THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. IX. HORIZONTAL BRANCH MORPHOLOGY AND THE SECOND PARAMETER PHENOMENON

By Aaron Dotter, A. Sarajedini, Jay Anderson, A. Aparicio, L. Bedin, B. Chaboyer, S. Majewski, A. Mar'in-Franch, A. Milone, N. Paust and 4 more. 2009

321 citations recorded by the source corpus.

The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon, first described in the 1960s, acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, astronomers noticed that 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 was required to characterize HB morphology. While the term “second parameter” has since come to be used in a broader context, its identity with respect to the original problem has not been conclusively determined. Here we analyze the median color difference between the HB and the red giant branch, hereafter denoted as Δ(V − I), measured from Hubble Space Telescope (HST) Advanced Camera for Surveys (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 from the data, the correlation between Δ(V − I) and age is stronger than that of any other parameter considered. Expanding the sample to include HST ACS and Wide Field Planetary Camera 2 photometry of the six most distant Galactic GCs lends additional support to the correlation between Δ(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. Furthermore, when a subset of GCs with similar metallicities and ages is considered, a correlation between Δ(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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/76e237e2cce7e11aed7d3154d9a5319595c87549

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. Aaron Dotter
2. Jay Anderson
3. G. Piotto
4. A. Milone
5. R. Gratton
6. E. Carretta
7. A. Bragaglia
8. L. Bedin

In Instrumentation

1. Jay Anderson
2. E. Carretta
3. R. Gratton
4. Aaron Dotter
5. A. Bragaglia
6. F. D’Antona
7. G. Piotto
8. L. Bedin

Citations in the corpus

• MODELING THE METALLICITY DISTRIBUTION OF GLOBULAR CLUSTERS (2010, background): “Unfortunately, even the most recent measurements of relative cluster ages in the Galaxy (De Angeli et al. 2005; Marı́n-Franch et al. 2009; Dotter et al. 2010) cannot detect age differences smaller than 9%, or about 1 Gyr, and therefore can neither support nor falsify the reionization scenario.”
• GLOBULAR CLUSTERS IN THE OUTER GALACTIC HALO: NEW HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS IMAGING OF SIX GLOBULAR CLUSTERS AND THE GALACTIC GLOBULAR CLUSTER AGE–METALLICITY RELATION (2011, methodology): “Palomar 15 and Pyxis are sparsely populated clusters; the red giant branch (RGB) of Pyxis loses coherence for F606W .”
• A SEARCH FOR STARS OF VERY LOW METAL ABUNDANCE. VI. DETAILED ABUNDANCES OF 313 METAL-POOR STARS (2014, background): “Stars with ages 12.0 ± 1.5 Gyr were formed at redshifts z > 2, so this range includes the ages of most metal-poor Milky Way globular clusters (e.g., Dotter et al. 2010), the ages of individual halo stars as computed from radioactive decay of heavy elements (e.g., Roederer et al. 2009), and the…”
• MODULES FOR EXPERIMENTS IN STELLAR ASTROPHYSICS (MESA) (2010, methodology): “…al. 2010), the color–magnitude diagrams of resolved stellar and sub-stellar populations in the Milky Way and nearby galaxies (e.g., VandenBerg 2000; Dotter et al. 2010), the integrated light of distant galaxies and star clusters via population synthesis techniques (e.g., Worthey 1994; Coelho et…”
• The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, methodology): “Forbes & Bridges (2010) Dotter et al. (2010, 2011) VandenBerg et al. (2013) Mean Name MV log M τ [Fe/H] τ [Fe/H] τ [Fe/H] τ [Fe/H] [mag] [M ] [Gyr] [Gyr] [Gyr] [Gyr] NGC104 −9.42 6.00 13.06 ± 0.90 −0.78 12.75 ± 0.50 −0.70 11.75 ± 1.03 −0.76 12.52 ± 0.49 −0.75 NGC288 −6.75 4.93 10.62 ± 0.51 −1.14…”
• Evidence for an Intermediate-mass Milky Way from Gaia DR2 Halo Globular Cluster Motions (2018, background): “We assume distance errors ∆D = 0.023D, which is equivalent to errors on the distance moduli of 0.05 mag, which is typical for GC distance errors (Dotter et al. 2010).”
• Constraining the mass and radius of neutron stars in globular clusters (2017, result): “We note that it is also in agreement with other estimates by, e.g., Dotter et al. (2010) (8.8 kpc).”
• THE EQUATION OF STATE FROM OBSERVED MASSES AND RADII OF NEUTRON STARS (2010, result): “A more recent determination(Dotter et al. 2010) gives an apparent distance of 10.2 kpc, which when corrected for extinction gives 8.1 kpc, consistent with our value described above.”
• The second and third parameters of the horizontal branch in globular clusters (2010, methodology): “It is then possible that M 3 is younger then M 13, a result found in some careful determinations (see Rosenberg et al. 1999, and De Angeli et al. 2005), but not in others (Marı́n-Franch et al. 2009; Dotter et al. 2010).”
• The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – IX. The Atlas of multiple stellar populations (2016, background): “The Dotter et al. (2010) sample includes 50 of the GCs”
• A catalogue of masses, structural parameters, and velocity dispersion profiles of 112 Milky Way globular clusters (2018, background): “…better agreement with other distance methods that generally find a cluster distance around d = 4.5 kpc (e.g. Gratton et al. 2003; Bono et al. 2008; Dotter et al. 2010; Woodley et al. 2012) than the kinematic distances derived by Baumgardt (2017) (d = 3.95±0.05 kpc) or Watkins et al. (2015b) (d =…”
• The ACS survey of Galactic globular clusters. XII. Photometric binaries along the main sequence (2011, methodology): “…of binaries and the HB shape we used three different parameters: 1) the median color difference between the HB and the RGB [∆(V − I)], measured by Dotter et al. (2010) for 60 GCs using the same CMDs of this paper; 2) The HB morphology index from Mackey & van den Bergh (2005); 3) the effective…”
• THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED METHOD ALONG WITH COLOR–MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES (2013, methodology): “On the other hand, the case shown in the right-hand panel bears considerable similarity to the fits of isochrones to the NGC 3201 and M 30 CMDs that were reported by Dotter et al. (2010).”