A very massive runaway star from Cygnus OB2
Astronomy & Astrophysics manuscript no. cygob2˙runaway c© ESO 2018
November 4, 2018
Letter to the Editor
A very massive runaway star from Cygnus OB2⋆
F. Comerón1 and A. Pasquali2
1 ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany
e-mail: fcomeron@eso.org
2 Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
e-mail: pasquali@mpia.de
Received; accepted
ABSTRACT
Aims. We analyze the available information on the star BD+43◦ 3654 to investigate the possibility that it may have had its origin in
the massive OB association Cygnus OB2.
Methods. We present new spectroscopic observations allowing a reliable spectral classification of the star, and discuss existing MSX
observations of its associated bow shock and astrometric information not previously studied.
Results. Our observations reveal that BD+43◦ 3654 is a very early and luminous star of spectral type O4If, with an estimated mass
of (70 ± 15) M⊙ and an age of ∼ 1.6 Myr. The high spatial resolution of the MSX observations allows us to determine its direction
of motion in the plane of the sky by means of the symmetry axis of the well-defined bow shock, which matches well the orientation
expected from the proper motion. Tracing back its path across the sky we find that BD+43◦ 3654 was located near the central, densest
region of Cygnus OB2 at a time in the past similar to its estimated age.
Conclusions. BD+43◦ 3654 turns out to be one of the three most massive runaway stars known, and it most likely formed in the
central region of Cygnus OB2. A runaway formation mechanism by means of dynamical ejection is consistent with our results.
Key words. Stars: early-type, kinematics. Stars: individual: BD+43◦ 3654. Galaxy: open clusters and associations: individual:
Cygnus OB2
1. Introduction
Current theories for the formation of massive stars stress the
importance of the dense cluster environment in which most
of them, if not all, form (Bonnell et al. 2007). Dynamical in-
teractions at the centers of massive star forming regions lead
to captures forming binary systems, ejections, mass segrega-
tion, and possibly coalescence. A remarkable byproduct of the
dynamical interactions in dense clusters of massive stars is
the relatively large abundance of runaway O-type stars, which
amount to almost ∼ 10% of the known O-type stars in the so-
lar vicinity (see Maı́z-Apellániz et al. (2004) for a recent cen-
sus). Runaway stars, characterized by their large spatial veloc-
ities, can form either by dynamical ejection from a dense clus-
ter (Poveda et al. 1967, Leonard & Duncan 1988, 1990) or by
the explosion as supernova of a member of a close massive
binary (Blaauw 1961, van Rensbergen et al. 1996, de Donder
et al. 1997). Evidence for actual examples resulting from both
mechanisms exists (Hoogerwerf et al. 2000, 2001), and both are
a consequence of the special conditions in which massive star
formation takes place. On the one hand, the high stellar density
of the parental cluster facilitates the dynamical ejection scenario.
On the other hand, the supernova scenario is favored by the high
frequency of binaries with high mass ratios among massive stars
(Garmany et al. 1982, Preibisch et al. 1999), which may be a
Send offprint requests to: F. Comerón
⋆ Based on observations collected at the Centro Astronómico
Hispano-Alemán (CAHA) at Calar Alto, operated jointly by the Max-
Planck Institut für Astronomie and the Instituto de Astrofı́sica de
Andalucı́a (CSIC).
consequence of dynamical capture followed by accretion and or-
bital evolution (Bate et al. 2003).
Cygnus OB2, the most massive OB association of the solar
neighbourhood (Knödlseder 2000, 2003, Comerón et al. 2002,
and references therein), should be the source of numerous run-
away stars given its rich content in massive stars, which in-
cludes the massive multiple system Cyg OB2 8 near its center.
Unfortunately, few studies to the date have addressed its possi-
ble runaway population, with the exception of the recent radial
velocity survey of Kiminki et al. (2007) in which no runaway
candidate has been identified until now. Comerón et al. (1994,
1998) pointed out the existence of large-scale kinematical pe-
culiarities in the Cygnus region, most likely related to the pres-
ence of Cygnus OB2, as shown by Hipparcos proper motions.
