First VLT/X-shooter spectroscopy of early-type stars outside the Local Group
Abstract
As part of the VLT/X-shooter science verification, we obtained the first optical medium-resolution spectrum of a previously identified bright O-type object in NGC55, an LMC-like galaxy at a distance of 2.0 Mpc. Based on the stellar and nebular spectrum, 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, NGC55C131, is a composite object, likely a stellar cluster, which contains one or several hot (Teff 50000 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 < 35000 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 ne < 102 cm-3 and an electron temperature T(OIII) 11500 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(OIII), ne or [O/H] on a scale of 73 pc extending in four directions from the ionising source. The properties of the HII region can be reproduced by a CLOUDY model which uses the central cluster as ionising source, thus providing a self-consistent interpretation of the data. We also report on the serendipitous discovery of HeII nebular emission associated with the nearby source NGC55C235, a feature usually associated with strong X-ray sources.