401
HIPPARCOS, VLA, AND CCD OBSERVATIONS OF CYG OB2 No. 5
1 1 2
M.E. Contreras , L.F. Ro driguez ,M.Tapia ,
3 3 3 3
D. Cardini , M. Badiali , A. Emanuele ,P.Persi
1
Instituto de Astronom a, UNAM, Ap do. Postal 70-264, 04510 M exico, D.F., M exico
2
Instituto de Astronom a, UNAM, Ap do. Postal 877, Ensenada, M exico
3
Istituto Astro sica Spaziale, CNR, C.P. 67, 00044 Frascati, Italy
uxes and mass loss values for 10 ob jects and deter- ABSTRACT
mined accurate radio p ositions. In particular, their
sample included Cyg OB2 No. 5 BD+40 4220; V729
Cyg which is known to b e a contact binary system
Cyg OB2 No. 5 BD+40 4220; V729 Cyg is known
formed bytwo O7I stars Torres-Do dgen et al. 1991.
to b e a contact binary system formed bytwo O-typ e
For this ob ject they detected emission at the optical
stars. For this ob ject a 6-cm radio emission has
p osition as well as an additional weak radio source
b een detected at the optical p osition as well as an
lo cated at 0:9 arcsec to the north east of the con-
additional weak non-thermal radio source lo cated at
tact binary. They suggested that this radio comp o-
0:9 arcsec to the north east of the contact binary.
nentwas p ossibly a third stellar ob ject in the system.
It has b een suggested that this radio comp onentwas
This was the rst time that a radio companion was
p ossibly a third stellar ob ject in the system. This was
observed to be asso ciated with the contact binary
the rst time that a radio companion was observed
system.
to b e asso ciated with the contact binary system. In
this work we present new radio, CCD and Hipparcos
Persi et al. 1985 reobserved this source at 2, 6 and
observations that provide the basis for a plausible ex-
20 cm. They found that the 6-cm ux density had
planation of the origin of the non-thermal radiation
increased by a factor of 3, with resp ect to the 1980
emitted by the `third' comp onent of this system and
observations made by Abb ott et al. 1981, and have
its nature.
found a very at sp ectral index of 0.2 0.1.
Later, Persi et al. 1990 continued monitoring Cyg
Key words: Radio continuum: general and stars;
OB2 No. 5 and found again that the emision had
early-typ e stars.
changed, this time decreasing backto the 1981 lev-
els. Persi et al. 1990 de ned what they called the
`high' and `low' states of emission. In the `high' state
the 6-cm measured ux densitywas of '5 to 7 mJy
1. INTRODUCTION
with a sp ectral index of ' 0.2 while in the
2 6cm
`low' state the 6-cm ux was of '1 to 2 mJy and the
sp ectral index had increased and b ecame consistent
One of the more reliable metho ds for the determina-
with the classical 0.6 value exp ected for a thermal
tion of the mass loss rate in massive stars is via the
wind. However, even in the low state of emission the
observation of the free-free radiation emitted by their
sp ectral index derived from the 6 and 20-cm uxes
ionized winds at radio wavelengths. The rst radio
had a negativevalue indicating the presence of a non-
survey was carried out by Abb ott et al. 1980. In
thermal comp onent. The presence of b oth thermal
this study they rep orted mass loss rate values for 15
and non-thermal radio emitting comp onents in Cyg
O, B and A-Typ e stars observed at 6-cm. Later stud-
OB2 No. 5 resembled the case of other Cyg OB2 as-
ies con rmed the detection of thermal radio emission
so ciation memb ers Cyg OB2 Nos. 8A, 9 and 12 and
at 2 and 6-cm Abb ott et al. 1981, 1984, 1986, Bieg-
some Wolf-Rayet stars WR 140 and WR 147.
ing et al. 1989 from OB stars. However, some early-
typ e stars show time-variable, non-thermal emission
Miralles et al. 1994 observed the source continuing
of p o orly understo o d origin Abb ott et al. 1984, Persi
the monitoring for ab out four more years. The source
et al. 1985, Williams et al. 1990, Miralles et al. 1994.
