In addition to the clearly observed WR star, it is evident from
Fig. 4 that there is a faint infrared companion located slightly to the
north. The slight east-west asymmetry in the halo of the image of
the WR star is a consequence of the alignment of the telescope
optics, and was observed in our other images taken during our
programme. We used the image of a single star to define the
telescope point-spread function, and applied the Starlink implementation (MEM2D) of the maximum entropy deconvolution algorithm to our image ofWR 147 to give the deconvolved image in Fig.
5, shown superimposed on the 5-GHz MERLIN observation. The
WR147
(K]
Figure 4. Near-infrared (2 fLm, K band) image of WR 147 showing the
position of a companion source (NlR) to the WN8 star. Each pixel is 57 x 57
mas2 •
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
16
P. M. Williams et al.
40 1038.6
o
38.4
..
'
G ()
38.2
•
38.0
z
0
~
z
37.8
-"
::::;
0
'"c
37.6
0
0
37.4
37.2
37.0
36.8
203453.90
53.88
53.86
53.84
RIGHT ASCENSION
53.82
Figure 5. Maximum entropy reconstruction of the 2.2-fLm image of WR
147, re-gridded to IS-mas pixels and superimposed on the S-GHz image
from Fig. 1(a). The infrared image of the WR star has been aligned with the
position of WR 147S. The FWHM of WR 147S in the IR image is 0.11
arcsec, the diffraction limit of UKIRT at this wavelength. WR 147NIR is
clearly visible to be slightly to the north ofWR 147N. The apparent infrared
emission to the south-east of the WR star is an artefact; a very similar feature
appeared in the reconstruction of another source observed at the same time.
which lies in the direction of the association Anon Cir (Lundstrom
& Stenholm 1984). The colour-magnitude diagram of this region
(Bassino et al. 1982) shows possible groupings at two different
distances and, as there is no external evidence that WR 66 belongs
to either, this does not provide a luminosity calibration for WN8
stars. In the absence of a better calibration, we follow Churchwell
et al. in adopting an absolute magnitude of K = -5.96 for
WR 147, which gives an absolute magnitude of K = -2.9 for
WR147NIR.
We can then determine the average absolute K magnitudes of hot
stars of different spectral type from the Mv given by Underhill &
Doazan (1982) and intrinsic (V - K) colours from Koornneef
(1983). This gives MK = -3.0 for BOV, -2.9 for BO.5V and -2.7
for B 1V. We therefore assume WR 147NIR to have the properties of
a BO.5V star. Of course, this is not a spectroscopic classification,
and we recognize that the uncertainty in the absolute magnitude of
the WN8 star implies that, if WR 147NIR is a main-sequence star,
its type could lie anywhere between late-O and mid-B. Given that
the visual (V) magnitude difference between the primary and
secondary is also -3 mag for a BO.5V companion, Lortet et al.
(1987) were unlucky that the brightness of WR 147NIR was at the
limit of their speckle observations.
It is also possible that the MK = -2.9 star is a late-type star:
about K7ID if a giant, an earlier type if a luminosity class IT star, but
not a supergiant (too faint) or main-sequence star (too bright). If
WR l47NIR were a K-type giant, the visible magnitude difference
between the two stars would be greater than 3 mag. However, if the
companion were coeval with the WN8 star, it is difficult to see how
the secondary could have evolved on to the giant branch in the
lifetime of the WR star primary. Therefore, we follow our hypothesis that WR 147NIR is a BO.5V star.
5
5.1
FWHM of the deconvolved image of the WN8 star is 2 pixel
(0.11 arcsec) in both coordinates, whereas the image of the companion appears to be slightly broader east-west. As this is similar to the
resolution of the telescope and may well be an artefact, we do not
regard this elongation as significant' and, for the present, consider
the companion to be point like.
The infrared companion is 11.0 ± 0.2 pixels (0.63 ± 0.01
arc sec) north and 1.7 ± 0.3 pixels (0.10 ± 0.02 arc sec) west of
the WN8 star, giving a total separation of 0.64 ± 0.02 arcsec, which
corresponds to 403 ± 13 au at a distance of 630 pc. This places the
companion, hereafter referred to as WR l47NIR, slightly more
distant from the WN8 star than is the peak of the non-thermal
emission (cf. Table 3) in both coordinates. If the southern, thermal
radio component S is centred on the infrared image of the WN8 star,
the peak of the non-thermal emission is not coincident with
WR 147NIR, but lies between the two stars (Fig. 5).
