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A Hipparcos census of the nearby OB associations
Zeeuw, P. T.; Hoogerwerf, R. D; Bruijne, J. H. J. de; Brown, A. G. A.; Blaauw, A.
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10.1086/300682
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Zeeuw, P. T., Hoogerwerf, R. D., Bruijne, J. H. J. D., Brown, A. G. A., & Blaauw, A. (1999). A Hipparcos
census of the nearby OB associations.
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A Hipparcos Census of the Nearby OB Asso ciations
1
P.T. de Zeeuw, R. Hoogerwerf, J.H.J. de Bruijne,
Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
A.G.A. Brown,
Instituto de Astronoma U.N.A.M., Apartado Postal 877, Ensenada, 22800 Ba ja California, Mexico
and
A. Blaauw
Kapteyn Instituut, Postbus 800, 9700 AV Groningen, The Netherlands and
Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
Accepted for publication in the Astronomical Journal, January 1999 issue
ABSTRACT
A comprehensive census of the stellar content of the OB asso ciations within 1 kp c from the Sun is presented, based on
Hipparcos p ositions, proper motions, and parallaxes. It is a key part of a long-term pro ject to study the formation,
structure, and evolution of nearbyyoung stellar groups and related star-forming regions.
OB associations are unbound `moving groups', which can be detected kinematically b ecause of their small internal
velocity dispersion. The nearby asso ciations have a large extent on the sky, which traditionally has limited astrometric
membership determination to bright stars (
V
<
6
m
), with spectral types earlier than
B5. The Hipparcos measurements
allow a ma jor improvement in this situation. Moving groups are identied in the Hipparcos Catalogue by combining
de Bruijne's refurbished convergent p oint metho d with the `Spaghetti metho d' of Ho ogerwerf & Aguilar. Astrometric
members are listed for 12 young stellar groups, out to a distance of
650 p c. These are the 3 subgroups Upp er Scorpius,
Upper Centaurus Lupus and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2,
Persei
(Per OB3), Cas{Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection pro cedure
corrects the list of previously known astrometric and photometric B- and A-type members in these groups, and identies
many new members, including a signicantnumber of F stars, as well as evolved stars, e.g., the Wolf{Rayet stars
2
Vel
(WR11) in Vel OB2 and EZ CMa (WR6) in Col 121, and the classical Cepheid
Cep in Cep OB6. Membership
probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the exp ected number of
interloper eld stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri ob jects
and other stars in the nal pre-main sequence phase. This provides a rm link b etween the classical high-mass stellar
content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding
interstellar medium, will b e presented elsewhere.
Astrometric evidence for moving groups in the elds of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3,
Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but
they are either at distances b eyond
500 p c where the Hipparcos parallaxes are of limited use, or they have unfavorable
kinematics, so that the group prop er motion does not distinguish it from the eld stars in the Galactic disk.
The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric
estimates. While part of this may be caused by the improved membership lists, a recalibration of the upp er main
sequence in the Hertzsprung{Russell diagram may b e called for. The mean motions display a systematic pattern, which
is discussed in relation to the Gould Belt.
Six of the 12 detected moving groups do not app ear in the classical list of nearby OB asso ciations. This is sometimes
caused by the absence of O stars, but in other cases a previously known op en cluster turns out to b e (part of ) an extended
OB association. The number of unb ound young stellar groups in the Solar neighbourho od may be signicantly larger
than thought previously.
1
Based on data from the Hipparcos astrometry satellite.
2
P.T. de Zeeuw et al.
1. INTRODUCTION
Ever since spectral classications for the bright stars b e-
came available, and in particular with the publication of
Cannon and Pickering's monumental Henry Draper Cat-
alog of stellar spectra in 1918{1924, it has b een evident
that O and B stars are not distributed randomly on the
sky | and hence not uniformly among the stellar p opu-
lation of the Galaxy | but instead are concentrated in
loose groups. This inspired research on their individual
properties, and on their motions and space distribution.
Studies of the stellar content, the internal velocity distri-
bution, and the common motion of the group members
with respect to the ambient stellar p opulation naturally
relied on the capability of identifying the stars belong-
ing to the group, their `members'. Among extensive early
investigations along these lines in the wake of work on
`moving groups' and compact stellar clusters (summarized
in Eddington 1914), we note Kapteyn's (1914, 1918) work
on `the brighter Galactic helium stars' | i.e., the B stars
| and the work by Rasmuson (1921, 1927). Among the
early investigations of the space distribution we note in
particular the comprehensivework byPannekoek (1929),
carried out at the time of the breakthrough of the mo d-
ern concept of the Galaxy as an isolated, rotating system.
