Kinematics and binaries in young stellar aggregates. II. NGC 6913 ≡ M 29
ABSTRACT Between 1996 and 2003 we obtained 226 high resolution spectra of 16 stars in the field of the young open cluster NGC 6913, to constrain its main properties and study its internal kinematics. Twelve of the program stars turned out to be members, one of them probably unbound. Nine are binaries (one eclipsing and another double lined) and for seven of them the observations allowed us to derive the orbital elements. All but two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already shows signs that could be interpreted as evidence of dynamical relaxation and mass segregation. However, they may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of possible cluster angular momentum. All the discovered binaries are hard enough to survive average close encounters within the cluster and do not yet show signs of relaxation of the orbital elements to values typical of field binaries. Table 2 is only available in electronic form at http://www.edpsciences.org Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/145
arXiv:astro-ph/0310090v1 3 Oct 2003
Astronomy & Astrophysics manuscript no. Boeche
(DOI: will be inserted by hand later)
February 2, 2008
Kinematics and binaries in young stellar aggregates.
II. NGC 6913≡M29
C. Boeche1, U. Munari1, L. Tomasella1, and R. Barbon2
1Osservatorio Astronomico di Padova, Sede di Asiago, I-36012 Asiago (VI), Italy
2Osservatorio Astrofisico del Dipartimento di Astronomia, Universit´ a di Padova, I-36012 Asiago (VI), Italy
Received date..............; accepted date................
Abstract. Between 1996 and 2003 we have obtained 226 high resolution spectra of 16 stars in the field of the young
open cluster NGC 6913, to the aim of constraining its main properties and study its internal kinematics. Twelve
of the program stars turned out to be members, one of them probably unbound. Nine are binaries (one eclipsing
and another double lined) and for seven of them the observations allowed to derive the orbital elements. All but
two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already
shows signs that could be interpreted as evidence of dynamical relaxation and mass segregation. However, they
may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from
the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination
of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide
binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of a
possible cluster angular momentum. All discovered binaries are hard enough to survive average close encounters
within the cluster and do not yet show sign of relaxation of the orbital elements to values typical of field binaries.
Key words. Binaries: spectroscopic – Stars: early type – ISM: bubbles – Open clusters and associations: general
– Open clusters and associations: individual (NGC 6913)
This is the second paper of a series devoted to the results
of a long term, high resolution spectroscopic study of early
type members of young open clusters, trapezium systems
and OB associations. The aims of this series are discussed
in Paper I (Munari and Tomasella 1999).
NGC 6913, the topic of this paper, is a young open
cluster harboring O-type members and lying close to the
plane of the Galaxy (α = 20h23m
l = 76◦
·92, b = +0◦
catalog as M29, few papers in literature deal with it,
furthermore showing some disagreement in the results.
Cluster distance is reported to be 2.2 kpc by Morgan
and Harris (1956) and Massey et al. (1995), 1.5 kpc by
Joshi et al. (1983), and 1.1 kpc by Hoag et al. (1961),
while Tifft (1958) suggested that NGC 6913 is indeed the
results of two separate groups of stars, one at 1.6 kpc
and the other somewhere between 1.9 and 2.4 kpc. The
mean and differential reddening span a range of values too:
< EB−V >=0.78, ∆EB−V=0.64 according to Joshi et al.
(1983), < EB−V >=0.71 and ∆EB−V=1.82 for Wang and
Hu (2000), and < EB−V >=1.03 following Massey et al.
·9, δ = +38◦32′(J2000);
·61). Despite appearing in the Messier
Send offprint requests to: U. Munari (firstname.lastname@example.org)
(1995). Similarly, estimated ages span from 0.3−1.75 Myr
of Joshi et al. (1983) to 10 Myr of Lyng˚ a (1987).
