Variable Stars in the Unusual, Metal-rich Globular Cluster NGC 6388

Abstract

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6441 using time-series BV photometry. The total number of variables found near NGC 6441 has been increased to ∼ 104, with 48 new variables being found in this survey. A significant number of the variables are RR Lyrae stars (∼ 46), most of which are probable cluster members. As was noted by Layden et al. (1999), the periods of the fundamental mode RR Lyrae are unusually long compared to field stars of similar metallicity. The existence of these long period RRab stars is consistent with Sweigart & Catelan's (1998) prediction that the horizontal branch of NGC 6441 is unusually bright. This result implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal. We discuss the difficulty in determining the Oosterhoff classification of NGC 6441 due to the unusual nature of its RR Lyrae. A number of ab-type RR Lyrae are found to be both brighter and redder than the other probable RRab found along the horizontal branch, which may be a result of blending with stars of redder color. A smaller than usual gap is found between the shortest period fundamental mode and the longest period first-overtone mode RR Lyrae. We determine the reddening of the cluster to be E(B −V) = 0.51 ± 0.02 mag, with substantial differential reddening across the face of the cluster. The mean V magnitude of the RR Lyrae is found to be 17.51 ± 0.02 mag resulting in a distance of 10.4 to 11.9 kpc, for a range of assumed values of M V for RR Lyrae stars. The possibility that stars in NGC 6441 may span a range in [Fe/H] is also discussed.

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

“…Long-period (P0.82-0.85 days) RRLs are quite rare in Galactic globulars. Several of them have also been identified in two peculiar bulge metal-rich globulars-NGC 6388, NGC 6441 (Pritzl et al 2001(Pritzl et al , 2002)-and in the Galactic field (Wallerstein et al 2009). Whether they are truly long-period RRLs or shortperiod Type II Cepheids (TIICs) is still a matter of lively debate (Marconi et al 2011;Soszyński et al 2011).…”

“…Moreover, they seem to belong to two different sub-groups (if we exclude a few long-and short-period outliers): (a) shortperiod-with periods ranging from ∼0.30 to ∼0.36 days and visual amplitudes ranging from a few hundredths of a magnitude to a few tenths; (b) long-period-with periods ranging from ∼0.36 to ∼0.45 days and amplitudes clustering around AV∼0.5 mag. With the only exception of the metalrich clusters NGC 6388 (Pritzl et al 2002), NGC 6441 (Pritzl et al 2001), and V70 in M3 (Jurcsik et al 2012), ω Cen is the only GGC where long-period RRc are found (Catelan 2004b). Theoretical and empirical evidence indicates that the RRc period distribution is affected by metallicity (Dall'Ora et al 2003).…”

An empirical mass-loss law for Population II giants from theSpitzer-IRAC survey of Galactic globular clusters

“…-M80, M13, M3 -This is the popular cluster triplet at intermediate metallicity with different HB morphologies, including red clumps, gaps, and blue tails (see, e.g., Ferraro et al 1997Ferraro et al , 1998Dalessandro et al 2013a (Rich et al 1997;Pritzl et al 2001Pritzl et al , 2002Pritzl et al , 2003Busso et al 2007;Dalessandro et al 2008). …”

Blue Straggler Stars in the Unusual Globular Cluster NGC 63881

“…Beside the red clump, which is a typical feature of metal-rich stellar populations, the HB clearly shows an extended blue tail (BT), first noticed by Rich et al (1997) and by Piotto et al (1997). Among a total of 1763 HB stars counted in the HST sample, five subpopulations can be distinguished (see xx 3.1.1 and 3.1.2 for details): (1) the red HB (RHB) population, consisting of 1418 stars grouped in the red clump; (2) 15 RR Lyrae variables, which we identified by cross-correlating the positions in our catalog with Lanzoni et al (2007c) those published by Pritzl et al (2002); 7 (3) 267 blue HB (BHB) stars; (4) 26 extreme HB (EHB) stars; and (5) 37 blue hook (BHk, to avoid confusion with BH for black hole) stars. Several previous works have shown that the HB morphology in NGC 6388 is complex.…”