A&A 396, 533-538 (2002)
DOI: 10.1051/0004-6361:20021373
A. Pasquali 1 - F. Comerón 2 - R. Gredel 3 - J. Torra 4 - F. Figueras 4
1 - ESO/ST-ECF,
Karl-Schwarzschild-Strasse 2, 85748
Garching bei München, Germany
2 -
ESO,
Karl-Schwarzschild-Strasse 2, 85748
Garching bei München, Germany
3 -
Max-Planck Institut für Astronomie,
Königstuhl 17, 69117 Heidelberg, Germany
4 -
Departament d'Astronomia i Meteorologia, Universitat de
Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain
Received 22 August 2002 / Accepted 19 September 2002
Abstract
We report the discovery of a new Wolf-Rayet star in the direction of
Cygnus. The star is strongly reddened but quite bright in the infrared, with
J = 9.22, H = 8.08 and
KS = 7.09 (2MASS). On the basis of its H + K
spectrum, we have classified WR 142a a WC8 star. We have estimated its
properties using as a reference those of other WC8 stars in the solar
neighbourhood as well as those of WR 135, whose near-infrared spectrum
is remarkably similar. We thus obtain a foreground reddening of
mag,
,
,
,
K,
,
yr-1. The
derived distance modulus,
mag, places it in a
region occupied by several OB associations in the Cygnus arm, and
particularly in the outskirts of both Cygnus OB2 and Cygnus OB9. The position
in the sky alone does not allow us to unambiguously assign the star to either
association, but based on the much richer massive star content of Cygnus OB2
membership in this latter association appears to be more likely.
Key words: stars: emission line, Be - stars: fundamental parameters - stars: Wolf-Rayet - Galaxy: open clusters and associations: general
Wolf-Rayet (WR) stars, first discovered by Wolf and Rayet in 1867,
are characterised by an emission-line spectrum dominated by strong
He, N and C features. Their observed properties are believed to be
the result of stellar evolution at high initial masses, during
which strong mass loss in winds of a few 10
yr-1 at terminal velocities between 400 and 5000 km s-1(cf. van der Hucht 2001) "peels'' the star until it comes to
expose at its surface the heavy elements synthesized in its core.
WR stars represent the last evolutionary stage of massive stars
before the final supernova explosion (Maeder & Conti 1994; Langer
& Woosley 1996; Langer & Hegel 1999). The flux ratios among the
He, N, and C lines allows the classification of WR stars into
subclasses, WN (where He and N lines dominate), WC and WO (when
He, C and O features become stronger), and subtypes (WN3-9 and
WC4-9 as the ionization degree increases).
WR stars significantly contribute to the energy budget and
chemical enrichment of their parent galaxy. Through their stellar
winds, they enrich the hosting environment with heavy elements and
release over time a kinetic energy comparable to that from
supernovae (Leitherer 1998). Given their high luminosities (few
)
individual WR stars are easily identified in
all the galaxies of the Local Group (Massey & Johnson 1998),
while the short lifetimes of such massive stars (several Myr) make
them appear generally near their birthplaces, tracing star
formation in spiral and starburst galaxies. Indeed, their
unmistakable features, detected in the integrated spectra of
galaxies undergoing intense bursts of star formation (cf. Conti
2000), make them useful to study starburst phenomena and star
formation processes on cosmological scales.
The latest census of WR stars detected in the Milky Way lists 227
such objects (van der Hucht 2001), or about 10
of the
estimated Galactic WR population (
1500-2500; Shara et al. 1999). The majority of Galactic WR stars has escaped detection
using traditional techniques (such as visible spectroscopy or
narrow-band photometry with filters centered on the characteristic
WR signature at
Å and its adjacent
continuum, cf. Massey & Conti 1983) which are naturally limited by
their unability to penetrate the large column densities of
absorbing dust along the lines of sight towards distant regions of
the galactic disk. Such limitation can be overcome by observing at
near-infrared wavelengths, where WR stars also display easily
identifiable spectral signatures and the extinction by dust is
only a fraction of that at shorter wavelengths.
