Red clump stars are commonly used to map the reddening and morphology of the inner regions of the Milky Way. We use the new photometric catalogues of the VISTA Variables in the Vía Láctea survey to achieve twice the spatial resolution of previous reddening maps for Galactic longitudes −10 • < l < 10 • and latitudes −1.5 • < b < 1.5 • . We use these de-reddened catalogues to construct the K s luminosity function around the red clump in the Galactic plane. We show that the secondary peak (fainter than the red clump) detected in these regions does not correspond to the bulge red-giant branch bump alone, as previously interpreted. Instead, this fainter clump corresponds largely to the over-density of red clump stars tracing the spiral arm structure behind the Galactic bar. This result suggests that studies aiming to characterise the bulge redgiant branch bump should avoid low galactic latitudes (|b| < 2 • ), where the background red clump population contributes significant contamination. It furthermore highlights the need to include this structural component in future modelling of the Galactic bar.

Context. A detailed study of the Galactic bulge stellar population necessarily requires an accurate representation of the interstellar extinction, particularly toward the Galactic plane and center, where severe and differential reddening is expected to vary on sub-arcmin scales. Although recent infrared surveys have addressed this problem by providing extinction maps across the whole Galactic bulge area, dereddened color-magnitude diagrams near the plane and center appear systematically undercorrected, prompting the need for higher resolution. These undercorrections affect any stellar study sensitive to color (e.g., star formation history analyses via color-magnitude diagram fitting), either making them inaccurate or limiting them to small and relatively stable extinction windows where this value is low and better constrained. Aims. This study is aimed at providing a high-resolution (2 arcmin to ∼10 arcsec) color excess map for the VVV bulge area in J − Ks color. Methods. We used the MW-BULGE-PSFPHOT catalogs, sampling ∼300 deg2 across the Galactic bulge (|l| < 10° and −10° < b <  5°) to isolate a sample of red clump and red giant branch stars, for which we calculated the average J − Ks color in a fine spatial grid in (l, b) space. Results. We obtained an E(J − Ks) map spanning the VVV bulge area of roughly 300 deg2, with the equivalent of a resolution between ∼1 arcmin for bulge outskirts (l <  6°) to below 20 arcsec within the central |l| < 1°, and below 10 arcsec for the innermost area (|l| < 1° and |b| < 3°).

Context. Thanks to the recent wide-area photometric surveys, the number of star cluster candidates have risen exponentially in the last few years. Most detections, however, are based only on the presence of an overdensity of stars in a given region, or an overdensity of variable stars, regardless of their distance. As candidates, their detection has not been dynamically confirmed. Therefore, it is currently unknown how many, and which ones, of the published candidates, are true clusters, and which ones are chance alignments. Aims. We present a method to detect and confirm star clusters based on the spatial distribution, coherence in motion and appearance on the color-magnitude diagram. We explain and apply it to one new star cluster, and several candidate star clusters published in the literature. Methods. The presented method is based on data from the Second Data Release of Gaia complemented with data from the VISTA Variables in the Vía Láctea survey for the innermost bulge regions. It consists of a nearest neighbors algorithm applied simultaneously over spatial coordinates, star color, and proper motions, in order to detect groups of stars that are close in the sky, move coherently and define narrow sequences in the color-magnitude diagram, such as a young main sequence or a red giant branch. Results. When tested in the bulge area (−10 < (deg) < +10; −10 < b (deg) < +10) the method successfully recovered several known young and old star clusters. We report here the detection of one new, likely old star cluster, while deferring the others to a forthcoming paper. Additionally, the code has been applied to the position of 93 candidate star clusters published in the literature. As a result, only two of them are confirmed as coherently moving groups of stars at their nominal positions.

Context. Multiple stellar populations of different ages and metallicities reside in the Galactic bulge tracing its structure and providing clues for its formation and evolution. Aims. We present the near-infrared observations of population II Cepheids in the Galactic bulge from VVV survey. The JHK s photometry together with optical data from OGLE survey provide an independent estimate of the distance to the Galactic center. The old, metal-poor and low-mass population II Cepheids are also investigated as useful tracers for the structure of the Galactic bulge. Methods. We identify 340 population II Cepheids in the VVV survey Galactic bulge catalogue based on their match with OGLE-III Catalogue. The single-epoch JH and multi-epoch K s observations complement the accurate periods and optical (V I) mean-magnitudes from OGLE. The sample consisting of BL Herculis and W Virginis subtypes is used to derive period-luminosity relations after correcting mean-magnitudes for the extinction. Our K s -band period-luminosity relation, K s = −2.189(0.056) [log(P) − 1] + 11.187(0.032), is consistent with published work for BL Herculis and W Virginis variables in the Large Magellanic Cloud. Results. We present a combined OGLE III and VVV catalogue with periods, classification, mean magnitudes and extinction for 264 Galactic bulge population II Cepheids having good-quality K s -band light curves. The absolute magnitudes for population II Cepheids and RR Lyraes calibrated using Gaia and Hubble Space Telescope parallaxes, together with calibrated magnitudes for Large Magellanic Cloud population II Cepheids, are used to obtain a distance to the Galactic center, R 0 = 8.34 ± 0.03(stat.) ± 0.41(syst.), which changes by +0.05 −0.25 with different extinction laws. While noting the limitation of small number statistics, we find that the present sample of population II Cepheids in the Galactic bulge shows a nearly spheroidal spatial distribution, similar to metal-poor RR Lyrae variables. We do not find evidence of the inclined bar as traced by the metal-rich red-clump stars. Conclusions. Population II Cepheid and RR Lyrae variables follow similar period-luminosity relations and trace the same metal-poor old population in the Galactic bulge. The number density for population II Cepheids is more limited as compared to abundant RR Lyraes but they are bright and exhibit a wide range in period that provides a robust period-luminosity relation for an accurate estimate of the distance to the Galactic center.