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112 results on '"Cassisi, S."'

1. Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). I. The final word on the origin of NGC 6388 and NGC 6441

Author

Massari, D., primary, Aguado-Agelet, F., additional, Monelli, M., additional, Cassisi, S., additional, Pancino, E., additional, Saracino, S., additional, Gallart, C., additional, Ruiz-Lara, T., additional, Fernandez-Alvar, E., additional, Surot, F., additional, Stokholm, A., additional, Salaris, M., additional, Miglio, A., additional, and Ceccarelli, E., additional

Published
2023
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2. Gaia Data Release 3

Author

Creevey, O. L., primary, Sordo, R., additional, Pailler, F., additional, Frémat, Y., additional, Heiter, U., additional, Thévenin, F., additional, Andrae, R., additional, Fouesneau, M., additional, Lobel, A., additional, Bailer-Jones, C. A. L., additional, Garabato, D., additional, Bellas-Velidis, I., additional, Brugaletta, E., additional, Lorca, A., additional, Ordenovic, C., additional, Palicio, P. A., additional, Sarro, L. M., additional, Delchambre, L., additional, Drimmel, R., additional, Rybizki, J., additional, Torralba Elipe, G., additional, Korn, A. J., additional, Recio-Blanco, A., additional, Schultheis, M. S., additional, De Angeli, F., additional, Montegriffo, P., additional, Abreu Aramburu, A., additional, Accart, S., additional, Álvarez, M. A., additional, Bakker, J., additional, Brouillet, N., additional, Burlacu, A., additional, Carballo, R., additional, Casamiquela, L., additional, Chiavassa, A., additional, Contursi, G., additional, Cooper, W. J., additional, Dafonte, C., additional, Dapergolas, A., additional, de Laverny, P., additional, Dharmawardena, T. E., additional, Edvardsson, B., additional, Le Fustec, Y., additional, García-Lario, P., additional, García-Torres, M., additional, Gomez, A., additional, González-Santamaría, I., additional, Hatzidimitriou, D., additional, Jean-Antoine Piccolo, A., additional, Kontiza, M., additional, Kordopatis, G., additional, Lanzafame, A. C., additional, Lebreton, Y., additional, Licata, E. L., additional, Lindstrøm, H. E. P., additional, Livanou, E., additional, Magdaleno Romeo, A., additional, Manteiga, M., additional, Marocco, F., additional, Marshall, D. J., additional, Mary, N., additional, Nicolas, C., additional, Pallas-Quintela, L., additional, Panem, C., additional, Pichon, B., additional, Poggio, E., additional, Riclet, F., additional, Robin, C., additional, Santoveña, R., additional, Silvelo, A., additional, Slezak, I., additional, Smart, R. L., additional, Soubiran, C., additional, Süveges, M., additional, Ulla, A., additional, Utrilla, E., additional, Vallenari, A., additional, Zhao, H., additional, Zorec, J., additional, Barrado, D., additional, Bijaoui, A., additional, Bouret, J.-C., additional, Blomme, R., additional, Brott, I., additional, Cassisi, S., additional, Kochukhov, O., additional, Martayan, C., additional, Shulyak, D., additional, and Silvester, J., additional

Published
2023
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6. The Aarhus red giants challenge: I. Stellar structures in the red giant branch phase