In this paper we report the discovery of a new nearby WR star, supporting the prediction by van der Hucht (2001) that many such stars in the solar neighbourhood still await discovery at near-infrared wavelengths. Indeed, the object reported here is bright at near-infrared wavelengths but has escaped recognition so far in the visible despite its proximity, due to the high extinction in its direction. The discovery of yet another nearby WR star deserves attention in several respects, as each WR star has its own special features and represents a unique contribution to our understanding of the late stages of massive stellar evolution. Moreover, its location and relation to other young stars in the region yields information on the star formation history in the complex where it formed. The near-infrared observations of the star, to which we will refer as WR 142a following the nomenclature in van der Hucht's (2001) catalogue, are described in Sect. 2. The results concerning its spectral classification and intrinsic properties can be found in Sect. 3. Finally, Sect. 4 discusses some implications of the WR population in Cygnus.
The selection of WR 142a as a target for spectroscopic
observations was based on its 2MASS photometry, from which it was
apparent that this star is a bright, red object with colours
significantly deviating from those of normal stars reddened by
foreground dust. Infrared magnitudes listed by 2MASS are J =
9.22, H = 8.08,
KS = 7.09. Its location at
,
places it in
the general direction of Cygnus, in a region where several OB
associations overlap (see Sect. 3.4). The USNO catalog lists a
star of B = 18.1, R = 14.7 at the position of this object. The
only previous reference to it in the literature seems to be
Melikian & Shevchenko (1990), who recognized it as a strong
emission-line star in their search for such objects in the
proximities of the cluster NGC 6910. However, its actual WR nature
has not been recognized until now.
We observed WR142a on June 24 and 25 2002, using the near-infrared
imager and spectrograph MAGIC mounted on the 1.23 m telescope of
the German-Spanish Astronomical Center in Calar Alto (Spain). This
instrument uses a NICMOS3
pixel2 detector
yielding a scale of 1
15 per pixel in imaging mode. JHKM imaging and H and K band spectroscopy were obtained and
reduced using the same instrumental setups and techniques
described in Comerón et al. (2002); the reader is thus referred
to that paper for details. Two spectra obtained on the night of 24
June 2002 at airmasses of 2.05 and 1.67 were individually reduced
to confirm the reality of individual features, especially in
regions of rapidly varying telluric absorption. The exposure time
of the combined spectrum is 12 min. As for the imaging, the
exposure times are 7.5 min in each of the J, H and KM filters. The images were obtained under conditions suspected to be
non-photometric, and flux calibration using a separate image of a
standard star was thus not attempted.
The low-resolution infrared spectrum of WR 142a (
)
is
plotted in Fig. 1. Emission lines have been identified by
cross-correlation with The Atomic Line List v2.04
(http://www.pa.uky.edu/~peter/atomic) and the K-band spectral
atlas of Figer et al. (1997). The most prominent features are due
to HeI and HeII, CIII and CIV. Their equivalent widths are listed
in Table 1.
![]() |
Figure 1:
The observed H-to-K spectrum of WR 142a. The
spectral resolution is
![]() ![]() |
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We have cross-checked the spectrum of WR 142a with the K-band
spectral atlas of Galactic WR stars by Figer et al. (1997) in
order to assign WR 142a a WR subtype. The line complex between 2
and 2.2 m has been particularly useful in the spectral
classification, since it is a specific signature (in line
wavelength and flux ratio) of WC8 stars (cf. Fig. 10 in Figer et al. 1997). Although the dispersion of Figer et al.'s (1997) data
is a factor of two higher (
against 240 of this
work), the comparison indicates that the spectrum of WR 142a is
almost identical to that of WR 135, a WC8 star in the Galactic OB
association Cygnus OB3. And similarly to WR 135, a weak absorption
can be detected blueward of the CIV + HeII emission line at
m suggesting a P Cygni profile for this
line blend (or for the HeII 2.06
m feature buried in this
line blend). Therefore, we classify WR 142a as a WC8 star.
Line (![]() |
EqW (Å) |
CIII + HeII 1.474 | 50 |
HeII 1.488 | 30 |
HeI 1.70 | 60 |
HeI + CIV 1.736 | 200 |
CIV 2.07 + HeII 2.076 + CIV 2.08 | 780 |
HeI 2.11 + CIII 2.108 | 160 |
HeI + HeII 2.16 | 40 |
HeII 2.18 | 70 |
CIV 2.278 | 30 |
HeII 2.31 + CIII 2.32 | 30 |
HeII 2.347 | 30 |
CIV 2.42 | 200 |
As a first step towards the estimate of the physical properties of WR 142a, we have compared its near-infrared photometry to that of six other nearby WC8 stars (WR 53, WR 60, WR 77, WR 101, WR 117, and WR 135) selected from Williams et al. (1987). Stars with detected companions were excluded. WR 101 and WR 135 have JHK photometry also from 2MASS, and WR 113 from Pitault et al. (1983); in all these cases, all published magnitudes agree to better than 0.1 mag. Extinctions in the visible, AV, are given for the six reference objects by van der Hucht (2001) based on their visible photometry. A seventh WC8 star, WR 102e, also has available 2MASS photometry. However, the fact that this star is located in the galactic center raises some concerns about its validity as a reference for comparison to the WR population in the solar neighbourhood. Besides the published JHK photometry we have also used the extinction given in van der Hucht catalogue, from which we derive intrinsic colours (J-H)0, (H-K)0 using the extinction law of Rieke & Lebofsky (1985).