Pannekoek's summarizing table 15 lists 37 groups of B
stars, among which several, called by him `Lacerta', `Cep
I', `Cep I I', `
Pers', `Lupus', `Scorpio', and others, may
be considered as `forerunners' of the ob jects of mo dern re-
search. His diagram of the groups of B stars, pro jected
on the Galactic plane (
loc. cit.
p. 65) is a remarkable fore-
shadow of some of the gures presented in this paper. In
the early 1950s the work of W.W. Morgan and collabora-
tors provided the rst identication of the Galactic spiral
structure in the distribution of stellar `aggregates' (Mor-
gan, Sharpless & Osterbrock 1952a, b; Morgan, Whitford
& Co de 1953). Herschel (1847) and Gould (1874) had al-
ready noticed that the brightest stars are not distributed
symmetrically with resp ect to the plane of the Milky Way,
but seemed to form a b elt that is inclined
18
to it. This
became known as the Gould Belt, and was subsequently
found to b e asso ciated with a signicant amountofinter-
stellar material (Lindblad 1967; Sandqvist, Tomboulides
& Lindblad 1988; Poppel 1997).
Ambartsumian (1947) introduced the term `asso cia-
tion' for the groups of OB stars; he pointed out that their
stellar mass density is usually less than 0.1 M
pc
3
. Bok
(1934) had already shown that such low-density stellar
groups are unstable against Galactic tidal forces, so that
OB asso ciations must b e young (Ambartsumian 1949), a
conclusion supported later by the ages derived from color-
magnitude diagrams. This is in harmony with the fact that
these groups are usually lo cated in or near star-forming
regions, and hence are prime sites for the study of star
formation processes and of the interaction of early-type
stars with the interstellar medium (see e.g., Blaauw 1964a,
1991 for reviews). Ruprecht (1966) compiled a list of OB
associations, with eld b oundaries, bright members, and
distance. He introduced a consistent nomenclature, ap-
proved by the IAU, whichwe adopt here. Ruprecht's list
is based on the massive `Catalogue of Star Clusters and
Associations', put together by him and his group, and
maintained for many years (Alter, Ruprecht & Vanysek
1970; Ruprecht, Balazs & White 1981).
Detailed knowledge of the stellar content, structure,
and kinematics of OB associations allows us to address
fundamental questions on the formation of stars. What is
the initial mass function? What are the characteristics of
the binary p opulation? What is the star formation rate
and eciency? Do all stars in a group form at the same
time? What causes the distinction between the formation
of bound open clusters and unbound asso ciations? What
is the connection between the stellar content of associa-
tions and the energetics and dynamical evolution of the
surrounding interstellar medium? What are the prop er-
ties of the ensemble of OB associations, and how do these
relate to the structure and evolution of the Galaxy? An-
swers to these questions are essential for the interpretation
of observations of extragalactic star-forming regions and
starburst galaxies.
Although OB asso ciations are unbound, their velo c-
ity dispersions are only a few km s
1
(e.g., Mathieu 1986;
Tian et al. 1996), and so they form coherent structures in
velocity space. The common space motion relative to the
Sun is p erceived as a convergence of the proper motions
of the members towards a single p oint on the sky (e.g.,
Blaauw 1946; Bertiau 1958). This can b e used to establish
membership based on measurements of proper motions.
Whereas many such astrometric membership studies have
been carried out for open clusters (e.g., van Leeuwen 1985,
1994; van Altena et al. 1993; Robichon et al. 1997), there
are few such studies for nearby OB associations b ecause
these generally cover tens to hundreds of square degrees
on the sky. Ground-based proper motion studies therefore
almost invariably have been conned to modest samples of
bright stars (
V
<
6
m
) in fundamental catalogs, or to small
areas covered by a single photographic plate. Photomet-
ric studies extended membership to later spectral types
(e.g., Warren & Hesser 1977a, b, 1978), but are less re-
liable due to, e.g., undetected duplicity, or the distance
spread within an association. As a result, membership
for many asso ciations has previously been determined un-
ambiguously only for spectral types earlier than B5 (e.g.,
Blaauw 1964a, 1991). Although this covers the imp ortant
upper main-sequence turno region, it is not known what
the lower mass limit is of the stars that b elong to the
association, so that our knowledge of these young stellar
groups has remained rather limited.
