Quantitative studies of the optical and UV spectra of Galactic early B supergiants

Abstract

Aims. We undertake an optical and ultraviolet spectroscopic analysis of a sample of 20 Galactic B0−B5 supergiants of luminosity classes Ia, Ib, Iab, and II. Fundamental stellar parameters are obtained from optical diagnostics and a critical comparison of the model predictions to observed UV spectral features is made. Methods. Fundamental parameters (e.g., T eff , log L * , mass-loss rates and CNO abundances) are derived for individual stars using CMFGEN, a nLTE, line-blanketed model atmosphere code. The impact of these newly derived parameters on the Galactic B supergiant T eff scale, mass discrepancy, and wind-momentum luminosity relation is examined. Results. The B supergiant temperature scale derived here shows a reduction of about 1000−3000 K compared to previous results using unblanketed codes. Mass-loss rate estimates are in good agreement with predicted theoretical values, and all of the 20 B0−B5 supergiants analysed show evidence of CNO processing. A mass discrepancy still exists between spectroscopic and evolutionary masses, with the largest discrepancy occurring at log (L/L ) ∼ 5.4. The observed WLR values calculated for B0−B0.7 supergiants are higher than predicted values, whereas the reverse is true for B1−B5 supergiants. This means that the discrepancy between observed and theoretical values cannot be resolved by adopting clumped (i.e., lower) mass-loss rates as for O stars. The most surprising result is that, although CMFGEN succeeds in reproducing the optical stellar spectrum accurately, it fails to precisely reproduce key UV diagnostics, such as the N v and C iv P Cygni profiles. This problem arises because the models are not ionised enough and fail to reproduce the full extent of the observed absorption trough of the P Cygni profiles. Conclusions. Newly-derived fundamental parameters for early B supergiants are in good agreement with similar work in the field. The most significant discovery, however, is the failure of CMFGEN to predict the correct ionisation fraction for some ions. Such findings add further support to revising the current standard model of massive star winds, as our understanding of these winds is incomplete without a precise knowledge of the ionisation structure and distribution of clumping in the wind.

Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem

“…First and foremost, contrary to the recently reported case of some B supergiants where a few key UV wind lines could not be reproduced by CMFGEN models (see Searle et al 2008; and also Crowther et al 2006) 6 , we find agreement with the observations (see Figs. A.1−A.10).…”

An astrosphere around the blue supergiant κ Cas: possible explanation of its filamentary structure

“…This distance is generally accepted in studies of κ Cas (e.g. Crowther, Lennon & Walborn 2006;Searle et al 2008). The presence of the astrosphere around κ Cas, however, suggests that this star is a runaway and that it might be formed far away from its present position on the sky (cf.…”

“…Taken at face value, these two distance estimates imply a too high bolometric luminosity of log(L * / L⊙) ≈ 5.8, which along with the effective temperature of κ Cas of T * = 23.5±1.5 kK (Searle et al 2008) would place this star on the S Doradus instability strip (Wolf 1989) in the HertzsprungRussell diagram. Since κ Cas does not show variability typical of stars in this region of the Hertzsprung-Russell diagram, it is likely that it is located at a shorter distance.…”

“…In the IRAS 60 µm image, the astrosphere has an arc-like shape, Table 1. Basic parameters of κ Cas (Crowther et al 2006;Searle et al 2008).…”

Measuring the shock-heating rate in the winds of O stars using X-ray line spectra

“…The bulk of the X-ray emission from EWS in massive stars comes from the first several stellar radii of the wind according to both theoretical calculations (Feldmeier et al 1997) and observational constraints from line profiles and forbidden-tointercombination line intensity ratios measured via X-ray spectroscopy (Leutenegger et al 2006). Thus the values of Repolust et al (2004); g Searle et al (2008); all terminal velocities from h Haser (1995), all mass-loss rates from Cohen et al (2014), and all ISM column densities from Fruscione et al (1994) Ra listed in the table justify the assumption of radiatively cooled shocks in the programme stars.…”