Author

Danish National Research Foundation, European Commission, Villum Fonden, Independent Research Fund Denmark, Ministerio de Ciencia e Innovación (España), Generalitat de Catalunya, Australian Research Council, National Science Foundation (US), Fundação para a Ciência e a Tecnologia (Portugal), Department of Atomic Energy (India), Silva Aguirre, Víctor, Christensen-Dalsgaard, Jørgen, Cassisi, S., Miller Bertolami, Marcelo M., Serenelli, Aldo, Stello, Dennis, Weiss, Achim, Angelou, George, Jiang, Chen, Lebreton, Yveline, Spada, Federico, Bellinger, E. P., Deheuvels, Sébastien, Ouazzani, Rhita-Maria, Pietrinferni, A., Mosumgaard, Jakob R., Townsend, Richard H. D., Battich, Tiara, Bossini, Diego, Constantino, Thomas N., Eggenberger, P., Hekker, Saskia, Mazumdar, S. P., Miglio, Andrea, Nielsen, K. B., Salaris, M., Danish National Research Foundation, European Commission, Villum Fonden, Independent Research Fund Denmark, Ministerio de Ciencia e Innovación (España), Generalitat de Catalunya, Australian Research Council, National Science Foundation (US), Fundação para a Ciência e a Tecnologia (Portugal), Department of Atomic Energy (India), Silva Aguirre, Víctor, Christensen-Dalsgaard, Jørgen, Cassisi, S., Miller Bertolami, Marcelo M., Serenelli, Aldo, Stello, Dennis, Weiss, Achim, Angelou, George, Jiang, Chen, Lebreton, Yveline, Spada, Federico, Bellinger, E. P., Deheuvels, Sébastien, Ouazzani, Rhita-Maria, Pietrinferni, A., Mosumgaard, Jakob R., Townsend, Richard H. D., Battich, Tiara, Bossini, Diego, Constantino, Thomas N., Eggenberger, P., Hekker, Saskia, Mazumdar, S. P., Miglio, Andrea, Nielsen, K. B., and Salaris, M.

Abstract

[Context] With the advent of space-based asteroseismology, determining accurate properties of red-giant stars using their observed oscillations has become the focus of many investigations due to their implications in a variety of fields in astrophysics. Stellar models are fundamental in predicting quantities such as stellar age, and their reliability critically depends on the numerical implementation of the physics at play in this evolutionary phase., [Aims] We introduce the Aarhus red giants challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes that aim to establish the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. We present the first set of results focusing on stellar evolution tracks and structures in the red-giant-branch (RGB) phase., [Methods] Using nine state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at a fixed radius along the red-giant branch for masses of 1.0 M⊙, 1.5 M⊙, 2.0 M⊙, and 2.5 M⊙, and compared the predicted stellar properties., [Results] Once models have been calibrated on the main sequence, we find a residual spread in the predicted effective temperatures across all codes of ∼20 K at solar radius and ∼30–40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2–5% (increasing with stellar mass), which we attribute to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ∼0.1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and find a spread of 0.1 dex or more for all considered masses., [Conclusions] Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and they call for further investigation on these matters in the context of using properties of red giants as benchmarks for astrophysical studies.

Published
2020

7. The Large Magellanic Cloud stellar content with SMASH: I. Assessing the stability of the Magellanic spiral arms

Author

Ministerio de Economía y Competitividad (España), European Commission, Ministerio de Ciencia, Innovación y Universidades (España), Gobierno de Canarias, National Science Foundation (US), European Research Council, Comisión Nacional de Investigación Científica y Tecnológica (Chile), Max Planck Society, Istituto Nazionale di Astrofisica, Ruiz-Lara, T., Gallart, C., Monelli, M., Nidever, D., Dorta, A., Choi, Y., Olsen, K., Besla, G., Bernard, E. J., Cassisi, S., Massana, Pol, Noël, N. E. D., Pérez, I., Rusakov, V., Cioni, M. -R. L., Majewski, Steven R., van der Marel, R. P., Martínez-Delgado, D., Monachesi, A., Monteagudo, L., Muñoz, R. R., Stringfellow, G. S., Surot, F., Vivas, A. K., Walker, A. R., Zaritsky, D., Ministerio de Economía y Competitividad (España), European Commission, Ministerio de Ciencia, Innovación y Universidades (España), Gobierno de Canarias, National Science Foundation (US), European Research Council, Comisión Nacional de Investigación Científica y Tecnológica (Chile), Max Planck Society, Istituto Nazionale di Astrofisica, Ruiz-Lara, T., Gallart, C., Monelli, M., Nidever, D., Dorta, A., Choi, Y., Olsen, K., Besla, G., Bernard, E. J., Cassisi, S., Massana, Pol, Noël, N. E. D., Pérez, I., Rusakov, V., Cioni, M. -R. L., Majewski, Steven R., van der Marel, R. P., Martínez-Delgado, D., Monachesi, A., Monteagudo, L., Muñoz, R. R., Stringfellow, G. S., Surot, F., Vivas, A. K., Walker, A. R., and Zaritsky, D.