Although they interpreted their results in terms of triggered star
formation (Elmegreen 1998), at least some of the stars that they
identified as moving away from Cygnus OB2 might be actual
runaways formed by either of the two mechanisms listed above.
In this paper we report the identification of a very high
mass runaway star, BD+43◦ 3654, very probably ejected from
Cygnus OB2. The star had been already identified as a likely
runaway by van Buren & McCray (1988) based on the existence
of an apparent bow shock in IRAS images, caused by the inter-
action of its stellar wind with the local interstellar medium. Here
we present the first spectroscopic observations of the star, which
show it to be a very early Of-type supergiant. We also present
proper motion data and higher resolution MSX images leading
to a more detailed analysis, which strongly supports an origin at
the core of Cygnus OB2.
http://arxiv.org/abs/0704.0676v1
2 F. Comerón and A. Pasquali: A very massive runaway star from Cygnus OB2
BD+43  3654
IS IS
Fig. 1. Spectrum of BD+43◦ 3654 showing the main absorption
lines used for spectral classification and the prominent emission
of NIII and HeII. The prominent, unlabeled features are Hγ, Hδ,
and Hǫ. Interstellar absorption features are indicated by dotted
lines. The locations where one might expect to detect NIV and
SiIV transitions are also indicated.
2. Observations
The spectrum presented here was obtained in the course of
a project aimed at producing spectral classifications of pre-
viously unknown, photometrically selected new OB stars in
the surroundings of Cygnus OB2. The photometry in the
BRJHKS bands was taken from the Naval Observatory Merged
Astrometric Dataset (NOMAD) catalog (Zacharias et al. 2004),
which combines astrometry and photometry from the Hipparcos,
Tycho-2, UCAC2, USNO-B1.0, and 2MASS catalogs. The spec-
troscopic observations were carried out with the 2.2m tele-
scope at the German-Spanish Astronomical Center on Calar Alto
(Spain) using the CAFOS visible imager and spectrograph. A
1”5 slit combined with the B-100 grism, providing a resolution
λ/∆λ = 800 in the blue part of the visible spectrum, were used.
The exposure time was 900 s. The spectrum was reduced, ex-
tracted, and wavelength calibrated using standard IRAF1 tasks
under the ONEDSPEC package, and it was ratioed by a sixth-
degree polynomial fit to the continuum in order to remove the
steep slope due to the strong extinction towards the star.
3. Results
3.1. Stellar classification, properties, and kinematics
Although the identification of BD+43◦ 3654 as a likely runaway
star dates back to van Buren & McCray (1988), no spectral clas-
sification is available in that work. Subsequent papers by van
Buren et al. (1995) and Noriega-Crespo et al. (1997) refer to
the star as a unspecified B-type but do not report dedicated ob-
servations, and no other spectroscopic classification appears to
be available in the literature apart from a generic classification
as ’OB reddened’ in the LS catalog (Hardorp et al. 1964). The
spectrum presented here is thus the first one allowing an accu-
rate spectral classification of BD+43◦ 3654 and the estimate of
its physical parameters.
1 IRAF is distributed by NOAO, which is operated by the Association
of Universities for Research in Astronomy, Inc., under contract to the
National Science Foundation.
Fig. 2. Comparison between the position of BD+43◦ 3654 and
the evolutionary tracks for very massive stars of Meynet et
al. (1994).
The most obvious spectroscopic feature of BD+43◦ 3654 is
the presence of intense emission in the NIII and HeII lines, and
possibly also in NIV and SiIV, clearly indicating that it is a Of
star. HeII lines are also prominent in absorption, and together
with the absence of HeI lines indicates a spectral type earlier
than O5. Absorption bands due to interstellar absorption, CaII
and diffuse interstellar bands, are also strong due to the high ex-
tinction towards the star. Based on comparison with the atlas
of Walborn & Kirkpatrick (1990), we classify the star as O4If.
Using intrinsic colors of early-type stars from Tokunaga (2000)
and the 2MASS HKS photometry from the NOMAD catalog, we
estimate a K-band extinction AK = 0.57 mag.
