Bimodality and Gaps on Globular Cluster Horizontal Branches. II. The Cases of NGC 6229, NGC 1851, and NGC 2808

Abstract

The outer halo globular cluster NGC 6229 has a peculiar horizontal-branch (HB) morphology, with clear indications of a bimodal HB and a "" gap ÏÏ on the blue HB. In this paper, we present extensive synthetic HB simulations to determine whether peculiar distributions in the underlying physical parameters are needed to explain the observed HB morphology. We Ðnd that a unimodal mass distribution along the HB can satisfactorily account for the observed HB bimodality, provided the mass dispersion is substantially larger than usually inferred for the Galactic globular clusters. In this case, NGC 6229 should have a well-populated, extended blue tail. A truly bimodal distribution in HB masses can also satisfactorily account for the observed HB morphology, although in this case the existence of an extended blue tail is not necessarily implied. The other two well-known bimodal-HB clusters, NGC 1851 and NGC 2808, are brieÑy analyzed. While the HB morphology of NGC 1851 can also be reproduced with a unimodal mass distribution assuming a large mass dispersion, the same is not true of NGC 2808, for which a bimodal, and possibly multimodal, mass distribution seems deÐnitely required.The problem of gaps on the blue HB is also discussed. Applying the standard Hawarden and Newell s2 test, we Ðnd that the NGC 6229 gap is signiÐcant at the 99.7% level. However, in a set of 1000 simulations, blue-HB gaps comparable to the observed one are present in D6%È9% of all cases. We employ a new and simple formalism, based on the binomial distribution, to explain the origin of this discrepancy, and conclude that HawardenÏs method, in general, substantially overestimates the statistical signiÐcance of gaps. Paper I). showed that the horizontal-branch (HB) morphology of this cluster is characterized by two peculiar properties : (1) A distinctly bimodal distribution in B[V colors, consistent with the relatively small number of RR Lyrae variables, compared to the blue HB and red HB populations ; and (2) at least one "" gap ÏÏ on the blue HB, as previously noted by Fullton, & Trammell Carney, (1991). Bimodal-HB clusters are relatively rare, as are clusters with gaps on the blue HB. NGC 6229 is one of only four known casesÈthe others being NGC 2808, NGC 6388, and NGC 6441Èwhere these two anomalies are simultaneously present. As argued by many authors, an understanding of the nature of the detected peculiarities in "" bimodal ÏÏ and "" gap ÏÏ clusters would be of paramount importance for understanding the nature of the second parameter phenomenon (see, e. for producing HB bimodality in the speciÐc cases of NGC 6229, NGC 1851, and NGC 2808 ? (3) Are gaps on the blue HB a natural consequence of stellar evolution, as claimed by some authors ? (4) What is the statistical signiÐcance of gaps ?In this paper, we present extensive synthetic HB simulations for NGC 6229. We begin in the next section by laying out a proposed nomenclature for both deÐning and di †er-entiating "" bimodal ÏÏ and "" gap ÏÏ clusters. In we describe°3, the theoretical proc...

The Evolutionary Status of M3 RR Lyrae Variable Stars: Breakdown of the Canonical Framework?

“…In particular, the evolutionary tracks are the same as those computed by Catelan et al (1998) and assume a main-sequence chemical composition Y MS ¼ 0:23, Z ¼ 0:001; the HB stars have an envelope helium abundance Y HB ¼ 0:2409, due to the extra helium brought to the surface during the first dredge-up. The HB synthesis code ''SINTDELPHI'' is an updated version of Catelan's (1993) code.…”

“…To compute a synthetic HB, the code assumes that the mass distribution on the zero-age HB (ZAHB) is a normal deviate (Rood 1973;Caputo et al 1987;Lee 1990;Catelan et al 1998). This standard assumption is now known to break down in the cases of at least some of the so-called bimodal HB clusters (e.g., Rood et al 1993;Catelan et al 1998Catelan et al , 2002a, as well as in clusters with significant populations of ''extreme HB stars'' (e.g., D'Cruz et al 1996)-but has generally been considered a reasonable approximation for clusters with ''well-behaved'' HBs such as the ones we model in the present paper (e.g., Lee 1990) and even in clusters with long blue tails such as M79 (Dixon et al 1996). Note, in addition, that the adoption of normal deviates, as opposed to strictly normal distributions, along with the fact that the HB synthesis technique adopts a Monte Carlo approach, implies that individual cases drawn from large pools of simulations will often bear little resemblance to an actual Gaussian distribution.…”

Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited

“…In the present study, stellar evolutionary tracks from Catelan et al (1998) and from for Z \ 0.001 and Z \ 0.002, respectively, have been employed. Unless otherwise stated, a main-sequence helium abundance was Y MS \ 0.23 adopted in all cases.…”

“…We also assume, in line with Paper I (see the extensive discussion in°2.1 therein, but also°4 below), that NGC 288 and 362 constitute a "" bona Ðde ÏÏ second-parameter pair, thus adopting the same metallicity for the two clusters. The underlying zero-age HB (ZAHB) mass distribution is approximated by a Gaussian deviate (see Catelan et al 1998 for a detailed discussion). The instability strip edges are the same as adopted in Catelan et al (2001). ""…”

Commission 37: Star Clusters and Associations: (Amas Stellaires et Associations)

“…A multi-mode HB mass distribution, of unknown origin, is required to explain this distribution (e.g. Catelan et al 1998). As a further complication, Behr et al (1999) have used high resolution spectra to reveal enormous anomalies (e.g.…”