The Astronomical Journal
We present new photometry for the Galactic thick-disk globular cluster Palomar 11 extending well past the main-sequence turnoff in the V and I bands. This photometry shows noticeable, but depleted, red giant and subgiant branches. The difference in magnitude between the red horizontal branch (red clump) and the subgiant branch is used to determine that Palomar 11 has an age of 10.4 ± 0.5 Gyr. The red clump is used to derive a distance d⊙ = 14.3 ± 0.4 kpc and a mean cluster reddening of E(V - I) = 0.40 ± 0.03. There is differential reddening across the cluster, of order δE(V - I) ~ 0.07. The color-magnitude diagram of Palomar 11 is virtually identical to that of the thick-disk globular cluster NGC 5927, implying that these two clusters have a similar age and metallicity. Palomar 11 has a slightly redder red giant branch than 47 Tuc, implying that Palomar 11 is 0.15 dex more metal-rich, or 1 Gyr older, than 47 Tuc. Ca II triplet observations, such as those of Rutledge and coworkers, favor the hypothesis that Palomar 11 is the same age as 47 Tuc, but slightly more metal-rich.
Lewis, Matthew S.; Liu, W. M.; Paust, N. E. Q.; and Chaboyer, Brian, "A New Color-Magnitude Diagram for Palomar 11" (2006). Open Dartmouth: Faculty Open Access Articles. 2095.