Assuming that the intrinsic colours of WR 142a are the same as
the average of those of the reference stars, we obtain
.
The uncertainty includes both the intrinsic
scatter in colours among WC8 stars and possible errors in
determining the extinction, which is generally done using BVcolours. The derived extinction for WR 142a is thus
,
or
.
Using the K band to estimate the
extinction introduces larger uncertainties due to the larger
scatter in derived intrinsic (H-K)0 colours among the reference
objects, and we have preferred to exclude it for this purpose.
A more accurate estimate is possible by using the measured B = 18.1in the USNO catalogue, keeping in mind that a possible error
may be introduced by variability in the non-simultaneous B and JHK observations. In that case, we have calculated
(B-V)0 = -0.14 using
the observed (B-V) of the reference stars and dereddening them with
the AV listed by van der Hucht (2001), using
AV/(AB-AV) = 3.1,
thus obtaining
(B-J)0 = 0.42. The observed
(B-J) = 8.9 thus implies
AV = 8.1, practically identical to the value derived from J-H but
based on a much longer wavelength baseline. The uncertainty, mostly due
to the scatter in (B-J)0, is accordingly smaller and estimated to
be 0.4 mag. We thus adopt
,
or
,
as the extinction towards WR 142a.
A similar procedure applied to the (V-J) photometry of the
reference stars yields an estimate of
for WR 142a, again with the caveat of the non-simultaneity of the
visual and infrared observations although the relatively
small scatter that we find in (V-J)0 suggests that this may not
be an important factor. Proceeding with the (V-J)0 given above
and assuming for the visual absolute magnitude of WR 142a the mean
value derived by van der Hucht (2001) for WC8 stars,
MV = -3.74, we
obtain
MJ = -4.30. In this case, to the already noted uncertainty
in (V-J)0 we must add the scatter in MV found for these stars,
which van der Hucht evaluates at 0.5 mag. Using the estimates of
the extinction and the absolute magnitude we obtain a distance
modulus of
,
corresponding to a distance of
1.8+0.6-0.5 kpc.
An alternative possibility consists of using the close similarity between the spectra of WR 142a and WR 135 to assume that the intrinsic properties of both are identical, including intrinsic colours and absolute magnitudes. WR 135 is the bluest star in the reference sample both in (J-H)0 and (V-J)0. The brighter MV listed by van der Hucht (2001) is balanced by the bluer colours to give MJ = -4.33, practically identical to the value used in the above estimate. However, fitting the colours requires a higher extinction ( AV = 11.0 or AJ = 3.1) and the distance modulus thus reduces to 10.2 mag, corresponding to a distance of 1.2 kpc, in the lower end of the range given above.
Despite of the fact that our observations were probably performed
under non-photometric conditions as noted in Sect. 2, it is
still possible to check for possible variability by comparing
WR 142a to other 2MASS stars in the field. Such comparison
suggests that WR 142a was slightly fainter when we observed it
than when it was observed by 2MASS, with
,
,
.
Only the significance of the
K variability is more than marginal, lying within the amplitude
range of the long-term variability of Wolf-Rayet stars reported by
Hackwell et al. (1979) and well below the rather extreme case
described by Danks et al. (1983).
The physical properties of WR 142a (mass M, radius R,
temperature T) may be estimated with simple
relations derived by Schaerer & Maeder (1992) from their models
of WNE/WC stars, with the caveat that these models
are quite sensitive to the assumed morphology of the stellar wind,
i.e. clumpiness, which may change the bolometric luminosity of
WRs. The luminosity can be estimated by
adopting for WR 142a
MV = -3.74 as determined by van der Hucht
(2001) for WC8 stars, and the average bolometric correction
BCV
= -4.12 found by Nugis & Lamers (2000) for the WC8 subgroup (taking
into account their Eq. (6) to transform from narrow- to broad-band
V magnitude). Since
,
the total
luminosity of WC8 stars turns out to be
.
