α Centauri A in the far infrared^⋆

First measurement of the temperature minimum of a star other than the Sun

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: rene.liseau@chalmers.se
² Departamento de Astrofísica, Centro de Astrobiología (CAB, CSIC-INTA), Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
⁴ Jet Propulsion Laboratory, M/S 169-506, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁵ Departamento de Física Teórica, C-XI, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
⁶ NASA Herschel Science Center, Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Herschel Science Center – C11, European Space Agency (ESA), European Space Astronomy Centre (ESAC), PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
⁸ European Southern Observatory, Casilla 1900, Santiago 19, Chile
⁹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹ Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Aptdo. Postal 51 y 216, 72000 Puebla, Pue., Mexico
¹² UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
¹³ Astrophysics Mission Division, Research and Scientific Support Department ESA, ESTEC, SRE-SA PO Box 299, Keplerlaan 1, 2200AG Noordwijk, The Netherlands
¹⁴ Astrophysikalisches Institut und Universitätssternwarte, Friedrich-Schiller-Universität Jena, Schillergäßchen 2–3, 07745 Jena, Germany
¹⁵ NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics Laboratory, Code 667, Greenbelt, MD 20771, USA
¹⁶ Laboratoire d’études spatiales et d’instrumentation en astrophysique, Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
¹⁷ Dept. of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹⁸ Space Science & Technology Department, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK

Received: 22 November 2012
Accepted: 14 December 2012

Abstract

Context. Chromospheres and coronae are common phenomena on solar-type stars. Understanding the energy transfer to these heated atmospheric layers requires direct access to the relevant empirical data. Study of these structures has, by and large, been limited to the Sun thus far.

Aims. The region of the temperature reversal can be directly observed only in the far infrared and submillimetre spectral regime. We aim at determining the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star α   Cen   A. As a bonus this will also provide a detailed mapping of the spectral energy distribution, i.e. knowledge that is crucial when searching for faint, Kuiper belt-like dust emission around other stars.

Methods. For the nearby binary system α   Cen, stellar parameters are known with high accuracy from measurements. For the basic model parameters T_eff, log g and [Fe/H], we interpolate stellar model atmospheres in the grid of Gaia/PHOENIX and compute the corresponding model for the G2 V star α   Cen   A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to Spitzer-MIPS, Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of α   Cen   A in the far infared.

Results. Similar to the Sun, the far infrared (FIR) emission of α   Cen   A originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of α   Cen   A appears marginally cooler, T_min ~ T_{160   μm} = 3920 ± 375 K. Beyond the minimum near 160 μm, the brightness temperatures increase, and this radiation very likely originates in warmer regions of the chromosphere of α   Cen   A.

Conclusions. To the best of our knowledge, this is the first time a temperature minimum has been directly measured on a main-sequence star other than the Sun.