Chemical composition of the young massive cluster NGC 1569-B

Gvozdenko, A.; Larsen, S. S.; Beasley, M. A.; Brodie, J.
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Astronomy and Astrophysics

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Context. The chemical composition of young massive clusters (YMCs) provides stellar population information on their host galaxy. As potential precursors of globular clusters (GCs), their properties can help us understand the origins of GCs and their evolution.
Aims: We present a detailed chemical abundance analysis of the YMC NGC 1569-B. The host galaxy, NGC 1569, is a dwarf irregular starburst galaxy at a distance of 3.36 ± 0.20 Mpc. We derive the abundances of the α, Fe-peak, and heavy elements.
Methods: We determined the abundance ratios from the analysis of an optical integrated-light (IL) spectrum of NGC 1569-B, obtained with the HIRES echelle spectrograph on the Keck I telescope. We considered different red-to-blue supergiant ratios (NRSG/NBSG), namely: the ratio obtained from a theoretical isochrone (NRSG/NBSG = 1.24), the ratio obtained from a resolved colour-magnitude diagram of the YMC (NRSG/NBSG = 1.53), and the ratio that minimises the χ2 when comparing our model spectra with the observations (NRSG/NBSG = 1.90). We adopted the latter ratio for our resulting chemical abundances.
Results: The derived iron abundance is sub-solar with [Fe/H] = −0.74 ± 0.05. In relation to the scaled solar composition, we find enhanced α-element abundances, [⟨Mg, Si, Ca, Ti⟩/Fe]= + 0.25 ± 0.11, with a particularly high Ti abundance of +0.49 ± 0.05. Other super-solar elements include [Cr/Fe] = +0.50 ± 0.11, [Sc/Fe] = +0.78 ± 0.20, and [Ba/Fe] = +1.28 ± 0.14, while other Fe-peak elements are close to scaled solar abundances: ([Mn/Fe] = −0.22 ± 0.12 and [Ni/Fe] = +0.13 ± 0.11).
Conclusions: The composition of NGC 1569-B resembles the stellar populations of the YMC NGC 1705-1, located in a blue compact dwarf galaxy. The two YMCs agree with regard to α-elements and the majority of the Fe-peak elements, except for Sc and Ba, which are extremely super-solar in NGC 1569-B - and higher than in any YMC studied so far. The blue part of the optical spectrum of a young population is still a very challenging wavelength region to analyse using IL spectroscopic studies. This is due to the uncertain contribution to the light from blue supergiant stars, which can be difficult to disentangle from turn-off stars, even when resolved photometry is available. We suggest that the comparison of model fits at different wavelengths offers a route to determining the red-to-blue supergiant ratio from IL spectroscopy.