The Hipparcos Catalogue (ESA 1997) contains accu-
rate positions, prop er motions, and trigonometric paral-
laxes, which are all tied to the same global reference sys-
tem (ICRS: see ESA 1997, Vol. 1
x
1.2.2). This makes
these measurements ideally suited for the identication
of astrometric memb ers of the nearby OB associations,
Hipparcos & the nearby OB asso ciations
3
with greater reliability, and to much fainter magnitudes
than accessible previously. Accordingly, the
SPECTER
con-
sortium was formed in Leiden in 1982, which successfully
proposed the observation by Hipparcos of candidate mem-
bers of nearby OB asso ciations. An extensive program of
ground-based observations was carried out in anticipation
of the release of the Hipparcos data (for a summary, see de
Zeeuw, Brown&Verschueren 1994;
x
2.3). Here we present
the results of our census of the nearby associations based
on the Hipparcos measurements.
We have developed a new pro cedure to identify mov-
ing groups in the Hipparcos Catalogue in an ob jective and
reliable way. It combines a refurbished convergent p oint
method (de Bruijne 1998) and the so-called `Spaghetti
method' of Ho ogerwerf & Aguilar (1998), and in this way
signicantly reduces the number of misidentications. We
have estimated the remaining number of interlopers by
means of Monte Carlo simulations. The Hipparcos mea-
surements are most valuable for the nearest asso ciations.
The individual parallaxes havetypical errors of
1 mas,
and hence are of little value beyond
500 p c. However,
the proper motions can be quite signicanttomuch larger
distances. For this reason we apply our procedure to elds
centered on the known associations and susp ected groups
at pre-Hipparcos estimated distances of less than 1 kp c
from the Sun.
This pap er is organized as follows. In
x
2we summarize
the original selection of elds, and the resulting Hippar-
cos sample, and in
x
3 we describ e our member selection
procedure. Then we discuss the results for each of the
elds, starting with the Scorpio{Centaurus{Lupus{Crux
complex in
x
4.
x
5 is devoted to the Vela region. In
x
6
we discuss Canis Ma jor, Mono ceros, and Orion. Then fol-
lowTaurus, Perseus, Cassiop eia, and Camelopardalis (
x
7),
and Lacerta, Cepheus, Cygnus, and Scutum (
x
8). In
x
9we
derive mean distances and motions for the nearby associ-
ations. We summarize our overall conclusions in
x
10, and
also outline the next steps. Appendices give details of
the member selection, of pitfalls in the determination of
mean distances, and lists of association members. The se-
lection procedure for the Scorpio{Centaurus{Lupus{Crux
complex (
x
4) is describ ed in detail. It provides an exam-
ple of howwehave analysed the other elds, for whichwe
restrict ourselves to a concise description of the pre- and
post-Hipparcos results.
Preliminary results of this census were rep orted in de
Bruijne et al. (1997), Ho ogerwerf et al. (1997) and de
Zeeuw et al. (1997). These are superseded by the results
presented here. Detailed studies of the physical properties
of the stars in the nearby OB asso ciations will be presented
elsewhere.
Figure 1:
Pre-Hipparcos locations of the OB asso ciations
within
1.5 kp c, pro jected onto the Galactic plane, as listed
by Ruprecht (1966). The Sun is at the center of the dashed
lines which give the principal directions in Galactic longi-
tude,
`
. The size of the circles represents the pro jected di-
mension of the associations, enlarged by a factor 2 with re-
spect to the distance scale. The size of the central dots in-
dicates the degree of current or recent star formation activ-
ity, as given by the number
N
of stars more luminous than
absolute magnitude
M
V
5
m
(Humphreys 1978). Asso cia-
tions discussed in this paper which are absent from Ruprecht's
list are represented as op en circles: Scorpius OB2 subgroups 3
and 4 (Blaauw 1946); Vela OB2 (Brandt et al. 1971); Trum-
pler 10 (Lynga 1959, 1962); Collinder 121 (Feinstein 1967);
Cassiopeia{Taurus (Blaauw 1956); Cepheus OB4 (MacConnell
1968); R Corona Australis (Marraco & Rydgren 1981). The
distribution of small dots indicates the Gould Belt (
x
9.2). Fig-
ure 29 presents the p ost-Hipparcos map of the nearby OB as-
sociations.
