How dusty isα Centauri?

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−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 10 2 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. < ∼ 4×10 −6 M 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 ∼10 −3 M 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.

Incidence of debris discs around FGK stars in the solar neighbourhood

“…The shift to the peak of the distribution to a negative value of χ 100 likely re-flects the fact that the extrapolation of the model photospheres to the PACS bands using a Rayleigh-Jeans approximation does not take into account the decrease in brightness temperature at ∼ 100 µm that occurs in late-type stars, and therefore overestimates the photospheric emission around that wavelength. That was directly observed using DUNES data for α Cen A (Liseau et al 2013) and α Cen B (Wiegert et al 2014). Note that the stars in the red histogram with χ 100 < 3.0 are those considered as harbouring a cold disc, the excess being present only at 160 µm.…”

ALMA observations ofαCentauri

“…In addition, one observation at 70 µm with Herschel-PACS (Molinari et al 2010;Wiegert et al 2014) nearly 2 CASA is an acronym for Common Astronomy Software Application. resolved the binary, which permitted individual fluxes to be measured.…”

“…In the particular case of the nearby binary α Cen A B (G2 V and K1 V), the far-infrared (FIR) spectral evidence for the phenomenon of a temperature minimum was quite convincing, although the pair was spatially unresolved by our longwavelength observations (Liseau et al 2013;Wiegert et al 2014). Furthermore, since adequate theoretical stellar model atmospheres longward of 40 µm were lacking, relative flux ratios had to be deduced by extrapolation from data obtained at shorter wavelengths.…”

The chemical composition of α Centauri AB revisited

“…The possible existence of a small-size transiting planet, α Cen Bc, was recently reported (Demory et al 2015), but needs confirmation. Infrared emission from dusty debris discs has not been detected in α Cen AB (Wiegert et al 2014). From the theoretical side, several studies have investigated the formation and dynamics of planets in the binary system, but quite often reached different conclusions owing to the complexity of the problem (e.g.…”