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THE RR LYRAE VARIABLES AND HORIZONTAL BRANCH OF NGC 6656 (M22)<sup>,</sup>
Kunder, Andrea,Stetson, Peter B.,Cassisi, Santi,Layden, Andrew,Bono, Giuseppe,Catelan, Má,rcio,Walker, Alistair R.,Paredes Alvarez, Leonardo,Clem, James L.,Matsunaga, Noriyuki,Salaris, Maurizio American Institute of Physics 2013 The Astronomical journal Vol.146 No.5
<P>The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC 6656 (M22), with observations spanning a range of 22 years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are 〈P〉<SUB>RR0</SUB> = 0.66 ± 0.02 days and 〈P〉<SUB>RR1</SUB> = 0.33 ± 0.01 days, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1-type to all RR-type variables is N<SUB>1</SUB>/N<SUB>RR</SUB> = 0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E( B – – V) = 0.36 ± 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R) = +0.97 ± 0.1 and at least one 'gap' located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.</P>
HESSER J. E.,STETSON P. B.,HARRISM W. E.,BOLTE M.,SMECKER-HANE T. A.,VANDENBERG D. A.,BELL R. A.,BOND H. E.,BERGH S. VAN DEN,MCCLURE R. D.,FAHLMAN G. G.,RICHER H. B. The Korean Astronomical Society 1996 Journal of The Korean Astronomical Society Vol.29 No.suppl1
We review observational evidence bearing on the formation of a prototypical large spiral galaxy, the Milky Way. New ground- and space-based studies of globular star clusters and dwarf spheroidal galaxies provide a wealth of information to constrain theories of galaxy formation. It appears likely that the Milky Way formed by an combination of rapid, dissipative collapse and mergers, but the relative contributions of these two mechanisms remain controversial. New evidence, however, indicates that initial star and star cluster formation occurred simultaneously over a volume that presently extends to twice the distance of the Magellanic Clouds.