Christian Moni Bidin

Context. O2 stars define the hottest end of the normal O-star sequence, but their identification requires blue-optical nitrogen diagnostics, which are absent in many surveys. Aims. We reassess three luminous Large Magellanic Cloud (LMC) sources with multi-epoch Sloan Digital Sky Survey V spectroscopy obtained with the Baryon Oscillation Spectroscopic Survey (BOSS) spectrographs: [WSI2008] 778, MGSD LH 117-43A/140, and Sk −69 135. Methods. We measured visit-resolved radial velocities and H, He, and N equivalent widths (EWs); compared robust combinations with LMC O2 templates at a common effective resolution; and used OSTAR2002 spectral energy distribution fits and a Gaia colour–magnitude diagram (CMD) as broadband checks. Results. The weighted log|EW(N IV λ 4058)/EW(N III λ 4640)| values are 0.64 ± 0.29, 0.48 ± 0.23, and 0.34 ± 0.44 for [WSI2008] 778, MGSD LH 117-43A/140, and Sk −69 135. The detection of N V λλ 4604, 4620 supports the O2 If ^* and O2 V–III classifications adopted for [WSI2008] 778 and MGSD LH 117-43A/140, respectively. For Sk −69 135, the uncertain N IV/N III ratio and non-detection of N V motivate O2–3 V–III. Weak He I λ 4471 may indicate unresolved later-type contamination in [WSI2008] 778 and Sk −69 135. The radial velocities of Sk −69 135 show suggestive but non-significant variability. OSTAR2002 fits over 45–55 kK and the Gaia CMD place all three sources in the luminous very early O regime; [WSI2008] 778 shows a strong mid-infrared excess, MGSD LH 117-43A/140 is nearly photospheric, and Sk −69 135 shows only a modest 3–8 μm excess. Conclusions. The BOSS spectra place [WSI2008] 778 and MGSD LH 117-43A/140 in the LMC very early O population and identify Sk −69 135 as an additional O2–3 source. These cases show how incomplete blue coverage and ambiguous catalogue associations can conceal the hottest massive stars.

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.

Mapping the local and distant Universe is key to our understanding of it. For decades, the Sloan Digital Sky Survey (SDSS) has made a concerted effort to map millions of celestial objects to constrain the physical processes that govern our Universe. The most recent and fifth generation of SDSS (SDSS-V) is organized into three scientific “mappers”: the Milky Way Mapper, which aims to chart the various components of the Milky Way and constrain its formation and assembly; the Black Hole Mapper, which focuses on understanding supermassive black holes in distant galaxies across the Universe; and the Local Volume Mapper, which uses integral field spectroscopy to map the ionized interstellar medium in the Local Group. This paper describes the scope and content for the nineteenth data release (DR19) of SDSS, which is the most substantial to date in SDSS-V. DR19 is the first to contain data from all three mappers. Additionally, we also describe nine value-added catalogs that enhance the science that can be conducted with the SDSS-V data. Finally, we discuss how to access SDSS DR19 and provide illustrative examples and tutorials.

Context . The characterization of globular clusters (GCs) in the Galactic bulge is a challenging task due to high extinction and severe stellar crowding. VVV-CL001 is a poorly studied GC located in the inner bulge, known for its extremely old age, extreme velocity, and low metallicity. Given its unique properties, a detailed study of this cluster can provide valuable insights into the early chemical and dynamical evolution of the Milky Way (MW). Aims . The aim of this study was to derive the fundamental parameters of VVV-CL001 including metallicity, heliocentric radial velocity (RV), proper motions (PMs), structural properties, orbit, and age, in order to improve our understanding of its origin and role in the early evolution of the MW. Methods . We combined spectroscopic, astrometric, and photometric data to characterize VVV-CL001. Metallicity and RV were determined from medium-resolution spectra obtained with FORS2 at the Very Large Telescope. PMs were derived using Gaia DR3 data. Near-infrared photometry from the FourStar instrument on Magellan was used to refine the cluster’s position, construct a radial density profile, and estimate its age, distance, and reddening. Results . Our results confirm that VVV-CL001 is an old 12.1 _−1.2 ^+1.0 Gyr), metal-poor ([Fe/H] = −2.25 ± 0.05 dex) globular cluster located at a heliocentric position of d _⊙ = 7.1 _−1.1 ^+1.3 , with a reddening of E( J − K _s ) = 1.40 _−0.02 ^+0.01 . Its mean PMs are μ _α ^* = −3.68 ± 0.09 mas yr ⁻¹ and µ _δ = −1.76 ± 0.10 mas yr ⁻¹ , and it exhibits a RV of −334 ± 4 km s ⁻¹ . The cluster follows a retrograde-prograde eccentric ( e = 0.76 _−0.14 ^+0.10 ) orbit, confined within the Galactic plane (|Z| _max = 1.0 _−0.32 ^+0.45 kpc) and inside the bar’s radius of influence ( R < 5 kpc), with a pericenter of r _peri = 0.6 _−0.2 ^+0.3 kpc and an apocenter of r _apo = 4.5 _−1.2 ^+2.5 kpc. Conclusions . These orbital properties, combined with its ancient age and low metallicity, strongly support an in situ origin for VVV-CL001 and likely membership of the disk GC system that was captured by the potential of the bar during its formation. Thus, VVV-CL001 emerges as a fossil remnant of the earliest phases of Galactic assembly and a valuable tracer of the population that contributed to the formation of the inner thick disk and bulge, which are likely part of the main progenitor of the MW. Our study highlights the relevance of detailed chemo-dynamical analyses in unveiling the origin of GCs in the inner Galaxy.

