How Dusty is Alpha Centauri?

How dusty is alpha Centauri? Excess or non-excess over the infrared photospheres of main-sequence stars

Authors:

Wiegert et al

Abstract:

Context.

Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby, solar-type binary Centauri 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 Cen system. This requires knowledge of their photospheres. Having already detected the temperature minimum, Tmin, of CenA at far-infrared wavelengths, we here attempt to do so also for the more active companion Cen B. Using the Cen stars as templates, we study possible e ects 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 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, are used to estimate the amount of debris around these stars.

Results.

For solar-type stars more distant than Cen, a fractional dust luminosity fd Ldust{Lstar 2 10 7 could account for SEDs that do not exhibit the Tmin-e ect. 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 2:5 level are observed at 24 m for both CenA and B, which, if interpreted to be due to zodiacal-type dust emission, would correspond to fd p1 3q 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 Cen stars, viz. 4 10 6 MK of 4 to 1000 m size grains, distributed according to npaq9a 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 CenA 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 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 Cen A-like temperature minimum may result in an erroneous apprehension about the presence of dust around other, more distant stars.