Is Arcturus a well-understood K giant?

Test of model atmospheres and potential companion detection by near-infrared interferometry

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: Tijl.verhoelst@ster.kuleuven.ac.be
² Observatoire de Paris-Meudon, LESIA, 5 place Jules Janssen, 92195 Meudon, France
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Institute for Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden
⁵ National Optical Astronomy Observatories, PO Box 26732, Tucson, AZ 85726, USA

Received: 10 November 2004
Accepted: 23 January 2005

Abstract

We present near-IR interferometric measurements of the K1.5 giant Arcturus (α Bootis), obtained at the IOTA interferometer with the FLUOR instrument, in four narrow filters with central wavelengths ranging from 2.03 μm to 2.39 μm. These observations were expected to allow us to quantify the wavelength dependence of the diameter of a typical K giant. They are compared to predictions from both plane-parallel and spherical model atmospheres. Unexpectedly, neither can explain the observed visibilities. We show that these data suggest the presence of a companion, in accordance with the Hipparcos data on this star, and discuss this solution with respect to Arcturus' single star status.