Presented at the
203rd Meeting of the American Astronomical Society in
Did the Globular Cluster NGC 6397 Trigger the
Formation of the Young Open Cluster NGC 6231?
Richard F. Rees Jr.
Department
of Physical Science,
and
Yerkes Observatory, The
University of Chicago,
Kyle M. Cudworth
Yerkes Observatory, The
University of Chicago,
Abstract
Maybe.
A
proper motion study of the globular cluster NGC 6397 (Rees et al., in preparation) found that the cluster passed through the
Galactic disk less than 5 Myr ago. The three-dimensional space velocity of NGC
6397 from that study allows us to determine where it hit the disk. We have used various measurements of the
absolute proper motion and radial velocity of the young open cluster NGC 6231
(often considered the core of the Sco OB1
association) from the literature to derive its three-dimensional space velocity
and find that it would have been near NGC 6397’s impact point at that
time. Age determinations for NGC 6231 in
the literature are consistent with its formation at the time of or soon after
the NGC 6397 disk crossing. We propose
that the disk passage of NGC 6397 may have triggered the formation of NGC 6231. If so, this would be the first observational
evidence for the disk passage of globular clusters as a dynamical trigger of
star formation, a mechanism proposed by Wallin et al. (1996, ApJ,
459, 555).
Introduction
NGC 6397 is a
nearby (d ≈ 2.3 kpc) globular cluster at (l,
b) = (338º, –12º), currently about 0.5 kpc
below the Galactic disk. A proper motion
study of this cluster (Rees et al.,
in preparation) combined the absolute proper motion derived in that paper with
the radial velocity from Meylan & Mayor (1981) to
derive a three-dimensional space velocity of (P,
Q, Z)LSR
= (+26, +123, –109) ± (6, 13, 13) km s–1. This indicates that the cluster passed
through the disk no more than 4.5 Myr ago. Since the mass of NGC 6397 is about 2.5 ´ 105 M¤
(Pryor & Meylan 1993), it would presumably have
some effect on the disk in the vicinity of its passage. Wallin et al. (1996) predicted that the passage
of a globular cluster could trigger star formation in marginally stable
interstellar clouds due to the gravitational compression provided by the
cluster potential. We decided to look
for such evidence of the passage and we may have found it.
Finding the Point of NGC 6397’s Disk Passage
At its current Z velocity, NGC 6397 would have taken
4.5 Myr to reach its current distance from the
disk. This is of course an upper limit,
since it has presumably been slowed since the passage by the gravitational
attraction of the disk. Calculations of
the deceleration using an infinite plane disk potential with a variety of
projected surface densities yields a disk passage about 4.0-4.5 Myr ago. (A more
detailed calculation by integrating the cluster’s Galactic orbit is beyond the
scope of this paper.) For the purposes
of determining a search area for evidence of the disk passage, we have
conservatively adopted a time of 4.3 ±
0.5 Myr.
We then extrapolated NGC 6397’s motion parallel to the
disk back to the time of disk passage.
Its velocity will not have been exactly constant over that time, so we
have conservatively adopted errors in P
and Q that are twice their formal errors. This has little effect on the results, since
the largest contribution in the uncertainty in the location of the point of
disk passage is the uncertainty in the cluster distance.
We have calculated the position of NGC
6397’s disk passage in a polar coordinate system with the origin at the
Galactic center as shown in Figure 1. We
adopt a solar Galactocentric distance of R0 = 8.5 kpc,
which is used in our space velocity derivation code. In this coordinate system, we find that NGC
6397 hit the disk at R = 6.4 ± 0.5 kpc and f = 12.3º ± 1.5º.
Because of Galactic rotation, objects near the NGC 6397
impact point at the time of the impact will no longer be there. Assuming a circular rotation for the Galactic
disk of 220 ± 20 km s–1,
we calculate that a disk object at the impact point would now be at a distance
of 2.2 ± 0.7 kpc from the Sun at a Galactic longitude
of l = 349º ± 7º. The error bars are quite generous, but the
purpose is to define an area of space to look for something interesting.
NGC 6231: Something Interesting
We began the
search by using the Voyager Planetarium Software to simply examine the area of
the sky indicated by the previous calculation.
Our attention was immediately drawn to the Sco
OB1 association, the core of which is usually identified as the open cluster
NGC 6231 (Perry et al. 1991). NGC 6231
is at (l, b) = (343.5º, +1.2º) at a distance of 2.0 ± 0.2 kpc (Perry et al.
1991, Baume et
al. 1999), although there is some debate in the literature over the
distance to it – for example, Balona & Laney
(1995) obtain a distance of 1.6 kpc. For the analysis which follows, we have used
both a “long” distance of 2.0 ± 0.2 kpc and a “short”
distance of 1.6 ± 0.2 kpc.
We
note in passing that Robichon et al. (1999) reported the Hipparcos parallaxes of six probable NGC 6231 members to be
negative – clearly a nonsensical result.
