Kalkayotl: A cluster distance inference code

Context. The high-precision parallax data of the Gaia mission allows for significant improvements in the distance determination to stellar clusters and their stars. In order to obtain accurate and precise distance determinations, systematics such as parallax spatial correlations need to be accounted for, especially with regard to stars in small sky regions. Aims. Our aim is to provide the astrophysical community with a free and open code designed to simultaneously infer cluster parameters (i.e., distance and size) and distances to the cluster stars using Gaia parallax measurements. The code includes cluster-oriented prior families and it is specifically designed to deal with the Gaia parallax spatial correlations. Methods. A Bayesian hierarchical model is created to allow for the inference of both the cluster parameters and distances to its stars. Results. Using synthetic data that mimics Gaia parallax uncertainties and spatial correlations, we observe that our cluster-oriented prior families result in distance estimates with smaller errors than those obtained with an exponentially decreasing space density prior. In addition, the treatment of the parallax spatial correlations minimizes errors in the estimated cluster size and stellar distances, and avoids the underestimation of uncertainties. Although neglecting the parallax spatial correlations has no impact on the accuracy of cluster distance determinations, it underestimates the uncertainties and may result in measurements that are incompatible with the true value (i.e., falling beyond the 2 sigma uncertainties). Conclusions. The combination of prior knowledge with the treatment of Gaia parallax spatial correlations produces accurate (error < 10%) and trustworthy estimates (i.e., true values contained within the 2 sigma uncertainties) of cluster distances for clusters up to similar to 5 kpc, along with cluster sizes for clusters up to similar to 1 kpc

Structural parameters of 389 local open clusters

Stars and planetary system

Unraveling UBC 274: a morphological, kinematical and chemical analysis of a disrupting open cluster

We do a morphological, kinematic and chemical analysis of the disrupting cluster UBC 274 (2.5 Gyr, $d=1778$ pc) to study its global properties. We use HDBSCAN to obtain a new membership list up to 50 pc from its centre and up to magnitude $G=19$ using Gaia EDR3 data. We use high resolution and high signal-to-noise spectra to obtain atmospheric parameters of 6 giants and subgiants, and individual abundances of 18 chemical species. The cluster has a highly eccentric (0.93) component, tilted $\sim$10 deg with respect to the plane of the Galaxy, which is morphologically compatible with the result of a test-particle simulation of a disrupting cluster. Our abundance analysis shows that the cluster has a subsolar metallicity of [Fe/H]$=-0.08\pm0.02$. Its chemical pattern is compatible with that of Ruprecht 147, of similar age but located closer to the Sun, with the remarkable exception of neutron-capture elements, which present an overabundance of $[n\mathrm{/Fe]}\sim0.1$. The cluster's elongated morphology is associated with the internal part of its tidal tail, following the expected dynamical process of disruption. We find a significant sign of mass segregation where the most massive stars appear 1.5 times more concentrated than other stars. The cluster's overabundance of neutron-capture elements can be related to the metallicity dependence of the neutron-capture yields due to the secondary nature of these elements, predicted by some models. UBC 274 presents a high chemical homogeneity at the level of $0.03$ dex in the sampled region of its tidal tails.Comment: Accepted by A&

Unravelling UBC 274: a morphological, kinematical, and chemical analysis of a disrupting open cluster

Stars and planetary system

Discovery of a low-mass companion inside the debris ring surrounding the F5V star HD 206893

Aims: Uncovering the ingredients and the architecture of planetary systems is a very active field of research that has fuelled many new theories on giant planet formation, migration, composition, and interaction with the circumstellar environment. We aim at discovering and studying new such systems, to further expand our knowledge of how low-mass companions form and evolve. Methods: We obtained high-contrast H-band images of the circumstellar environment of the F5V star HD 206893, known to host a debris disc never detected in scattered light. These observations are part of the SPHERE High Angular Resolution Debris Disc Survey (SHARDDS) using the InfraRed Dual-band Imager and Spectrograph (IRDIS) installed on VLT/SPHERE. Results: We report the detection of a source with a contrast of 3.6 × 10[SUP]-5[/SUP] in the H-band, orbiting at a projected separation of 270 milliarcsec or 10 au, corresponding to a mass in the range 24 to 73 M[SUB]Jup[/SUB] for an age of the system in the range 0.2 to 2 Gyr. The detection was confirmed ten months later with VLT/NaCo, ruling out a background object with no proper motion. A faint extended emission compatible with the disc scattered light signal is also observed. Conclusions: The detection of a low-mass companion inside a massive debris disc makes this system an analog of other young planetary systems such as β Pictoris, HR 8799 or HD 95086 and requires now further characterisation of both components to understand their interactions.Peer reviewe

Discovery of a low-mass companion inside the debris ring surrounding the F5V star HD 206893

