Blue Straggler Stars (BSSs) are an exotic population of stars more massive than any main sequence (MS) object in their host systems, observed from Globular Clusters, to open star clusters, the Galactic field, and dwarf spheroidal galaxies. They are believed to form either from stellar collisions, mergers or through mass transfer (MT) in a binary system; but discerning their formation process has proven to be not an easy feat. Two signatures have been identified for the MT process: CO depletion on the surface of the BSS, and the detection of UV emission from its peeled companion. Employing deep far-UV observations secured with the Solar Blind Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope of five BSSs showing significant surface depletion of carbon (C) and oxygen (O) in the Galactic globular cluster 47 Tucanae, we observed for the first time both these signatures in the same BSS. The hot surface temperature of the companion star (T > 20'000 K) indicates that it likely formed through mass transfer less than 12 Myr ago. 

The second data release for the Survey of Surveys (SoS) is out!

The SoS is a project to critically compile survey results into a single catalog to facilitate the scientific use of the available information. In this second release, we present two new catalogs of stellar parameters (Teff, log g, and [Fe/H]). Part of the data release contains measurements calibrated from five spectroscopic surveys (APOGEE, GALAH, Gaia-ESO, RAVE, and LAMOST). A second part of this release is obtained by using SoS-Spectro as a reference to train a multilayer perceptron that predicts stellar parameters based on two photometric surveys, SDSS and SkyMapper, and Gaia as a baseline reference. In the end, we obtained a catalog of stellar parameters for about 23 million stars that we make publicly available. We validated our results with several comparisons with other machine-learning catalogs, stellar clusters, and asteroseismic samples. We found substantial improvements in the parameter estimates compared to other machine-learning methods in terms of precision and accuracy, especially in the metal-poor range. This was particularly evident when we validated our results with globular clusters.

Omega Centauri (ω Cen) is one of the most complex star clusters in the Milky Way. In this study we looked at the properties of its binary systems via multi-epoch MUSE spectroscopic observations spanning over eight years and covering much of its central regions (i.e. core radius). We did not detect any stellar-mass BHs candidates orbiting luminous stars. This suggests that BHs orbiting stars may be rare in ω Cen or in wide orbits around low-mass companions or that the periods of such systems are longer than expected from cluster dynamics. Additionally, we constrained the orbital properties of 19 binary systems in the cluster, with periods ranging from fractions of a day up to several hundred days. We observe an excess of binaries with P≥10 d and find evidence that the intrinsic period distribution of binaries in ω Cen differs from those predicted by cluster evolutionary models.

Building on the Stetson photometry for 48 globular clusters (Stetson et al. 2019), we derived accurate differential reddening maps that can be used to correct any photometric catalog in those areas. We used the maps to estimate the amount of intracluster medium within each cluster and we found it to be in agreement with theoretical expectations. In the past, the upper limits derived from non-detections of infrared emission were used to claim that the intracluster medium was orders of magnitude less abundant than expected. Our maps support recent discoveries about the spatial distribution which, together with new data on the dust chemical composition, show of the missing intracluster medium, the the problem was indeed a false problem.