If the individual luminosity of WR 135 is adopted instead as more
appropriate for WR 142a, we obtain
.
Using Schaerer and Maeder's mass-luminosity relation,
we obtain a mass between 7.9 and
(assuming
respectively
and 5.24 as described
above). Likewise, the stellar radius R and temperature T are
given approximated by
and
from where we obtain
,
K, with little dependence on the choice between 5.04 or
5.24 for
.
Finally, adopting Nugis & Lamers'
equation (24), which gives the stellar mass loss as a function of
the stellar present mass, we derive for WR142a a mass-loss rate of
yr-1 or
yr-1, depending on which one of the masses
given above is adopted. It must be stressed that the values above
are a simple estimate of the stellar properties of WR142a based on
average properties of WC8 stars, and an accurate model atmosphere
fitting of its IR spectrum is indeed needed.
Although the winds of late-type WC stars are generally a source of warm dust (Smith & Houck 2001; Chiar et al. 2001), no IRAS mid-infrared point source is detected near the position of WR 142a. This is somewhat unexpected, as the sensitivity of IRAS should enable detection of a typical late WC star within 7 kpc from the Sun (Cohen 1995). Mid-infrared observations provide no evidence either for any extended structure at larger scales around WR 142a, as is sometimes found the neighbourhood of other WR stars (e.g. Miller & Chu 1993; Saken et al. 1995; Pineault & Terebey 1997; Gervais & St-Louis 1999, and references therein). This does not completely rule out the possible existence of an extended nebula, as the wide range of ring nebula sizes around WR stars (Miller & Chu 1993) and the intricate structure of thermal infrared emission in the general direction of Cygnus would naturally make the detection of such a nebula elusive. Near-infrared images of the immediate surroundings of WR 142a (Fig. 2) also show it to be rather inconspicuous, with no obvious nebulosity associated to it within the 5' field of view of our images.
![]() |
Figure 2: A view of the field around WR 142a (the bright star at the center of the image) showing no evidence for extended emission. The printed version of this paper shows the K-band image, while a JHK colour composite is presented in the electronic version. |
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At the estimated distance of 1.8 kpc, WR 142a lies in
the extended complex of massive star forming regions composed by
several OB associations and part of the molecular complex Cygnus X
(e.g. Odenwald & Schwartz 1989). Its location with respect to the
three most nearby associations and the other WR stars identified
within
of it are shown in Fig. 3.
The overlap among the associations and the difficulty in precisely
establishing their boundaries (or even in establishing them as
different entities) makes it difficult to unambiguously assign
WR 142a to one of them. Adopting the distribution of members given
by Garmany & Stencel (1992), WR 142a lies near the boundaries of both
Cygnus OB2 and Cygnus OB9. Association to the latter may be suggested
in view of its proximity (less than )
to the open cluster
NGC 6910. The average extinction towards Cygnus OB9, where members
obscured by AV > 6 are listed by Garmany & Stencel (1992),
does not conflict with this assignment. It may be interesting to
note that WR 142a appears in the sky about 10' West of the
compact HII region DWB87 (Dickel et al. 1969) and may be obscured
by the molecular cloud surrounding it. No detailed studies of this
nebula seem to exist in the literature, preventing us from
ascertaining whether a physical connection between WR 142a and
DWB87 may exist. On the other hand, it is also possible that
WR 142a is actually a member of Cygnus OB2. Cygnus OB2
is a compact OB association (see Knödlseder 2000; Comerón et al. 2002, and references therein) rich in very massive members
which are on the average more reddened than those of Cygnus OB9.
Cygnus OB2 has four other WR stars, two of which also belong to the
WC class, indicating progenitors with initial masses above
(Crowther et al. 1995a), and in this respect the
higher content of very massive stars of Cygnus OB2 favors the
hypothesis that WR 142a is a member of this association lying in
its outskirts. Given the considerable uncertainties affecting the
definition of the membership, the boundaries, and the distances of
each of these associations (see Comerón et al. 1998 for an
additional discussion), the assignment of WR 142a to either
Cygnus OB2 or Cygnus OB9 must remain as only tentative for the time
being.
