Another relic bulge globular cluster: ESO 456-SC38 (Djorgovski 2)

Abstract

Context. 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. Aims. 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. Methods. 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.Results. The isochrone fitting was carried out with the statistical Markov chain Monte Carlo method. We derive an accurate distance of d = 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 . Conclusions. 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.

Discovery of an Outbursting 12.8 Minute Ultracompact X-Ray Binary∗

“…It may belong to this cluster. Assuming the distance to Djorg 2 is 8.75 kpc (Ortolani et al 2019) and approximate extinction in this direction A V ≈ 2.4 mag (based on the interstellar calculator pro-vided in Nataf et al 2013), we find that the star with the observed brightness F606W ≈ 21.2 mag (broad V band) would have an absolute brightness M V ≈ +4.1 mag, which is a typical value for UCXBs (Nelemans et al 2010).…”

“…The X-ray modeling was performed using the Sherpa package (Freeman et al 2001). The total unabsorbed flux in the range 0.5-10 keV is 4.8 +0.5 −0.6 × 10 −13 erg s −1 cm −2 , which corresponds to the luminosity of 4.4±0.5×10 33 erg s −1 , assuming the source is associated with the cluster at the distance of 8.75 kpc (Ortolani et al 2019). In the lower panel of Fig.…”

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XX. Ages of Single and Multiple Stellar Populations in Seven Bulge Globular Clusters

“…For instance, Kerber et al (2018) applied a similar likelihood function, but comparing synthetic and observed fiducial lines instead of considering all stars. SIRIUS was already applied in Kerber et al (2019) and Ortolani et al (2019).…”

Towards a fully consistent Milky Way disk model

“…The parameter t t1 5 determines the starting value of the SFR as sf r t (0 Gyr) = t γ t1 (1 − e −βtt2 ), and t p − t t2 is the maximum age of the thick-disk population with t p being the disk's present-day age. The value of t p is set to 13 Gyr to be consistent with the typical age estimates derived for the oldest globular clusters in the Galactic bulge and halo (de la Fuente Marcos et al 2015;Ortolani et al 2019). With a time resolution of ∆t = 25 Myr, this results in 160 isothermal subpopulations for the thick disk.…”