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

Aims. 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 moreover is often inferred to be transient from models for planetesimal belt evolution. In this paper we examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. Methods. We use MIDI on the VLTI to observe the debris discs around η Corvi and HD69830 using baseline lengths from 44–130 m. New VISIR observations of HD69830 at 18.7 μm are also presented. These observations are compared with disc models to place limits on disc size. Results. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8–13 μm in a manner consistent with the disc flux being well resolved, notably with a dip at 10–11.5 μm due to the silicate emission feature. The average ratio of visibilities measured between 10–11.5 μm and 8–9 μ mi s 0.934 ± 0.015 for HD69830 and 0.880 ± 0.013 for η Corvi over the four baselines for each source, a departure of 4 and 9σ from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7 μm. The combined limits from MIDI and 8 m imaging constrain the warm dust to lie within 0.05–2.4 AU for HD69830 and 0.16–2.98 AU for η Corvi. Conclusions. These results represent the first resolution of dust around main sequence stars using mid-infrared interferometry. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling (1.0 AU for HD69830 and 1.7 AU for η Corvi). Tentative evidence for a common position angle for the dust at 1.7 AU with that at 150 AU around η 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.