Abstract

The Large Magellanic Cloud (LMC) is the closest and most studied example of an irregular galaxy. Among its principal defining morphological features, its off-centred bar and single spiral arm stand out, defining a whole family of galaxies known as the Magellanic spirals (Sm). These structures are thought to be triggered by tidal interactions and possibly maintained via gas accretion. However, it is still unknown whether they are long-lived stable structures. In this work, by combining photometry that reaches down to the oldest main sequence turn-off in the colour-magnitude diagrams (CMD, up to a distance of ∼4.4 kpc from the LMC centre) from the SMASH survey and CMD fitting techniques, we find compelling evidence supporting the long-term stability of the LMC spiral arm, dating the origin of this structure to more than 2 Gyr ago. The evidence suggests that the close encounter between the LMC and the Small Magellanic Cloud (SMC) that produced the gaseous Magellanic Stream and its Leading Arm also triggered the formation of the LMC's spiral arm. Given the mass difference between the Clouds and the notable consequences of this interaction, we can speculate that this should have been one of their closest encounters. These results set important constraints on the timing of LMC-SMC collisions, as well as on the physics behind star formation induced by tidal encounters. © ESO 2020.

Published
2020

8. The Large Magellanic Cloud stellar content with SMASH

Author

Ruiz-Lara, T., primary, Gallart, C., additional, Monelli, M., additional, Nidever, D., additional, Dorta, A., additional, Choi, Y., additional, Olsen, K., additional, Besla, G., additional, Bernard, E. J., additional, Cassisi, S., additional, Massana, P., additional, Noël, N. E. D., additional, Pérez, I., additional, Rusakov, V., additional, Cioni, M.-R. L., additional, Majewski, S. R., additional, van der Marel, R. P., additional, Martínez-Delgado, D., additional, Monachesi, A., additional, Monteagudo, L., additional, Muñoz, R. R., additional, Stringfellow, G. S., additional, Surot, F., additional, Vivas, A. K., additional, Walker, A. R., additional, and Zaritsky, D., additional

Published
2020
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9. The Aarhus red giants challenge

Author

Silva Aguirre, V., primary, Christensen-Dalsgaard, J., additional, Cassisi, S., additional, Miller Bertolami, M., additional, Serenelli, A., additional, Stello, D., additional, Weiss, A., additional, Angelou, G., additional, Jiang, C., additional, Lebreton, Y., additional, Spada, F., additional, Bellinger, E. P., additional, Deheuvels, S., additional, Ouazzani, R. M., additional, Pietrinferni, A., additional, Mosumgaard, J. R., additional, Townsend, R. H. D., additional, Battich, T., additional, Bossini, D., additional, Constantino, T., additional, Eggenberger, P., additional, Hekker, S., additional, Mazumdar, A., additional, Miglio, A., additional, Nielsen, K. B., additional, and Salaris, M., additional

Published
2020
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10. The Aarhus red giants challenge

Author

Christensen-Dalsgaard, J., primary, Silva Aguirre, V., additional, Cassisi, S., additional, Miller Bertolami, M., additional, Serenelli, A., additional, Stello, D., additional, Weiss, A., additional, Angelou, G., additional, Jiang, C., additional, Lebreton, Y., additional, Spada, F., additional, Bellinger, E. P., additional, Deheuvels, S., additional, Ouazzani, R. M., additional, Pietrinferni, A., additional, Mosumgaard, J. R., additional, Townsend, R. H. D., additional, Battich, T., additional, Bossini, D., additional, Constantino, T., additional, Eggenberger, P., additional, Hekker, S., additional, Mazumdar, A., additional, Miglio, A., additional, Nielsen, K. B., additional, and Salaris, M., additional

Published
2020
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11. Precise distances from OGLE-IV member RR Lyrae stars in six bulge globular clusters.