2. THE NEARBY OB ASSOCIATIONS
2.1. The Solar neighbourho od
Properties of the OB associations within 1.5 kpc from
the Sun were reviewed by Blaauw (1964a, 1991). Fig-
ure 1 shows the pre-Hipparcos locations of these asso cia-
tions, as given in the ocial IAU list (Ruprecht 1966). It
also includes a few asso ciations and subgroups discussed
here, but absent from Ruprecht's list. The large variety
in stellar content, pro jected dimension, clumpiness, age,
and connection to the interstellar medium introduces bi-
ases in the identication of the asso ciations. For exam-
ple, associations containing several luminous supergiants
can be detected out to several kp c (e.g., Cep OB1 in
x
8.2; Humphreys 1978; Garmany & Stencel 1992), whereas
associations without such stars can easily escap e atten-
tion. Ruprecht's list is based on Ambartsumian's deni-
tion which requires the presence of O stars and an op en
4
P.T. de Zeeuw et al.
cluster as `nucleus' (e.g., Alter et al. 1970). As a result the
list is incomplete, e.g., twoofthe subgroups of Sco OB2
are not included (
x
4). Many of the groups in Figure 1 are
associated with concentrations of molecular material (e.g.,
Dame et al. 1987). Most asso ciations within 500 p c from
the Sun b elong to the Gould Belt system (
x
9.2).
2.2. Hipparcos Prop osal 141
When the call for prop osals for the Hipparcos mission was
released in 1982, the pre-launch sp ecications indicated
absolute prop er motions and parallaxes with 1
accura-
cies of 1.5{2 mas (yr
1
), a limiting magnitude of
V
11
m
,
and a total mean stellar density of ab out 3 stars p er square
degree. This clearly promised a ma jor step forward in the
study of the nearby OB asso ciations. Accordingly, the
SPECTER
consortium submitted a prop osal to ESA to in-
clude in the Hipparcos Input Catalogue candidate mem-
bers of the known OB associations (or subgroups) out to
a distance of 1 kp c. We took generous b oundaries around
the known locations. Table 1 lists the asso ciations, gives
their pre-Hipparcos distance
D
clas
, the eld b oundaries
in Galactic coordinates (
`; b
), and the proposed number
N
prop
of candidate members p er asso ciation or subgroup.
We selected all O and B stars from the Catalogue of
Stellar Identications (CSI: Jung & Bischo 1971) within
the asso cation b oundaries, as well as later-type stars within
certain magnitude limits dened by the (pre-Hipparcos)
distances of the asso ciations, so as to exclude foreground
and background stars. This resulted in 13961 candidate
member stars. We divided this sample initially into three
categories. Priority1contained stars of sp ectral type O
and B and stars that were considered as established or
probable members of the asso ciations based on proper mo-
tion, radial velocity or photometric work by previous au-
thors. The remaining stars were divided into two nearly
equal groups, by assigning them priority 2 or 3 dep ending
on whether their CSI number is odd or even. We to ok this
seemingly p eculiar step because the number densityofour
proposed ob jects was uncomfortably close to the limit of
3 p er square degree. We therefore suggested to the Input
Consortium that in order to satisfy the constraintonthe
number density of ob jects, while preserving the statistical
completeness of our sample, all priority3stars could be
dropped, if necessary.
The
SPECTER
proposal was approved as number 141,
with the comment that for groups b eyond 600 p c the par-
allax measurements would not b e very signicant, and that
|aswe had exp ected | the number density of the fainter
proposed stars was larger than was p ossible to accept for
the Hipparcos Input Catalogue. This resulted in the in-
clusion of 9150 of our candidate members in the Hipparcos
Input Catalogue (Turon et al. 1992), amongst whichwere
nearly all the priority 1 stars. Table 1 lists the number
N
HIC
of the originally proposed stars that were accepted
per eld, together with the total number
N
HIP
of stars
observed by Hipparcos in the same eld.
Table 1. The nearby OB asso ciations
Name
D
clas
`
`
+
b
b
+
N
prop
N
HIC
N
HIP
Sco OB2 1 170 330 3
19 7 2522 1650 3320
Sco OB2
2
?
160 337 3 7 32 1233 908 1796
Sco OB2
3
?
170 313 337 5 31 1737 1202 2215
Sco OB2
4
?
