Danilo González-Díaz

Context . Most Galactic stars are born in large groups or stellar clusters, whose final stage before complete dissolution into the stellar field is a collection of a handful of stars, most of which are binaries, called open cluster remnants (OCR). Aims . This study aims to determine whether the potential OCR candidates Ruprecht 3, Ruprecht 31, and ESO 426 SC26 are true star clusters by characterising their member stars based on kinematic and parallax data and studying their behaviour relative to the stellar background. Methods . We used independent radial velocity measurements of thirty-eight stars from the FEROS and FIDEOS spectrographs, as well as precise astrometry from Gaia DR3. Results . The radial velocity analysis shows a relatively low number of binary stars among our selected targets. Furthermore, the analysis of parallaxes, proper motions, and radial velocities shows no evidence of a kinematically and spatially consistent stellar group in the analysed fields. Additionally, we find that the original overdensity measurements are driven only by bright stars, as there is no excess of faint stars within the observed regions. We tested our methods using the well-known OCR NGC 1901. Conclusions . We conclude that none of our three candidates are true OCRs but most likely asterisms. NGC 1901 is a 500 ± 100 Myr-old cluster remnant located 418 ± 6 pc from the Sun composed of no more than 20–25 stars within a 10′ radius from the centre.

Context. The bulge globular cluster (BGC) NGC 6316 has been the subject of few previous chemical studies beyond metallicity. Thanks to the bulge Cluster APOgee Survey (CAPOS), we can now improve our knowledge of the chemistry and nature of this cluster. CAPOS makes use of high-resolution, high signal-to-noise ratio near-infrared spectroscopy, which is capable of penetrating the substantial dust towards the Galactic bulge (a significant optical obstacle for this cluster due to its high reddening). Aims. We aim to conduct the most robust chemical study to date for NGC 6316 by deriving abundances for a number of elements with a variety of nucleosynthetic origins, most of which have never been studied before in this cluster. Methods. We used the Brussels Automatic Code for Characterizing High accuracy Spectra (BACCHUS) with atmospheric parameters photometrically obtained in order to determine the abundances Results. We determined, for the first time, high-resolution spectroscopic abundances for C, N, O, Mg, Al, Si, P, K, Ca, Ti, V, Cr, Mn, Fe, Ni, and Ce for this cluster. We obtained a mean [Fe/H] = −0.87 ± 0.02, finding no indication of an intrinsic metallicity spread. Our metallicity agrees with the most recent values from other studies, revising earlier values that were ∼0.5 dex more metal-rich. With this new value, this cluster, long believed to be a member of the classical metal-rich group of BGCs around −0.5, now falls in the dominant BGC peak around [Fe/H] = −1. The cluster presents a clear C–N anti-correlation and [α/Fe] = 0.31 ± 0.02. Our abundances show similar behaviour to other in situ globular clusters with comparable metallicity. An isochrone fitting gave us E(B–V) = 0.71, a higher value than any other from the literature for this cluster since we also estimated R_V = 2.7, which is in good agreement with determinations from other works; (M–m)₀ = 15.32 ± 0.05. We derive an overall metallicity of [M/H] = −0.6 ± 0.05, which is in agreement with our abundance determination.