A summary of previous distance determinations to
Cygnus OB2 has been presented by Hanson (2003). Based on
her results, we adopt her favored distance modulus DM = 10.8
corresponding to a distance of 1450 pc, with an estimated un-
certainty of ±0.4 based on the results of previous determinations
summarized in that work. Assuming that BD+43◦ 3654 is ap-
proximately at the same distance from the Sun as Cygnus OB2,
we derive its absolute magnitude as
MV = KS − AK + (V − KS )0 − DM = −6.3 ± 0.5 (1)
where the 0.5 mag uncertainty includes as the dominant source
the quoted uncertainty in the distance modulus and the contribu-
tion of error in the derivation of the extinction. We estimate the
latter to be 0.2 mag based on the quality of the fit of a reddened
O4-type spectral energy distribution to the measured BRJHKS
photometry. The contribution of the errors in the broad-band
photometry is negligible as compared to those other two sources.
Different calibrations of the stellar parameters of O stars
can be found in literature to estimate the mass and the age of
BD+43◦ 3654. These calibrations are based on a different treat-
ment of the stellar model atmospheres, depending on whether
non-LTE conditions, line-blanketing effects and stellar winds
are taken into account. For an O4 supergiant in the Milky Way
(i.e. of solar metallicity), Martins et al. (2005) provide an ef-
fective temperature Te f f = 40702 K; Repolust et al. (2004) es-
timate a colder Te f f = 39000 K, while Vacca et al. (1996) give
Te f f = 47690 K. We have adopted the average of those three cal-
ibrations, 42464 K, as the temperature for BD+43◦ 3654, con-
sidering as the uncertainty the range of temperatures spanned by
F. Comerón and A. Pasquali: A very massive runaway star from Cygnus OB2 3
those models. This uncertainty is larger than the temperature dif-
ference between the subtypes O4 and O5, and between types O4I
and O4V, for any given calibration (see e.g. Martins et al. 2005).
The same is true for the effects of metallicity, which are hardly
noticeable even when metal abundances change by a factor of
10. This is clearly shown in Fig. 15 of Heap et al. (2006), where
the temperature-spectral type relationships from different cali-
brations involving both galactic and Small Magellanic Cloud O
stars are compared. Therefore, plausible uncertainties in either
our spectral classification or in our assumption of solar metal-
licity for BD+43◦ 3654 do not significantly alter the size of the
error bars in Fig. 2. The absolute magnitudes MV obtained by all
three models are very similar, with an average of MV = −6.36
and individual determinations deviating by less than 0.05 mag
from that value. This is remarkably close to the value that we ob-
tain from the photometry of BD+43◦ 3654 and the assumption
that its distance modulus is the same obtained by Hanson (2003)
for Cygnus OB2, thus supporting our choice of that distance for
the star.
The position of BD+43◦ 3654 on the Herzsprung-Russell
(HR) diagram is shown in Fig. 2, together with the isochrones
computed by Meynet et al. (1994) for solar metallicity and with
enhanced stellar mass loss. These evolutionary tracks are prefer-
able because they better reproduce the low-luminosity observed
for some Wolf-Rayet stars, the surface chemical composition of
WC and WO stars and the ratio of blue to red supergiants in the
star clusters of the Magellanic Clouds. The isochrones plotted
in Fig. 2 refer to a stellar mass of the progenitor on the main
sequence of Mi = 20, 25, 40 M⊙ (in grey) and Mi = 60, 85 M⊙
(in black). The comparison between the observed properties of
BD+43◦ 3654 and the isochrones allows us to estimate an initial
mass Mi ≃ (70 ± 15) M⊙ and an approximate age of 1.6 Myr.
The isochrones do not take into account stellar rotation, which
many studies in the past decade have found to greatly affect mix-
ing and mass loss, and to be an important ingredient for stellar
evolution (Meynet & Maeder 1997, Langer et al. 1998, Heger &
Langer 2000, Meynet & Maeder 2000, and references therein).