To get an idea of what type of star WR 147NIR could be, we
estimate its absolute magnitude from the magnitude difference
(AK = 3.04 ± 0.09) derived from our image and the absolute
magnitude of the WN8 star. We retain the distance (630 pc) and
absolute magnitude determined by Churchwell et al. based on
comparative infrared photometry ofWR 147 and WR 105, despite
the fact that the latter has recently been re-classified as a WN9h star
by Smith, Shara & Moffat (1996). The only WN8 star that might
have a distance and absolute magnitude determinable from
membership of a stellar association is WR 66 (HD 134877),
DISCUSSION
WR 147 as a colliding-wind binary
The discovery of an infrared stellar companion to the WN8 star
leads us to consider the possibility that WR 147 is a colliding-wind
binary (CWB) system, where the non-thermal emission arises in a
wind-collision region, as inferred in WR 140 and 146.
First we address the question whether a BO.5 main-sequence star
could have a substantial stellar wind. The onset of mass loss,
observable in the ultraviolet, by stellar winds from stars on
the main sequence is quite sudden at Teff = 27500 K, log LlLo =
4.4 ± 0.1 (Grigsby & Morrison 1995). From Underhill & Doazan
(1982), we see that the effective temperature of a BO.5V star is
29 000 K. From the relation between bolometric correction and
temperature (B.C. = -0.5 - 0.08TIl03 ) given by Howarth &
Prinja (1989), we estimate a bolometric correction of -2.8 and
hence a bolometric magnitude of -6.5 for WR 147NIR. The
corresponding luminosity is log LlLo = 4.5. From this, it is evident
that WR 147NIR is just sufficiently hot and luminous to have a
substantial stellar wind which can collide with that of the WN8 star.
However, we should not lose sight of the dependence of this result
on the adopted luminosity of the WN8 star: WR l47NIR has a
luminosity close to the borderline for stellar wind, and a small
uncertainty in luminosity translates into a large uncertainty in massloss rate. In any event, we should expect the mass-loss rate of
WR 147NIR to be much lower than that of the WN8 star: Runacres
& Blomme (1996) derive mass-loss rates of 5 - 6 X 10- 8 Mo yr- 1
for the BO.5IVand BO.2V stars A Lep and T Sco, which also have
luminosities log LlLo - 4.5.
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
The colliding-wind Wolf-Rayet system WR 147
N
17
The scale of the system and position of the contact discontinuity
are related to the separation of the two stars, D, through the ratio of
wind momenta, 1/, by
1/2
rOB
o
Figure 6. Sketch of WN and B star components showing the location of the
shocked gas in the wind-interaction region (shaded) on either side of the
contact discontinuity (solid line). The intersection of the contact discontinuity and the line between the stars is referred to as the stagnation point.
The winds of the WN8 and BO.5 stars are expected to collide
where their momenta balance, forming a contact discontinuity. This
is expected to occur relatively close to the BO.5 star owing to its
lower mass-loss rate. The scale of the wind-interaction region is
determined by the distance rOB of the contact discontinuity from
the BO.5 star (Eichler & Usov 1993). The interaction region
(Fig. 6) has the form of a cap of height rOB and diameter 1TrOB
centred on the BO.5 star, facing the WN8 star, and a cone facing
away from the WN8 star. If the non-thermal emission were
produced by electrons accelerated here, we would expect the
source to resemble the interaction region both in its proximity to
the BO.5 companion and in its shape - elongated perpendicular to
the line between the components with an aspect ratio ~1T,
depending on the inclination and extent to which emission
arises 'behind' the BO.5 star. Our observations (Table 3 and
Fig. 5) fit well in both respects, giving support to the CWB
interpretation of the non-thermal emission.
We therefore estimate the scale of the interaction region by
relating the observed width of the source, ~267 mas, to the crosssection 1TrOB' This would give rOB ~ 85 mas, an overestimate if the
emission arises from 'outside' the contact discontinuity. From the
difference between the infrared and radio separations, we have the
projected value of rOB, rOB cos i = 60 ± 30 mas. This value of rOB is
quite uncertain since it comes from the difference of two much
larger quantities and we do not know which part of WR 147N
coincides with the position of the contact discontinuity. It also
depends on the (unknown) inclination but is consistent with the
value of rOB estimated from the width of the source, so that the 50
per cent uncertainty is pessimistic. For further discussion of the
system, we will adopt rOB = 80 mas and rOB cos i = 60 mas,
recognizing their uncertainty.
