First Very Large Telescope/X-shooter spectroscopy of early-type stars outside the Local Group

Abstract

As part of the Very Large Telescope (VLT)/X-shooter science verification, we obtained the first optical medium-resolution spectrum of a previously identified bright O-type object in NGC 55, a Large Magellanic Cloud (LMC)-like galaxy at a distance of ∼2.0 Mpc. Based on the stellar and nebular spectra, we investigate the nature and evolutionary status of the central object(s) and its influence on the surrounding interstellar medium. We conclude that the source, NGC 55 C1_31, is a composite object, likely a stellar cluster, which contains one or several hot (Teff≃ 50 000 K) WN stars with a high mass-loss rate (∼3 × 10−5 M⊙ yr−1) and a helium-rich composition (NHe/NH= 0.8). The visual flux is dominated by OB-type (super)giant stars with Teff≲ 35 000 K, solar helium abundance (NHe/NH= 0.1) and mass-loss rate ∼2 × 10−6 M⊙ yr−1. The surrounding H ii region has an electron density of ne≤ 102 cm−3 and an electron temperature of T(O iii) ≃ 11 500 ± 600 K. The oxygen abundance of this region is [O/H] = 8.18 ± 0.03, which corresponds to Z= 0.31 ± 0.04  Z⊙. We observed no significant gradients in T(O iii), ne or [O/H] on a scale of 73 pc extending in four directions from the ionizing source. The properties of the H ii region can be reproduced by a cloudy model which uses the central cluster as ionizing source, thus providing a self-consistent interpretation of the data. We also report on the serendipitous discovery of He ii nebular emission associated with the nearby source NGC 55 C2_35, a feature usually associated with strong X-ray sources