Bibcode
Piotto, G.; Bellini, A.; Sarajedini, A.; Pietrinferni, A.; Nardiello, D.; Bedin, L. R.; Anderson, J.; Cassisi, S.; Renzini, A.; King, I. R.; Milone, A. P.
Referencia bibliográfica
The Astrophysical Journal, Volume 765, Issue 1, article id. 32, 27 pp. (2013).
Fecha de publicación:
3
2013
Revista
Número de citas
69
Número de citas referidas
64
Descripción
NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters
that share many properties, including the presence of an extended
horizontal branch (HB), quite unexpected because of their high metal
content. In this paper we use Hubble Space Telescope's WFPC2, ACS, and
WFC3 images and present a broad multicolor study of their stellar
content, covering all main evolutionary branches. The color-magnitude
diagrams (CMDs) give compelling evidence that both clusters host at
least two stellar populations, which manifest themselves in different
ways. NGC 6388 has a broadened main sequence (MS), a split sub-giant
branch (SGB), and a split red giant branch (RGB) that becomes evident
above the HB in our data set; its red HB is also split into two
branches. NGC 6441 has a split MS, but only an indication of two SGB
populations, while the RGB clearly splits in two from the SGB level
upward, and no red HB structure. The multicolor analysis of the CMDs
confirms that the He difference between the two main stellar populations
in the two clusters must be similar. This is observationally supported
by the HB morphology, but also confirmed by the color distribution of
the stars in the MS optical band CMDs. However, a MS split becomes
evident in NGC 6441 using UV colors, but not in NGC 6388, indicating
that the chemical patterns of the different populations are different in
the two clusters, with C, N, and O abundance differences likely playing
a major role. We also analyze the radial distribution of the two
populations.
Based on proprietary and archival observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by AURA, Inc., under NASA contract NAS 5-26555.
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