1/
D.
= --II2-
1+1/
(1)
The infrared image gives the projected value of D,
Dcos i = 635 ± 20 mas, directly. Comparing this with rOB cos i =
60 ± 30 mas derived above gives rOB ID ~ 0.09 ± 0.05, and
1/ ~ 0.011~~, a value that is independent of the inclination angle
of the system.
For spherical winds, the wind-momentum ratio is given by
1/ = (MV~)OB / (MV~)WN' Here, we will use our observed value of
1/ and the wind momentum of the WN8 star to estimate that of the
BO.5 star. Churchwell et al. derived a wind velocity of 900 kIn S-I
for the WN8 star from the displacement of the absorption component of the 2.058-j.Lm He I line, and used this, together with the flux
from the southern component and the composition of the wind, to
derive a mass-loss rate of 4.2 x 10-5 Mo yr-I. Eenens & Williams
(1994) derived a higher wind velocity of 1100 kIns- 1 by fitting the
P Cygni profile of the 1.083- j.Lffi He I line, and van der Hucht et al.
(1996) derived a terminal velocity of 960 kIn s-I from the widths of
the [Nem], [S N] and [CaN] fine-structure lines observed with/SO.
We adopt 1000 kIn S-I and revise the mass-loss rate of the WN8 star
to 4.6 x 10-5 Mo yr- I •
To convert the wind momentum of the BO.5 star to a mass-loss
rate, we adopt a terminal velocity of 800 kIn s-I, slightly greater
than the escape velocity (Prioja 1989). Using these parameters and
the value of the momentum ratio, we estimate a mass-loss rate of
6 x 10-7 Mo yr-I for the BO.5 star. The formal uncertainty on 1/
gives a range of (1-15) xlO- 7 but the concurrence of the two
estimates of rOB suggests a narrower range. IfWR 147NIR and the
interaction region lie in the equatorial wind-compressed zone
(WCZ) of the WN8 star, the mass-loss rate derived for the BO.5
star should be scaled up by a factor of ~ a few, depending on the
rotational velocity of the WN8 star and the wind acceleration (cf.
Ignace et al. 1996). Similarly, if WR 147NIR lies at a high latitude
on the WN8 star, the mass-loss rate derived for the BO.5 star will be
lower. However, as it is more likely that the interaction region lies at
an intermediate latitude on the WN8 star, we will retain the massloss rate derived. It is rather higher than the mass-loss rates for the
BO.2V and BO.5IV stars T Sco and A Lep (Runacares & Blomme
1996), and comparable to those of the BOe stars 'Y Cas and 0
Pup (Waters, Core & Lamers 1987) determined from infrared
observations - although the luminosities given for these BOe stars
are about twice that we adopted for WR l47NIR.
5.2 X-rays from the colliding winds
We can now ask how much of the X-ray luminosity observed with
Einstein (0.13 ± 0.02 Lo) could be provided by the colliding
winds. Certainly, there is ample energy in the winds. The fraction
of the WN8 stellar wind colliding with the interaction region is
0.006 for a spherically symmetric WN8 wind. This corresponds to
~21 Lo. Assuming about one-third of the BO.5 stellar wind to be
involved in the interaction, we would have an additional ~ 11 Lo of
wind luminosity. These luminosities would be factors of ~ a few
higher or lower if the wind of the WN8 star were flattened and the
interaction region lay at equatorial or high latitudes of the WN8 star
respectively.
We consider two mechanisms for the production of X-ray
photons: thermal bremsstrahlung from the shock-heated gas and
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
18
P. M. Williams et ai.
inverse Compton scattering of stellar photons by the accelerated
electrons. Following Usov (1992), the thermal X-ray luminosity is
calculated from the wind parameters and source size derived above.
The wind from the WNS star can be shown to be heated to -4 x 107
K near the stagnation point. For the shocked WNS wind, external to
the contact discontinuity, we get a thermal X-ray luminosity of
1.4 x 1031 erg S-I. A similar calculation for the BO.5 wind (the
'inner shock') yields 2.2x 1032 ergs-I. The total thermal X-ray
luminosity is therefore expected to be Lx = 0.06 L 8 , which is about
half of the observed luminosity. This result is very sensitive to the
mass-loss rates, varying approximately as (MOB )1.5 x (MWN )0.5
(Usov 1992).
