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

J. Wiegert, R. Liseau+20 more|Chalmers Publication Library (Chalmers University of Technology)|Jan 27, 2014

Stellar, planetary, and galactic studies84 references9 citations

TL;DR

This study investigates the dust content around the α Centauri A and B binary stars using Herschel and APEX observations, combining far-infrared and submillimeter photometry with radiative transfer models. It finds marginal 2.5σ excesses at 24 μm indicating possible zodical-like dust belts with fractional luminosities ~10–100× higher than the solar system’s zodiacal cloud, while far-infrared data constrain upper limits to dust emission, suggesting low dust levels consistent with Kuiper belt-like systems.

ABSTRACT

[Abridged] Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby binary aCentauri have higher than solar metallicities, which is thought to promote giant planet formation. We aim to determine the level of emission from debris in the aCen system. Having already detected the temperature minimum, Tmin, of aCenA, we here attempt to do so also for the companion aCenB. Using the aCen stars as templates, we study possible effects Tmin may have on the detectability of unresolved dust discs around other stars. We use Herschel and APEX photometry to determine the stellar spectral energy distributions. In addition, we use APEX for spectral line mapping to study the complex background around aCen seen in the photometric images. Models of stellar atmospheres and discs are used to estimate the amount of debris around these stars. For solar-type stars, a fractional dust luminosity fd 2e-7 could account for SEDs that do not exhibit the Tmin-effect. Slight excesses at the 2.5 sigma level are observed at 24 mu for both stars, which, if interpreted to be due to dust, would correspond to fd (1-3)e-5. Dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the aCen stars, viz.

Motivation & Objective

• To determine the level of circumstellar dust emission around the solar-type stars α Centauri A and B using far-infrared and submillimeter photometry.
• To assess the impact of the stellar temperature minimum (T_min) effect on the detectability of unresolved debris discs around main-sequence stars.
• To model the dust distribution and estimate dust masses assuming power-law size distributions and dynamical disc models.
• To compare inferred dust luminosities with those of the solar system’s Edgeworth-Kuiper belt and zodiacal cloud for context.
• To evaluate the detectability of low-mass dust discs around stars with and without the T_min phenomenon in their spectral energy distributions.

Proposed method

• Acquired photometric data at 70–870 μm using Herschel-PACS and Herschel-SPIRE instruments on the Herschel space observatory.
• Complemented with ground-based submillimeter observations at 350–870 μm using APEX-LABOCA and APEX-SHeFI for spectral line mapping.
• Constructed stellar spectral energy distributions (SEDs) from 24 μm to 870 μm, incorporating the T_min effect observed in α Cen A and marginally in α Cen B.
• Applied radiative transfer models using RADMC-3D to simulate dust emission from particulate discs with power-law size distributions (n(a) ∝ a^−q).
• Used dynamical disc models with inner and outer radii (R_in = 0.1 AU, R_out = 0.5 AU) and temperature profiles T(r) ∝ r^−0.55 to derive dust mass and fractional luminosity (f_d = L_dust / L_star).
• Constrained model parameters including grain size limits (4–1000 μm), power-law exponents (q = 2.0–4.0, γ = 0–10), and mass density (ρ = 2.5 g cm⁻³).

Experimental results

Research questions

• RQ1 What is the level of circumstellar dust emission around α Centauri A and B, as inferred from far-infrared and submillimeter photometry?
• RQ2 To what extent does the T_min effect in the SEDs of α Cen A and B influence the detectability of unresolved dust discs around solar-type stars?
• RQ3 How do the inferred dust luminosities and masses compare to those of the solar system’s Edgeworth-Kuiper belt and zodiacal cloud?
• RQ4 What are the upper limits on dust mass and fractional luminosity f_d for α Cen A and B based on non-detection of excess emission at long wavelengths?
• RQ5 How do power-law size distributions and dynamical disc models affect the derived dust mass estimates for potential Zodi-like belts around α Cen?

Key findings

• α Centauri A exhibits a clear temperature minimum (T_min) in its SED at 100–160 μm, indicating emission from the chromospheric base, while α Cen B shows a marginal T_min signal.
• For solar-type stars without a T_min effect, a fractional dust luminosity f_d ≈ 2 × 10⁻⁷ could account for missing flux, comparable to the Edgeworth-Kuiper belt in the solar system.
• Marginal 2.5σ excesses at 24 μm are detected in the SEDs of both α Cen A and B, suggesting possible dust emission with f_d ≈ (1–3) × 10⁻⁵, or 10² times the solar zodiacal cloud.
• Dynamical disc modeling with a q = 3.5 power-law size distribution yields a dust mass estimate of ≤ 4 × 10⁻⁶ M_⊕ (≈ 3 × 10²⁰ g) for grains between 4 μm and 1 mm.
• For filled-in T_min emission, an Edgeworth-Kuiper belt-like disc would require ≈ 10⁻³ M_⊕ of dust, consistent with the observed long-wavelength SEDs.
• The inferred dust masses and f_d values for α Cen A and B are consistent with Zodi-like belts that are 10 to 100 times more massive than the solar zodiacal cloud, based on mid-infrared excesses.