The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon

Abstract

The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, the outer Galactic halo contains GCs with redder HBs at a given metallicity than are found inside the Solar circle. Thus, at least a second parameter is required to characterize HB morphology. Here we analyze the median color difference between the HB and the red giant branch (RGB), d(V-I), measured from HST ACS photometry of 60 GCs within ~20 kpc of the Galactic Center. Analysis of this homogeneous data set reveals that, after the influence of metallicity has been removed, the correlation between d(V-I) and age is stronger than that of any other parameter considered. Expanding the sample to include HST photometry of the 6 most distant Galactic GCs lends additional support to the correlation between d(V-I) and age. This result is robust with respect to the adopted metallicity scale and the method of age determination, but must bear the caveat that high quality, detailed abundance information is not available for a significant fraction of the sample. When a subset of GCs with similar metallicities and ages are considered, a correlation between d(V-I) and central luminosity density is exposed. With respect to the existence of GCs with anomalously red HBs at a given metallicity, we conclude that age is the second parameter and central density is most likely the third. Important problems related to HB morphology in GCs, notably multi-modal distributions and faint blue tails, remain to be explained. (Abridged)

Accepted for publication in ApJ; 49 pages, 19 figures

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• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XXII. Relative ages of multiple populations in five Globular Clusters
• Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)
• The temperature distribution of horizontal branch stars: methods and first results
• Models for Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. III. Grids of Isochrones for $-2.5\le$ [Fe/H] $\le -0.5$ and Helium Abundances $Y=0.25$ and $0.29$ at Each Metallicity
• Millisecond Pulsars in Dense Star Clusters: Evolution, Scaling Relations, and the Galactic-Center Gamma-ray Excess
• On the original composition of the gas forming first-generation stars in clusters: insights from HST and JWST
• Core collapse and horizontal-branch morphology in galactic globular clusters
• A Comparative Study Between M30 and M92 : M92 Is A Merger Remnant With A Large Helium Enhancement
• Analysis of integrated-light spectra of Galactic globular clusters
• Milky Way Globular Cluster Metallicity and Low-Mass X-ray Binaries: The Red Giant Influence
• Inferring the Helium abundance of extragalactic Globular Clusters using Integrated Spectra
• Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group
• 2MASS photometry and age estimate of globular clusters in the outer halo of M31
• Study of UV bright stellar populations in the globular cluster NGC 1261 using AstroSat
• Time-Series BVI Photometry for the Globular Cluster NGC 6981 (M72)
• A Three-Dimensional Analytical Model of the Interstellar Extinction within the Nearest Kiloparsec
• The formation and disruption of globular cluster populations in simulations of present-day $L^{\ast }$ galaxies with controlled assembly histories
• Effect of Binary Fraction on Horizontal Branch Morphology under Tidally Enhanced Stellar Wind
• Distances to Local Group Galaxies via Population II, Stellar Distance Indicators I: The Sculptor Dwarf Spheroidal
• The dynamical state of the globular clusters Rup 106 and IC 4499
• A Differential RR Lyrae Line-of-Sight Distance Between M31 and M33
• The "canonical" White Dwarf Cooling Sequence of M5
• Uncertainties in asteroseismic grid-based estimates of the ages of halo stars
• The Populations of Carina. I. Decoding the Color--Magnitude Diagram
• Discovery of an extended Horizontal Branch in the Large Magellanic Cloud globular cluster NGC1835
• CAPOS: The bulge Cluster APOgee Survey VII. First detailed chemical analysis of NGC 6316
• The ACS survey of globular clusters. XIII. Photometric calibration in comparison with Stetson standards
• The GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) II: Characterization of 47 Tuc with Bayesian Statistics
• Searching for exotic object companions in the dense core of NGC 362: a multi-wavelength and multi-epoch photometric analysis
• Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud: III. Horizontal Branch Morphology
• Role of host galaxy in the formation of multiple stellar populations: Analysis of NGC 1786 and NGC 1898
• Synthetic stellar spectra to study multiple populations in globular clusters: an extended grid and the effects on the integrated light
• Observational dynamics of low-mass stellar systems
• The mass-metallicity relation at z>3 down to M_*= 10^4 M_Sun. A local perspective using the metallicity distribution of RR Lyrae stars
• Variable Stars in Sagittarius Globular Clusters I. Arp~2
• Inferring the efficiency of convective-envelope overshooting in Red Giant Branch stars
• Identification and characterization of optical companions to the population of millisecond pulsars in the globular cluster M3
• Modeling Formation of Globular Clusters: Beacons of Galactic Star Formation
• New Way to Date Globular Clusters: Brown Dwarf Cooling Sequences
• WFPC2 UV survey of Galactic Globular Clusters. The Horizontal Branch temperature distribution
• Chemo-dynamical reconstruction of Milky Way globular cluster progenitors: Age-metallicity relations and the universality of multiple stellar populations
• A test of Ca ii H & K photometry for isolating massive globular clusters below the metallicity floor
• Revealing the stellar population of the ultra-obscured Galactic globular cluster Glimpse-C02
• Old stellar systems in UV: resolved and integrated properties
• Constraining the original composition of the gas forming first-generation stars in globular clusters
• Globular Clusters in the Sagittarius stream. Revising members and candidates with Gaia DR2
• Self-similar orbit-averaged Fokker-Planck equation for isotropic spherical dense clusters (iii) application of pre-collapse solution to Galactic globular clusters
• Gravitational Microlensing Rates in Milky Way Globular Clusters