Author

Oliveira, R. A. P., Ortolani, S., Barbuy, B., Kerber, L. O., Maia, F. F. S., Bica, E., Cassisi, S., Souza, S. O., and Pérez-Villegas, A.

Subjects
RR Lyrae stars, GLOBULAR clusters, VARIABLE stars, STAR clusters, STELLAR parallax, DATA release
Abstract

Context. RR Lyrae stars are useful standard candles allowing one to derive accurate distances for old star clusters. Based on the recent catalogues from OGLE-IV and Gaia Early Data Release 3 (EDR3), the distances can be improved for a few bulge globular clusters. Aims. The aim of this work is to derive an accurate distance for the following six moderately metal-poor, relatively high-reddening bulge globular clusters: NGC 6266, NGC 6441, NGC 6626, NGC 6638, NGC 6642, and NGC 6717. Methods. We combined newly available OGLE-IV catalogues of variable stars containing mean I magnitudes, with Clement's previous catalogues containing mean V magnitudes, and with precise proper motions from Gaia EDR3. Astrometric membership probabilities were computed for each RR Lyrae, in order to select those compatible with the cluster proper motions. Applying luminosity–metallicity relations derived from BaSTI α-enhanced models (He-enhanced for NGC 6441 and canonical He for the other clusters), we updated the distances with relatively low uncertainties. Results. Distances were derived with the I and V bands, with a 5 − 8% precision. We obtained 6.6 kpc, 13.1 kpc, 5.6 kpc, 9.6 kpc, 8.2 kpc, and 7.3 kpc for NGC 6266, NGC 6441, NGC 6626, NGC 6638, NGC 6642, and NGC 6717, respectively. The results are in excellent agreement with the literature for all sample clusters, considering the uncertainties. Conclusions. The present method of distance derivation, based on recent data of member RR Lyrae stars, updated BaSTI models, and robust statistical methods, proved to be consistent. A larger sample of clusters will be investigated in a future work. [ABSTRACT FROM AUTHOR]

Published
2022
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15. Mapping the stellar age of the Milky Way bulge with the VVV

Author

Surot, F., primary, Valenti, E., additional, Hidalgo, S. L., additional, Zoccali, M., additional, Sökmen, E., additional, Rejkuba, M., additional, Minniti, D., additional, Gonzalez, O. A., additional, Cassisi, S., additional, Renzini, A., additional, and Weiss, A., additional

Published
2019
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16. Star formation history and metallicity in the Galactic inner bulge revealed by the red giant branch bump

Author

Nogueras-Lara, F., primary, Schödel, R., additional, Dong, H., additional, Najarro, F., additional, Gallego-Calvente, A. T., additional, Hilker, M., additional, Gallego-Cano, E., additional, Nishiyama, S., additional, Neumayer, N., additional, Feldmeier-Krause, A., additional, Girard, J. H. V., additional, Cassisi, S., additional, and Pietrinferni, A., additional

Published
2018
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17. The Aarhus red giants challenge I. Stellar structures in the red giant branch phase.

Author

Aguirre, V. Silva, Christensen-Dalsgaard, J., Cassisi, S., Bertolami, M. Miller, Serenelli, A., Stello, D., Weiss, A., Angelou, G., Jiang, C., Lebreton, Y., Spada, F., Bellinger, E. P., Deheuvels, S., Ouazzani, R. M., Pietrinferni, A., Mosumgaard, J. R., Townsend, R. H. D., Battich, T., Bossini, D., and Constantino, T.

