Lifting the dust veil from the globular cluster Palomar 2

Abstract

This work employs high-quality Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) F606W and F814W photometry to correct for the differential reddening affecting the colour–magnitude diagram (CMD) of the poorly studied globular cluster (GC) Palomar 2. Differential reddening is taken into account by assuming that morphological differences among CMDs extracted across the field of view of Palomar 2 correspond essentially to shifts (quantified in terms of δE(B − V)) along the reddening vector due to a non-uniform dust distribution. The average reddening difference over all partial CMDs is δE(B−V)=0.24±0.08, with the highest reaching δE(B − V) = 0.52. The corrected CMD displays well-defined and relatively narrow evolutionary sequences, especially for the evolved stars, i.e. the red giant, horizontal, and asymptotic giant branches (RGB, HB, and AGB, respectively). The average width of the upper main sequence and RGB profiles of the corrected CMD corresponds to 56 per cent of the original one. Parameters measured on this CMD show that Palomar 2 is ≈13.25 Gyr old, has the mass M∼1.4×105M⊙ stored in stars, is affected by the foreground E(B − V) ≈ 0.93, is located at d⊙ ≈ 26 kpc from the Sun, and is characterized by the global metallicity Z/Z⊙ ≈ 0.03, which corresponds to the range −1.9 ≤ [Fe/H] ≤ −1.6 (for 0.0 ≤ [α/Fe] ≤ +0.4), quite consistent with other outer halo GCs. Additional parameters are the absolute magnitude MV ≈ −7.8, and the core and half-light radii rC ≈ 2.6 pc and RHL ≈ 4.7 pc, respectively.