The VLT FLAMES Survey of Massive Stars: Rotation and Nitrogen Enrichment as the Key to Understanding Massive Star Evolution

Abstract

Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here we study a sample of stars, including rapid rotators, to constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg, and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to ∼300 km s using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational Ϫ1 mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen-enriched intrinsic slow rotators ( km s ) incompatible with our models (∼20% of the sample). Furthermore, while we find fastϪ1 v sin i Շ 50 rotators with enrichments in agreement with the models, the observation of evolved ( dex) fast rotators log g ! 3.7 that are relatively unenriched (a further ∼20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen-burning main sequence. We are left with a picture where invoking binarity and perhaps fossil magnetic fields is required to understand the surface properties of a population of massive main-sequence stars.

The VLT-FLAMES Tarantula Survey

“…Four model atmosphere grids have been generated with metallicities representative of the Galaxy ((log ( Fe /H ) + 12) = 7.5 dex, and other abundances scaled accordingly), the LMC (7.2 dex), the SMC (6.8 dex), and a lower metallicity regime (6.4 dex). Model atmospheres have been calculated to cover a range of effective temperatures from 12 000 K to 35 000 K in steps not greater than 1500 K, and surface gravities ranging from near to the Eddington limit up to 4.5 dex in steps of 0.15 dex (Hunter et al 2008a). These codes make the classical non-LTE assumptions including that the atmospheres can be considered plane parallel and that winds do not have a significant effect on the optical spectrum.…”

“…However all but two (VFTS 027 and VFTS 293) were subsequently excluded as their estimated logarithmic surface gravity (see Sect. 3) was greater than 3.2 dex, which has been used previously to delineate the end of hydrogen core burning (see, for example, Hunter et al 2008aHunter et al , 2009a.…”

“…A critical examination by Maeder et al (2014) of the nitrogen abundances estimated in the FLAMES survey (Evans et al 2005;Hunter et al 2008a) advocated the use of N/C against N/O abundance ratio diagrams. In theory, this could provide a powerful diagnostic.…”

“…Quantitative analysis of these objects was presented in a series of articles by Dufton et al (2006), Trundle et al (2007), Hunter et al (2007Hunter et al ( , 2008aHunter et al ( ,b, 2009a, and Dunstall et al (2011).…”

A peculiar Of star in the Local Group galaxy IC 1613

“…S. Smartt) was a project aimed precisely at testing this prediction, which was nicely confirmed by Mokiem et al (2007b) by analysing O-type stars in the Milky Way, the Large Magellanic Cloud (LMC), and the Small Magellanic Cloud (SMC). However, the second part of the project, the comparison of the predictions of stellar evolutionary models including massloss and rotation with the stellar properties, revealed problems when comparing the nitrogen abundances (Hunter et al 2008), indicating that other physical processes may be at work. This started the VLT Tarantula FLAMES Survey (VFTS), a detailed A&A 543, A85 (2012) investigation of the massive stellar population of 30 Doradus in the LMC currently under way (Evans et al 2011).…”