Subjects
STELLAR structure, RED giants, STELLAR oscillations, STELLAR evolution, STELLAR mass, PHYSICAL constants
Abstract

Context. With the advent of space-based asteroseismology, determining accurate properties of red-giant stars using their observed oscillations has become the focus of many investigations due to their implications in a variety of fields in astrophysics. Stellar models are fundamental in predicting quantities such as stellar age, and their reliability critically depends on the numerical implementation of the physics at play in this evolutionary phase. Aims. We introduce the Aarhus red giants challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes that aim to establish the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. We present the first set of results focusing on stellar evolution tracks and structures in the red-giant-branch (RGB) phase. Methods. Using nine state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at a fixed radius along the red-giant branch for masses of 1:0 M, 1:5 M, 2:0 M®, and 2:5 M®, and compared the predicted stellar properties. Results. Once models have been calibrated on the main sequence, we find a residual spread in the predicted effective temperatures across all codes of ~20K at solar radius and ~30-40K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2-5% (increasing with stellar mass), which we attribute to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ~0:1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and find a spread of 0.1 dex or more for all considered masses. Conclusions. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and they call for further investigation on these matters in the context of using properties of red giants as benchmarks for astrophysical studies. [ABSTRACT FROM AUTHOR]

Published
2020
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19. Asiago eclipsing binaries program

Author

Tomasella, L., Munari, U., Cassisi, S., Siviero, A., Dallaporta, S., Sordo, R., and Zwitter, T.

Subjects
Space and Planetary Science, Astrophysics (astro-ph), FOS: Physical sciences, Astronomy and Astrophysics, Astrophysics
Abstract

The orbit and physical parameters of the previously unsolved SB2 EB V570 Per are derived using high resolution Asiago Echelle spectroscopy and B, V photo-electric photometry. The metallicity from chi^2 analysis is [M/H]=+0.02 +/- 0.03, and reddening from interstellar NaI and KI absorption lines is E(B-V) =0.023 +/- 0.007. The two components have masses of 1.449 +/- 0.006 and 1.350 +/- 0.006 Msun and spectral types F3 and F5, respectively. They are both still within the Main Sequence band (T_1 =6842 +/- 25 K, T_2 =6562 +/- 25 K from chi^2 analysis, R_1 =1.523 +/- 0.030, R_2 =1.388 +/- 0.019 Rsun) and are dynamically relaxed to co-rotation with the orbital motion (Vrot sin i_{1,2} =40 and 36 (+/-1) km/sec). The distance to V570 Per obtained from the orbital solution is 123 +/- 2 pc, in excellent agreement with the revised Hipparcos distance of 123 +/- 11 pc. The observed properties of V570 Per components are compared to BaSTI models computed on purpose for exactly the observed masses and varied chemical compositions. This system is interesting since both components have their masses in the range where the efficiency of convective core overshooting has to decrease with the total mass as a consequence of the decreasing size of the convective core during the central H-burning stage. Our numerical simulations show that, a small but not null overshooting is required, with efficiencies lambda_{OV} =0.14 and 0.11 for the 1.449 and 1.350 Msun components, respectively. This confirms the finding of Paper II on the similar system V505 Per. At the approx 0.8 Gyr age of the system, the element diffusion has reduced the surface metallicity of the models from the initial [M/H]=+0.17 to [M/H]=+0.02, in perfect agreement with the spectroscopically derived [M/H]=+0.02 +/- 0.03 value., Comment: accepted by A&A. This revised upload to astro-ph correct a formatting error generated by uncorrect A&A style file

