Bibcode
Dufton, P. L.; Thompson, A.; Crowther, P. A.; Evans, C. J.; Schneider, F. R. N.; de Koter, A.; de Mink, S. E.; Garland, R.; Langer, N.; Lennon, D. J.; McEvoy, C. M.; Ramírez-Agudelo, O. H.; Sana, H.; Símon Díaz, S.; Taylor, W. D.; Vink, J. S.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 615, id.A101, 21 pp.
Fecha de publicación:
7
2018
Revista
Número de citas
24
Número de citas referidas
24
Descripción
Previous analyses of the spectra of OB-type stars in the Magellanic
Clouds have identified targets with low projected rotational velocities
and relatively high nitrogen abundances; the evolutionary status of
these objects remains unclear. The VLT-FLAMES Tarantula Survey obtained
spectroscopy for over 800 early-type stars in 30 Doradus of which 434
stars were classified as B-type. We have estimated atmospheric
parameters and nitrogen abundances using TLUSTY model atmospheres for 54
B-type targets that appear to be single, have projected rotational
velocities, ve sin i ≤ 80 km s-1 and were not
classified as supergiants. In addition, nitrogen abundances for 34
similar stars observed in a previous FLAMES survey of the Large
Magellanic Cloud have been re-evaluated. For both samples, approximately
75-80% of the targets have nitrogen enhancements of less than 0.3 dex,
consistent with them having experienced only small amounts of mixing.
However, stars with low projected rotational velocities, ve
sini ≤ 40 km s-1 and significant nitrogen enrichments are
found in both our samples and simulations imply that these cannot all be
rapidly rotating objects observed near pole-on. For example, adopting an
enhancement threshold of 0.6 dex, we observed five and four stars in our
VFTS and previous FLAMES survey samples, yet stellar evolution models
with rotation predict only 1.25 ± 1.11 and 0.26 ± 0.51
based on our sample sizes and random stellar viewing inclinations. The
excess of such objects is estimated to be 20-30% of all stars with
current rotational velocities of less than 40 km s-1. This
would correspond to 2-4% of the total non-supergiant single B-type
sample. Given the relatively large nitrogen enhancement adopted, these
estimates constitute lower limits for stars that appear inconsistent
with current grids of stellar evolutionary models. Including targets
with smaller nitrogen enhancements of greater than 0.2 dex implies
larger percentages of targets that are inconsistent with current
evolutionary models, viz. 70% of the stars with rotational velocities
less than 40 km s-1 and 6-8% of the total single stellar
population. We consider possible explanations of which the most
promising would appear to be breaking due to magnetic fields or stellar
mergers with subsequent magnetic braking.
Based on observations at the European Southern Observatory in programmes
171.D0237, 073.D0234 and 182.D-0222.
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