Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-05-03
Mon.Not.Roy.Astron.Soc.370:799-818,2006
Astronomy and Astrophysics
Astrophysics
22 pages, 15 figures and 8 tables. Accepted by MNRAS on 2006 April 28
Scientific paper
10.1111/j.1365-2966.2006.10504.x
We present new high spectral resolution VLT/UVES spectroscopy and archival HST/STIS imaging and spectroscopy of the giant HII region Tol89 in NGC5398. From optical and 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 Starburst99 (Leitherer et al.) UV spectral modelling, and nebular H beta 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). An LMC metallicity is measured for both knots, while nebular HeII 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. We estimate that there are ~95 early WN stars and ~35 early WC stars in Tol89-A, using empirical template spectra of LMC WR stars from Crowther and Hadfield. Remarkably, we also detect ~ three mid WNs in the smallest (mass) cluster in Tol89-A, A4. From the strength of nebular H beta, we obtain N(O) ~690 and 2800 for knots A and B. 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. [abridged]
Crowther Paul A.
Sidoli F.
Smith Lleweilun J.
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