Study of molecular layers in the atmosphere of the supergiant star μ Cep by interferometry in the K band

¹ Observatoire de Paris, LESIA, UMR 8109, 92190 Meudon, France e-mail: guy.perrin@obspm.fr
² National Optical Astronomy Observatories, Tucson, AZ 85726-6732, USA
³ Instituut voor Sterrenkunde, KU Leuven, Belgium
⁴ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁵ Caltech/Michelson Science Center, Pasadena, CA 91125, USA

Received: 4 November 2004
Accepted: 15 February 2005

Abstract

Infrared interferometry of supergiant and Mira stars has recently been reinterpreted as revealing the presence of deep molecular layers. Empirical models for a photosphere surrounded by a simple molecular layer or envelope have led to a consistent interpretation of previously inconsistent data. The stellar photospheres are found to be smaller than previously understood, and the molecular layer is much higher and denser than predicted by hydrostatic equilibrium. However, the analysis was based on spatial observations with medium-band optical filters, which mixed the visibilities of different spatial structures. This paper reports spatial interferometry with narrow spectral bands, isolating near-continuum and strong molecular features, obtained for the supergiant μ Cep. The measurements confirm strong variation of apparent diameter across the K-band. A layer model shows that a stellar photosphere of angular diameter  mas is surrounded by a molecular layer of diameter  mas, with an optical thickness varying from nearly zero at 2.15 μm to >1 at 2.39 μm. Although μ Cep and α Ori have a similar spectral type, interferometry shows that they differ in their radiative properties. Comparison with previous broad-band measurements shows the importance of narrow spectral bands. The molecular layer or envelope appears to be a common feature of cool supergiants.