The massive star population in the giant H II region Tol 89 in NGC 5398

Abstract

We present new high spectral resolution Very Large Telescope (VLT)/UV–Visual Echelle Spectrograph (UVES) spectroscopy and archival Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) imaging and spectroscopy of the giant H ii region Tol 89 in NGC 5398. From optical and ultraviolet (UV) HST images, we find that the star-forming complex as a whole contains at least seven young compact massive clusters. We resolve the two brightest optical knots, A and B, into five individual young massive clusters along our slit, A1–4 and B1, respectively. From UV spectral modelling using the starburst99 code of Leitherer et al., and nebular Hβ equivalent widths in the optical, we derive ages that are consistent with the formation of two separate burst events, of ∼4 ± 1 Myr and <3 Myr for knots A (A1–4) and B (B1), respectively. A Large Magellanic Cloud (LMC) metallicity is measured for both knots from a nebular line analysis, while nebular He ii 4686 is observed in knot B and perhaps in knot A. We detect underlying broad wings on the strongest nebular emission lines indicating velocities up to 600 km s−1. From UV and optical spectroscopy, we estimate that there are ∼95 early WN stars and ∼35 early WC stars in Tol 89-A, using empirical template spectra of LMC Wolf–Rayet (WR) stars from Crowther & Hadfield, with the WC population confined to cluster A2. Remarkably, we also detect a small number of approximately three mid WNs in the smallest (mass) cluster in Tol 89-A, A4, whose spectral energy output in the UV is entirely dominated by the WN stars. From the strength of nebular Hβ, we obtain N(O) ∼ 690 and 2800 for knots A and B, respectively, which implies N(WR)/N(O) ∼ 0.2 for knot A. We also employ a complementary approach using starburst99 models, in which the O star content is inferred from the stellar continuum, and the WR population is obtained from spectral synthesis of optical WR features using the grids from Smith et al. We find reasonable agreement between the two methods for the O star content and the N(WR)/N(O) ratio but find that the WR subtype distribution is in error in the starburst99 models, with far too few WN stars being predicted. We attribute this failure to the neglect of rotational mixing in evolutionary models. Our various modelling approaches allow us to measure the cluster masses. We identify A1 as a super star cluster (SSC) candidate with a mass of ∼1–2 × 105 M⊙. A total mass of ∼6 × 105 M⊙ is inferred for the ionizing sources within Tol 89-B.