The internal and galactic kinematics of NGC 6913 has
not been so far investigated in literature. The cluster ra-
dial velocity used by Hron (1987) in modeling the rotation
curve of the Galaxy, −25 km sec−1, was assembled by
scanty literature data that apparently missed all bright-
est cluster members, and is largely off our much more
accurate and representative −16.9(±0.6) km sec−1value
(see sect. 3.2). Internal kinematics and binary content of
NGC 6913 are unknown because no detailed radial ve-
locity study of its members has been ever pursued, and
proper motions investigations (Sanders 1973, Dias et al.
2002) are not deep and accurate enough for a firm mem-
bership segregation over a wide range of magnitudes, do
not cover all candidate members and do not allow resolu-
tion of the internal kinematics.
In this paper we aim to look in more details to
NGC 6913 general properties (like astrometric member-
ship, photometry, reddening, distance, mass and age) and
to present and discuss the results of our extensive spec-
troscopic study of NGC 6913 based on 226 high resolution
spectra monitoring of 16 stars in the field of the cluster
over the time span 1996-2003. These observations are used
to constrain the internal velocity dispersion, the cluster
2 C. Boeche et al.: Kinematics and binaries in young stellar aggregates. II. NGC 6913≡M29
Fig.1. Finding chart for NGC 6913 program stars.
galactic motion, the individual rotational velocities, and
the internal kinematical and evolutionary status of the
cluster. Spectroscopic orbits are calculated for the discov-
ered binary stars.
2. Spectroscopic observations
Table 1 summarizes the main properties of the 16 se-
lected program stars, and Figure 1 provides a finding
chart for them. The program stars have been spectro-
scopically observed over the period 1996-2003 with the
1.82 cm telescope and Echelle+CCD spectrograph of the
Astronomical Observatory of Padova at Asiago (Italy).
Table 2 provides the journal of observations. The instru-
mental set-up, spectra extraction and calibration, accura-
cies, etc. are identical to Paper I and the reader is referred
to it for details.
2.1. Spectral classification and radial velocities
Scanty information exists on the spectral classification of
the program stars. Wang and Hu (2000) derived spectral
types from low resolution spectra (5.3˚ A/pix) covering
the range 4200-6900˚ A. Kazlauskas and Jasevicius (1986)
obtained photoelectric photometry in the Vilnius system,
that we have converted into spectral types using the red-
dening free color parameters Q defined by Strayˇ zis (1977)
appropriate for the RV = AV/EB−V=3.6 reddening law
that applies to NGC 6913 according to Johnson (1962).
We have also derived spectral classification of the pro-
gram stars using our Echelle spectra, classified against
the Yamashita et al. (1977) spectral atlas. Even if spec-
tral classification of Echelle spectra has to be carried out
with care (lines to be compared normally fall on different
Fig.2. The program stars on the reddening corrected V◦,
(B−V )◦diagram using EB−V from Table 3 and V , B−V
photometry from Table 1 for RV = AV/EB−V =3.6 ap-
propriate for NGC 6913 according to Johnson (1962). The
isochrone for solar metallicity and 5 Myr is from Bertelli
et al. (1994) and it is scaled to m−M=10.5 (dotted line),
m − M=11.0 (solid line) and m − M=11.7 (dashed line).
It is evident how the cluster distance cannot be well con-
strained. In this paper we adopt a 1.6 kpc distance.
Echelle orders), nevertheless the resulting spectral types
look quite reasonable, and, given the far superior spec-
tral resolution and high S/N, also possibly more accurate
than those of Wang and Hu (2000). The three estimates
of the spectral type are compared in Table 3. The last two
columns of the table give the reddening and distance when
Fitzgerald (1970) intrinsic colors and our spectral classifi-
cation are compared to V , B − V photometry in Table 1.
The positions of the program stars on the reddening cor-
rected HR diagram are shown in Figure 2.
Radial velocities from individual observations (here-
after referred to as epoch radial velocities) of the program
stars are given in Table 4. For O and B type program
stars they rest on individual measurement of HeI and HeII
lines. For the other, cooler program stars the radial veloc-
ities come from measurement of the metallic absorptions
lines (mainly Fe I, Mg I, Ti II). The radial velocities of the
Be program star #5 pertain to the emission lines, which
completely fill helium and hydrogen absorption lines.