As shown by Meynet & Maeder (2000), for an initial rotational
velocity of 200-300 km s−1 and solar metallicity isochrones be-
come brighter by a few tenths of a magnitude and the lifetime
in the H-burning phase increases by 20-30%. Given the obser-
vational errors on BD+43◦ 3654, these changes do no affect sig-
nificantly our estimates of the initial mass and age of the star.
Proper motions for BD+43◦ 3654 are available from the
NOMAD catalog, based on measurements by the Hipparcos
satellite in the Tycho catalog further refined with previous
ground-based observations. The values listed are µα cos δ =
(−0.4 ± 0.7) mas yr−1 and µδ = (+1.3 ± 1.0) mas yr
−1. The
corresponding values expressed in galactic coordinates, which
are more convenient to derive the spatial velocity of the star
with respect to its local interstellar medium, are µl cos b =
(+0.8 ± 0.9) mas yr−1, µb = (+1.1 ± 0.8) mas yr
Assuming that the local interestellar medium in the sur-
roundings of BD+43◦ 3654 moves in a circular orbit around
the galactic center, its proper motion (µl cos b)0, (µb)0 can be
described by the first-order approximation to the local galactic
velocity field; see e.g. Scheffler & Elsässer (1987):
(µl cos b)0 = 0.211[A cos 2l cos b + B cos b
sin l −
cos l] (2a)
(µb)0 = 0.211[−A sin 2l sin b cos b
Fig. 3. Image obtained in the Midcourse Space Experiment
(MSX) galactic plane survey in the D medium-infrared band
(13.5 µm - 15.9 µm). The position of BD+43◦ 3654 is marked
with a grey circle. The galactic North is up and the direction of
growing galactic longitude to the left.
cos l sin b +
sin l sin b −
cos b] (2b)
where A and B are the Oort constants in units of km s−1 kpc−1;
U, V , and W are the components of the solar peculiar mo-
tion in the directions toward the galactic center, the direction
of circular galactic rotation, and the North galactic pole re-
spectively, in km s−1, and D is the distance to the Sun in
kpc. We have adopted A = −B = 12.5 km s−1 kpc−1, cor-
responding to a flat rotation curve with an angular velocity of
25 km s−1 kpc−1 and (U,V,W) = (7, 14, 7) km s−1. The proper
motion of BD+43◦ 3654 with respect to its local interstellar
medium is then
∆µl cos b = µl cos b − (µl cos b)0 = (5.3 ± 1.1) mas yr
−1 (3a)
∆µb = µb − (µb)0 = (2.0 ± 0.9) mas yr
−1 (3b)
where the uncertainty allows for an error of 2 km s−1 kpc−1 in
each of A, B, and 2 km s−1 in each of U, V , and W. The position
angle θ of the residual proper motion with respect to the North
galactic pole, counted as positive in the direction of increasing
galactic latitude, is then
θ = tan−1
∆µl cos b
= 69◦3 ± 9◦4 (4)
The component of the spatial velocity on the plane of the sky
that we derive from the residual proper motion at the adopted
distance of 1450 pc is (39.8±9.8) km s−1, which is several times
the sound speed in a warm neutral interstellar medium at a tem-
perature of ∼ 8000 K, as expected from the fact that a clear bow
shock is observed ahead of the star in the direction of its motion.
3.2. The bow shock
The original identification of a possible bow shock associated
to BD+43◦ 3654 was reported by van Buren & McCray (1988)
4 F. Comerón and A. Pasquali: A very massive runaway star from Cygnus OB2
based on 60 µm IRAS maps, and further details were given
by van Buren et al. (1995) and Noriega-Crespo et al. (1997).
While indeed suggestive of a bow shock, the resolution of the
IRAS 60 µm images presented by van Buren et al. (1995) is not
sufficient to accurately determine the shape of the bow shock and
its position with respect to the star.
Much improved images of the region around BD+43◦ 3654
have been provided by the Midcourse Space Experiment (MSX)
satellite (Price et al. 2001). The BD+43◦ 3654 bow shock ap-
pears in them as a neat, well defined arc-shaped nebula in the
D (13.5-15.9 µm; see Fig. 3) and E (18.2-25.1 µm) bands, and
is absent in the A (6.8-10.8µm) and C (11.1-13.2 µm) bands.
