A survey of the linear polarization of directly imaged exoplanets and brown dwarf companions with SPHERE-IRDIS

Abstract

Context. Young giant planets and brown dwarf companions emit near-infrared radiation that can be linearly polarized up to several percent. This polarization can reveal the presence of an (unresolved) circumsubstellar accretion disk, rotation-induced oblateness of the atmosphere, or an inhomogeneous distribution of atmospheric dust clouds. Aims. We aim to measure the near-infrared linear polarization of 20 known directly imaged exoplanets and brown dwarf companions. Methods. We observed the companions with the high-contrast imaging polarimeter SPHERE-IRDIS at the Very Large Telescope. We reduced the data using the IRDAP pipeline to correct for the instrumental polarization and crosstalk of the optical system with an absolute polarimetric accuracy <0.1% in the degree of polarization. We employed aperture photometry, angular differential imaging, and point-spread-function fitting to retrieve the polarization of the companions. Results. We report the first detection of polarization originating from substellar companions, with a polarization of several tenths of a percent for DH Tau B and GSC 6214-210 B in H-band. By comparing the measured polarization with that of nearby stars, we find that the polarization is unlikely to be caused by interstellar dust. Because the companions have previously measured hydrogen emission lines and red colors, the polarization most likely originates from circumsubstellar disks. Through radiative transfer modeling, we constrain the position angles of the disks and find that the disks must have high inclinations. For the 18 other companions, we do not detect significant polarization and place subpercent upper limits on their degree of polarization. We also present images of the circumstellar disks of DH Tau, GQ Lup, PDS 70, β Pic, and HD 106906. We detect a highly asymmetric disk around GQ Lup and find evidence for multiple scattering in the disk of PDS 70. Both disks show spiral-like features that are potentially induced by GQ Lup B and PDS 70 b, respectively. Conclusions. The presence of the disks around DH Tau B and GSC 6214-210 B as well as the misalignment of the disk of DH Tau B with the disk around its primary star suggest in situ formation of the companions. The non-detections of polarization for the other companions may indicate the absence of circumsubstellar disks, a slow rotation rate of young companions, the upper atmospheres containing primarily submicron-sized dust grains, and/or limited cloud inhomogeneity.

RHAPSODIE: Reconstruction of High-contrAst Polarized SOurces and Deconvolution for cIrcumstellar Environments

“…It also takes proper account of bad pixels by using weighted maps instead of interpolating them. These bad pixels can cause systematic errors of several tenths of a percent in the polarization measurements as shown by (van Holstein et al 2021) In addition, the effect of bad pixel interpolation could also have some impact when reaching 0.1% polarimetric accuracy. We validated the method on both simulated and archive data from SPHERE/IRDIS and compared its performance with state-of-the-art methods.…”

A SPHERE survey of self-shadowed planet-forming disks

“…All the observations studied in this work were obtained in the NIR with the Infra-Red Dual Imaging and Spectrograph (IRDIS, Dohlen et al 2008) of SPHERE (Beuzit et al 2019) in dual-beam polarimetric imaging mode (DPI, Langlois et al 2014;van Holstein et al 2020;de Boer et al 2020). This mode allows us to separate the dominant NIR stellar light and the polarized light from the stellar surrounding producing high-resolution and highcontrast maps of the light scattered by the disk.…”

Full characterization of the instrumental polarization effects of the spectropolarimetric mode of SCExAO/CHARIS

“…4 Using polarimetric differential imaging (PDI), the NIR polarimetric modes of the high-contrast imaging instruments SPHERE-IRDIS [7][8][9][10] at the Very Large Telescope, Germini Planetary Imager 5,11 at the Gemini South Telescope, and HiCIAO 12 at the Subaru telescope have been able to successfully image circumstellar disk of various ages. [13][14][15][16][17] SPHERE-IRDIS and the Gemini Planet imager have also been used to search for polarization signals from substellar and planetary objects, [18][19][20][21][22] leading to the polarimetric detections of two spatially unresolved circumsubstellar disks. 22 In 2017, the HiCIAO instrument at the Subaru telescope was decommissioned and replaced with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system.…”

“…[13][14][15][16][17] SPHERE-IRDIS and the Gemini Planet imager have also been used to search for polarization signals from substellar and planetary objects, [18][19][20][21][22] leading to the polarimetric detections of two spatially unresolved circumsubstellar disks. 22 In 2017, the HiCIAO instrument at the Subaru telescope was decommissioned and replaced with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. 23 SCExAO is a multi-band instrument that operates at wavelengths from 600 to 2500 nm.…”

“…Furthermore, these accuracies suffice for quantitative polarimetric observations of circumstellar disks. 22,28 The outline of this work is as follows. In section 2, we give a brief description of polarized light.…”