2008MNRAS.383..263L

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 ¹²C/¹³C ~ 5±1 isotopic ratio. Using archival imaging from the Advanced Camera for Surveys onboard the Hubble Space Telescope (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 estimates based on integrated broad-band colours. We derive a radial velocity of <v_r> = -78±3 km/s and a velocity dispersion of 9.6±0.3 km/s. In combination with an estimate of the half-light radius of 0.20±0.05arcsec from the HST data, this leads to a dynamical mass of (4.4±1.1)x10⁵M_☉. The dynamical mass agrees very well with the mass predicted by simple stellar population models for a cluster of this age and luminosity, assuming a normal stellar initial mass function. The cluster core radius appears smaller at longer wavelengths, as has previously been found in other extragalactic young star clusters.