The nature of mid-infrared excesses from hot dust around Sun-like stars

Abstract

Aims. Studies of the debris disk phenomenon have shown that most systems are analogous to the Edgeworth-Kuiper Belt (EKB). However a rare subset of sun-like stars possess dust which lies, in contrast, in the terrestrial planet region. In this study we aim to determine how many sources with apparent mid-infrared excess are truly hosts of warm dust, and investigate where the dust in these systems must lie. Methods. We observed using ground-based mid-infrared imaging with TIMMI2, VISIR and MICHELLE a sample of FGK main sequence stars previously reported to have hot dust. A new modelling approach was developed to determine the constraints that can be set on the radial extent of excess emission in such observations by demonstrating how the detectability of a disk of a given flux as a fraction of the total flux from the system (F disk /F total ) depends primarily on the ratio of disk radius to PSF width and on the uncertainty on that PSF width. Results. We confirm the presence of warm dust around three of the candidates; η Corvi, HD 145263 and HD 202406. For η Corvi modelling constrains the dust to lie in regions smaller than ∼3.5 AU. The modelling constrains the dust to regions smaller than 80−100 AU for HD 145263 and HD 202406, with SED fitting suggesting the dust lies at a few tens of AU. Of two alternative models for the η Corvi excess emission, we find that a model with one hot dust component at less than 0. 164 (<3 AU) (combined with the known submm dust population at ∼150 AU) fits all the data better at the 2.6σ level than an alternative model with two populations of dust emitting in the mid-infrared: hot dust at less than 0. 19 (<3.5 AU) and a mid-temperature component at ∼0. 66 (12 AU). We identify several systems which have a companion (HD 65277 and HD 79873) or background object (HD 53246, HD 123356 and HD 128400) responsible for their mid-infrared excess, and for three other systems we were able to rule out a point-like mid-infrared source near the star at the level of excess observed in lower resolution observations (HD 12039, HD 69830 and HD 191089). Conclusions. Hot dust sources are either young and possibly primordial or transitional in their emission, or have relatively small radius steady-state planetesimal belts, or they are old and luminous with transient emission. High resolution imaging can be used to constrain the location of the disk and help to discriminate between different models of disk emission. For some small disks, interferometry is needed to resolve the disk location.

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

“…For both sources we found there were no background or companion objects in the Spitzer aperture that might be excluded by the MIDI aperture (Smith et al 2008). The 3 Neptune mass planets discovered within 1AU of HD69830 (Lovis et al 2006) are not expected to be brighter in the mid-infrared than 10 −3 F (Burrows et al 2004), and so would only contribute a discrepancy of at most ∼1 mJy at 10 μm to our total intensity measurement.…”

“…Following the 8 m limits on the η Corvi emission presented in Smith et al (2008), we have new data obtained on VISIR at the VLT under proposal 079.C-0259 on 6th April 2007 on the emission around HD69830. The data were all taken in filter Q2 (λ c = 18.72 μm, Δλ = 0.88 μm) with pixel scale 0.…”

“…The data were reduced using custom routines described in detail in Smith et al (2008). In summary, data reduction involved determination of a gain map using the mean values of each frame to determine pixel responsivity (masking off pixels on which emission from the source could fall, equivalent to a sky flat).…”

“…We followed the procedure of Smith et al (2008) to place limits on the size of the disc after no extension was detected. This procedure is described in detail in Smith et al (2008), and involves convolving the PSF model with different disc + star models to determine what combinations of disc parameters (geometry and emission level) would have resulted in a detection of extended emission.…”

“…The predicted dust locations around both sources is 80-90 mas (from SED modelling of the IRS spectra of the targets, Beichman et al 2005;Chen et al 2006), although there remains considerable uncertainty in the true radial location of the emission. Thus far 8 m imaging of η Corvi suggests the dust lies within 3 AU of the star (Smith et al 2008). Here we present VISIR observations of HD69830 and MIDI observations of both targets.…”