Hubble Space TelescopeSnapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441

Abstract

We present the results of a Hubble Space Telescope snapshot program to survey the inner region of the metal-rich globular cluster NGC 6441 for its variable stars. A total of 57 variable stars were found, including 38 RR Lyrae stars, six Population II Cepheids, and 12 long-period variables. Twenty-four of the RR Lyrae stars and all of the Population II Cepheids were previously undiscovered in ground-based surveys. Of the RR Lyrae stars observed in this survey, 26 are pulsating in the fundamental mode with a mean period of 0.753 days and 12 are first-overtone-mode pulsators with a mean period of 0.365 days. These values match up very well with those found in ground-based surveys. Combining all the available data for NGC 6441, we find mean periods of 0.759 and 0.375 days for the RRab and RRc stars, respectively. We also find that the RR Lyrae stars in this survey are located in the same regions of a period-amplitude diagram as those found in ground-based surveys. The overall ratio of RRc to total RR Lyrae stars is 0.33. Although NGC 6441 is a metal-rich globular cluster and would, on that ground, be expected either to have few RR Lyrae stars or to be an Oosterhoff type I system, its RR Lyrae stars more closely resemble those in Oosterhoff type II globular clusters. However, even compared with typical Oosterhoff type II systems, the mean period of its RRab stars is unusually long. We also derived I-band period-luminosity relations for the RR Lyrae stars. Of the six Population II Cepheids, five are of W Virginis type and one is a BL Herculis variable star. This makes NGC 6441, along with NGC 6388, the most metal-rich globular cluster known to contain these types of variable stars. Another variable, V118, may also be a Population II Cepheid, given its long period and its separation in magnitude from the RR Lyrae stars. We examine the period-luminosity relation for these Population II Cepheids and compare it with those in other globular clusters and in the Large Magellanic Cloud. We argue that there does not appear to be a change in the period-luminosity relation slope between the BL Herculis and W Virginis stars, but that a change of slope does occur when the RV Tauri stars are added to the period-luminosity relation.

The VMC Survey – XIII. Type II Cepheids in the Large Magellanic Cloud★

“…Nemec, Nemec, & Lutz (1994) derived metal-dependent period-luminosity (P L) relations in various optical photometric bands both in the fundamental and in the first overtone modes but subsequently Kubiak & Udalski (2003) found that all the observed T2CEPs in the OGLE II (Optical Gravitational Lensing Experiment; Udalski et al 1992) sample, with periods in the range ∼0.7 to about 10 days, satisfy the same P L relation. This result was then confirmed by Pritzl et al (2003) and Matsunaga et al (2006) for GGCs, by Groenewegen, Udalski, & Bono (2008) for the Galactic Bulge and again by Soszyński et al (2008) on the basis of OGLE III data.…”

The Catalina Surveys Southern periodic variable star catalogue

“…Nevertheless, like Classical Cepheids, these variables can be useful for measuring distances since they too obey a period-luminosity relationship (e.g. McNamara 1995; Pritzl et al 2003;Soszyński et al 2008). …”

On THE RR LYRAE STARS IN GLOBULARS. IV. Ω CENTAURI OPTICAL UBVRI PHOTOMETRY*

“…These are 16 out of the ∼100 GGCs hosting RRLs . To characterize the role played by the metallicity in shaping their pulsation properties they were divided, according to their metal content (Harris 1996) -including 60RRab, 1RRd and 41RRc belonging to the two metal-rich globulars (Pritzl et al 2001(Pritzl et al , 2002(Pritzl et al , 2003 . From left to right UBVRI light curves of the RRab variable V100.…”