![]() |
Figure 3: Location of WR 142a with respect to the three OB associations in its neighbourhood. Also shown are the positions of other WR stars, with full circles representing WC stars and empty circles WN stars. The boundaries of the associations are a rough approximation to the distribution of their members as plotted in Garmany & Stencel (1992). |
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Our near-infrared observations of a source in Cygnus with infrared colours deviating from those of normal, reddened stars has brought up the serendipitous discovery of another WR star in this complex of rich OB associations. Table 2 summarizes the parameters of this star, WR 142a.
The detection of WR 142a adds one more example of a late evolved star
to the massive star content of this region. It is a remarkable
fact that Cygnus OB2 contains examples of all the stages in the
evolutionary path of the most massive stars,
O main sequence
Of/WN
LBV
WN
WC for initial masses about
(Langer et al. 1994). The addition of one more
member of the rare class of stars with such high initial masses is
thus a valuable one to a region that can be considered as the best
avaliable laboratory for the observational study of stellar
evolution at the upper end of the main sequence.
The abundant massive stars content of the complex Cygnus OB2/OB9,
where over 100 O-type stars and descendants have been identified
(Garmany & Stencel 1992; Massey et al. 1995; Knödlseder 2000;
Comerón et al. 2002), leads to the expectation that this complex
may be at present or in the near future the scenario of intense
supernova activity. Taking 7.1 Myr as the expected lifetime of a
late O-type star (Meynet et al. 1994; Schaerer & de Koter 1997),
the content of Cygnus OB2/OB9 would lead to the expectation of a rate
of about 1 SN every less than 70 000 yr. The observability of the
radio-continuum signature of individual supernova remnants is
limited in time and highly dependent on selection effects that are
difficult to model (Green 1991), but the inferred interval between
supernovae in Cygnus is rather short as compared to the estimated
age of many supernova remnants (Matthews et al. 1998). Thus, the
fact that none has been observed
within 2 degrees from the centre of Cygnus OB2 (cf. Green 2001)
suggests that no evolved massive star has gone through the
supernova phase yet. This poses some constraints on the age of
Cygnus OB2. According to the evolutionary track for an initial
mass of
computed by Langer et al. (1994), a star
enters the WC phase when its mass has decreased to
.
Only when its present mass is as small as
,
the star can be considered a probable candidate for a
supernova explosion. This happens after
yrs
from the main-sequence phase. Moreover, according to Langer et al.
(1994), WC stars pop up around 3 million years. Hence, we may argue
that Cygnus OB2 is as young as 3-4 million years, basically in
agreement with Massey et al.
(2001) who derived, from photometry of OB main-sequence stars, a
mean age of
yrs.
![]() |
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J (2MASS) | 9.22 |
H (2MASS) | 8.08 |
K (2MASS) | 7.09 |
J (June 2002) | 9.31 |
H (June 2002) | 8.15 |
K (June 2002) | 7.34 |
spectral type | WC8 |
r (kpc) | 1.8+0.6-0.5 |
AV (mag) |
![]() |
![]() |
5.041 |
M (![]() |
7.91 |
R (![]() |
0.81 |
T (K) | 125,0001 |
An inspection of van der Hucht's (2001) catalogue reveals that
Cygnus OB2 is the richest association in number of WC stars compared
to WN stars, and the discovery of the new WC star reported here
underlines this difference with respect to other associations.
This could be interpreted as the result of a shallower IMF combined
with the young age of the association. Indeed, an IMF slope of -0.9has been measured for Cygnus OB2 by Massey (1998) and the cluster
turn-off mass has been determined of about
by
Massey et al. (2001).
The discovery of a new Wolf-Rayet at a distance less than 2 kpc from the Sun reported here dramatically illustrates the incompleteness that affects the census of even the most massive stars in our galactic neighbourhood, while stressing the need for dedicated infrared surveys to improve the sample. Such efforts will be certainly worthwhile, given the critical dependence that our understanding of the star formation history and stellar mass function in OB associations has on working on complete samples, especially when the most massive and rarest members are concerned.
Acknowledgements
We would like to thank an anonymous referee whose comments improved this manuscript. Also, it is a pleasure to thank the staff of the Calar Alto observatory, especially Mr. Santos Pedraz, for their assistance during the observing run in which the observations presented here were obtained. We also appreciate the help of Ms. Uta Grothkopf in making available to us the original article of Melikian and Shevchenko, and of Ms. Petia Andreeva in translating it from Russian. This paper made use of data obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation; and of the SIMBAD database, operated at CDS, Strasbourg, France.