The non-thermal radio luminosity 7 x 1028 erg s -I derived in
Section 3 may be considered to arise from a source of radius
-('IT/2)rOB mas and filling factor 0.1 (to allow for its hollow bowl
shape). This yields an equipartition magnetic field strength Beq - 9
mG. Although the source size is uncertain, and Beq DC R-6n , the
minimum magnetic field strength has to be -1 mG, otherwise the
Razin-Tsytovich effect (e.g. Pacholczyk 1970) will suppress
1.6-GHz emission from WR 147N. For a field of a few milligauss,
the radio luminosity requires a total energy in relativistic electrons
of -5 x 1039 erg.
Can these electrons scatter the ultraviolet photons of the BO.5
star to produce the observed X-ray flux? The final energy, Ef , of
an inverse-Compton-scattered photon is related to its initial
energy, E j , by Ef = Ej, where 'Y is the Lorenz factor. At a
stellar temperature of -30000 K, the peak in the stellar photon
distribution occurs at about 10 eV. Therefore, inverse-Compton
X -ray photons at energies near 1 keV arise primarily from scattering
off electrons with 'Y = 10, or with energies of S x 10-6 erg. A
relativistic electron with energy E, created at a distance r from a
source of photons with luminosity L*, will have a lifetime to
inverse-Compton scattering of
star (J Car, and up to 0.07 L 8 , depending on the model, for the BOV
star T Sco (Cassinelli et al. 1994).
5.3 Comparison with other non-thermal-emitting WR
systems
Our conclusion that WR 147 is a CWB non-thermal radio source
invites comparison with the three other WR systems whose nonthermal radio emission has been ascribed to stellar wind collision.
Some properties of the four systems are collected and compared in
Table 7. The spectral types of the companions to the WR stars are
uncertain. Apart from that of WR 147 inferred from its infrared
luminosity above, they come from composite spectra dominated by
the emission lines of the WR stars which mask most of the lines
used for the classification of OB-type spectra.
The quantities listed as 'sizes' are estimates of the separations of
the WR stars and non-thermal sources. The separations come from
the observed projected separations of the sources in WR 146
(Dougherty et al.) and WR 147 (this paper) and are therefore
lower limits. The size of WR 140 is from the orbit determination
of Williams et al. (1990a) and is the separation at the phase
(t/J - 0.7) of radio maximum. In the case ofWR 125, we estimate
a lower limit to the orbital size from the inference, from the infrared
variations, that the period of WR 125 is at least twice that of
WR 140 (Williams 1996).
For comparison, the radii of the 5-GHz 'photospheres', the wind
radii from which the 5-GHz continua of the winds of the WR stars
may be considered to arise (Wright & Barlow 1975), are -36 au for
the three WC stars and -75 au for WR 147. For systems smaller
than this radius, e.g. WR 140, the 5-GHz non-thermal emission is
observable only if the wind of the WR star is not spherically
symmetric and only when the geometry is favourable. On the
other hand, the non-thermal emission from widely separated
systems like WR 146 and 147 is readily observable because the
non-thermal emission arises further out in the WR winds, close to
the companions.
The 5-GHz flux densities of the non-thermal components in
WR 125 and 140 are those at radio maxima, while those of WR 146
and 147 are average values, excluding the MERLIN data for the
latter. The most striking difference between WR 147 and the other
systems in Table 7 is the relative weakness of its non-thermal
emission. The 5-GHz luminosities of the three WC systems are
similar, and all - four times stronger than that of WR 147.