Published
2008

21. The Carina Project

Author

Fabrizio, M., primary, Nonino, M., additional, Bono, G., additional, Primas, F., additional, Thévenin, F., additional, Stetson, P. B., additional, Cassisi, S., additional, Buonanno, R., additional, Coppola, G., additional, da Silva, R. O., additional, Dall’Ora, M., additional, Ferraro, I., additional, Genovali, K., additional, Gilmozzi, R., additional, Iannicola, G., additional, Marconi, M., additional, Monelli, M., additional, Romaniello, M., additional, and Walker, A. R., additional

Published
2015
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22. The Na-O anticorrelation in horizontal branch stars

Author

Gratton, R. G., primary, Lucatello, S., additional, Sollima, A., additional, Carretta, E., additional, Bragaglia, A., additional, Momany, Y., additional, D’Orazi, V., additional, Salaris, M., additional, Cassisi, S., additional, and Stetson, P. B., additional

Published
2014
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28. The Na-O anticorrelation in horizontal branch stars

Author

Gratton, R. G., primary, Lucatello, S., additional, Sollima, A., additional, Carretta, E., additional, Bragaglia, A., additional, Momany, Y., additional, D’Orazi, V., additional, Cassisi, S., additional, Pietrinferni, A., additional, and Salaris, M., additional

Published
2012
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32. The double sub-giant branch of NGC 6656 (M 22): a chemical characterization

Author

Marino, A. F., primary, Milone, A. P., additional, Sneden, C., additional, Bergemann, M., additional, Kraft, R. P., additional, Wallerstein, G., additional, Cassisi, S., additional, Aparicio, A., additional, Asplund, M., additional, Bedin, R. L., additional, Hilker, M., additional, Lind, K., additional, Momany, Y., additional, Piotto, G., additional, Roederer, I. U., additional, Stetson, P. B., additional, and Zoccali, M., additional

Published
2012
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40. Li-rich red giant branch stars in the Galactic bulge

Author

Gonzalez, O. A., primary, Zoccali, M., additional, Monaco, L., additional, Hill, V., additional, Cassisi, S., additional, Minniti, D., additional, Renzini, A., additional, Barbuy, B., additional, Ortolani, S., additional, and Gomez, A., additional

Published
2009
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41. On the metallicity gradient of the Galactic disk

Author

Pedicelli, S., primary, Bono, G., additional, Lemasle, B., additional, François, P., additional, Groenewegen, M., additional, Lub, J., additional, Pel, J. W., additional, Laney, D., additional, Piersimoni, A., additional, Romaniello, M., additional, Buonanno, R., additional, Caputo, F., additional, Cassisi, S., additional, Castelli, F., additional, Leurini, S., additional, Pietrinferni, A., additional, Primas, F., additional, and Pritchard, J., additional

Published
2009
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44. Ground-based CCD astrometry with wide-field imagers

Author

Yadav, R. K. S., primary, Bedin, L. R., additional, Piotto, G., additional, Anderson, J., additional, Cassisi, S., additional, Villanova, S., additional, Platais, I., additional, Pasquini, L., additional, Momany, Y., additional, and Sagar, R., additional

Published
2008
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45. The peculiar horizontal branch morphology of the Galactic globular clusters NGC 6388 and NGC 6441: new insights from UV observations

Author

Busso, G., primary, Cassisi, S., additional, Piotto, G., additional, Castellani, M., additional, Romaniello, M., additional, Catelan, M., additional, Djorgovski, S. G., additional, Recio Blanco, A., additional, Renzini, A., additional, Rich, M. R., additional, Sweigart, A. V., additional, and Zoccali, M., additional

Published
2007
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47. Na-O anticorrelation and horizontal branches

Author

Carretta, E., primary, Bragaglia, A., additional, Gratton, R. G., additional, Momany, Y., additional, Recio-Blanco, A., additional, Cassisi, S., additional, François, P., additional, James, G., additional, Lucatello, S., additional, and Moehler, S., additional

Published
2007
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48. Na-O anticorrelation and horizontal branches

Author

Gratton, R. G., primary, Lucatello, S., additional, Bragaglia, A., additional, Carretta, E., additional, Cassisi, S., additional, Momany, Y., additional, Pancino, E., additional, Valenti, E., additional, Caloi, V., additional, Claudi, R., additional, D'Antona, F., additional, Desidera, S., additional, François, P., additional, James, G., additional, Moehler, S., additional, Ortolani, S., additional, Pasquini, L., additional, Piotto, G., additional, and Recio-Blanco, A., additional

Published
2007
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49. The Na-O anticorrelation in horizontal branch stars.

