How dusty is α Centauri?^⋆,⋆⋆

Excess or non-excess over the infrared photospheres of main-sequence stars

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: wiegert@chalmers.se
² Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, Laboratoire d’études spatiales et d’instrumentation en astrophysique, 92195 Meudon Cedex, France
³ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
⁴ ESA – ESAC Gaia SOC. PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁵ Jet Propulsion Laboratory, M/S 169-506, 4800 Oak Grove Drive, Pasadena CA 91109, USA
⁶ Departamento de Física Teórica, C-XI, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
⁷ Departamento de Astrofísica, Centro de Astrobiología (CAB, CSIC-INTA), Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁸ NASA Herschel Science Center, Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena CA 91125, USA
⁹ Herschel Science Center – C11, European Space Agency (ESA), European Space Astronomy Centre (ESAC), PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
¹⁰ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
¹¹ European Southern Observatory, Casilla 1900, Santiago 19, Chile
¹² Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA
¹⁴ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Sta. Ma. Tonantzintla, Puebla, Mexico
¹⁵ Institute of planetary Research, German Aerospace Center, Rutherfordstrasse 2, 124 89 Berlin, Germany
¹⁶ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁷ Astrophysikalisches Institut und Universitätssternwarte, Friedrich-Schiller-Universität Jena, Schillergäßchen 2-3, 07745 Jena, Germany
¹⁸ Astrophysics Mission Division, Research and Scientific Support Department ESA, ESTEC, SRE-SA PO Box 299, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
¹⁹ NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics Laboratory, Code 667, Greenbelt MD 20771, USA
²⁰ Dept. of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
²¹ Space Science & Technology Department, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK
²² Institute for Theoretical Physics and Astrophysics, University of Kiel, Leibnizstraße 15, 24098 Kiel, Germany

Received: 13 May 2013
Accepted: 21 January 2014

Abstract

Context. Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby, solar-type binary α Centauri have metallicities that are higher than solar, which is thought to promote giant planet formation.

Aims. We aim to determine the level of emission from debris around the stars in the α Cen system. This requires knowledge of their photospheres. Having already detected the temperature minimum, T_min, of α Cen A at far-infrared wavelengths, we here attempt to do the same for the more active companion α Cen B. Using the α Cen stars as templates, we study the possible effects that T_min may have on the detectability of unresolved dust discs around other stars.

Methods. We used Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry to determine the stellar spectral energy distributions in the far infrared and submillimetre. In addition, we used APEX-SHeFI observations for spectral line mapping to study the complex background around α Cen seen in the photometric images. Models of stellar atmospheres and of particulate discs, based on particle simulations and in conjunction with radiative transfer calculations, were used to estimate the amount of debris around these stars.

Results. For solar-type stars more distant than α Cen, a fractional dust luminosity f_d ≡ L_dust/L_star ~ 2 × 10^-7 could account for SEDs that do not exhibit the T_min effect. This is comparable to estimates of f_d for the Edgeworth-Kuiper belt of the solar system. In contrast to the far infrared, slight excesses at the 2.5σ level are observed at 24 μm for both α Cen A and B, which, if interpreted as due to zodiacal-type dust emission, would correspond to f_d ~ (1−3) × 10^-5, i.e. some 10² times that of the local zodiacal cloud. Assuming simple power-law size distributions of the dust grains, dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the α Cen stars, viz. ≲ of 4 to 1000 μm size grains, distributed according to n(a) ∝ a^−3.5. Similarly, for filled-in T_min emission, corresponding Edgeworth-Kuiper belts could account for of dust.

Conclusions. Our far-infrared observations lead to estimates of upper limits to the amount of circumstellar dust around the stars α Cen A and B. Light scattered and/or thermally emitted by exo-Zodi discs will have profound implications for future spectroscopic missions designed to search for biomarkers in the atmospheres of Earth-like planets. The far-infrared spectral energy distribution of α Cen B is marginally consistent with the presence of a minimum temperature region in the upper atmosphere of the star. We also show that an α Cen A-like temperature minimum may result in an erroneous apprehension about the presence of dust around other, more distant stars.