  The object ESO456-SC38 (Djorgovski 2) is one of the globular clusters that is
closest to the Galactic center. It is on the blue horizontal branch and has a
moderate metallicity of [Fe/H]~-1.0. It is thus similar to the very old inner
bulge globular clusters NGC 6522, NGC 6558, and HP 1, and therefore appears to
be part of the primeval formation stages of the Milky Way. The aim of this work
is to determine an accurate distance and metallicity for ESO456-SC38, as well
as orbital parameters, in order to check similarities with other clusters in
the inner bulge that have previously been well studied in terms of spectroscopy
and photometry. This is a considerably fainter cluster that is contaminated by
a rich stellar field; it is also quite absorbed by the dusty foreground. We
analyzed ESO456-SC38 based on Hubble Space Telescope photometry, with the
filters F606W from ACS, F110W and F160W from WFC3,and photometry in V and I
from FORS2 at the VLT. We combined this with identified stars that are covered
by Gaia Data Release 2. The isochrone fitting was carried out with the
statistical Markov chain Monte Carlo method. We derive an accurate distance of
dSun = 8.75+-0.12 kpc and a reddening of E(B-V)=0.81^+0.02_-0.02. The
best-fitting BaSTI isochrones correspond to an age of 12.70^{+0.72}_{-0.69} Gyr
and a metallicity of [Fe/H]=-1.11^{+0.03}_{-0.03}. ESO 456-SC38 adds to the
list of moderately metal-poor globular clusters located in the inner bulge. It
is on the blue horizontal branch and is very old. The cluster is confined to
the bulge and bar region, but it does not support the Galactic bar structure.
The old stellar population represented by clusters like this has to be taken
into account in models of Galactic bulge formation. Studying them also provides
indications on the formation times of the globular clusters themselves.