The position of the apsis of the bow shock with respect to
BD+43◦ 3654 can be well determined from those images, be-
ing located at 3.4 arcmin from the star in a direction that forms
an angle of 62◦5±10◦ with the direction towards the north galac-
tic pole. This position angle is in very good agreement with the
position angle of the residual velocity vector of the star (Eq. (4)).
The position of the bow shock with respect to the star allows
us to estimate the density of the interstellar medium through
which BD+43◦ 3654 is moving. The apsis of the bow shock is
approximately located at the stagnation radius, which is the dis-
tance from the star where the ram pressure of the interstellar gas
equals that of the stellar wind, given by (e.g. Wilkin 1996):
Ṁwvw
4πρav2∗
where Ṁw and vw are respectively the mass loss rate and termi-
nal wind velocity of the star, ρa is the ambient gas density, and
v∗ is the spatial velocity of the star. The distance given in Eq. (5)
assumes that the bow shock is bound by shock fronts on both
sides. In reality, the non-zero cooling time of the shocked stel-
lar wind builds up a thick layer of low-density, high-temperature
gas between the reverse shock on the stellar wind and the bow
shock. The existence of this thick layer moves the position of the
apsis of bow shock to a greater distance from the star than that
given by Eq. (5). This expression actually gives the position of
the reverse shock ahead of the star, as shown in numerical sim-
ulations by Comerón & Kaper (1998), and the actual position of
the bow shock is normally ∼ 1.5R0, the precise distance depend-
ing on the quantities entering the right-hand side of Eq. (5) and
the cooling curve of the stellar wind gas. Concerning the stellar
wind, we have adopted Ṁ = 10−5 M⊙ yr
−1 and v∗ = 2300 km s
as typical values derived by Markova et al. (2004) and Repolust
et al. (2004) for the O4I stars in their samples. Finally, we use
v∗ = 39.8 km s
−1 as derived in the previous Section, assum-
ing for simplicity that most of the velocity of BD+43◦ 3654 is
on the plane of the sky and that there are no projection effects
on the position of the bow shock. Introducing these values in
Eq. (5), we obtain a number density of the local interstellar gas
nH ≃ 6 cm
−3. It must be kept in mind that this is only a rough es-
timate of the density, mainly due to the large uncertainties in the
values adopted for the different quantities intervening in Eq. (5)
and the assumption that the residual motion of the star is in the
plane of the sky. In particular, we note that Bouret et al. (2005)
find mass loss rates smaller by a factor of ∼ 3 for the galactic
O4If+ star HD190429A when taking into account wind clump-
ing with respect to the homogeneous wind case, which may im-
ply an overestimate of nH by a similar factor due to our adopted
values. In any case, the estimated density clearly indicates that
the star is moving in a tenuous medium whose density matches
well that typical of the warm HI gas in the vicinity of the galactic
midplane (e.g. Dickey & Lockman 1990).
4. Discussion: the origin of BD+43◦ 3654
The spectral type and estimated mass of BD+43◦ 3654 places it
among the three most massive runaway stars known to date. The
only other two comparable stars are ζ Pup and λ Cep (spectral
types O4I(n)f and O6I(n)fp, respectively; Maı́z Apellániz 2004),
whose masses (65-70 M⊙, as estimated by Hoogerwerf et
al. (2001) from evolutionary models by Vanbeveren et al. (1998))
are similar to the one that we estimate for BD+43◦ 3654.
Although currently placed near the boundary separating
Cygnus OB1 and OB9 (to the extent that this boundary may
be real; see Schneider et al. (2007)), the proper motion of
BD+43◦ 3654 points away from the core of Cygnus OB2, which
is approximately marked by the location of the multiple system
of O stars Cyg OB2 8A-D. Other early O-type stars, most no-
tably Cyg OB2 22A (O3If*), Cyg OB2 22B (O6V((f))), and
Cyg OB2 9 (O5If+), are also within few arcminutes of that lo-
cation. The position angle of BD+43◦ 3654 with respect to this
system is 58◦84, very similar to the position angle of its resid-
ual proper motion vector (Sect. 3.1) and of the axis of the bow
shock (Sect. 3.2). In view of the high density of very massive
OB stars found in the central regions of Cygnus OB2 (Massey
& Thompson 1991), we thus consider as a very likely possibility
that BD+43◦ 3654 was formed there and subsequently expelled.
