The VLT-FLAMES Tarantula Survey

Abstract

Context. Model atmosphere analyses have been previously undertaken for both Galactic and extragalactic B-type supergiants. By contrast, little attention has been given to a comparison of the properties of single supergiants and those that are members of multiple systems. Aims. Atmospheric parameters and nitrogen abundances have been estimated for all the B-type supergiants identified in the VLT-FLAMES Tarantula survey. These include both single targets and binary candidates. The results have been analysed to investigate the role of binarity in the evolutionary history of supergiants.Methods.  non-local thermodynamic equilibrium (LTE) model atmosphere calculations have been used to determine atmospheric parameters and nitrogen abundances for 34 single and 18 binary supergiants. Effective temperatures were deduced using the silicon balance technique, complemented by the helium ionisation in the hotter spectra. Surface gravities were estimated using Balmer line profiles and microturbulent velocities deduced using the silicon spectrum. Nitrogen abundances or upper limits were estimated from the N  spectrum. The effects of a flux contribution from an unseen secondary were considered for the binary sample. Results. We present the first systematic study of the incidence of binarity for a sample of B-type supergiants across the theoretical terminal age main sequence (TAMS). To account for the distribution of effective temperatures of the B-type supergiants it may be necessary to extend the TAMS to lower temperatures. This is also consistent with the derived distribution of mass discrepancies, projected rotational velocities and nitrogen abundances, provided that stars cooler than this temperature are post-red supergiant objects. For all the supergiants in the Tarantula and in a previous FLAMES survey, the majority have small projected rotational velocities. The distribution peaks at about 50 km s −1 with 65% in the range 30 km s −1 ≤ v e sin i ≤ 60 km s −1 . About ten per cent have larger v e sin i (≥100 km s −1 ), but surprisingly these show little or no nitrogen enhancement. All the cooler supergiants have low projected rotational velocities of ≤70 km s −1 and high nitrogen abundance estimates, implying that either bi-stability braking or evolution on a blue loop may be important. Additionally, there is a lack of cooler binaries, possibly reflecting the small sample sizes. Single-star evolutionary models, which include rotation, can account for all of the nitrogen enhancement in both the single and binary samples. The detailed distribution of nitrogen abundances in the single and binary samples may be different, possibly reflecting differences in their evolutionary history. Conclusions. The first comparative study of single and binary B-type supergiants has revealed that the main sequence may be significantly wider than previously assumed, extending to T eff = 20 000 K. Some marginal differences in single and binary atmospheric parameters and abundances have been identified, possibly implying non-stand...

The VLT-FLAMES Tarantula Survey

“…This is done in McEvoy et al (2015) for two O9.7 sources, VFTS 087 (O9.7 Ib-II) and 165 (O9.7 Iab), where fastwind analyses are compared to tlusty analyses (Hubeny 1988;Hubeny & Lanz 1995;Lanz & Hubeny 2007). Here fastwind settles on temperatures that are 1000 K higher, which is within the uncertainties quoted.…”

“…4.2) one might perceive by eye a trend similar to that reported by Trundle & Lennon (2005) and McEvoy et al (2015). However, the Pearson correlation coefficient does not allow us to accept the presence of such a linear trend at the 5% significance level.…”

“…For early-B stars, one also relies on Si iii and iv lines as a temperature diagnostic (see, e.g., McEvoy et al 2015). We performed several checks to assess the reliability of our parameters for the late O stars.…”

Hubble Tarantula Treasury Project. V. The Star Cluster Hodge 301: The Old Face of 30 Doradus*

“…In clusters of this approximate age the PMSG is populated by bright B-type giants and supergiants (of similar color to the MS dwarfs and sub-giants), in the case of Hodge 301 these two stars are confirmed cluster members with spectral types B3 Ib and B2 II-III(n)e (see Evans et al 2015). While the fact that the precursor of SN1987A was a B3 I star (Walborn et al 1989) indicates that core He burning stars can inhabit this part of the CMD/Hertzsprung Russel diagram (HRD), there is as yet no unambiguous method for distinguishing between core H and He burning blue stars (see Grebel et al 1996, Hunter et al 2008, Vink et al 2010, and McEvoy et al 2015 for discussions of how rotational velocity distributions, mass-loss considerations and binary frequency might define the width of the MS for B-stars in the LMC). For now we will assume that the MSTO is as shown in Figure 4 but will return to this point in the discussion.…”

Two bi-stability jumps in theoretical wind models for massive stars and the implications for luminous blue variable supernovae

“…Unless otherwise stated, our cmfgen models have been computed for 1/2 solar metal abundances 1 , in order to be relevant for results of the VLT-FLAMES Tarantula Survey (Evans et al 2011;Bestenlehner et al 2014;McEvoy et al 2015). We adopt the parameters summarised in Table 1.…”