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SCYON Issue 79 edited by Giovanni Carraro, Martin Netopil, and Ernst Paunzen https://www.univie.ac.at/scyon/ email: [email protected] The official Newsletter of the IAU Commission H4. SCYON Issue No. 79 February 21st, 2020 Dear Colleagues, Looking in the recent published papers about star clusters, space based data such as from Gaia, Kepler, and TESS bring valuable new insights. But also ground-based spectroscopic and polarimetric data are still most important to get the whole picture. With future releases of the Gaia satellite mission, the question how to interpret color-magnitude diagrams together with astrometric data will become an essential topic. Here, a joint effort is needed in order to estimate reliable cluster parameters including also the metallicity. We would like to draw attention to the announced conference "Wheel of Star Formation" which will take place in September in Prague, Czech Republic. It will cover many important topics associ- ated to star clusters. The SCYON editor team: Giovanni Carraro, Martin Netopil, and Ernst Paunzen CONTENTS About the Newsletter Abstracts of refereed papers . 2 SCYON publishes abstracts from any Star Forming Regions ..................2 area in astronomy, which are relevant to Galactic Open Clusters .................3 research on star clusters. We welcome all Galactic Globular Clusters .............6 kinds of submitted contributions (abstracts Clusters in the Magellanic clouds . .10 of refereed papers or conference proceedings, Dynamical evolution - Simulations . 11 PhD summaries, and general announcements Miscellaneous .........................13 of e.g. conferences, databases, tools, etc.) Proceedings abstracts ....................14 Conferences and Announcements .........15 The mission of this newsletter is to help all the researchers in the field with a quick and efficient link to the scientific activity in the field. We encourage everybody to contribute to the new releases! New abstracts can be submitted at any time using the webform on the SCYON homepage. https://www.univie.ac.at/scyon Star Forming Regions 2 §Star Forming Regions ¤ ¦ ¥ The density gradient inside molecular-gas clumps as a booster of their star formation activity G. Parmentier 1 (1) Zentrum f¨urAstronomie der Universit¨atHeidelberg, Heidelberg, Germany Star-forming regions presenting a density gradient experience a higher star formation rate than if they were of uniform density. We refer to the ratio between the star formation rate of a spherical centrally-concentrated gas clump and the star formation rate that this clump would experience if it were of uniform density as the magnification factor ζ. We map ζ as a function of clump mass, radius, initial volume density profile and star formation time-span. For clumps with a steep density profile (i.e. power-law slope ranging from −3 to −4, as observed in some high-density regions of Galactic molecular clouds), we find the star formation rate to be at least an order of magnitude higher than its top-hat equivalent. This implies that such clumps experience faster and more efficient star formation than expected based on their mean free-fall time. This also implies that measurements of the star formation efficiency per free-fall time of clumps based on their global properties, namely, mass, mean volume density and star formation rate, present wide fluctuations. These reflect the diversity in the density profile of star-forming clumps, not necessarily variations in the physics of star formation. Steep density profiles inside star-cluster progenitors may be instrumental in the formation of multiple stellar populations, such as those routinely observed in old globular clusters. Accepted by: Astrophysical Journal https://ui.adsabs.harvard.edu/abs/2019ApJ...887..179P/abstract Galactic Open Clusters 3 §Galactic Open Clusters ¤ ¦ ¥ On the Metallicity Gradient in the Galactic Disk A. V. Loktin 1, M. E. Popova 1 (1) Astronomical Observatory, Ural Federal University, Ekaterinburg, Russia The problem of the chemical composition gradient in the galactic disk is studied based on a sample of metallicity estimates of open star clusters, using Gaia DR2-improved distance estimates. A clearly non-monotonic variation was observed in the average metallicity of clusters with increasing galactocentric distance. One can clearly see the metallicity jump of 0.22 in [Fe/H] at a Galactocentric distance of about 9.5 kpc, which appears to be linked to the outer boundary of the thinnest and youngest component of the galactic disk. The absence of a significant metallicity gradient in the internal (R < 9 kpc) and external (R > 10 kpc) regions of the disk demonstrates the absence of noticeable metal enrichment at times of the order of the ages corresponding to those of the disk regions under consideration. Observational data show that the disk experiences noticeable metal enrichment only during the starburst epochs. No significant dependence was found between the average metallicity and the age of the clusters. Accepted by: Research in Astronomy and Astrophysics https://ui.adsabs.harvard.edu/abs/2020arXiv200102916L/abstract .................................................................................................... Variability in the Massive Open Cluster NGC 1817 from K2: A Rich Population of Asteroseismic Red Clump, Eclipsing Binary, and Main Sequence Pulsating Stars E. Sandquist 1, D. Stello 2, T. Arentoft 3, and 13 co-authors (1) San Diego State University, Department of Astronomy, San Diego, USA; (2) School of Physics, The University of New South Wales, Sydney, Australia; (3) Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark We present a survey of variable stars detected in K2 Campaign 13 within the massive intermediate age (∼ 1 Gyr) open cluster NGC 1817. We identify a complete sample of 44 red clump stars in the cluster, and have measured asteroseismic quantities (νmax and/or ∆ν) for 29 of them. Five stars showed suppressed dipole modes, and the occurrence rates indicate that mode suppression is unaffected by evolution through core helium burning. A subset of the giants in NGC 1817 (and in the similarly aged cluster NGC 6811) have νmax and ∆ν values at or near the maximum observed for core helium burning stars, indicating they have core masses near the minimum for fully non-degenerate helium ignition. Further asteroseismic study of these stars can constrain the minimum helium core mass in red clump stars and the physics that determines this limit. Two giant stars show photometric variations on timescales similar to previously measured spectroscopic orbits. Thirteen systems in the field show eclipses, but only five are probable cluster members. We identify 32 δ Sct pulsators, 27 γ Dor candidates, and 7 hybrids that are probable cluster members, with most new detections. We used the ensemble properties of the δ Sct stars to identify stars with possible radial pulsation modes. Among the oddities we have uncovered are: an eccentric orbit for a short-period binary containing a δ Sct pulsating star; a rare subgiant within the Hertzsprung gap showing δ Sct pulsations; and two hot γ Dor pulsating star candidates. Accepted by: Astronomical Journal https://ui.adsabs.harvard.edu/abs/2020arXiv200101839S/abstract Galactic Open Clusters 4 Hunting for open clusters in Gaia DR2: 582 new OCs in the Galactic disc A. Castro-Ginard 1, C. Jordi 1, X. Luri 1, and 8 co-authors (1) Dept. F´ısicaQu`antica i Astrof´ısica,Institut de Ci`enciesdel Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Barcelona, Spain Open clusters are key targets for studies of Galaxy structure and evolution, and stellar physics. Since the Gaia data release 2 (DR2), the discovery of undetected clusters has shown that previous surveys were incomplete. Our aim is to exploit the Big Data capabilities of machine learning to detect new open clusters in Gaia DR2, and to complete the open cluster sample to enable further studies of the Galactic disc. We use a machine-learning based methodology to systematically search the Galactic disc for overdensities in the astrometric space and identify the open clusters using photometric information. First, we used an unsupervised clustering algorithm, DBSCAN, to blindly search for these overdensities in Gaia DR2 (l; b; $; µα∗ ; µδ), and then we used a deep learning artificial neural network trained on colour{magnitude diagrams to identify isochrone patterns in these overdensities, and to confirm them as open clusters. We find 582 new open clusters distributed along the Galactic disc in the region jbj < 20 deg. We detect substructure in complex regions, and identify the tidal tails of a disrupting cluster UBC 274 of ∼ 3 Gyr located at ∼ 2 kpc. Adapting the mentioned methodology to a Big Data environment allows us to target the search using the physical properties of open clusters instead of being driven by computational limitations. This blind search for open clusters in the Galactic disc increases the number of known open clusters by 45%. Accepted by: Astronomy & Astrophysics https://ui.adsabs.harvard.edu/abs/2020arXiv200107122C/abstract .................................................................................................... A Study of the Blue Straggler Population of the Old Open Cluster Collinder 261 M. J. Rain 1, G. Carraro 1, J. A. Ahumada 2, and 4 co-authors (1) Dipartimento di Fisica e Astronomia, Universita di Padova, Padova, Italy; (2) Observatorio Astron´omico, Universidad Nacional de C´ordoba,C´ordoba,Argentina Blue stragglers (BSs) are stars located in an unexpected region of the color-magnitude diagram (CMD) of a stellar population, as they appear bluer and more luminous than the stars in the turn-off region. They are ubiquitous, since they have been found among Milky Way field stars, in open and globular clusters, and also in other galaxies of the Local Group. Here we present a study on the BS population of the old and metal-rich open cluster Collinder 261, based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted with Fibre Large Array Multi Element Spectrograph (FLAMES) at the Very Large Telescope (VLT). We also analyze the radial distribution of the BS population to probe the dynamical status of the cluster.
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