Issue |
A&A
Volume 516, June-July 2010
|
|
---|---|---|
Article Number | A55 | |
Number of page(s) | 29 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200913451 | |
Published online | 24 June 2010 |
Online Material
Appendix A: Classification of Galactic globular clusters
Since the work by Zinn (1985) a few progresses were done with respect to his criteria for the separation of the subpopulations of Galactic GCs. Many classification schemes rest on the appearance of the clusters' CMD and related parameters (namely metallicity, age and HBR index). However, one of the main aims of our project is to explain the HB morphology and its relations with chemical signatures of stellar generations in GC, so we cannot use the distribution of stars along the HB as a separation criterion. In Table A.1 we list the quantities used in Sect. 3.3 to separate disk/bulge clusters from the halo ones according to the combination of their location in the Galaxy and their kinematics.
Table A.1: Classification and main parameters adopted for GCs.
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Figure A.1: Classification of disk (red squares) and inner halo (blue dots) clusters. The curve is the discriminating line as obtained from our selection criteria. |
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Figure A.2: Absolute values of the difference dV between the observed radial velocity of GCs and the one expected from the Galactic rotation curve, as a function of the rotational velocity given by Dinescu et al. (1999) and Casetti-Dinescu et al. (2007). The dotted line indicates one-to-one correlation, while the red solid lines indicate the linear regression. |
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Table A.2: Comparison of our classification with the one by Mackey & van den Bergh (2005).
Table A.3: Globular clusters of LMC, SMC, and Fornax dSph and their properties.
As said in Sect. 3.3,
outer halo clusters were simply classified as those currently located
at more than 15 kpc from the centre of the Galaxy (see Carollo
et al. 2008).
Clusters with
below 3.5 kpc were
considered as bulge GCs, even though some of them might be halo
clusters on
very elongated orbits presently close to the pericentres. To separate
the
remaining clusters into inner halo and disk GCs, we computed the
differences
(dV, Col. 8 in Table A.1) between the
observed radial velocity (corrected to the LSR) and the one expected
from the Galactic rotation curve (see Clemens 1985). In the dV-Z
plane (where Z is the clusters' distance from the
Galactic plane, in kpc, see Col. 10 of Table A.1), we defined an
ellipse with equation

Clusters with y<1 were classified as disk GCs, while we considered as halo GCs the ones with y>1, see Fig A.1. Of course, a better estimate of GC kinematic is possible when the whole orbit is available. Given the quite good correlation (see Fig. A.2) between dV and the rotational velocity (Col. 9 of Table A.1, where



It should, however, be clear that dV is not at all a synonymous of (

Column 11 in Table A.1 shows our
classification for the population of
the Galactic GCs in disk/bulge (D/B), inner halo (IH), outer
halo (OH), and GCs of dSphs. For each cluster we report the
integrated magnitude (,
Col. 2), the HB morphology parameter (HBR, Col. 6),
and the Galactocentric distance (
,
Col. 7)
as directly retrieved from the Harris catalogue. The metallicity values
([Fe/H], Col. 3) were instead replaced with the determinations
by Carretta et al. (2009c).
Additionally, we list in Col. 4
the [
/Fe] ratios
(with corresponding references given in the table
notes). Column 5 displays the age parameter; more in detail,
we computed an
average value between the two different estimates by Marin-Franch
et al. (2009)
and De Angeli et al. (2005), after
applying a correction of 0.08 to the second ones for GCs with
[Fe/H] ranging from -1.8 to -1.1 dex, as suggested by a
cluster-to-cluster comparison. When neither of these estimates was
available, we adopted the ones calculated by Vandenberg (2000),
normalised to the Marin-Franch scale by assuming that
13.5 Gyr = 1.00. We then corrected the
values so obtained for the difference between the metallicities
considered in those
papers and those listed in Carretta et al. (2009c),
transformed into [M/H] using an average [
/Fe] of +0.4 (see
Table 2).
This correction was made using the sensitivity of age on metallicity
given by
Marin-Franch et al. (2009).
First, we decided to compare our new classification with the
previous ones
relying only on metallicity and HB morphology (see Sect. 3.3). As
representative of this approach, we chose the work by Mackey &
van den Bergh
(2005).
Briefly, they defined as disk component all the GCs with
[Fe/H]>-0.8 dex; the so-called ``old'' halo and
``young'' halo clusters were then divided following Zinn (1993), namely
by computing the offset in HB type -at a given metallicity- with
respect to the fiducial line of the inner halo clusters. GCs with an offset larger
and smaller than -0.3 in HB type were classified as old halo
and young
halo, respectively. The Table A.2 schematically
shows the comparison and emphasises the different natures of the two
classifications. As (partially) expected, the matrix is not diagonal;
i.e., there is not a one-to-one correlation
between old halo (young halo) and inner halo (outer halo) subgroups.
More in
detail, the metallicity criterion is largely responsible for such a
discrepancy: had we adopted the requirement of
0.8 dex,
35 of the 36 clusters that we classified as disk/bulge and
Mackey & van den Bergh as old
halo should be moved into inner halo+old halo cell, while all the 5 GCs
placed in the disk+young halo box should become inner halo+young halo
clusters
thanks to their low metallicity. However, and most important, even
taking these changes into account in the relative population of the
matrix cells, the
resulting correspondence is not yet one-to-one. As to the inner halo
GCs, 80% constitute the old halo and the remaining 20% the
young halo clusters; for the outer halo GCs, the promiscuity is even
greater, resulting in 40% and
60%, respectively, for old and young haloes. This is direct evidence of
the strong difference between kinematics (and/or positional) criteria
and the
ones based only on metallicity and HB type.
One last word on the classification. While this work was in
preparation, a paper by Fraix-Burnet et al. (2009) appeared,
where they use a cladistic technique to divide a sample of
54 GCs into three subsamples (called Groups 1, 2,
and 3 and later identify with inner halo, outer halo, and
disk, respectively) on the basis of [Fe/H],
MV,
,
and age. We cross-checked the assignments for the clusters in common
and found good agreement only for disk clusters, and this in a limited
sense. When they classify a cluster as disk, we agree (in
17 cases out of 18), but we have many other disk
clusters that they instead
classify in the halo subsamples. In particular, for the 19 GCs
in our FLAMES
sample, the two classifications agree for seven clusters and disagree
for seven
others (five GCs are not present in their data set). We think that the
main
factor producing this difference is that they ignored the
kinematics, although the information is present for their sample, while
our
method rests on that.
To conclude, we report in Table A.3 the analogoues
of
Table A.1,
but for the LMC, SMC, and Fornax GC systems; as
in the previous case, the number in brackets corresponds to the
reference (for
[/Fe],
[Fe/H], age, and HBR) whose decoding is given in the Notes. The
integrated magnitudes MV
for the LMC and Fornax GCs are taken from
van den Bergh & Mackey (2000), while
for the SMC ones were computed from the apparent magnitudes UBV
by van den Bergh (1981),
along with the distance moduli as given in those papers providing the
clusters' age (Refs. 15-17 see
Table A.3).
As to age, for LMC and SMC GCs, since absolute values
were available (see ref. given in Table A.3), we report
them to our
relative scale, adopting the previous conversion of
13.5 Gyr = 1.00. For the
Fornax clusters, the ages were instead derived starting from the
cluster-to-cluster relative differences (
Age) obtained by Buonanno
et al. (1998,
1999) and
assuming that 1.05 =
14.2 Gyr.
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