160 292 313
10 16 1531 1161 2787
Sco OB2
5 170 273 292
20 5 1337 973 2225
Col 121 630 222 244
15
3 562 424 1001
Ori OB1 500 197 215
26
12 1318 750 968
Mon OB1 715 201 205
3 3 86 46 94
Per OB2 400 156 164
22
13 207 130 218
Persei
?
170 140 155
11
3 619 361 517
Cam OB1 900 130 153
3 8 862 320 919
Cep OB4 845 116 120 3 7 46 18 60
Cep OB3 960 108 113 1 7 169 97 157
Cep OB2 700 96 108
1 12 651 345 676
Lac OB1 600 94 107
19
7 561 344 709
Cyg OB7 740 84 96
5 9 249 181 680
Cyg OB4 1000 81 85
9
6 15 10 51
Sct OB2 730 20 26
3 2 45 28 82
Cas{Tau
??
140 all sky 49 49
Runaways all sky 162 153
Total 13961 9150 18475
Note. | Pre-Hipparcos distance estimate
D
clas
(from Ruprecht 1966;
Sco OB2
1, 2 3, 2 4, and 2 5 from Blaauw 1946; Col 121 from Feinstein
1967; Cep OB4 from MacConnell 1968; Cas{Tau from Blaauw 1956), eld
boundaries in Galactic coordinates (
`; b
), and number
N
prop
of candidate
members prop osed for observation with Hipparcos in 1982.
N
HIC
is the
numb er of candidates accepted for inclusion in the Hipparcos Input Catalogue.
N
HIP
is the total number of stars in the Hipparcos Catalogue within the eld
boundaries. Some of the elds contain more than one association. In this pap er,
we also study elds in Corona Australis (
x
4.6) and in Vela (
x
5).
?
Sco OB2
2:
Upper Scorpius (US); Sco OB2
3: Upp er Centaurus Lupus (UCL); Sco OB2 4:
Lower Centaurus Crux (LCC);
Persei: Per OB3.
??
Ruprecht lists Cas{Tau
as a semi-doubtful asso ciation, and gives a distance of 1400 pc.
Even though we had distinguished priority 2 and 3
stars to allow cutting of the sample by a factor 2 in a sta-
tistically unbiased way, the nal selection turned out to
contain b oth, so that statistical inferences, especially for
stars fainter than the completeness limit, should b e made
with care. For example, in some areas the inclusion of
many B-type stars resulted in a bias against stars of type
A and later, so as to stay within the limits on number
density (Figure 2).
We reprop osed our program in 1992, and took the op-
portunity to mo dify the eld boundaries slightly, and to
include the groups R CrA and Vel OB2. The preliminary
results of our census, reported in de Bruijne et al. (1997),
Hoogerwerf et al. (1997), and de Zeeuw et al. (1997), are
based on the resulting sample of 12842 stars, made avail-
able to us in late 1996. During this investigation it b ecame
clear that in some cases the associations seemed to `spill
over' the previously chosen boundaries in
`
,
b
, and
V
. Now
that the entire Hipparcos Catalogue is available, we can in-
vestigate the full set of stars in the association elds, and
also study the surrounding areas, and we do so here.
The Hipparcos Catalogue also includes 153 of our origi-
nal 162 prop osed candidate runaway OB stars. The study
of these stars is of interest for the question of their ori-
gin: sup ernova explosions in high-mass binaries, or dy-
namical ejection, or both (Blaauw 1961, 1993; Poveda,
Ruiz & Allen 1967). We are in the process of retracing
the three-dimensional paths of the runaways and the OB
associations in the Galactic p otential in order to identify
the parent asso ciations, and the age of the runaways. This
![Figure 15: Positions and proper motions (left) and parallaxes (right) of the classical Monoceros OB1 members in the Hipparcos Catalogue ( lled symbols), and all stars earlier than A0 with parallaxes < 10 mas (open circles). One star lies outside the plotted parallax range ( = 5:93 6:94 mas). The classical NGC2264 members (Walker 1956) are denoted by lled circles. Their parallaxes, 3 mas, do not t the distance estimate of 1 kpc (e.g., Walker 1956; P erez 1991). The lled squares are the classical Mon R1 members (Racine 1968, Herbst et al. 1982). The proper motion and parallax of HIP31065 ([`; b]](/figures/figure-15-positions-and-proper-motions-left-and-parallaxes-1lv6lfp0.webp)