Given that WR 147 is the most widely separated system and that
the projected separation of the components is about five times
greater than the 5-GHz 'photosphere', we cannot ascribe this
difference to greater circumstellar extinction in the WNS wind of
WR 147. The difference must lie in the intrinsic luminosities of the
non-thermal sources.
l
4'ITr2 c
tiC
(2)
=---,
alcL*E
where alC = 3.97 x 10-2 • Since WR 147N is at least S x 1014 cm
from the BO.5 star, whose luminosity is 1.S x 1038 ergs-I, electrons
with energy -10-5 erg have tiC - 1010 s. For a total electron energy
-5 x 1039 erg, the inverse-Compton X-ray luminosity is _1030
erg S-I. Clearly, inverse-Compton scattering of non-thermal electrons does not contribute significantly to the observed 1-keV X-ray
luminosity
The difference between the observed total X-ray luminosity and
the theoretical estimate of that from the collision region can be
accounted for by X-rays from the two stars. Pollock (19S7) found
that the mean X-ray luminosity of single WN stars is 0.06 L 8 . In
addition, the luminosity of the BO.5 star could be of similar
magnitude, as seen by comparison with 0.007 L8 for the BO.5V
Table 7. Properties of WR systems exhibiting non-thermal emission.
WR
Spectra
s: (obs)
(rnJy)
125
140
146
147
WC7+09
WC7+04-5
WC6+09
WN8+BO.5
1.5
22.5
28.S
12
Size
(au)
~42
~
~
27
139
378
Dist.
(kpc)
4.7
1.3
1.1
0.63
LsGHz
MWR
(erg s-I)
<M0yr- l )
3.9 x
4.6 x
4.1 x
1.0 x
1022
1022
1022
1022
5.7 X
5.7 X
3X
4.6x
10-5
10-5
10-5
10-5
vWR
krns- I
2700
2860
2900
1000
¥:MV 2 )WR
Reference
(erg S-I)
1.3 x 1038
1.5 x 1038
8.0x 1037
1.5 X 1037
Williams et al. 1992
Williams et al. 1990a
Dougherty et al. 1996
this paper
a5-GHz flux of the non-thermal component; using the data of Churchwell et al. for WR 147.
© 1997 RAS, MNRAS 289,10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
The colliding-wind Wolf-Rayet system WR 147
The most significant differences in the other properties of the
systems are in the wind terminal velocities and kinetic energies. The
terminal velocities are well determined observationally, and that of
WR 147 is a factor of ~2.8 lower than those of the three other
systems. The kinetic energy is a factor of ~ 8 lower for a spherically
symmetric WR 147 wind, with the possible modifications referred
to above if the wind is flattened. We expect the available WR wind
luminosity to scale as the product of wind energy and angular crosssection of the interaction region. The last scales as (rOB / D)2, i.e.
approximately as (MV~)OB / (MV~)WN for smallTJ (equation 1). This
suggests that the non-thermal luminosity should scale approximately as the product of the WR wind velocity and the OB
wind momentum for equal efficiencies of conversion of wind
luminosity.
Within the uncertainties, this accounts for the relatively low nonthermal luminosity ofWR 147 relative to WR 125 and 146, assuming the wind velocities of the late-subtype 0 stars in the latter
systems to be in the range lOOO-2000kms- 1 (Prinja, Barlow &
Howarth 1990). The companion to the WC7 star in WR 140 is of an
earlier subtype (04-5) whose velocity has been measured
(3100kms- 1 : Fitzpatrick, Savage & Sitko 1982) to be rather
higher than those expected for the other systems, but the nonthermal luminosity of WR 140 is not proportionally higher. This
may be a consequence of the non-spherical geometry of this
system (cf. White & Becker 1995), and this source awaits detailed
modelling.
It appears from the similarity of the non-thermal luminosities of
the three WC-star-based systems that the separation of the components is unimportant, which might seem surprising for interacting
systems. The fact that we have only lower limits to the separations
of the more widely separated systems reinforces this result. The
explanation lies in the fact that, for a given wind-momentum ratio,
the size of the interaction region scales with the separation of the
components (equation 1) and intercepts the same fraction of the WR
wind and hence the same kinetic energy if the wind has accelerated
to its terminal velocity.
6
CONCLUSIONS
High-resolution infrared and radio images of the X-ray-luminous
Wolf-Rayet system WR 147 (AS 431) have been observed. The
new radio image resolves both components of this north-south
double source and confirms the presence of a 'bridge' of emission
joining them. The emission from the southern component, which is
associated with the wind of the WN8 star, is not circular but sized
~ 170 x 253 mas2 , suggesting that the stellar wind is not spherically
symmetric. Almost half the flux is resolved out by MERLIN and the
data need augmentation with contemporaneous observations on
shorter baselines to observe the extended emission. The nonthermal component, ~0.6 arc sec north of the WN8 star, is broadened east-west by ~267 mas. The infrared image reveals the
presence of a companion to the WN8 star, close to the non-thermal
radio source but slightly (~60 mas) more distant from the WN8
star. The companion is til( ~ 3 mag fainter than the WN8 star and
has the luminosity of a BO.5V star, just sufficiently luminous
(log LlLo = 4.5) to possess a stellar wind capable of colliding
with that of the WN8 star and allowing us to treat the system as a
colliding-wind binary.