Author

Gratton, R. G., Lucatello, S., Sollima, A., Carretta, E., Bragaglia, A., Momany, Y., D'Orazi, V., Salaris, M., Cassisi, S., and Stetson, P. B.

Subjects
GLOBULAR clusters, STELLAR populations, STELLAR evolution, MASS loss (Astrophysics), SURFACE temperature
Abstract

We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae variables, and 17 blue horizontal branch (BHB) stars in the low-concentration, moderately metal-rich globular cluster NGC 6723 ([Fe/H] = -1.22 ± 0.08 from our present sample). The spectra were optimized to derive O and Na abundances. In addition, we obtained abundances for other elements, including N, Fe, Mg, Ca, Ni, and Ba. We used these data to discuss the evidence of a connection between the distribution of stars along the horizontal branch (HB) and the multiple populations that are typically present in globular clusters. We found that all RHB and most (13 out of 17) BHB stars are O-rich, Na-poor, and N-poor; these stars probably belong to the first stellar generation in this cluster. Only the four warmest observed stars are (moderately) O-poor, Na-rich, and N-rich, and they probably belong to the second generation. While our sample is not fully representative of the whole HB population in NGC 6723, our data suggest that in this cluster only HB stars warmer than ~9000 K, that is one fourth of the total, belong to the second generation, if at all. Since in many other clusters this fraction is about two thirds, we conclude that the fraction of first/second generation in globular clusters may be strongly variable. In addition, the wide range in colour of chemically homogeneous first-generation HB stars requires a considerable spread in mass loss (>0.10 M⊙). The reason for this spread is yet to be understood. Finally, we found a high Ba abundance, with a statistically significant radial abundance gradient. [ABSTRACT FROM AUTHOR]

Published
2015
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50. Observing multiple stellar populations with VLT/FORS2.

Author

Nardiello, D., Milone, A. P., Piotto, G., Marino, A. F., Bellini, A., and Cassisi, S.

Subjects
STELLAR populations, OPEN clusters of stars, PHOTOMETRY, METAL-poor stars, HELIUM
Abstract

Aims. We present the photometric analysis of the external regions of three Galactic globular clusters: NGC 6121, NGC 6397, and NGC 6752. The main goal is the characterization of multiple stellar populations along the main sequence (MS) and the study of the radial trend of different populations hosted by the target clusters. Methods. The data have been collected using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) mounted at the Very Large Telescope (VLT) UT1 of the European Southern Observatory (ESO) in UBVI filters. From these data sets we extracted high-accuracy photometry and constructed color-magnitude diagrams (CMDs). We exploit appropriate combinations of colors and magnitudes, which are powerful tools for identifying multiple stellar populations, such as B versus U - B and V versus cU,B,I = (U - B) - (B - I) CMDs. Results. We confirm previous findings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of ωCentauri, this is the first detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121, we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction of first and second generation stars measured in the central regions. There is no evidence for significant radial trend. The MS of NGC 6397 is consistent with a simple stellar population. We propose that the lack of multiple sequences is due both to observational errors and to the limited sensitivity of U, B,V, I photometry to multiple stellar populations in metal-poor GCs. Finally, we compute the helium abundance for the stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (ΔY ~ 0.02) difference between stars in the two sequences. [ABSTRACT FROM AUTHOR]

Published
2015
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