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

Context. Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby, solar-type binary αCentauri have higher than solar metallicities, which is thought to promote giant planet formation. Aims. We aim to determine the level of emission from debris around the stars in the αCen system. This requires knowledge of their photospheres. Having already detected the temperature minimum, Tmin, of αCen A at far-infrared wavelengths, we here attempt to do so also for the more active companion αCen B. Using the αCen stars as templates, we study possible effects Tmin may have on the detectability of unresolved dust discs around other stars. Methods. We use Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry to determine the stellar spectral energy distributions in the far infrared and submillimetre. In addition, we use APEX-SHeFI observations for spectral line mapping to study the complex background around αCen seen in the photometric images. Models of stellar atmospheres and of particulate discs, based on particle simulations and in conjunction with radiative transfer calculations, are used to estimate the amount of debris around these stars. Results. For solar-type stars more distant than αCen, a fractional dust luminosity fd ” Ldust{Lstar „ 2 ˆ 10 ́7 could account for SEDs that do not exhibit the Tmin-effect. This is comparable to estimates of fd 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 αCen A and B, which, if interpreted to be due to zodiacal-type dust emission, would correspond to fd „ p1 ́ 3q ˆ 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 αCen stars, viz. ă „ 4ˆ 10 ́6 MK of 4 to 1000 μm size grains, distributed according to npaq9 a ́3.5. Similarly, for filled-in Tmin emission, corresponding Edgeworth-Kuiper belts could account for „ 10 ́3 MK of dust. Conclusions. Our far-infrared observations lead to estimates of upper limits to the amount of circumstellar dust around the stars αCen 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 αCen B is marginally consistent with the presence of a minimum temperature region in the upper atmosphere of the star. We also show that an αCen A-like temperature minimum may result in an erroneous apprehension about the presence of dust around other, more distant stars.