Variable Stars in the Unusual, Metal-rich, Globular Cluster NGC 6441

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 D104, with 48 new variables being found in this survey. A signiÐcant number of the variables are RR Lyrae stars (D46), most of which are probable cluster members. As was noted by Layden et al. in 1999, the periods of the fundamental-mode RR Lyrae stars are unusually long compared with Ðeld stars of similar metallicity. The existence of these long-period RRab stars is consistent with Sweigart & CatelanÏs 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 Oosterho † classiÐcation of NGC 6441 because of the unusual nature of its RR Lyrae stars. A number of ab-type RR Lyrae stars 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 Ðrst-overtoneÈmode RR Lyrae stars. We determine the reddening of the cluster to be E(B[V ) \ 0.51^0.02 mag, with substantial di †erential 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 10.4È11.9 kpc, for a range of assumed values of for RR Lyrae stars. The possibility that stars in NGC 6441 SM V T may span a range in [Fe/H] is also discussed.

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). …”

Na-O anticorrelation and horizontal branches

“…In fact, roughly speaking, A(K) = 0.4 E(B − V), hence for a differential reddening rms of 0.05 mag (Layden et al 1999;Pritzl et al 2001), the different K − T eff relations differ from the average one by some 0.02 mag rms; since the slope of the K − T eff relation is 332 K/mag, the errors made by adopting a unique average relation rather than one appropriate for each star is 0.02 × 332 = 7 K. The internal errors in the temperatures of individual stars are actually larger, because the photometric errors in the 2MASS K magnitudes (∼0.03 mag) should also be taken into account: they cause an error of about 10 K in the adopted temperatures. While the two errors should be summed in quadrature, we conservatively simply added them and attribute an internal error of ±17 K to the individual T eff 's.…”

Variable Stars in Metal-rich Globular Clusters. II. NGC 6316

“…In this calculation, we used the integrated V-band apparent magnitude and interstellar reddening values from Harris (1996) along with the distance modulus determined herein to obtain M V = À8.65 mag. The S RR value for NGC 6316 is much smaller than was found for the unusual clusters NGC 6388 and NGC 6441, which have S RR = 1.7 (Pritzl et al 2002) and 7.5 (Pritzl et al 2001), respectively. By comparison, 47 Tuc has S RR = 0.3 (Harris 1996).…”

“…This argument favors membership in the foreground bulge. However, the RRLs in the metal-rich globulars NGC 6388 and NGC 6441 are unusually luminous (Pritzl et al 2002;Layden et al 1999;Pritzl et al 2001), and their '' horizontal '' branches slope upward toward the blue end (Rich et al 1997). Thus, it is possible that V17 is a luminous member of NGC 6316.…”