Figure 19.
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.
44
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 fit-
ted two different sets of model isochrones
to the color-magnitude diagrams (CMDs)
to determine the stellar population param-
eters, distance, and reddening. Figure 19
shows the results 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 old-
est 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 stel-
lar clusters.” The study appears in Monthly
Notices 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]
January 2020 / 2019 Year in Review