Resolving the hot dust around HD69830 and eta Corvi with MIDI and VISIR

Most of the known debris discs exhibit cool dust in regions analogous to the Edgeworth-Kuiper Belt. However, a rare subset show hot excess from within a few AU, which is often inferred to be transient. We examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. We use MIDI on the VLTI to observe the debris discs around eta Corvi and HD69830 using baseline lengths from 44-130m. New VISIR observations of HD69830 at 18.7um are also presented. These observations are compared with disc models to place limits on disc size. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8-13um in a manner consistent with the disc flux being well resolved, notably with a dip at 10-11.5um due to the silicate emission feature. The average ratio of visibilities measured between 10-11.5um and 8-9um is 0.934+/-0.015 for HD69830 and 0.880+/-0.013 for eta Corvi over the 4 baselines for each source, a departure of 4 and 9sigma from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7um. The combined limits from MIDI and 8m imaging constrain the warm dust to lie within 0.05-2.4AU for HD69830 and 0.16-2.98AU for eta Corvi. These results represent the first resolution in the mid-IR of dust around main sequence stars. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling. Tentative evidence for a common position angle for the dust at 1.7AU with that at 150AU around eta Corvi, which might be expected if the hot dust is fed from the outer disc, demonstrates the potential of this technique for constraining the origin of the dust and more generally for the study of dust in the terrestrial regions of main sequence stars.

💡 Research Summary

The paper presents the first direct spatial resolution of warm (∼300 K) dust located within a few astronomical units of two main‑sequence stars, HD 69830 and η Corvi, using mid‑infrared interferometry (MIDI on the VLTI) complemented by high‑resolution imaging (VISIR on the VLT). Both stars are known from spectral energy distribution (SED) analyses to host unusually hot excess emission, characterized by a strong 10 µm silicate feature, which is rare among debris‑disc systems that typically show cool, Kuiper‑belt‑like dust. The authors aim to constrain the radial location of this hot dust to discriminate between competing origin scenarios, such as a transient massive collision versus a steady supply from an outer reservoir.

MIDI observations were carried out on baselines ranging from 44 m to 130 m, providing visibility (V²) measurements across the 8–13 µm atmospheric window. The visibility curves for both targets display a clear wavelength‑dependent dip between 10 and 11.5 µm, coincident with the silicate emission peak. By comparing the average visibility in the silicate band (10–11.5 µm) with that in a nearby continuum window (8–9 µm), the authors find ratios of 0.934 ± 0.015 for HD 69830 and 0.880 ± 0.013 for η Corvi. These values deviate from the ratio of unity expected for an unresolved excess by 4σ and 9σ respectively, demonstrating that the dust emission is at least partially resolved on the interferometric baselines.

To translate the visibility deficits into physical size constraints, simple geometric models (circular Gaussian distributions and thin rings) were fitted to the data, taking into account the baseline lengths and the spectral dependence of the visibility. The resulting allowed radial ranges are 0.05–2.4 AU for HD 69830 and 0.16–2.98 AU for η Corvi. The authors also obtained a new VISIR image of HD 69830 at 18.7 µm; the source remained unresolved, providing an independent upper limit of ≈2.4 AU at this longer wavelength, consistent with the MIDI constraints.

The derived radii agree well with those inferred from SED modelling, confirming that the hot dust resides well inside the canonical “habitable zone” of these stars. For η Corvi, the interferometric data hint at a position angle for the inner dust that aligns with the known outer belt at ∼150 AU, suggesting a possible dynamical link: material could be transported inward via scattering by unseen planets, resonant interactions, or Poynting‑Robertson drag, feeding the inner warm component.

The study demonstrates several key points. First, mid‑infrared interferometry can resolve dust structures on sub‑AU scales around nearby main‑sequence stars, opening a new observational window on the terrestrial planet‑forming region. Second, the silicate emission feature not only provides compositional information but also serves as a diagnostic of spatial resolution because the visibility dip directly traces the fraction of flux arising from the resolved component. Third, the presence of warm dust at radii of 0.1–3 AU does not necessarily require a recent catastrophic collision; instead, a steady-state supply from an outer reservoir remains viable, especially when inner and outer disc geometries appear aligned.

In conclusion, the combined MIDI and VISIR observations place robust constraints on the location of hot dust around HD 69830 and η Corvi, confirming that the excess originates from within a few AU of the host stars. These results validate the use of mid‑infrared interferometry for probing the inner regions of debris‑disc systems and provide essential empirical input for models of dust production, transport, and planetary system evolution in the terrestrial zones of Sun‑like stars.