Figure 5.
Results of fitting the
GeMS near-infrared
CMD of HP 1 using
the Dartmouth Stellar
Evolutionary Database
(DSED) models. Left
panel: CMD showing
all likely member stars
(grey) and those used in
the fit (black). The best-fit
isochrone is indicated
by a thick green line; the
green shading shows
the uncertainty range.
The red arrow indicates
a change in reddening
of ΔE(B − V) = 0.10 mag.
Right panels: The result-
ing one- and two-dimen-
sional constraints for all
model parameters. The
contours correspond to
confidence levels of 0.5σ,
1.0σ, 1.5σ, and 2.0σ.
[Figure reproduced from
Kerber et al., Monthly
Notices of the Royal
Astronomical Society,
484: 5530, 2019.]
12
low the MSTO. The study was led by Leandro
Kerber of the Universidade de São Paulo and
Universidade Estadual de Santa Cruz in Brazil.
The team combined their GSAOI data with
archival F606W (wide V) images from the
HST’s Advanced Camera for Surveys to de-
termine relative proper motions and select
bona fide cluster members. They then fitted
two different sets of model isochrones to the
color-magnitude diagrams (CMDs) to deter-
mine the stellar population parameters, dis-
tance, and reddening. Figure 5 shows the re-
sults for one set of isochrones using only the
GeMS/GSAOI data; the team also performed
fits to CMDs made with a combination of
HST and GeMS data. The analysis indicates
an age near 13 billion years, confirming that
HP 1 is one of the oldest globular clusters in
the Milky Way and likely formed less than a
billion years after the Big Bang.
The heliocentric distance of 6.6 kiloparsecs
(kpc) estimated from the isochrone fitting
agrees well with the distance implied by
the extinction-corrected brightnesses of 11
RR Lyrae stars identified within the cluster.
The team combined this distance with the
GeminiFocus
measured radial velocity and the absolute
proper motion given by Gaia (Data Release
2) in order to constrain the cluster’s orbit.
They find that HP 1 passes just 0.12 kpc from
the Galactic Center at closest approach
and reaches a maximum distance of about
3 kpc. It is likely that many of the cluster’s
stars have been stripped away as it has re-
peatedly plunged through the bulge during
the course of its long history.
“HP 1 is one of the surviving members of
the fundamental building blocks that as-
sembled our Galaxy’s inner bulge,” said
Kerber. Added coauthor Mattia Libralato of
the Space Telescope Science Institute, “The
combination of high angular resolution and
near-infrared sensitivity makes GeMS/GS-
AOI an extremely powerful tool for study-
ing these compact, dust-enshrouded stellar
clusters.” The study appears in Monthly No-
tices of the Royal Astronomical Society.
John Blakeslee is the Chief Scientist at Gemini
Observatory and located at Gemini South in
Chile. He can be reached at:
[email protected]
April 2019