showed again its radio variations b etween the `high'
This synchrotron radiation can be comparable or
and `low' states with ux densities and sp ectral in-
even larger than the exp ected free-free emission and
dices similar to those found by Persi et al. 1990.
this non-thermal contamination limits the accuracy
The radio emission seems to switch between these
of the metho d.
two states in a p erio d of ab out 7years. They have
mo deled the emission as due to a synchrotron enve-
lop e created by a p erio dic pro duction of relativistic Abb ott et al. 1981 selected the most luminous stars
electrons in the wind. Miralles et al. 1994 con rmed in the Galaxy, most of their sample residing in the
the coincidence b etween the radio maximum emission Cyg OB2 asso ciation. They rep orted 6-cm radio
402
and the optical p osition for the binary system and re-
vered the weak radio companion at 3.6 and 6-
disco CYGOB5 3.6-cm Pesado=NA Haz=0.3x0.3 arcsec
cm. They determined a separation of 0:8 0:1 arc-
sec to the NE of the main comp onent and supp orted
the idea that this source was a third comp onentofthe
and not a background ob ject. The sp ectral
system 41 08 05.0
index derived for the companion was = 0:3 0:5.
However with such a large error it was not p ossible
establish if the radio companion was thermal or
to 04.5
non-thermal. Herbig 1967 had rep orted a star of
magnitude 13 or 14 lo cated at ab out 1:5 arcsec
a p osition angle of 60 70 from the optical
with 04.0
p osition of the brightest comp onent. Miralles et al.
1994 suggested the p ossibility that this could b e the
DECLINATION (B1950)
optical counterpart of the weak radio companion.
03.5
In a recent study Contreras et al. 1996 rep orted two
sets of observations for Cyg OB2 No. 5 obtained in
03.0
1994 and 1995. They found that the ux densities
app eared to b e increasing b etween the two observing
runs at the di erent frequencies observed, 6, 3.5 and This tendency and the fact that the sp ectral 0.7 cm. 02.5
20 30 34.95 34.90 34.85 34.80 34.75
ed were decreasing was indicativeofthe indices deriv RIGHT ASCENSION (B1950) Cont peak flux = 7.4066E-03 JY/BEAM onset of the `high' emission state predicted to o ccur in Levs = 1.8000E-05 * ( -4.00, 4.000, 5.000,
6.000, 7.000, 8.000, 10.00, 20.00, 30.00,
y Miralles et al. 1994. Besides, in their 1994
1996 b 60.00, 100.0, 200.0, 350.0)
observations they detected the radio companion at 6-
cm and 3.6-cm. With these two observed uxes they
derived an upp er limit value for the sp ectral index,
Figure1. Cleaned, natural-weight map of Cyg OB2 No. 5
= 2:4 0:6, indicating beyond doubt that the
from self-calibrated uv data at 3.6 cm. The crosses mark
companion source was clearly non-thermal at least
the position of the optical stars see text.
during that ep o ch. The separation b etween the main
comp onent and the companion was 0:8 0:1 arcsec.
Hence, separation values determined in the radio by
di erent authors Abb ott et al. 1981, Miralles et al.
2.2. CCD Observations
1994, Contreras et al. 1996 indicated that the sep-
aration between the two comp onents was less than
1:0 arcsec, apparently smaller than the value rep orted
by Herbig 1967 in the optical. In this work we
CCD images were obtained through the broad-band
present new radio and optical observations that de-
lters B , V and R Tek4 800800 CCD on the 1-m
termine this separation accurately and provide the
Johannes Kepler Telescop e JKT at the Observato-
basis for a plausible explanation of the origin of the
rio Ro que de los Muchachos in La Palma, Canarias,
non-thermal radiation emitted by the `third' comp o-
Spain, on the nights 6/7 and 15/16 Novemb er 1994
nent of this system and its nature.
as part of the Service Observing program. The plate
scale was 0.33 arcsec/pixel and the mean seeing was
around one arcsec during b oth nights. The exp osure
times were b etween 1{10 s but even with the short-
2. OBSERVATIONS
est exp osure, Cyg OB2 No. 5 was saturated in the I
frames.
