[Paper Review] Resolving the cold debris disc around a planet-hosting star . PACS photometric imaging observations of q<SUP>1</SUP> Eridani (HD 10647, HR 506)

R. Liseau, C. Eiroa+33 more|arXiv (Cornell University)|Jan 1, 2010

Stellar, planetary, and galactic studies14 references31 citations

TL;DR

Using Herschel PACS photometric imaging, this study resolves the cold debris disc around the planet-hosting star q1 Eri at far-infrared wavelengths for the first time, revealing a narrow, 40 AU-wide ring at ~85 AU with a surface density profile consistent with a ring-like structure. This ring is the first observed analogue of the Edgeworth-Kuiper Belt in an exoplanetary system, suggesting the potential presence of an additional, undetected planet, q1 Eri c.

ABSTRACT

Context. About two dozen exo-solar debris systems have been spatially resolved. These debris discs commonly display a variety of structural features such as clumps, rings, belts, excentric distributions and spiral patterns. In most cases, these features are believed to be formed, shaped and maintained by the dynamical influence of planets orbiting the host stars. In very few cases has the presence of the dynamically important planet(s) been inferred from direct observation. Aims. The solar-type star q(1) Eri is known to be surrounded by debris, extended on scales of less than or similar to 30 ''. The star is also known to host at least one planet, albeit on an orbit far too small to make it responsible for structures at distances of tens to hundreds of AU. The aim of the present investigation is twofold: to determine the optical and material properties of the debris and to infer the spatial distribution of the dust, which may hint at the presence of additional planets. Methods. The Photodetector Array Camera and Spectrometer (PACS) aboard the Herschel Space Observatory allows imaging observations in the far infrared at unprecedented resolution, i.e. at better than 6 '' to 12 '' over the wavelength range of 60 mu m to 210 mu m. Together with the results from ground-based observations, these spatially resolved data can be modelled to determine the nature of the debris and its evolution more reliably than what would be possible from unresolved data alone. Results. For the first time has the q(1) Eri disc been resolved at far infrared wavelengths. The PACS observations at 70 mu m, 100 mu m and 160 mu m reveal an oval image showing a disc-like structure in all bands, the size of which increases with wavelength. Assuming a circular shape yields the inclination of its equatorial plane with respect to that of the sky, i > 53 degrees. The results of image de-convolution indicate that i likely is larger than 63 degrees, where 90 degrees corresponds to an edge-on disc. Conclusions. The observed emission is thermal and optically thin. The resolved data are consistent with debris at temperatures below 30 K at radii larger than 120 AU. From image de-convolution, we find that q(1) Eri is surrounded by an about 40 AU wide ring at the radial distance of similar to 85 AU. This is the first real Edgeworth-Kuiper Belt analogue ever observed.

Motivation & Objective

• To determine the optical and material properties of the debris disc around q1 Eri, a planet-hosting star.
• To infer the spatial distribution of dust in the disc using spatially resolved far-infrared data.
• To test whether the observed disc structure is consistent with dynamical sculpting by unseen planets.
• To characterize the disc as a potential analogue of the Solar System's Edgeworth-Kuiper Belt.
• To improve constraints on the disc's physical properties using high-sensitivity, high-resolution PACS observations.

Proposed method

• PACS photometric imaging on the Herschel Space Observatory at 70 µm, 100 µm, and 160 µm with angular resolutions of 6′′, 7′′, and 11.5′′, respectively.
• Use of chop-nod and scan-map observation modes to optimize signal-to-noise and minimize striping in extended source imaging.
• Image deconvolution using the maximum entropy method (MEM) with a PSF derived from α Boo, applied to enhance structural detail.
• Subtraction of the stellar photospheric flux (8 mJy at 100 µm) from the 100 µm scan map prior to deconvolution to isolate the disc emission.
• Modeling of surface brightness profiles along major and minor axes to infer inclination, radial extent, and surface density distribution.
• Comparison of observed fluxes with theoretical models, including single-temperature blackbody and ring-belt composite models.

Experimental results

Research questions

• RQ1 What is the spatial structure and radial extent of the cold debris disc around q1 Eri at far-infrared wavelengths?
• RQ2 Is the observed disc morphology consistent with a ring-like structure, and what are its physical parameters?
• RQ3 Can image deconvolution techniques reveal sub-structure such as a narrow ring or belt in the disc?
• RQ4 Does the disc structure suggest the presence of additional, undetected planets beyond the known planet q1 Eri b?
• RQ5 How does the disc's thermal emission compare with models of a cold, extended debris disc?

Key findings

• The debris disc around q1 Eri is resolved at 70 µm, 100 µm, and 160 µm, showing an oval-shaped brightness distribution that increases in size with wavelength.
• The disc is best described as a narrow ring or belt with a width of approximately 40 AU, centered at a radial distance of ~85 AU from the star.
• Image deconvolution reveals a central depression of ~2% depth and a broadened plateau, consistent with a ring-like structure rather than a smooth disc.
• The disc inclination is constrained to i > 63° from deconvolved images, with a best-fit model suggesting i ≈ 70°, consistent with the edge-on geometry inferred from optical scattered light.
• The dust temperature is below 30 K at radii larger than 120 AU, indicating a cold, outer debris disc.
• The observed disc structure is the first direct analogue of the Edgeworth-Kuiper Belt in an exoplanetary system, suggesting the potential presence of an additional planet, q1 Eri c, at smaller radii.