Aims. Uncovering the ingredients and the architecture of planetary systems is a very active field of research that has fuelled many new theories on giant planet formation, migration, composition, and interaction with the circumstellar environment. We aim at discovering and studying new such systems, to further expand our knowledge of how low-mass companions form and evolve. Methods. We obtained high-contrast H-band images of the circumstellar environment of the F5V star HD 206893, known to host a debris disc never detected in scattered light. These observations are part of the SPHERE High Angular Resolution Debris Disc Survey (SHARDDS) using the InfraRed Dual-band Imager and Spectrograph (IRDIS) installed on VLT/SPHERE. Results. We report the detection of a source with a contrast of 3.6 × 10-5 in the H-band, orbiting at a projected separation of 270 milliarcsec or 10 au, corresponding to a mass in the range 24 to 73 MJup for an age of the system in the range 0.2 to 2 Gyr. The detection was confirmed ten months later with VLT/NaCo, ruling out a background object with no proper motion. A faint extended emission compatible with the disc scattered light signal is also observed. Conclusions. The detection of a low-mass companion inside a massive debris disc makes this system an analog of other young planetary systems such as β Pictoris, HR 8799 or HD 95086 and requires now further characterisation of both components to understand their interactions.J.M. is supported by the ESO fellowship programme. E.C. is supported by NASA through Hubble Fellowship grant HST-HF2-51355 and HST-AR-12652 awarded by STScI, operated by the AURA, Inc., for NASA under contract NAS5-26555. O.A. is a F.R.S.-FNRS Research Associate. The research leading to these results was partly funded by the European Research Council under the European Union’s Seventh Framework Programme (ERC Grant Agreement No. 337569), and by the French Community of Belgium through an ARC grant for Concerted Research Action. G.M.K. is supported by the Royal Society as a Royal Society University Research Fellow. M.C.W. and L.M. are supported by the European Union through ERC grant 279973. V.C. is supported by the Millennium Science Initiative (Chilean Ministry of Economy) through grant RC130007. C.d.B. acknowledges support from the Mexican CONACyT research grant CB-2012-183007

DIFFERENTIAL CHEMICAL ABUNDANCES OF BENCHMARK OPEN CLUSTERS

International audienceOpen Clusters (OCs) are very good laboratories to test theories of stellar evolution. They have also been widely used as tracers of the formation and evolution of the Galactic disk. Nearby OCs with different ages and metallicities are commonly used as benchmark objects for galactic and stellar studies because high precision measurements are usually achievable. We aim to perform a detailed study of ∼30 nearby (closer than 500 pc) and old (age>300 Myr) OCs. Thanks to Gaia DR2, accurate memberships, distances, motions and ages are available for a large amount of OCs, allowing us to revise their physical properties. Here we show the preliminary results of the strictly line-by-line differential abundances of three targets: the Hyades, NGC 2632 (Praesepe) and Ruprecht 147

Chemically characterising nearby evolved Open Clusters

Open Clusters (OCs) are excellent tracers of the formation and evolution of the Galaxy, as well as an ideal laboratory to test theories of stellar evolution. In particular, nearby OCs spanning a wide age range are commonly used as benchmark objects for galactic and stellar studies owing to the highest precision that is achievable. We have designed a project to perform an in-depth study of 45 nearby (closer than 500 pc) and old (age>300Myr) OCs. After Gaia DR2, very accurate memberships, distances, motions and ages are available for these OCs, allowing us to revise their physical properties. In particular, we are determining their chemical composition in a homogeneous way using a strictly line-by-line differential method with high-resolution, high signal-to-noise spectra, either retrieved from public archives or from our own observations. We present the first results of this project where we mesure the internal dispersions of abundances within the studied OCs and compare their chemical signatures

NEARBY OPEN CLUSTERS SHAPE : THREE-DIMENSIONAL GAUSSIAN MIXTURE MODEL AND TWO-DIMENSIONAL DENSITY MAPS

We use new memberships from Gaia DR2 to investigate the shape of the nearest open clusters. For open clusters closer than 250 pc from the Sun, we fit 3D ellipsoids on the spatial distribution of the members. At higher distances, the radial effect due to the uncertainties on the parallax measurements introduces a strong bias in the spatial distribution of members. Therefore, for more distant open clusters, we compute density maps on the sky to study their spatial properties and search for correlations with age and galactic environments of the clusters

The shape of nearby Open Clusters

International audienceWe use new memberships from Gaia DR2 to investigate the shape of the nearest open clusters. We investigate their 2D distribution projected on the sky and fit two gaussians components on the central part and external envelope. For open clusters closer than 200 pc from the Sun, we fit 3D ellipsoids on the spatial distribution of the members. We look for correlations of the spatial properties with age and galactic environment