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Million Star Cluster NGC 2808

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NGC 2808 (also ESO 91-SC1 and GCl 13) is a magnitude +10.4 globular cluster located 31300 light-years away in the constellation Centaurus.

The globular was discovered by Scottish astronomer James Dunlop using a 22.86 cm (9 inch) f/12 speculum Newtonian reflector in Parramatta, New South Wales, on the 7th May 1826.

Right Ascension 09h 12m 03.10s, Declination -64 51' 48.6"

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Title: Variable stars in the Globular Cluster NGC 2808
Authors: Andrea Kunder, Peter B. Stetson, Marcio Catelan, Alistair R. Walker, Pia Amigo

The first calibrated broadband BVI time-series photometry is presented for the variable stars in NGC 2808, with observations spanning a range of twenty-eight years. We have also redetermined the variability types and periods for the variable stars identified previously by Corwin et al, revising the number of probable fundamental-mode RR Lyrae variables (RR0) to 11 and the number of first-overtone variables (RR1) to five. Our observations were insufficient to discern the nature of the previously identified RR1 star, V24, and the tentatively identified RR1 star, V13. These two variables are ~0.8 mag brighter than the RR Lyrae variables, appear to have somewhat erratic period and/or luminosity changes, and lie inside the RR Lyrae instability strip. Curiously, all but one of the RR Lyrae stars studied in this relatively metal-rich cluster exhibit the Blazhko phenomenon, an effect thought to occur with higher frequency in metal-poor environments. The mean periods of the RR0 and RR1 variables are <P>_RR0=0.56 0.01 d and <P>_RR1=0.30 0.02 d, respectively, supporting an Oosterhoff I classification of the cluster. On the other hand, the number ratio of RR1- to RR0-type variables is high, though not unprecedented, for an Oosterhoff I cluster. The RR Lyrae variables have no period shifts at a given amplitude as compared to the M3 variables, making it unlikely that these variables are He-enhanced. Using the recent recalibration of the RR Lyrae luminosity scale by Catelan & Cortes, a mean distance modulus of (m-M)_V= 15.57 0.13 mag for NGC 2808 is obtained, in good agreement with that determined here from its type II Cepheid and SX Phoenicis population. Our data have also allowed the discovery of two new candidate SX Phoenicis stars and an eclipsing binary in the blue straggler region of the NGC 2808 colour-magnitude diagram.

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Title: Multiple Sequences of M-dwarfs in NGC 2808 and Omega Centauri
Authors: A. P. Milone

The infrared channel of the Wide-Field Camera 3 on the Hubble Space Telescope revealed multiple main sequences of very low-mass stars in the globular clusters NGC 2808 and Omega Centauri. In this paper I summarize the observational facts and provide a possible interpretation.

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Title: The infrared eye of the Wide-Field Camera 3 on the Hubble Space Telescope reveals multiple main sequences of very low-mass stars in NGC 2808
Authors: A. P. Milone, A. F. Marino, S. Cassisi, G. Piotto, L. R. Bedin, J. Anderson, F. Allard, A. Aparicio, A. Bellini, R. Buonanno, M. Monelli, A. Pietrinferni

We use images taken with the infrared channel of the Wide Field Camera 3 on the Hubble Space Telescope (HST) to study the multiple main sequences (MSs) of NGC 2808. Below the turn off, the red, the middle, and the blue MS, previously detected from visual-band photometry, are visible over an interval of about 3.5 F160W magnitudes. The three MSs merge together at the level of the MS bend. At fainter magnitudes, the MS again splits into two components containing ~65% and ~35% of stars, with the most-populated MS being the bluest one. Theoretical isochrones suggest that the latter is connected to the red MS discovered in the optical colour-magnitude diagram (CMD), and hence corresponds to the first stellar generation, having primordial helium and enhanced carbon and oxygen abundances. The less-populated MS in the faint part of the near-IR CMD is helium-rich and poor in carbon and oxygen, and it can be associated with the middle and the blue MS of the optical CMD. The finding that the photometric signature of abundance anticorrelation are also present in fully convective MS stars reinforces the inference that they have a primordial origin.

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