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Figure 8:
Various line indices as a function of galactocentric
radius
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In Fig. 8 we plot some Lick indices as a function of
galactocentric radius
.
To increase the range in radius,
we again merge our sample with the data for metal-poor halo globular
cluster of Trager et al. (1998). The galactocentric radius was taken from
the 1999 update of the McMaster catalog of Milky Way globular clusters
(Harris 1996). Our compilation includes now both bulge and halo
globular clusters and spans a range
1-40 kpc in galactocentric
distance.
All metal indices show a gradually declining index strength as a
function of
.
The inner globular clusters show a strong
decrease in each index out to
10 kpc. The sequence continues at
apparently constant low values out to large radii. Furthermore, some
indices (CN, Mgb, and
Fe
)
show a dichotomy between
the bulge and the halo globular cluster system. While the Mgb and
Fe
indices clearly reflect the bimodality in the
metallicity distribution of Milky Way globular clusters, the striking
bimodality in the CN index is more difficult to understand. In the
context of Sect. 4.2 this may well be explained by
evolutionary differences between metal-rich bulge and metal-poor halo
globular clusters.
The behavior of H
differs from that of the other indices.
There is no clear sequence of a decreasing index as a function of
,
as for the metal-sensitive indices. Instead we measure a
mean H
index with
Å. The strength of the Balmer
series is a function of
.
In old stellar populations,
relatively hot stars, which contribute significantly to the
Balmer-line strength of the integrated light, are found at the main
sequence turn-off and on the horizontal branch. The temperature of the
turn-off is a function of age and metallicity while the temperature of
the horizontal branch is primarily a function of metallicity and, with
exceptions, of the so-called "second parameter''.
In the following we focus on the correlation of the horizontal branch
morphology on the H
index. We use the horizontal branch ratio
HBR from the McMaster catalog (
:
B and R are the
number of stars bluewards and redwards of the instability strip; V is
the number of variable stars inside the instability strip) to
parameterize the horizontal branch morphology. Figure 9 shows that the HBR parameter vs.
follows a similar trend as H
vs.
in Fig. 8. This supports the idea that the change in H
(as a function of
)
is mainly driven by the change of the
horizontal branch morphology as one goes to more distant halo globular
clusters with lower metallicities. Indeed, the lower panel in Fig. 9 shows that HBR is correlated with the H
index (Spearman rank coefficient 0.77). The functional form of this
correlation is
Figure 9 implies that the change of H
is
mainly driven by the horizontal branch morphology which itself is
influenced by the mean globular cluster metallicity. However, we know
of globular cluster pairs - so-called "second parameter'' pairs -,
such as the metal-poor halo globular clusters NGC 288 and NGC 362
(
,
Catelan et al. 2001) and the metal-rich bulge
clusters NGC 6388 and NGC 6624 (
,
Rich et al. 1997; Zoccali et al. 2000), with very similar metallicities and different horizontal
branch morphologies. In fact, NGC 6388 (and NGC 6441, another
metal-rich cluster in our sample also featuring a blue horizontal
branch) shows a stronger H
index than other sample globular
clusters at similar metallicities (see
Sect. 4.3). Clearly, metallicity cannot be the only
parameter which governs the horizontal branch morphology. In the
context of the "second-parameter effect'' other global and non-global
cluster properties (Freeman & Norris 1981) impinging on the horizontal
branch morphology have been discussed of which the cluster age and/or
several other structural and dynamical cluster properties are
suspected to be the best candidates (e.g. Fusi Pecci et al. 1993; Rich et al. 1997). Our sample does not contain enough "second parameter'' pairs
to study the systematic effects these "second parameters'' might have
on H
,
such as the correlation of the residuals of the
HBR-H
relation as a function of globular cluster age or
internal kinematics. A larger data set would help to solve this issue.
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Figure 9:
Horizontal branch morphology in terms of the HBR parameter
as a function of galactocentric radius
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Copyright ESO 2002