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

The central (‘bulge’) region of the Milky Way is teeming with a significant fraction of mildly metal-deficient stars with atmospheres that are strongly enriched in cyanogen (¹²C¹⁴N). Some of these objects, which are also known as nitrogen-enhanced stars, are hypothesised to be relics of the ancient assembly history of the Milky Way. Although the chemical similarity of nitrogen-enhanced stars to the unique chemical patterns observed in globular clusters has been observed, a direct connection between field stars and globular clusters has not yet been proven. In this work, we report on high-resolution, near-infrared spectroscopic observations of the bulge globular cluster NGC 6723, and the serendipitous discovery of a star, 2M18594405−3651518, located outside the cluster (near the tidal radius) but moving on a similar orbit, providing the first clear piece of evidence of a star that was very likely once a cluster member and has recently been ejected. Its nitrogen abundance ratio ([N/Fe] ≳ + 0.94) is well above the typical Galactic field-star levels, and it exhibits noticeable enrichment in the heavys-process elements (Ce, Nd, and Yb), along with moderate carbon enrichment; all characteristics are known examples in globular clusters. This result suggests that some of the nitrogen-enhanced stars in the bulge likely originated from the tidal disruption of globular clusters.

We conduct a quantitative analysis of the star formation history (SFH) of the Milky Way’s (MW) bulge by exploiting the constraining power of its stellar [Fe/H] and [Mg/Fe] distribution functions. Using Apache Point Observatory Galactic Evolution Experiment survey data, we confirm the previously established bimodal [Mg/Fe]–[Fe/H] distribution within 3 kpc of the inner Galaxy. To fit the chemical bimodal distribution, we use a simple but flexible star formation framework, which assumes two distinct stages of gas accretion and star formation, and systematically evaluate a wide multidimensional parameter space. We find that the data favour a three-phase SFH that consists of an initial starburst, followed by a rapid star formation quenching episode, and a lengthy, quiescent secular evolution phase. The metal-poor, high-α bulge stars ([Fe/H] < 0.0 and [Mg/Fe] > 0.15) are formed rapidly ( 0.0 and [Mg/Fe] < 0.15) then accumulates gradually through inefficient star formation during the secular phase. This is qualitatively consistent with the early SFH of the inner disc. Given this scenario, a notable fraction of young stars (age <5 Gyr) is expected to persist in the bulge. Combined with extragalactic observations, these results suggest that a rapid star formation quenching process is responsible for bimodal distributions in both the MW’s stellar populations and in the general galaxy population and thus plays a critical role in galaxy evolution.

In this article, we present a detailed chemical analysis of seven red giant members of NGC 6553 using high-resolution spectroscopy from VLT FLAMES. We obtained the stellar parameters (Teff, Log(g), vt, [Fe/H]) of these stars from the spectra, and we measured the chemical abundance for 20 elements, including light elements, iron-peak elements, α-elements, and neutron-capture elements. The metallicities in our sample stars are consistent with a homogeneous distribution. We found a mean of [Fe/H] = −0.14 ± 0.07 dex, in agreement with other studies. Using the α-elements Mg, Si, Ca, and Ti, we obtain the mean of [α/Fe] = 0.11 ± 0.05. We found a vertical relation between Na and O, characterized by a significant spread in Na and an almost non-existent spread in O. In fact, Na and Al are the only two light elements with a large intrinsic spread, which demonstrates the presence of multiple populations (MPs). An intrinsic spread in Mg is not detected in this study. The α, iron-peak, and neutron-capture elements show good agreement with the trend of the bulge field stars, indicating similar origin and evolution, in concordance with our previous studies for two other bulge globular clusters (NGC 6440 and NGC 6528).