The presence of the non-thermal emission between the two stars,
much closer to that with the weaker wind and having a shape
consistent with that of a wind-interaction region, is direct evidence
for a colliding-wind origin for the emission. About half of the X-ray
19
emission can also be accounted for by the release of energy in the
wind collision.
Comparison of the non-thermal luminosity with those of the three
non-thermal source systems incorporating WC-type stars (WR 125,
140 and 146) shows a correlation with the velocity of the WR wind.
ACKNOWLEDGMENTS
It is a pleasure to thank Mark Casali for taking the UKIRT images,
and Ger de Bruyn, Andy Pollock, Russ Taylor and Vladimir Usov
for stimulating discussions and correspondence. The MultiElement Radio-Linked Interferometer Network (MERLIN) at
Jodrell Bank is operated by the University of Manchester for the
Particle Physics and Astronomy Research Council (PPARC). The
United Kingdom Infrared Telescope (UKIRT) and James Clerk
Maxwell Telescope (JCMT) on Mauna Kea, Hawaii, are operated
by the Joint Astronomy Centre for PPARC. The Westerbork
Synthesis Radio Telescope (WSRT) is operated by the Netherlands
Foundation for Research in Astronomy with financial support from
the Netherlands Organization for Scientific Research (NWO).
REFERENCES
Abbott D. C., Bieging J. H., Churchwell E., Torres A. v., 1986, ApJ, 303, 239
AltenhoffW. J., Thurn C., Wendker H. J., 1994, A&A, 281,161
Aspin C. A., Puxley P. J., Hawarden T. G., Patterson M. J., Pickup D. A.,
1997,MNRAS,284,257
Baars J. W. M., Genzel R, Pauliny-Toth 1.1. K, Witzel A., 1977, A&A, 61,
99
Bassino L. P., Dessaunet V. H., Muzzio J. C., Waldhausen S., 1982, MNRAS,
201,885
Beichman C. A., Neugebauer G., Habing H. J., Clegg P. E., Chester T. J.,
1988, Infrared Astronomical Satellite (lRAS'), Catalogs and Atlases,
Explanatory Supplement, NASA RP-1190, Vol. 1
Caillault J-P., Chanan G. A., Helfiand D. J., Patterson J., Nousek J. A.,
Takalo L. 0., Bothun G. 0., Becker R H., 1985, Nat, 313, 376
Cassinelli J. P., Cohen D. H., MacFarlane J. J., Sanders W. T., Welsh B. Y.,
1994, ApJ, 421, 705
Cassinelli J. P., Ignace R, Bjorkman J. E., 1995, in van der Hucht K. A.,
Williams P. M., eds, Wolf-Rayet Stars: Binaries, Colliding Winds,
Evolution. Kluwer, Dordrecht, p. 191
Chen w., White R L., 1994, Ap&SS, 221, 259
Churchwell E., Bieging J. H., van der Hucht K A., Williams P. M., Spoelstra
T. A.Th., Abbott D. c., 1992, ApJ, 393, 329
Cohen M., 1995, ApJS, 100,413
Conti P. S., Massey P., 1989, ApJ, 337, 251
Contreras M. E., Rodriguez L. E, Gomez Y., Velazquez A., 1996, ApJ, 469,
329
Cornwell T. J., Brown R, 1989, in Perley R A., Schwab E R, Bridle A. H.,
eds, ASP Conf. Ser. 6, Synthesis Imaging in Radio Astronomy. Astron.
Soc. Pac., San Francisco, p. 167
DavisRJ.,BodeM. E, van derHuchtK A., WilliamsP. M., 1995,invander
Hucht K A., Williams P. M., eds, Wolf-Rayet Stars: Binaries, Colliding
Winds, Evolution. Kluwer, Dordrecht, p. 547
Dougherty S. M., Williams P. M., van der Hucht K A., Bode M., Davis R J.,
1996,MNRAS, 280,963
Duncan W. D., Robson E. I., Ade P. A. R, Griffin M. J., Sandell G., 1990,
MNRAS,243,126
Eenens P. R J., Williams P. M., 1994, MNRAS, 269, 1082
Eichler D., Usov v., 1993, ApJ, 402, 271
Fitzpatrick E. L., Savage B. D., Sitko M. L., 1982, ApJ, 256, 578
Grigsby J. A., Morrison N. D., 1995, ApJ, 442, 794
Hogg D. E., 1985, in Hjellming R M., Gibson D. M., eds, Radio Stars.