Marakov (2003) reanalyzed the Hipparcos data
and derived an average distance of 0.6 ± 0.2 kpc for
these six stars – but the recomputed parallaxes vary by nearly a factor of 10,
are less than 1-s for two stars and less than 1.5-s for a third; none are as
good as 3-s. We do not therefore believe that even the
recomputed Hipparcos
parallaxes for this cluster are reliable.
There is also
some discrepancy in age determinations of NGC 6231 in the literature, but
determinations include 3-5 Myr (Baume
et al. 1999), 3.6 ± 0.6 Myr (van Genderen et al. 1984), 4.5 Myr (Santos & Bica
1993) and 5 ± 1 Myr (Balona
& Laney 1995). van Genderen
et al. (1994) also derive ages of
3.9-4.5 Myr for the Wolf-Rayet
star Br114 = HR 6249 = HD151932 and 3-4.5 Myr for the
hypergiant z1
Sco. Pinning
down the distance and age of NGC 6231 is complicated by the very high (~80%)
binary frequency on the upper main sequence (García
& Mermilliod 2001) and differential reddening at
least partially due to intracluster dust (Santos
& Bica 1993).
Still, many age determinations in the literature are consistent with NGC
6231 forming at the time of or shortly after the disk passage of NGC 6397.
Where was NGC 6231 4.3 ± 0.5 Myr
ago?
There are at least six published absolute proper motions for NGC 6231 (Baumgardt et al. 2000, Glushkova et al. 1997; Dias et al. 2002; Rastorguev et al. 1999, Robichon et al. 1999), based on Hipparcos, Tycho, and 4M data. We have used each of the above proper motions and the NGC 6231 radial velocity of –30.7 ± 0.9 km s–1 from García & Mermilliod (2001) to calculate space velocities for both the “long” and “short” distances. These calculations use the same software used for NGC 6397, which is described by Cudworth & Hanson (1993). The results are given in Table 1. Although the proper motions are discrepant, the proper motion of a disk object in the direction of NGC 6231 – nearly toward the Galactic center – largely represents the difference in its Galactic motion from the Sun’s motion.
|
Table 1.
NGC 6231 absolute proper motions from the literature and our derived
space velocities. |
|||||||||
|
|
|
|
|
Long NGC 6231 distance |
Short NGC 6231 distance |
||||
|
Paper |
m data |
ma arcsec
yr–1 |
md arcsec yr–1 |
P km s–1 |
Q km s–1 |
Z km s–1 |
P km s–1 |
Q km s–1 |
Z km s–1 |
|
Baumgardt
et al. (2000) |
Hipparcos |
–0.0003 ±0.0006 |
–0.0023 ±0.0005 |
6.5 ±1.8 |
222.9 ±5.1 |
–6.1 ±5.4 |
9.8 ±1.6 |
226.2 ±4.3 |
–3.8 ±5.4 |
|
Glushkova
et al. (1997) |
4M |
–0.0035 ±0.0002 |
–0.0051 ±0.0002 |
21.0 ±2.0 |
186.5 ±5.9 |
0.3 ±2.0 |
20.9 ±2.0 |
196.7 ±5.8 |
1.3 ±1.6 |
|
Dias et al.
(2002) |
Tycho |
–0.0017 ±0.0014 |
–0.0011 ±0.0014 |
6.6 ±4.1 |
223.7 ±13.1 |
11.5 ±13.6 |
9.9 ±3.3 |
226.9 ±10.6 |
10.3 ±10.9 |
|
Rastorguev
et al. (1999) |
Hipparcos |
+0.0021 ±0.0015 |
–0.0049 ±0.0015 |
7.4 ±4.3 |
218.8 ±13.7 |
–39.3 ±14.8 |
10.5 ±3.5 |
222.9 ±11.1 |
–30.5 ±12.2 |
|
Rastorguev
et al. (1999) |
Tycho |
–0.0015 ±0.0015 |
–0.0011 ±0.0015 |
6.1 ±4.3 |
224.8 ±13.6 |
9.7 ±14.1 |
9.6 ±3.5 |
227.8 ±11.0 |
8.8 ±11.4 |
|
Robichon
et al. (1999) |
Hipparcos |
0.0000 ±0.0005 |
–0.0019 ±0.0003 |
4.7 ±1.5 |
227.6 ±3.8 |
–6.6 ±4.2 |
8.4 ±1.3 |
230.0 ±3.2 |
–4.2 ±3.4 |
We
have determined the NGC 6231 (R, f)
coordinates at the time of the NGC 6397 disk passage by extrapolating its
motion backwards in time as we did for NGC 6397. The results are given in Table 2. For every case – for long and short distances
– the error boxes for NGC 6231 and NGC 6397 show considerable overlap. Figures 2 and 3 show the Galaxy with the
current projected positions of the clusters and the error boxes for where they
were 4.3 ± 0.5 Myr
ago. The averages of the long (Figure 2)
and short (Figure 3) distances are shown for NGC 6231. Since the six NGC 6231 proper motions are not all independent of each other, the
smallest errors in R and f of the six
individual cases were used to determine the size of the error box around the
average. The long NGC 6231 distance
obviously gives better overlap, but there is significant overlap even in the
short distance case.
