A populous intermediate-age open cluster and evidence of an embedded cluster among the FSR globular cluster candidates

Abstract

We study the nature of the globular cluster (GC) candidates FSR 1603 and FSR 1755 selected from the catalogue of Froebrich, Scholz & Raftery. Their properties are investigated with Two-Micron All-Sky Survey field-star decontaminated photometry, which is used to build colour–magnitude diagrams (CMDs) and stellar radial density profiles. FSR 1603 has the open cluster Ruprecht 101 as optical counterpart, and we show it to be a massive intermediate-age cluster. Relevant parameters of FSR 1603 are the age ≈1 Gyr, distance from the Sun d⊙≈ 2.7 kpc, Galactocentric distance RGC≈ 6.4 kpc, core radius RC≈ 1.1 pc, mass function slope χ≈ 1.8, observed stellar mass (for stars with mass in the range 1.27 ≤m≤ 2.03 M⊙) Mobs≈ 500 M⊙ and a total (extrapolated to m= 0.08 M⊙) stellar mass Mtot≈ 2300 M⊙. FSR 1755, on the other hand, is not a populous cluster. It may be a sparse young cluster embedded in the H ii region Sh 2-3, subject to an absorption AV≈ 4.1, located at d⊙≈ 1.3 kpc. Important field-star contamination, spatially variable heavy dust obscuration, even in Ks, and gas emission characterize its field. A nearly vertical, sparse blue stellar sequence shows up in the CMDs.