2.1. Very Large Array Observations
A Gaussian Point Spread Function was tted to iso-
lated stars on each frame and was then subtracted, af-
New 3.6 and 6-cm observations were made with the
ter scaling, from the sp ot centred on Cyg OB2 No. 5.
1
Very Large Array VLA of NRAO . The 3.6-cm ob-
The residual image clearly showed the fainter isolated
servations were made during 1996 Decemb er 27 and
companion star. Its relative p osition was measured
28, while the 6-cm observations were made during
on all frames with an average value for the sepa-
1997 January 4. During b oth observing runs the ar-
ration to primary binary of 0.98 0.06 arcsec and
raywas in the A con guration giving resolutions of
p osition angle 61 7 degrees. The di erential pho-
0:2 arcsec and 0:3 arcsec, at 3.6 and 6-cm, re-
tometry Cyg OB2 No. 5 minus companion yielded
sp ectively. The amplitude calibrator was 1328+307
V = 3:96 and B V =0:11.
and the phase calibrator 2005+403 in b oth cases.
The b o otstrapp ed ux densities of 2005+403 were
Assuming the values of the photometry for the com-
2.980.01 and 3.240.01 Jy at 3.6 and 6-cm, resp ec-
p osite system to be those given by Torres-Do dgen
tively. Editing, calibration, imaging, and analysis of
et al. 1991, the individual colours of the sp ectro-
the data was p erformed using the AIPS software of
scopic binary Cyg OB2 No. 5 are V = 9:18 and
NRAO. Cleaned maps made from self-calibrated uv
B V =1:81, while those of the stellar compan-
data at 3.6 and 6-cm are shown in Figures 1 and 2.
ion are V =13:14 and B V =1:70. As the star
1
NRAO is op erated by Asso ciated Universities, Inc., under
was saturated, no photometry was p erformed in the
co op erative agreement with the National Science Foundation.
I band.
403
2.3. Hipparcos Observations sho cks, giving rise to the observed synchrotron non-
thermal radiation.
A complete, detailed description of the Hipparcos
Assuming the absolute V magnitude and intrinsic
mission and of the derivation of the astrometric pa-
B V colour of two O7Ia stars for the contact
rameters is given in the Hipparcos Catalogue ESA
binary primary and the same distance and redden-
1997. During the 37 months of the Hipparcos mis-
ing Torres-Do dgen et al. 1991, then we inferred a
sion, the ob ject Cyg OB2 No. 5, identi ed by the
sp ectral typ e B2V for the companion star.
Hipparcos Catalogue number HIP 101341, fell 113
times in the eld of view of the rotating telescop e,
Avery similar situation has b een recently found for
each time p erforming an apparent transit through
WR 147 by Williams et al. 1997.
the mo dulating grid of its fo cal plane. By analyising
this set of signals with a pro cessing algorithm de-
scrib ed in Mignard et al. 1995, it has b een p ossible
REFERENCES
to obtain the separation 0:948 0:043 arcsec and
the p osition angle 54 3 . These data are in go o d
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3. DISCUSSION
ApJ, 250, 645
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02.5 20 30 34.95 34.90 34.85 34.80 34.75 RIGHT ASCENSION (B1950) Cont peak flux = 6.9363E-03 JY/BEAM Levs = 1.8000E-05 * ( -4.00, 4.000, 6.000, 8.000, 10.00, 15.00, 20.00, 30.00, 60.00,
100.0, 200.0, 300.0, 400.0)
Figure2. Cleaned, uniform-weight map of Cyg OB2 No. 5
from self-calibrated uv data at 6-cm. The crosses mark the
position of the optical stars see text.
These results indicate that the radio `companion' is
not a star, but most likelyabow sho ck pro duced at
the p osition where the winds of the contact binary
and the secondary star collide. Acceleration of elec-
trons to relativistic sp eeds can take place in strong