B-type supergiants in the Small Magellanic Cloud: rotational velocities and implications for evolutionary models

¹ Department of Pure & Applied Physics, The Queen's University of Belfast, BT7 1NN, Northern Ireland, UK e-mail: p.dufton@qub.ac.uk
² Isaac Newton Group of Telescopes, Apartado de Correos 368, 38700 Santa Cruz de La Palma, Canary Islands, Spain
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain

Received: 28 November 2005
Accepted: 27 January 2006

Abstract

High-resolution spectra for 24 SMC and Galactic B-type supergiants have been analysed to estimate the contributions of both macroturbulence and rotation to the broadening of their metal lines. Two different methodologies are considered, viz. goodness-of-fit comparisons between observed and theoretical line profiles and identifying zeros in the Fourier transforms of the observed profiles. The advantages and limitations of the two methods are briefly discussed with the latter techniques being adopted for estimating projected rotational velocities (v sin i) but the former being used to estimate macroturbulent velocities. The projected rotational velocity estimates range from approximately 20 to 60 km s^-1, apart from one SMC supergiant, Sk 191, with a v sin i  90 km s^-1. Apart from Sk 191, the distribution of projected rotational velocities as a function of spectral type are similar in both our Galactic and SMC samples with larger values being found at earlier spectral types. There is marginal evidence for the projected rotational velocities in the SMC being higher than those in the Galactic targets but any differences are only of the order of 5–10 km s^-1, whilst evolutionary models predict differences in this effective temperature range of typically 20 to 70 km s^-1. The combined sample is consistent with a linear variation of projected rotational velocity with effective temperature, which would imply rotational velocities for supergiants of 70 km s^-1 at an effective temperature of 28 000 K (approximately B0 spectral type) decreasing to 32 km s^-1 at 12 000 K (B8 spectral type). For all targets, the macroturbulent broadening would appear to be consistent with a Gaussian distribution (although other distributions cannot be discounted) with an  half-width varying from approximately 20 km s^-1 at B8 to 60 km s^-1 at B0 spectral types.