B-type supergiants in the SMC: Chemical compositions and comparison of static and unified models

Abstract

Abstract. High-resolution UCLES/AAT spectra are presented for nine B-type supergiants in the SMC, chosen on the basis that they may show varying amounts of nuclear-synthetically processed material mixed to their surface. These spectra have been analysed using a new grid of approximately 12 000 non-LTE line blanketed  model atmospheres to estimate atmospheric parameters and chemical composition. The abundance estimates for O, Mg and Si are in excellent agreement with those deduced from other studies, whilst the low estimate for C may reflect the use of the C  doublet at 4267 Å. The N estimates are approximately an order of magnitude greater than those found in unevolved B-type stars or H  regions but are consistent with the other estimates in AB-type supergiants. These results have been combined with results from a unified model atmosphere analysis of UVES/VLT spectra of B-type supergiants (Trundle et al. 2004, A&A, 417, 217) to discuss the evolutionary status of these objects. For two stars that are in common with those discussed by Trundle et al., we have undertaken a careful comparison in order to try to understand the relative importance of the different uncertainties present in such analyses, including observational errors and the use of static or unified models. We find that even for these relatively luminous supergiants,  models yield atmospheric parameters and chemical compositions similar to those deduced from the unified code .

The VLT-FLAMES Tarantula Survey

“…Details of the methods adopted can be found in Hunter et al (2007), while a more detailed discussion of the grids can be found in Ryans et al (2003), Dufton et al (2005) 2 . Here only a brief summary will be given, with further details available as discussed above.…”

“…The validity of such an approach was investigated by Dufton et al (2005), who analysed the spectra of B-type Ia supergiants in the SMC using the current grid and also theoretical spectra generated by the  code (Santolaya-Rey et al 1997;Puls et al 2005), that incorporates wind effects. Dufton et al (2005) found excellent agreement in the atmospheric parameters estimated from the two methods. Effective temperature estimates agreed to within typically 500 K, logarithmic gravities to 0.1 dex and microturbulent velocities to 2 km s −1 .…”

“…For nitrogen, the differences were less than 0.2 dex, although there did appear to be a systematic difference of 0.1 dex between the two approaches. Dufton et al (2005) investigated this in detail and suggested that this arose from differences in the adopted nitrogen model atoms and wind effects.…”

“…The Si  triplet (4552, 4567, and 4574 Å) was adopted as it is observed in effectively all our spectra and as they arise from the same multiplet, the effects of errors in the absolute oscillator strengths and non-LTE effects should be minimised. This method has been used previously by, for example, Dufton et al (2005), Hunter et al (2007), and Fraser et al (2010), who noted that it is sensitive to errors in the equivalent width measurements of the Si  lines. Therefore the microturbulence was also estimated from requiring that the silicon abundance was consistent with that found in the LMC.…”

Neutral material around the B[e] supergiant star LHA 115-S 65

“…Assuming that all elements with an ionization potential lower than that of hydrogen and oxygen are fully ionized delivers an upper limit of q Metals < 9.5×10 −5 . For this, we used an SMC metallicity of 20% solar (see, e.g., Dufton et al 2005), and solar abundance values for the metals from Grevesse & Sauval (1998). This very low value obtained from the metals means that if hydrogen is ionized by only 1%, it still provides the dominant quantity of free electrons.…”