|
Table 2. Galactocentric
polar coordinates at time of NGC 6397 disk crossing |
|||||
|
|
|
Long NGC 6231 distance |
Short NGC 6231 distance |
||
|
Cluster |
m source |
R kpc |
f º |
R kpc |
f º |
|
NGC 6397 |
Rees et al., in preparation |
6.4 ±0.5 |
12.3 ±1.5 |
6.4 ±0.5 |
12.3 ±1.5 |
|
NGC 6231 |
Baumgardt et al. (2000) |
6.7 ±0.5 |
13.3 ±1.4 |
7.0 ±0.5 |
11.6 ±1.3 |
|
NGC 6231 |
Gluskova et al. (1997) |
6.6 ±0.5 |
12.0 ±1.3 |
6.9 ±0.5 |
10.9 ±1.2 |
|
NGC 6231 |
Dias et al. (2002) |
6.7 ±0.5 |
13.4 ±1.7 |
7.0 ±0.5 |
11.9 ±1.5 |
|
NGC 6231 |
Rastorguev et al. (1999) (Hipparcos) |
6.7 ±0.5 |
13.2 ±1.7 |
7.0 ±0.5 |
11.7 ±1.5 |
|
NGC 6231 |
Rastorguev et al. (1999) (Tycho) |
6.7 ±0.5 |
13.4 ±1.7 |
7.0 ±0.5 |
11.9 ±1.5 |
|
NGC 6231 |
Robichon et al. (1999) |
6.7 ±0.5 |
13.5 ±1.4 |
7.0 ±0.5 |
12.0 ±1.3 |
Figure 2(a).
The Galaxy as seen from above. The box is shown in detail in Figure 2(b). The position of NGC 6231 is for the “long”
distance. Figure by Richard Powell and
Richard Rees.
Figure 2(b).
View of the Galaxy from above (detail). This
is for the average results of the “long” distance for NGC 6231. The Sun is
located at the yellow dot, NGC 6231 is at the green dot, and NGC 6397 is about
0.5 kpc below the red dot. Less than five million years ago, NGC 6397
passed through the Galaxy’s disk in the red error box; NGC 6231 formed at about
the same time in the green error box.
The yellow shows the large overlap between the two regions, indicating
that NGC 6231 formed where NGC 6397 passed through the disk. Figure by Richard Powell and Richard Rees.
Figure 3(a).
As Figure 2(a), but for the “short” NGC 6397 distance. The box is shown in detail in Figure 3(b). Figure by Richard Powell and Richard Rees.
Figure 3(b). As Figure 2(b), but for the “short” NGC 6231
distance. The ovelap
in the error boxes is not as much in this case but is still significant. Figure by Richard Powell and Richard Rees.
Can We Convict NGC 6397
of Triggering NGC 6231’s Formation?
We submit that NGC 6397 may have had the
opportunity to trigger the formation of NGC 6231, and Wallin
et al. (1996) have described a possible means.
We leave it to others to speculate upon NGC 6397’s motive.
To determine if this case is really
winnable, the large uncertainties in the locations of NGC 6397 and NGC 6231 at
the time of the crime need to be better constrained. The biggest source of uncertainty is in the
distances to both clusters, but especially the distance to NGC 6231. We note that NASA’s Space Interferometry Mission (SIM) and ESA’s
Gaia – scheduled for launch near the
end of the decade – are expected to provide parallaxes to ~ 1% precision at
distance of 2 kpc.
These missions will also be capable of much more precise proper motions
of the clusters, so it should be possible to determine if they were indeed both
at the scene of the crime. An improved
NGC 6231 distance should also help constrain its age sufficiently to ensure
that it did in fact form at or after the time of NGC 6397’s disk passage.
In the meantime, determinations of the
Galactic orbits of the clusters could be used to constrain how close they came
in the past, but care should be taken to avoid model-dependent errors due to
any particular choice of Galactic potential.
If archival plate material of NGC 6231 is sufficient, it may be possible
to pin down its space velocity better.
(We would welcome knowledge of any old plates of NGC 6231 of which the
reader is aware.) On the theoretical
front, more detailed modeling of what a globular cluster does to the disk when
it goes through it would further illuminate the triggering mechanism.
Wallin et al.
(1996) noted that a globular cluster passes through the disk once every Myr or so. We shall
be reexamining the globular clusters that have been part of the Yerkes program to determine if any of them may have also
triggered star formation at the last disk passage.
Acknowledgements
We thank Harvard for the loan of the
old NGC 6397 plates and the observers of 1893-1944 who took them, whose efforts
made this study possible. We also thank
Richard Powell for allowing us to adopt his Milky Way drawings, Steve Majewski and
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