Cluster Horizontal Branch Research Articles

Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13

High-resolution optical spectra of 13 blue horizontal-branch stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (Teff > 12,000 K), helium is depleted by factors of 10-100 below solar, while iron is enhanced to 3 times the solar abundance, 2 orders of magnitude above the canonical metallicity of [Fe/H]-1.5 dex for this globular cluster. Nitrogen, phosphorus, and chromium exhibit even more pronounced enhancements, and other metals are also mildly overabundant, with the exception of magnesium, which stays very near the expected cluster metallicity. These photospheric anomalies are most likely due to diffusion—the gravitational settling of helium and the radiative levitation of the other elements—in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the cluster's horizontal branch and on estimates of its age and distance.

Peculiar Multimodality on the Horizontal Branch of the Globular Cluster NGC 2808

We present distributions of colors of stars along the horizontal branch (HB) of the globular cluster NGC 2808, from Hubble Space Telescope Wide Field Planetary Camera 2 imaging in B, V, and an ultraviolet filter (F218W). This cluster's HB is already known to be strongly bimodal, with approximately equal-sized HB populations widely separated in the color-magnitude diagram. Our images reveal a long blue tail with two gaps, for a total of four nearly distinct HB groups. These gaps are very narrow, corresponding to envelope-mass differences of only ~0.01 M☉. This remarkable multimodality may be a signature of mass-loss processes, subtle composition variations, or dynamical effects; we briefly summarize the possibilities. The existence of narrow gaps between distinct clumps on the HB presents a challenge for models that attempt to explain HB bimodality or other peculiar HB structures.

Infrared Photometry of the Globular Cluster Terzan 6

view Abstract Citations (6) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared Photometry of the Globular Cluster Terzan 6 Fahlman, G. G. ; Douglas, K. A. ; Thompson, I. B. Abstract The highly reddened globular cluster Terzan 6 lies in the direction of the galactic center. Published distance estimates, based on very uncertain values for the visual magnitude of the cluster horizontal branch and the interstellar reddening place the cluster either on the far side of the galactic center at distance of 12.8 kpc or at distance of only 1.5 kpc. If at the near distance, Ter 6 is the closest globular cluster to the Sun. The cluster exhibits a collapsed core and contains a ROSAT x-ray source. We present new infrared photometry obtained with the Las Campanas NICMOS3 Camera. The (K, J-K) color-magnitude diagram clearly reveals the location of the horizontal branch. Through a comparison with IR photometry of stars in the similar metallicity cluster M7 1, we calculate that the reddening to Ter 6 is E(B - V) = 2.04 mag and determine the cluster distance to be 6.80±0.46 kpc. Publication: The Astronomical Journal Pub Date: November 1995 DOI: 10.1086/117678 Bibcode: 1995AJ....110.2189F Keywords: GLOBULAR CLUSTERS: INDIVIDUAL: TERZAN 6 full text sources ADS | data products SIMBAD (2)

Blue horizontal branch stars in NGC 6522 and Baade's Window

view Abstract Citations (28) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Blue Horizontal Branch Stars in NGC 6522 And Baade's Window Terndrup, Donald M. ; Walker, Alistair R. Abstract We present color-magnitude diagrams in BVI (Cousins) of the globular cluster NGC 6522 in Baade's Window. Our photometry, which extends to V approximately equals 19, shows that NGC 6522 has an extended blue horizontal branch (BHB), typical of clusters of moderate metallicity close to the galactic center. We derive (Fe/H) = -1.6 +/- 0.2, E(B-V)zero = 0.52 +/- 0.05 for the cluster from comparison to the giant branches of other globulars and from the level of the cluster's horizontal branch. We identify a region in the color-magnitude diagrams which is populated mainly by BHB stars, and show that most of these are spatially concentrated around the cluster and so are not members of the bulge; the spatial density of bulge BHB stars is 0.4 arcmin-2. If the bulge has the same (1750 A--V) color as the bulge of M31, then about 20% the light at 1750 A is from BHB stars. Publication: The Astronomical Journal Pub Date: May 1994 DOI: 10.1086/116986 Bibcode: 1994AJ....107.1786T Keywords: Abundance; Astronomical Photometry; Blue Stars; Color-Magnitude Diagram; Galactic Bulge; Globular Clusters; Horizontal Branch Stars; Metallicity; Spatial Distribution; Calibrating; Charge Coupled Devices; Data Reduction; High Resolution; Optical Filters; Spatial Resolution; Telescopes; Astronomy; GLOBULAR CLUSTERS: INDIVIDUAL: NGC 6522; STARS: HORIZONTAL-BRANCH full text sources ADS | data products SIMBAD (6)

Third parameter effects in globular clusters

We show that the apparent correlations between the length of the globular cluster horizontal branch and cluster luminosity and density [Fusi Pecci et al. AJ, 105, 1145 (1993)] may be affected by observational bias. This bias results from the fact that one can trace horizontal branches over greater lengths in rich clusters than in poor ones. The existence of a correlation between the length of the horizontal branch and the cluster central density is confirmed at about the 2σ level of confidence. The strength of an apparent correlation between cluster luminosity M v and the length of the horizontal branch is found to depend critically on the choice of sample used to analyze the data

The Oosterhoff Period Effect and the Age of the Galactic Globular Cluster System

AbstractThe Oosterhoff division of globular clusters into two dichotomous mean period groups is a result of the variation with metallicity of the combined effects of (1) a mean increase in period with decreasing metallicity, and (2) the change of globular cluster horizontal branch (HB) morphology from the M3 to the M13 HB type within the instability strip in the metallicity range of [Fe/H] between -1.7 and -1.9. A new representation of the Oosterhoff period effect showing this property is made from the individual cluster data in Figure 1. The relation between period and metallicity for cluster and for field RR Lyraes at the blue fundamental edge of the instability strip in the HR diagram as read from this figure islog Pab = -0.122(±0.02)([Fe/H]) - 0.500(±0.01)using the metallicity scale of Butler.The high slope coefficient is consistent with the extant models of the HB when they are read at the varing temperature of the fundamental blue edge given by equation (3) of the text. Most of the current literature treats only the constant temperature condition, which is manifestly incorrect. It is this temperature effect that reconciles the observations and the models.A new calibration of the absolute magnitudes of RR Lyrae stars as a function of metallicity, combined with new oxygen enhanced isochrones for globular clusters (Bergbusch & VandenBerg 1992) reduces the age of the Galactic globular cluster system to 14.1 ± 0.3 Gyr (internal error). The resulting lower age of the universe which, when combined with a Hubble constant near 50 km s-1 Mpc-1 determined from type I supernovae, shows that the cosmological expansion has been decelerated by an amount consistent with the closure density, permitting Ω ∼ 1 now from the timing test.