Anatomy of a young massive star cluster: NGC 1569-B

We present new H-band echelle spectra, obtained with the NIRSPEC spectrograph at Keck II, for the massive star cluster ‘B’ in the nearby dwarf irregular galaxy NGC 1569. From spectral synthesis and equivalent width measurements, we obtain abundances and abundance patterns. We derive an Fe abundance of [Fe/H] =− 0.63 ± 0.08, a supersolar [α/Fe] abundance ratio of +0.31 ± 0.09, and an O abundance of [O/H] =− 0.29 ± 0.07. We also measure a low 12 C/ 13 C ≈ 5 ± 1 isotopic ratio. Using archival imaging from the Advanced Camera for Surveys onboard theHubbleSpaceTelescope (HST), we construct a colour–magnitude diagram (CMD) for the cluster in which we identify about 60 red supergiant (RSG) stars, consistent with the strong RSG features seen in the H-band spectrum. The mean effective temperature of these RSGs, derived from their observed colours and weighted by their estimated H-band luminosities, is 3790 K, in excellent agreement with our spectroscopic estimate of T eff = 3800 ± 200 K. From the CMD, we derive an age of 15–25 Myr, slightly older than previous