[ITAL]HUBBLE SPACE TELESCOPE[/ITAL] Observations of Galactic Globular Cluster Cores. II. NGC 6273 and the Problem of Horizontal-Branch Gaps

Abstract

We present observations of the center of the Galactic globular cluster NGC 6273, obtained with the Hubble Space T elescope Wide Field Planetary Camera 2 as part of the snapshot program GO-7470. A BV color-magnitude diagram (CMD) for D28,000 stars is presented and discussed. The most prominent feature of the CMD, identiÐed for the Ðrst time in this paper, is the extended horizontal-branch blue tail (EBT) with a clear double-peaked distribution and a signiÐcant gap. The EBT of NGC 6273 is compared with the EBTs of seven other globular clusters for which we have a CMD in the same photometric system. From this comparison, we conclude that all the globular clusters in our sample with an EBT show at least one gap along the horizontal branch, which could have similar origins. A comparison with theoretical models suggests that at least some of these gaps may be occurring at a particular value of the stellar mass, common to a number of di †erent clusters. From the CMD of NGC 6273 we obtain a distance modulusWe also estimate an average reddening \ 16.27^0.20. though the CMD is strongly a †ected by di †erential reddening, with the relative reddening spanning a *E(B[V ) D 0.2 mag in the WFPC2 Ðeld. A luminosity function for the evolved stars in NGC 6273 is also presented and compared with the most recent evolutionary models.

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

“…Besides u Cen and NGC 2808, the only other globular cluster reported to have a signiÐcant EHB population fainter than mag is NGC 6273 (Piotto et al 1999). M V \ 4.5 Thus, Ñash-mixed stars probably do not comprise a signiÐ-cant fraction of the hot HB stars in classical EHB globular clusters such as NGC 6752 or M13, in which nearly all the EHB stars have mag.…”

“…Other globular clusters with long blue HB tails, e.g., M13, M80 and NGC 6752, do not contain a sufficient number of the faintest EHB stars mag) to deter-(M V Z 4.5 mine if an EHB gap is present (see, e.g., Fig. 7 of Piotto et al 1999). Thus, NGC 2808 stands out as the globular cluster in which an EHB gap is most easily detected observationally.…”

“…Quite possibly, any EHB gap in u Cen has been blurred by the metallicity distribution in the cluster. Another candidate for an EHB gap is NGC 6273, which has one of the longest blue HB tails of any globular cluster (Piotto et al 1999). Unfortunately, the CMD of NGC 6273 is a †ected by large di †erential reddening [*E(B[V ) D 0.2 mag ; Piotto et al 1999], which would likewise obscure any EHB gap.…”

“…Another candidate for an EHB gap is NGC 6273, which has one of the longest blue HB tails of any globular cluster (Piotto et al 1999). Unfortunately, the CMD of NGC 6273 is a †ected by large di †erential reddening [*E(B[V ) D 0.2 mag ; Piotto et al 1999], which would likewise obscure any EHB gap. Other globular clusters with long blue HB tails, e.g., M13, M80 and NGC 6752, do not contain a sufficient number of the faintest EHB stars mag) to deter-(M V Z 4.5 mine if an EHB gap is present (see, e.g., Fig.…”

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

“…(1999) and Bono etal. (2001) have discussed the red giant branch (RGB) bump, and compared the predicted position and dimension of this feature with the observed ones; have used the star counts on the HB and on the RGB to gather information on the helium content and on the dependence of the helium content on the cluster metallicity; Zoccali & Piotto (2000) have made the most extensive comparison so far available between the model evolutionary times away from the main sequence and the actual star counts on the subgiant branch (SGB) and RGB; Cassisi et al (2001) have compared the observed and theoretical properties of the asymptotic giant branches; Piotto et al (2000) have used the color-magnitude diagrams in this database to study the Galactic globular cluster relative ages, and Piotto et al (1999b) have started to investigate the Galactic globular cluster blue straggler (BS) population, and have shown how the BS color-magnitude diagram and luminosity function can differ in clusters with rather different morphologies.…”