HSTANDSPITZEROBSERVATIONS OF THE HD 207129 DEBRIS RING

Abstract

A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in scattered visible light with the ACS coronagraph on the Hubble Space Telescope (HST) and in thermal emission using MIPS on the Spitzer Space Telescope at λ = 70 μm (resolved) and 160 μm (unresolved). Spitzer IRS (λ = 7-35 μm) and MIPS (λ = 55-90 μm) spectrographs measured disk emission at λ > 28 μm. In the HST image the disk appears as a ∼30 AU wide ring with a mean radius of ∼163 AU and is inclined by 60• from pole-on. At 70 μm, it appears partially resolved and is elongated in the same direction and with nearly the same size as seen with HST in scattered light. At 0.6 μm, the ring shows no significant brightness asymmetry, implying little or no forward scattering by its constituent dust. With a mean surface brightness of V = 23.7 mag arcsec −2 , it is the faintest disk imaged to date in scattered light. We model the ring's infrared spectral energy distribution (SED) using a dust population fixed at the location where HST detects the scattered light. The observed SED is well fit by this model, with no requirement for additional unseen debris zones. The firm constraint on the dust radial distance breaks the usual grain size-distance degeneracy that exists in modeling of spatially unresolved disks, and allows us to infer a minimum grain size of ∼2.8 μm and a dust size distribution power-law spectral index of −3.9. An albedo of ∼5% is inferred from the integrated brightness of the ring in scattered light. The low-albedo and isotropic scattering properties are inconsistent with Mie theory for astronomical silicates with the inferred grain size and show the need for further modeling using more complex grain shapes or compositions. Brightness limits are also presented for six other main-sequence stars with strong Spitzer excess around which HST detects no circumstellar nebulosity

Modelling the huge,Herschel-resolved debris ring around HD 207129

“…In our power-law SAnD model, the best-fit slope of the size distribution is κ ≈ −3.8, which is in good agreement with the value of Krist et al (2010) and slightly steeper than both the canonical value of −3.5 (Dohnanyi 1969) regime (O'Brien & Greenberg 2003). The peak geometrical optical depth is τ = 4 × 10 −4 and the dust amounts to a total of 7.2 × 10 −3 M ⊕ in objects with radii smaller than 1 mm.…”

“…When comparing with the model proposed by Krist et al (2010) for Spitzer images and spectra, the Herschel images provide stronger contraints than the SED (Figs. 5 and 6).…”

“…Shortward of about 25 µm, only the HST detected excess emission (Krist et al 2010). The Marshall et al (2011) 350 44.3 ± 9 Herschel/SPIRE, Marshall et al (2011) 500 25.9 ± 8 Herschel/SPIRE, Marshall et al (2011) 870 5 ± 3 APEX/LABOCA, Nilsson et al (2010) …”

“…Excess emission was first detected by IRAS (Walker & Wolstencroft 1988), and later observed by ISO (Jourdain de Muizon et al 1999), Spitzer (Trilling et al 2008), and APEX (Nilsson et al 2010). Extended emission is seen and particularly the observations of scattered light with the Hubble Space Telescope (HST) (Krist et al 2010) have shown the disk to feature an inclined ring with a reported radius of 163 AU. The presumed underlying belt of planetesimals marks the upper end of the range of radial extents of known debris disks.…”

“…(a) The given confidence intervals include calibration uncertainties. See Marshall et al (2011, their Spitzer/IRS spectrum then clearly samples the rise of the SED (see, e.g., Krist et al 2010). To homogenise the data for our modelling purposes, we extracted a single photometric point at 32 µm.…”