StarDance Results
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Recent results
Not all nitrogen-rich field stars originate from globular clusters
Not all nitrogen-rich field stars originate from globular clusters
The primordial (blue) and enriched (red) field stars in the Kepler sample, with the sample of GC stars identified by Schiavon et al. (2024) (green), both using APOGEE DR17 abundances.
By combining asteroseismology from Kepler and chemical abundances from APOGEE DR17, we present a sample of low-metallicity red giants in the Kepler field which show abundance signatures consistent with originating from Milky Way globular clusters. The sample is separated into primordial stars and enriched stars, with enriched stars showing nitrogen enrichment, along with carbon and oxygen depletion. We discovered for the first time that the asteroseismic masses and ages of the majority of the enriched stars in our sample are apparently too young to have originated from globular clusters, with some showing evidence of being products of mass-transfer within a binary system. This result leads us to the conclusion that not all enriched field stars originated from globular clusters as previously thought, with binaries emerging as the leading enrichment mechanism for follow-up investigations.
Robust ellipticity measurements of 29 Galactic globular clusters
Using combined ground-based and HST photometry, we present a new robust method to measure the ellipticity of Galactic globular clusters and apply it to a sample of 29 systems. We show that standard approaches, such as ellipse fitting of density contours and principal component analysis, can significantly overestimate flattening when clusters are nearly round or when only small stellar samples are available. Our new method overcomes these biases and reveals that more than half of the sample shows measurable flattening, with 16 clusters having ellipticity above 0.05. By comparing the derived shapes with cluster kinematics through the V/σ framework, we find that rotation is the main driver of flattening in several clusters, while velocity anisotropy and Galactic tidal effects likely also contribute in specific cases. These results highlight the importance of accounting for non-sphericity in models of globular clusters and provide a reliable tool for future studies of their internal structure, including multiple stellar populations.
The rotation strength as a function of the flattening.
The Preliminary Mauve Science Programme: Science themes identified for the first year of operations
Candidate target list coloured by spectral type across the Mauve field of regard.
Mauve is a newly launched small UV-visible satellite designed to obtain low-resolution spectra over the 200–700 nm range. Its first-year science programme includes 10 core themes, around 290 candidate targets, and 5000 observing hours devoted to key questions in stellar astrophysics, including stellar flares and CMEs, quiescent UV emission in low-mass stars, young exoplanet hosts and HWO targets, classical Be and Herbig Ae/Be stars, and binaries in exotic stellar populations.
2025 results
Detection of a white dwarf orbiting a carbon-oxygen-depleted blue straggler in 47 Tucanae
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.
SED fitting of the BSS4 observed photometry through the combination of the SED that best fits the points at λ > 4000 Å and the SED of and the SED of a 0.55 M⊙ CO-WD.
Survey of Surveys: II. The Stellar parameters for 23 million stars
Survey of Surveys: II. The Stellar parameters for 23 million stars
Kiel diagram for the final catalog, colored with the estimated errors on [Fe/H].
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.
AstroSat/UVIT Study of NGC 663: first detection of Be+sdOB systems in a young star cluster
Be stars are rapidly rotating stars surrounded by a disc; however, the origin of these remains unclear. The Mass and angular momentum transfer in close binaries account for the rapid rotation of a major fraction of Be stars, supported by the previous detection of low-mass stripped companions to them. The stripped companions can be helium burning subdwarf OB-type stars (sdOBs), and white dwarfs (WDs). In our search for hot companions in a sample of Be stars in NGC 663, we found that that 19 out of 23 candiates possess a UV excess, which can be unambiguously attiributed to the presence of hot companions. We report the first detection of high-mass sdOB companions to Be stars, with 69.5% of them found in binaries within a cluster, offering direct evidence of binary interactions and their role in shaping the properties of Be stars.
Optical colour–magnitude diagram of NGC 663, based on Gaia DR3 photometry.
Studying binary systems in Omega Centauri with MUSE – II. Observational constraints on the orbital period distribution
Colour–magnitude diagram of ω Cen, where all stars with MUSE spectra are presented, colour-coded for their probability to be in binary systems
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.
2024 results
Differential reddening in 48 globular clusters: An end to the quest for the intracluster medium
Differential reddening map of NGC 3201 obtained in this work.
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.
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