Category Archives: NGC6441

Papers on Omega Cen, NGC6440, NGC6388, NGC6441, M4…

Hello, here is a brief list (not sure if complete…) with the latest preprints regarding Galactic Globular Clusters, which appeared this month on astro-ph:

NGC 6441: another indication of very high helium content in GGCs stars

NGC 6441 :: new paper

    Caloi, V.; D’Antona, F.

NGC 6441: another indication of very high helium content in globular cluster stars

The metal-rich bulge globular cluster NGC 6441 shows a well developed blue horizontal branch (Rich et al.), together with a strong slope upward from the red clump to the blue of the RR Lyrae region. Both features, the former corresponding to the well-known second parameter problem, are not explained by conventional evolutionary models. Helium self-enrichment is proposed as a possible solution to both questions, a mechanism already invoked for the interpretation of the peculiarities in NGC 2808 and M13. We make use of horizontal branch simulations, covering a wide range in main sequence helium abundance, to investigate whether the main features of NGC 6441 horizontal branch population, including the RR Lyrae variables period, can be reproduced. To describe the horizontal branch of NGC 6441, the helium content Y in the red clump must reach at least 0.35; values up to Y~0.37 are necessary to populate the RR Lyr region, reproducing also the observed mean long periods; depending on the dispersion in mass loss assumed in the simulations, values up to Y~0.38–0.40 are necessary to populate the blue HB. The total self–enriched population amounts to ~60% of the whole stellar content. Self-enrichment and multiple star formation episodes in the early evolution of globular clusters appear more and more able to account for many of the chemical and population peculiarities observed in these systems. The very large helium abundances (Y>0.35) required for ~14% of the horizontal branch population pose some problem on the enrichment mechanisms.
Astronomy and Astrophysics, Volume 463, Issue 3, March I 2007, pp.949-955 (A&A Homepage)

ADS Link

Image-Subtraction Photometry of Variable Stars in NGC 6388 and NGC 6441

New paper (“The Astronomical Journal”, Volume 132):

    Corwin, T. Michael; Sumerel, Andrew N.; Pritzl, Barton J.; Smith, Horace A.; Catelan, M.; Sweigart, Allen V.; Stetson, Peter B.

Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441

We have applied Alard’s image-subtraction method (ISIS ver. 2.1) to the observations of the globular clusters NGC 6388 and NGC 6441 previously analyzed using standard photometric techniques (DAOPHOT and ALLFRAME). In this reanalysis of observations obtained at the Cerro Tololo Inter-American Observatory, besides recovering the variables previously detected on the basis of our ground-based images, we have also been able to recover most of the RR Lyrae variables previously detected only in the analysis of Hubble Space Telescope WFPC2 observations of the inner region of NGC 6441. In addition, we report five possible new variables not found in the analysis of the Hubble Space Telescope observations of NGC 6441. This dramatically illustrates the capabilities of image-subtraction techniques applied to ground-based data to recover variables in extremely crowded fields. We have also detected 12 new variables and 6 possible variables in NGC 6388 not found in our previous ground-based studies. The revised mean period for RRab stars in NGC 6388 is 0.676 days, while the mean period of RRab stars in NGC 6441 is unchanged at 0.759 days. These values are among the largest known for any Galactic globular cluster. Additional probable type II Cepheids were identified in NGC 6388, confirming its status as a metal-rich globular cluster rich in Cepheids.

ADS link to the paper