Reidel, Dordrecht, p. 117
Howarth I. D., Prinja R K., 1989, ApJS, 69, 527
© 1997 RAS, MNRAS 289, 10-20
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.289...10W
20
P. M. Williams et al.
Ignace R., Cassinelli J. P., Bjorkman J. E., 1996, ApJ, 459, 671
Jardine M., Allen H. R., Pollock A. M. T., 1996, A&A, 314, 594
Koornneef J., 1983, A&A, 128, 84
Leitherer c., Robert C., 1991, ApJ, 377, 629
Lortet M. c., Blazit A., Bonneau D., Foy R., 1987, A&A, 180, 111
Lundstrom 1., Stenholm B., 1984, A&AS, 58, 163
Moran J. P., Davis R. 1., Bode M. F., Taylor A. R., Spencer R. E., Argue A.
N., Irwin M. J., Shanklin J. D., 1989, Nat, 340, 449
Pacholczyk A. G., 1970, Radio astrophysics. Freeman, San Francisco
Pollock A. M. T, 1987, ApJ, 320, 283
Pollock A. M. T., Haberl F., Corcoran M. F., 1995, in van der Hucht K. A.,
Williams P. M., eds, Wolf-Rayet Stars: Binaries, Colliding Winds,
Evolution. Kluwer, Dordrecht, p. 512
Prinja R. K., 1989, MNRAS, 241, 721
Prinja R. K., Barlow M. J., Howarth 1. D., 1990, ApJ, 361, 607
Runacres M. c., Blomme R., 1996, A&A, 309, 544
Sandell G., 1994, MNRAS, 271, 75
Schulte-Ladbeck R. E., 1995, in van der Hucht K. A., Williams P. M., eds,
Wolf-Rayet Stars: Binaries, Colliding Winds, Evolution. Kluwer,
Dordrecht, p. 176
Smith L. F., Shara M. M., Moffat A. F. J., 1996, MNRAS, 281, 163
Stevens J. A., Robson E. I., 1994, MNRAS, 270, L75
Underhill A., Doazan v., 1982, B Stars with and without Emission Lines.
NASA SP-456, Washington
Usov v. v., 1992, ApJ, 389, 635
van der Hucht K. A., Williams P. M., Spoelstra T. A. Th., Swaanenvelt J. P.,
1995, in van der Hucht K. A., Williams P. M., eds, Wolf-Rayet Stars:
Binaries, Colliding Winds, Evolution. Kluwer, Dordrecht, p. 559
van der Hucht K. A. et aI., 1996, A&A, 315, Ll93
Waters L. B. F. M., Cote J., Lamers H. J. G. L. M., 1987, A&A, 185,206
White R. L., Becker R. H., 1995, ApJ, 289, 698
Williams P. M., 1996, in Vreux J.-M., Detal A., Fraipont D., Gosset E.,
Rauw G., eds, Wolf-Rayet stars in the framework of stellar evolution,
Proc. 33rd Liege Int. Astrophys. Colloq. Universite de Liege, p. 135
Williams P. M., van der Hucht K. A., Pollock A. M. T, F1orkowski D. R.,
van der Woerd H., Wamsteker W. M., 1990a, MNRAS, 243, 662
Williams P. M., van der Hucht K. A., Sandell G., The P. S., 1990b, MNRAS,
244,101
Williams P. M., van der Hucht K. A., Bouchet P., Spoelstra T A.Th., Eenens
P. R. J., Geballe T. R., Kidger M. R., Churchwell, E., 1992, MNRAS,
258,461
Williams P. M., van der Hucht K. A., Spoelstra T A.Th., 1994a, A&A, 291,
805
Williams P. M., van der Hucht K. A., Kidger M. R., Geballe T R., Bouchet
P., 1994b, MNRAS, 266, 247
Wright A. E., Barlow M. J., 1975, MNRAS, 170,41
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