Assuming that BD+43◦ 3654 was born in the close vicinity
of Cyg OB2 8, of which is currently separated by an angular
distance δ = 2◦67, the travel time to its current position is τ =
(∆µl cos b)2 + (∆µb)2 = 1.7 ± 0.4 Myr, which is close to the
age of the star inferred from the evolutionary tracks and the posi-
tion in the H-R diagram (Sect. 3.1). The coincidence between the
age and the travel time supports dynamical ejection early in its
life as the cause for its runaway velocity, since there would have
been no time for a hypothetical massive companion to evolve, go
through different mass transfer episodes (Vanbeveren et al. 1998)
and then explode as supernova. The spatial velocities of the other
two massive runaways noted above are probably higher, unless
the radial velocity of BD+43◦ 3654 exceeds the projected veloc-
ity on the plane of the sky: Hoogerwerf et al. (2001) measure
a velocity of 62.4 km s−1 for ζ Pup, and 74.0 for λ Cep. High
spatial velocities may be the signature of an origin by supernova
ejection, since high ejection velocities by dynamical interaction
become increasingly unlikely as the mass of the ejected star in-
creases. The observed mass-velocity relationship for runaway
stars (Gies & Bolton 1986) clearly shows this trend. Hoogerwerf
et al. (2001) favor a supernova scenario for λ Cep on the basis
of the difference between its age and that of the likeliest parental
association. The birthplace of ζ Pup is more uncertain according
to Hoogerwerf et al. (2001), but van Rensbergen et al. (1996)
also favor a supernova scenario. We note however that the ve-
locities of all three stars is well below the upper limit for the
ejection of very massive stars in encounters with massive bina-
ries (Leonard 1991). We thus consider the similarity between the
estimated age of BD+43◦ 3654, and its kinematic age if it was
born near the center of Cygnus OB2, as the strongest argument in
support of a dynamical ejection, possibly from an original clus-
ter containing in addition Cyg OB2 8, 9, and 22.
BD+43◦ 3654 is the first runaway star from Cygnus OB2
identified thus far, but most likely it is not the only one in such a
rich association. If the fraction of runaways among O-type stars
is the same for Cygnus OB2 as for the more nearby population
of O stars, we estimate that about ten more Cygnus OB2 run-
aways may remain to be discovered, having the potential of pro-
viding new information on their formation environments and on
the mechanisms leading to the runaway ejection.
F. Comerón and A. Pasquali: A very massive runaway star from Cygnus OB2 5
Acknowledgements. It is as always a pleasure to acknowledge the support of the
staff of the Calar Alto observatory during the execution of our observations. We
also thank the detailed and constructive comments of the referee, Dr. Dave van
Buren. This research has made use of the SIMBAD database operated at CDS,
Strasbourg, France. It also makes use of data products from the Two Micron
All Sky Survey, which is 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, as well as of data products from the Midcourse Space
Experiment (MSX). Processing of the MSX data was funded by the Ballistic
Missile Defense Organization with additional support from NASA Office of
Space Science. This research has also made use of the NASA/ IPAC Infrared
Science Archive, which is operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics and Space
Administration.
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List of Objects
‘Cygnus OB2’ on page 1
‘BD+43◦ 3654’ on page 1
‘HD190429A’ on page 4
‘ζ Pup’ on page 4
‘λ Cep’ on page 4
‘Cyg OB2 8A-D’ on page 4
‘Cyg OB2 22A’ on page 4
‘Cyg OB2 22B’ on page 4
‘Cyg OB2 9’ on page 4
	Introduction
	Observations
	Results
	Stellar classification, properties, and kinematics
	The bow shock
	Discussion: the origin of BD+43 3654
