The Quintuplet cluster
III. Hertzsprung-Russell diagram and cluster age
A. Liermann, W.-R. Hamann, and L. M. Oskinova
The Quintuplet, one of three massive stellar clusters in the Galactic
center (GC), is located about 30 pc in projection from
Sagittarius A*. We aim at the construction of the Hertzsprung-Russell
diagram (HRD) of the cluster to study its evolution and to
constrain its star-formation history. For this purpose we use the most
complete spectral catalog of the Quintuplet stars. Based on
the K-band spectra we determine stellar temperatures and luminosities
for all stars in the catalog under the assumption of a uniform
reddening towards the cluster. We find two groups in the resulting HRD:
early-type OB stars and late-type KM stars, well separated
from each other. By comparison with Geneva stellar evolution models we
derive initial masses exceeding 8 M_sun for the OB stars. In
the HRD these stars are located along an isochrone corresponding to an
age of about 4 Myr. This confirms previous considerations,
where a similar age estimate was based on the presence of evolved
Wolf-Rayet stars in the cluster. We derive number ratios for the
various spectral subtype groups (e.g. NWR/NO,
NWC/NWN) and compare them
with predictions of population synthesis models. We
find that an instantaneous burst of star formation at about 3.3 to 3.6
Myr ago is the most likely scenario to form the Quintuplet cluster.
Furthermore, we apply a mass-luminosity relation to construct the
initial mass function (IMF) of the cluster. We find indications for a
slightly top-heavy IMF. The late-type stars in the LHO catalog are red
giant branch (RGB) stars or red supergiants (RSGs) according
to their spectral signatures. Under the assumption that they are located
at about the distance of the Galactic center we can derive their
luminosities. The comparison with stellar evolution models reveals that
the initial masses of these stars are lower than 15 M_sun implying
that they needed about 15 Myr (RSG) or even more than 30 Myr (RGB) to
evolve into their present stage. It might be suspected that
these late-type stars do not physically belong to the Quintuplet
cluster. Indeed, most of them disqualify as cluster members because
their radial velocities differ too much from the cluster average.
Nevertheless, five of the brightest RGB/RSG stars from the LHO
catalog share the mean radial velocity of the Quintuplet, and thus
remain highly suspect for being gravitationally bound members. If
so, this would challenge the cluster formation and evolution scenario.
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