2.2. Binaries and orbital solutions
About half of the program stars have turned out to be
spectroscopic binaries. Table 5 summarizes the barycen-
tric velocity, the membership and the binary status based
on epoch radial velocities in Table 4. Table 6 gives the
spectroscopic orbits computed for all the binary stars but
C. Boeche et al.: Kinematics and binaries in young stellar aggregates. II. NGC 6913≡M293
Table 1. Program stars. The first four columns give our identification number (cf. finding chart in Figure 1), and
that assigned by Hoag et al. (1961), Sanders (1973) and Kazlauskas and Jasevicius (1986). V and B −V are Tycho-2
VT and (B −V )T transformed into Johnson system following Bessell (2000) prescriptions. U −B is the median of the
measurements by Massey et al. (1995), Joshi et al. (1983) and Hoag et al. (1961). Star #10 is reported as a short
period variable by Pe˜ na et al. 2001.
VB − VU − B
TYC 3152 1325 1
TYC 3152 1369 1
TYC 3152 1451 1
TYC 3152 236 1
TYC 3152 1309 1
TYC 3152 1137 1
TYC 3152 1415 1
TYC 3152 1019 1
TYC 3152 676 1
TYC 3152 1467 1
TYC 3152 1423 1
TYC 3152 1453 1
#6 and 16, which are clearly binaries but the available
radial velocities are not enough to determine the orbital
period and thus to allow to derive an orbital solution.
Therefore the RV⊙quoted for stars #6 and 16 in Table 5
is the mean of the measurements, not the barycentric ve-
locity, and the two velocities tend to differ with increasing
eccentricity and paucity of measurements. Consequently,
the RV⊙of stars #6 and 16 quoted in Table 5 which differ
by slightly more than 3σ from the cluster mean velocity
cannot be considered as a firm indication that stars #6
and 16 are field stars.
Similarly to Paper I, the spectroscopic orbits have
been obtained with a Fortran code written by Roger F.
Griffin (Cambridge University) and adapted to run under
GNU/Linux by us.
Program stars #1–7 have been observed also by Liu et
al. (1989, 1991) who reported some epoch radial velocities
for them. Such data appear affected by large errors for the
O and B stars (program stars #2–7), which make them
useless in our analysis. They are instead in good agree-
ment with our velocities for star #1, much cooler having a
spectral type F0 III. The reason for the poor quality of the
Liu et al. radial velocities of hot stars probably lies in the
shortness of the wavelength range they observed (∼150˚ A)
and by the fact that it is dominated by Hδ, which we ig-
nored in our analysis given the Balmer progression and its
excessive scatter compared to the much more performing
HeI and HeII lines. It is also worth noticing that Liu et
al. did not recognized star #7 as double lined, in spite
having observed it at orbital phase 0.66 when the velocity
separation between the components is ∼140 km sec−1(cf.
Figure 3) and therefore outstanding.
The spectroscopic orbits of stars #2 and 11 in Figure 3
and Table 6 are to be considered quite preliminary, given
their small amplitude, high eccentricity and limited num-
ber of observations. Further observations are obviously en-
couraged for these two stars. Photometric observations of
the double lined star #7 are in progress to the aim of
constraining the orbital inclination and derive individual
masses, and they will reported elsewhere when completed.
2.3. Rotation velocities
Rotational velocities for the program stars are given in
Table 5. They have been derived from HeI lines for stars
#2, 3, 4, 6, 7, 8, 10, 12, 15, 16 and FeI lines for the remain-
ing ones, following the numerical relations for the Asiago
Echelle spectrograph calibrated in Paper I (its Figure 7).
No rotation velocity is derived for the Be program star #5
because all HeI lines are badly affected by emissions. The
correspondence of the rotational velocity scale between
HeI and FeI lines (which we have been forced to used in
all program stars with a spectral type later than B) has
been carefully checked on a grid of Kurucz rotationally
broadened spectra we have calculated on purpose.
