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Titolo:
The Large Magellanic Cloud globular cluster NGC 1866: New data, new models, new analysis
Autore:
Testa, V; Ferraro, FR; Chieffi, A; Straniero, O; Limongi, M; Pecci, FF;
Indirizzi:
Osservatorio Astron Roma, I-00040 Monteporzio, Italy Osservatorio Astron Roma Monteporzio Italy I-00040 40 Monteporzio, Italy European So Observ, D-85748 Garching, Germany European So Observ Garching Germany D-85748 v, D-85748 Garching, Germany CNR, Ist Astrofis Spaziale, I-00100 Rome, Italy CNR Rome Italy I-00100CNR, Ist Astrofis Spaziale, I-00100 Rome, Italy Osserv Astron Collurania, I-64100 Teramo, Italy Osserv Astron Collurania Teramo Italy I-64100 nia, I-64100 Teramo, Italy Osservatorio Astron Roma, I-000040 Monteporzio, Italy Osservatorio Astron Roma Monteporzio Italy I-000040 0 Monteporzio, Italy Staz Astron Cagliari, I-09012 Capoterra, Italy Staz Astron Cagliari Capoterra Italy I-09012 i, I-09012 Capoterra, Italy
Titolo Testata:
ASTRONOMICAL JOURNAL
fascicolo: 6, volume: 118, anno: 1999,
pagine: 2839 - 2864
SICI:
0004-6256(199912)118:6<2839:TLMCGC>2.0.ZU;2-R
Fonte:
ISI
Lingua:
ENG
Soggetto:
INTERMEDIATE-MASS STARS; ASYMPTOTIC GIANT BRANCH; NGC-1866; LMC; PHOTOMETRY; EVOLUTION; METALLICITY; ISOCHRONES; POPULATION; OVERSHOOT;
Keywords:
color-magnitude diagrams; galaxies : star clusters; Magellanic Clouds;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
40
Recensione:
Indirizzi per estratti:
Indirizzo: Testa, V Osservatorio Astron Roma, Via Frascati 33, I-00040 Monteporzio, Italy Osservatorio Astron Roma Via Frascati 33 Monteporzio Italy I-00040
Citazione:
V. Testa et al., "The Large Magellanic Cloud globular cluster NGC 1866: New data, new models, new analysis", ASTRONOM J, 118(6), 1999, pp. 2839-2864

Abstract

We present a new deep (down to V similar to 24) photometry of a wide region (similar to 6' x 6') around the Large Magellanic Cloud globular cluster NGC 1866. Our sample is much larger (by more than a factor of 3) than any previous photometry and with a main sequence (MS) that may be considered complete, down to at least 3 mag below the brightest MS star; such an occurrence allows a meaningful and robust comparison with various theoretical scenarios produced by means of models computed with the evolutionary code FRANEC. Both age and present-day mass function slope, a, are derived by a fit to the available MS and by the use of the parameter Delta sigma, which is simply the difference, in a, between the observed and the predicted integrated MS luminosity functions. Our main conclusions are as follows: (1) The adoption of standard models (i.e., computed by adopting the Schwarzschild criterion to fix the border of the convective core) allows a fair fit to the MS for an age of the order of 100-140 Myr and a present-day mass function havinga slope alpha between 2.3 and 1.9, the exact values depending on the adopted distance modulus, It is moreover possible to reproduce the average He clump luminosity while the total number of stars predicted in the He clump istwice the observed value; this means that we reobtain and confirm the first finding of Becker & Mathews, according to whom the simple adoption of a "classical" scenario leads to a neat discrepancy concerning the prediction of the number of stars in the He clump. (2) The adoption of models computed by increasing the size of the convective core by a certain amount-i.e., 0.25H(p)-leads to a fair fit to the MS only for a visual distance modulus (m -M)(v) similar or equal to 18.6, an age t similar or equal to 200 Myr, and a mass function slope alpha similar or equal to 2.2. In this case, the total number of He clump stars is well reproduced, although the luminosity function of the He clump itself is predicted to be systematically less luminousthan observed. The previous conclusions are based on the assumption that there is no appreciable population of binaries in NGC 1866. Though there arenot yet sufficient data on the frequency of binary systems in these clusters, we analyze how the previous scenarios would change if a consistent (similar or equal to 30%) population of binary systems were present in the cluster. This choice is based on the fact that a fraction of binaries of the order of 30% has already been found in NGC 1818, a cluster similar to NGC 1866. The inclusion of a 30% binary population leads to the following additional conclusions: (3) The adoption of the standard models now leads to a goodfit to the entire luminosity function-i.e., MS, turnoff, and He clump stars-for a visual distance modulus (m - M)(v) = 18.8, an age t similar or equal to 100 Myr, and a mass function slope alpha similar or equal to 2.4, thuslargely removing the "classical" discrepancy between observed and predicted number of stars in the He-burning clump. The quoted visual distance modulus constrains the unreddened distance modulus (m - M)(0), within 18.50 and 18.62, depending on the reddening (whose most common values available in the literature range from 0.05 to 0.10). (4) At variance with the last point,the fit obtained by using models computed with an enlarged convective coregets worse when a binary component is taken into account. This is because of the fact that the presence of binary systems increases the existing discrepancy between the observed and predicted clump luminosity, since the He clump is predicted to be even less luminous than in absence of binaries. As a consequence of this analysis, we think that the next step toward a proper understanding of NGC 1866 and similar clusters, must include the accurate determination of the frequency of binary systems, which we hope will beperformed with the incoming Cycle 8 Hubble Space Telescope observations ofNGC 1866.

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Documento generato il 04/12/20 alle ore 13:09:01