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

TL;DR

This study characterizes two FSR globular cluster candidates, revealing FSR 1603 as a massive intermediate-age open cluster and suggesting FSR 1755 is a young embedded cluster within an H II region, based on photometric analysis.

Contribution

The paper provides detailed physical parameters for FSR 1603 and proposes FSR 1755 as a young embedded cluster, clarifying their nature and properties through 2MASS photometry.

Findings

FSR 1603 is an intermediate-age open cluster (~1 Gyr) with ~2300 solar masses.

FSR 1755 is likely a young, embedded cluster in an H II region.

FSR 1603 has a core radius of about 1.1 pc and a mass function slope of 1.8.

Abstract

We study the nature of the globular cluster (GC) candidates FSR 1603 and FSR 1755 selected from the catalogue of \citet{FSRcat}. Their properties are investigated with 2MASS field-star decontaminated photometry, which is used to build colour-magnitude diagrams (CMDs), and stellar radial density profiles (RDPs). FSR 1603 has the open cluster (OC) Ruprecht 101 as optical counterpart, and we show it to be a massive intermediate age cluster (IAC). Relevant parameters of FSR 1603 are the age $\approx1$ Gyr, distance from the Sun $\ds\approx2.7$ kpc, Galactocentric distance $\dgc\approx6.4$ kpc, core radius $\rc\approx1.1$ pc, mass function slope $\chi\approx1.8$, observed stellar mass (for stars with mass in the range $\rm 1.27 \ms\leq m\leq2.03 \ms$) $\mObs\approx500 \ms$, and a total (extrapolated to $\rm m=0.08 \ms$) stellar mass $\mTot\approx2300 \ms$. FSR 1755, on the other hand, is not a populous cluster. It may be a sparse young cluster embedded in the H II region Sh2-3, subject to an absorption $\aV\approx4.1$, located at $\ds\approx1.3$ kpc. Important field-star contamination, spatially-variable heavy dust obscuration, even in \ks, and gas emission characterise its field. A nearly vertical, sparse blue stellar sequence shows up in the CMDs.