VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), σ Oph, BM Sco, and HD 206859 ^⋆

¹ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: mwittkow@eso.org
² Dpt. Astronomia i Astrofísica, Universitat de València, C/Dr. Moliner 50, 46100 Burjassot, Spain
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía S/N, 18008 Granada, Spain
⁴ Centro Astronómico Hispano Alemán, Calar Alto, (CSIC-MPG), Sierra de los Filabres, 04550 Gergal, Spain
⁵ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, BP. 4229, 06304 Nice Cedex 4, France
⁶ Observatorio Astronómico, Universidad de València, 46980 Paterna, València, Spain

Received: 20 July 2016
Accepted: 3 October 2016

Abstract

Aims. We add four warmer late-type supergiants to our previous spectro-interferometric studies of red giants and supergiants.

Methods. We measure the near-continuum angular diameter, derive fundamental parameters, discuss the evolutionary stage, and study extended atmospheric atomic and molecular layers.

Results. V766 Cen (=HR 5171 A) is found to be a high-luminosity (log L/L_⊙ = 5.8 ± 0.4) source of effective temperature 4290 ± 760 K and radius 1490 ± 540 R_⊙, located in the Hertzsprung-Russell (HR) diagram close to both the Hayashi limit and Eddington limit; this source is consistent with a 40 M_⊙ evolutionary track without rotation and current mass 27–36 M_⊙. V766 Cen exhibits Na i in emission arising from a shell of radius 1.5 R_Phot and a photocenter displacement of about 0.1 R_Phot. It shows strong extended molecular (CO) layers and a dusty circumstellar background component. The other three sources are found to have lower luminosities of about log L/L_⊙ = 3.4–3.5, corresponding to 5–9 M_⊙ evolutionary tracks. They cover effective temperatures of 3900 K to 5300 K and radii of 60–120 R_⊙. They do not show extended molecular layers as observed for higher luminosity RSGs of our sample. BM Sco shows an unusually strong contribution by an over-resolved circumstellar dust component.

Conclusions. V766 Cen is a red supergiant located close to the Hayashi limit instead of a yellow hypergiant already evolving back toward warmer effective temperatures as discussed in the literature. Our observations of the Na i line and the extended molecular layers suggest an optically thick pseudo-photosphere at about 1.5 R_Phot at the onset of the wind. The stars σ Oph, BM Sco, and HD 206859 are more likely high-mass red giants instead of RSGs as implied by their luminosity class Ib. This leaves us with an unsampled locus in the HR diagram corresponding to luminosities log L/L_⊙ ~ 3.8–4.8 or masses 10–13 M_⊙, possibly corresponding to the mass region where stars explode as (type II-P) supernovae during the red supergiant stage. With V766 Cen, we now confirm that our previously found relation of increasing strength of extended molecular layers with increasing luminosities extends to double our previous luminosities and up to the Eddington limit. This might further point to steadily increasing radiative winds with increasing luminosity.