The HIP 79977 debris disk in polarized light

Abstract

Context. Debris disks are observed around 10 to 20 % of FGK main-sequence stars as infrared excess emission. They are important signposts for the presence of colliding planetesimals and therefore provide important information about the evolution of planetary systems. Direct imaging of such disks reveals their geometric structure and constrains their dust-particle properties. Aims. We present observations of the known edge-on debris disk around HIP 79977 (HD 146897) taken with the ZIMPOL differential polarimeter of the SPHERE instrument. We measure the observed polarization signal and investigate the diagnostic potential of such data with model simulations.Methods. SPHERE-ZIMPOL polarimetric data of the 15 Myr-old F star HIP 79977 (Upper Sco, 123 pc) were taken in the Very Broad Band (VBB) filter (λ c = 735 nm, ∆λ = 290 nm) with a spatial resolution of about 25 mas. Imaging polarimetry efficiently suppresses the residual speckle noise from the AO system and provides a differential signal with relatively small systematic measuring uncertainties. We measure the polarization flux along and perpendicular to the disk spine of the highly inclined disk for projected separations between 0.2 (25 AU) and 1.6 (200 AU). We perform model calculations for the polarized flux of an optically thin debris disk which are used to determine or constrain the disk parameters of HIP 79977. Results. We measure a polarized flux contrast ratio for the disk of (F pol ) disk /F * = (5.5 ± 0.9) · 10 −4 in the VBB filter. The surface brightness of the polarized flux reaches a maximum of SB max = 16.2 mag arcsec −2 at a separation of 0.2 − 0.5 along the disk spine with a maximum surface brightness contrast of 7.64 mag arcsec −2 . The polarized flux has a minimum near the star < 0.2 because no or only little polarization is produced by forward or backward scattering in the disk section lying in front of or behind the star. The width of the disk perpendicular to the spine shows a systematic increase in FWHM from 0.1 (12 AU) to 0.3 − 0.5 , when going from a separation of 0.2 to > 1 . This can be explained by a radial blow-out of small grains. The data are modelled as a circular dust belt with a well defined disk inclination i = 85(±1.5) • and a radius between r 0 = 60 and 90 AU. The radial density dependence is described by (r/r 0 ) α with a steep (positive) power law index α = 5 inside r 0 and a more shallow (negative) index α = −2.5 outside r 0 . The scattering asymmetry factor lies between g = 0.2 and 0.6 (forward scattering) adopting a scattering-angle dependence for the fractional polarization such as that for Rayleigh scattering. Conclusions. Polarimetric imaging with SPHERE-ZIMPOL of the edge-on debris disk around HIP 79977 provides accurate profiles for the polarized flux. Our data are qualitatively very similar to the case of AU Mic and they confirm that edge-on debris disks have a polarization minimum at a position near the star and a maximum near the projected separation of the main debris belt. The comparison of the polarized flux contr...

HD 117214 debris disk: scattered-light images and constraints on the presence of planets

“…In Sect. 6 we compare the disk HD 117214 with another Sco-Cen debris disk, HIP 79977 (Engler et al 2017), discuss the similarity between the scattering phase function (SPF) that we obtained for the HD 117214 disk and SPFs measured for other debris disks, and investigate the possible presence of giant planets inside the debris belt. We conclude and summarize our results in Sect.…”

“…C.3b). Therefore we tested a second model with another phase function given by a linear combination of two HG scattering functions (Engler et al 2017):…”

“…As the polarized phase function we used the product of the scattering function f (θ, g 1 , g 2 ) (Eq. 7) with the polarization fraction (Engler et al 2017)…”

Optical polarised phase function of the HR 4796A dust ring

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Investigating the presence of two belts in the HD 15115 system

“…The Stokes Q and U parameters were obtained with the double-difference method from calibrated data using a custom routine. We applied the same basic data-reduction steps as described in Engler et al (2017) where the procedure is explained in detail. For the frame centering we have used the same method as for the centering of the total intensity data set (see Sect.…”