From the spectral classification in Table 3, the stel-
lar radii over the HR diagram as tabulated by Straiˇ zys
and Kuriliene (1981) and the observed Vrotsini projected
rotation velocity, we have derived in the last column of
Table 5 the projected rotation period (Prot/sini) for the
binaries with an orbital solution in Table 6. The projected
rotation period is obviously an upper limit to the true ro-
tation period. Compared to the orbital period in Table 6,
4C. Boeche et al.: Kinematics and binaries in young stellar aggregates. II. NGC 6913≡M29
Table 2. Journal of observations. D is the dispersions
(˚ A/pix) at Hα (0.19 corresponding to unbinned spectra,
0.38 to 2× binned spectra), and ∆λ is the wavelength cov-
erage. The last column gives the program stars observed
in each given run.
dateD∆λ (˚ A)star #
it can be used to infer about the co-rotation status of the
Table 3. Spectral types of the program stars from Wang
& Hu (2000), from photometry in the Vilnius system by
Kazlauskas and Jasevicius (1986) transformed by us into
spectral types following Strayˇ zis (1977), and from classi-
fication of our Echelle spectra against the Yamashita et
al. (1977) reference spectral atlas. The last columns give
the EB−V (from Fitzgerald 1970 intrinsic colors) and the
spectro-photometric distances for our spectral classifica-
tion and the photometry in Table 1.
Table 4. Example of the Table containing the epoch ra-
dial velocities (and their errors) for the program stars,
available in full in electronic form only.
Star #1 is an SB1 eclipsing binary and therefore the
sini projection factor converge toward unity, which allows
a direct comparison between rotation and orbital periods,
the former being twice longer. The lack of synchronicity
could be related to the primary evolving away from the
main sequence and the time scales of the two processes.
C. Boeche et al.: Kinematics and binaries in young stellar aggregates. II. NGC 6913≡M295
Fig.3. Orbital solutions for the binary program stars (cf.
Given the masses estimated from the spectral type and
the amplitude of radial velocity variation, star #7 prob-
ably has a high inclination too, possibly being eclipsing
itself. The rotational velocity in Table 5 pertains to the
B5 IV primary, the measurement of the secondary being
Table 5. Heliocentric radial velocity (with its standard er-
ror) of the program stars, binary status and cluster mem-
bership according to radial velocities, and projected ro-
tational velocity (with its standard error). The radial ve-
locity of the binary stars is the barycentric velocity from
the orbital solutions in Table 6. The last column gives the
projected rotation period Prot/sini of the solved binaries
to be compared with the orbital period.
binary member Vrotsini Prot/sini
(km s−1) (days)
9 –5.6 ±0.5
too uncertain given the difference in brightness. The ro-
tation and orbital periods are quite close, and in view of
the uncertainties at play, the primary in star #7 looks
Stars #3 and 12 are evidently not co-rotating, be-
cause the projected rotation period is at least several times
shorter than the orbital period in Table 6, and working on
sini can only enlarge the difference. For the remaining bi-
nary stars #2, 4 and 11, no conclusion can be drawn about
the co-rotation status, the projected rotation period being
longer than the orbital one.
2.4. Cluster membership
Sanders (1973, hereafter S73) has published an astromet-
ric investigation of 228 stars in the field of NGC 6913,
identifying 92 possible members. He has howeverused only
one plate pair, with an epoch separation of just 22 yr,
with moreover the first epoch plate “severely blackened
by the moon”. Consequently, noting the too large frac-
tion of detected members among the measured stars, he
warned that the cluster separation from the field is not
satisfactory, and that the member/non-member status he
assigned may be frequently in error. S73 limiting mag-
nitude is V =13.8, with a completness limit not fainter
than V =13.0 that corresponds to 1.2 M⊙ on the main
sequence of NGC 6913. Dias et al. (2002, hereafter D02)