Pre-Gaia distances for the open cluster Pismis 19 disagree with Gaia parallaxes. A Two Micron All Sky Survey (2MASS) JK _s red clump distance was therefore established for Pismis 19 (2.90 ± 0.15 kpc), which reaffirms that zero-point corrections for Gaia are required. OGLE GD-CEP-1864 is confirmed as a member of Pismis 19 on the basis of DR3 proper motions and its 2MASS+Vista Variables in the Vía Láctea color–magnitude position near the tip of the turnoff. That 0.3 day variable star is likely a δ Scuti rather than a classical Cepheid. The case revealed a pertinent criterion to segregate those two populations in tandem with the break in the Wesenheit Leavitt Law (≃0.5 day). Just shortward of that period discontinuity are δ Scutis, whereas beyond the break lie first overtone classical Cepheids mostly observed beyond the first crossing of the instability strip.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context. The abundances of the α-elements are key for understanding the early chemical enrichment of the Galactic bulge. The elements of interest present lines in different wavelength regions, and some of them show lines only in part of the spectra. In the present work, the CNO trio, the a-elements Mg, Si, Ca, and Ti, and odd-Z Na and Al are examined as measured from optical and H-band lines. Aims. The aim of this work is to carry out a detailed comparison of stellar parameters and abundances derived in the optical and near-infrared (H-band). We also inspect the best available lines for a list of bulge stars previously analyzed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) team in the H-band and by our group in the optical. This work is mainly of interest to spectroscopists. Methods. In the present work, we compared the stellar parameters and abundance results derived from APOGEE H-band spectra with optical analyses based on Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope (VLT/UVES) data for eight bulge stars. Results. We point out the most suitable wavelength region for each of the studied elements, and highlight difficulties in the derivation of stellar parameters both in the optical and H-band. The near-infrared will allow observations of a large number of stars in the near future given new instruments soon to be available. The identification of spectral lines in this spectral region and the investigation of their reliability are ongoing efforts worldwide. New instruments will also allow simultaneous observation of H-band and optical.

This study presents the results concerning six red giant stars members of the globular cluster NGC 6558. Our analysis utilized high-resolution near-infrared spectra obtained through the CAPOS initiative (the APOgee Survey of Clusters in the Galactic Bulge), which focuses on surveying clusters within the Galactic Bulge, as a component of the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2). We employ the BACCHUS (Brussels Automatic Code for Characterizing High accUracy Spectra) code to provide line-by-line elemental-abundances for Fe-peak (Fe, Ni), α-(O, Mg, Si, Ca, Ti), light-(C, N), odd-Z (Al), and the s-process element (Ce) for the 4 stars with high signal-to-noise ratios. This is the first reliable measure of the CNO abundances for NGC 6558. Our analysis yields a mean metallicity for NGC 6558 of 〈[Fe/H]〉 = −1.15 ± 0.08, with no evidence for a metallicity spread. We find a Solar Ni abundance, 〈[Ni/Fe]〉 ∼ +0.01, and a moderate enhancement of α-elements, ranging between +0.16 to <+0.42, and a slight enhancement of the s-process element 〈[Ce/Fe]〉 ∼ +0.19. We also found low levels of 〈[Al/Fe]〉 ∼ +0.09, but with a strong enrichment of nitrogen, [N/Fe]>+0.99, along with a low level of carbon, [C/Fe]<−0.12. This behaviour of Nitrogen-Carbon is a typical chemical signature for the presence of multiple stellar populations in virtually all GCs; this is the first time that it is reported in NGC 6558. We also observed a remarkable consistency in the behaviour of all the chemical species compared to the other CAPOS bulge GCs of the same metallicity.

Context. The open cluster (OC) NGC 2453 is of particular importance since it has been considered to host the planetary nebula (PN) NGC 2452, however their distances and radial velocities are strongly contested. Aims. In order to obtain a complete picture of the fundamental parameters of the OC NGC 2453, 11 potential members were studied. The results allowed us to resolve the PN NGC 2452 membership debate. Methods. Radial velocities for the 11 stars in NGC 2453 and the PN were measured and matched with Gaia data release 2 (DR2) to estimate the cluster distance. In addition, we used deep multi-band UBVRI photometry to get fundamental parameters of the cluster via isochrone fitting on the most likely cluster members, reducing inaccuracies due to field stars. Results. The distance of the OC NGC 2453 (4.7 ± 0.2 kpc) was obtained with an independent method solving the discrepancy reported in the literature. This result is in good agreement with an isochrone fitting of 40–50 Myr. On the other hand, the radial velocity of NGC 2453 (78 ± 3 km s⁻¹) disagrees with the velocity of NGC 2452 (62 ± 2 km s⁻¹). Our results show that the PN is a foreground object in the line of sight. Conclusions. Due to the discrepancies found in the parameters studied, we conclude that the PN NGC 2452 is not a member of the OC NGC 2453.