Revealing asymmetrical dust distribution in the inner regions of HD 141569

Abstract

Context. The combination of high-contrast imaging with spectroscopy and polarimetry offers a pathway to studying the grain distribution and properties of debris disks in exquisite detail. Here, we focus on the case of a gas-rich debris disk around HD 141569A, which features a multiple-ring morphology first identified with SPHERE in the near-infrared. Aims. We obtained polarimetric differential imaging with SPHERE in the H-band to compare the scattering properties of the innermost ring at 44 au with former observations in total intensity with the same instrument. In polarimetric imaging, we observed that the intensity of the ring peaks in the south-east, mostly in the forward direction, whereas in total intensity imaging, the ring is detected only at the south. This noticeable characteristic suggests a non-uniform dust density in the ring. With these two sets of images, we aim to study the distribution of the dust to solve for the actual dust distribution. Methods. We implemented a density function varying azimuthally along the ring and generated synthetic images both in polarimetry and in total intensity, which are then compared to the actual data. The search for the best-fit model was performed both with a grid-based and an MCMC approach. Using the outcome of this modelization, we further measured the polarized scattering phase function for the observed scattering angle between 33° and 147° as well as the spectral reflectance of the southern part of the ring between 0.98 and 2.1 μm. We tentatively derived the grain properties by comparing these quantities with MCFOST models and assuming Mie scattering. Results. We find that the dust density peaks in the south-west at an azimuthal angle of 220°~238° with a rather broad width of 61°~127°. The difference in the intensity distributions observed in polarimetry and total intensity is the result of this particular morphology. Although there are still uncertainties that remain in the determination of the anisotropic scattering factor, the implementation of an azimuthal density variation to fit the data proved to be robust. Upon elaborating on the origin of this dust density distribution, we conclude that it could be the result of a massive collision when we account for the effect of the high gas mass that is present in the system on the dynamics of grains. In terms of grain composition, our preliminary interpretation indicates a mixture of porous sub-micron sized astro-silicate and carbonaceous grains. Conclusions. The SPHERE observations have allowed, for the first time, for meaningful constraints to be placed on the dust distribution beyond the standard picture of a uniform ring-like debris disk. However, future studies with a multiwavelength approach and additional detailed modeling would be required to better characterize the grain properties in the HD 141569 system.

Reference-star differential imaging on SPHERE/IRDIS

“…In HD 141569, we detect more disk emission in the west side of the ring R3 than the ADI detection in Singh et al (2021). We also detect a large fraction of the full ring including the backside for AK Sco, while the previously published ADI result (Janson et al 2016) was only able to see the forward-scattered side.…”

Observations of circumstellar disks in scattered light with SPHERE at the VLT

“…One plausible explanation is that such multiple-ring geometries are generated by instabilities rather than shaped by planets, for instance based on the so-called "photo electric" effect [107], which implies that the gas compound is not negligible in these system with gas to dust ratio close to unity. CO gas was detected at least in HIP 73145 and HD 141569, with the latter being categorized as a gas-rich debris disk and showing a very strong match between the gas and dust distribution [108,109]. To explain this it has been shown that collisions within planetesimals in a belt can release both second generation dust and gas explaining their colocation [110].…”

The JWST Early Release Science Program for the Direct Imaging & Spectroscopy of Exoplanetary Systems

“…Finally, in all NIRCam and MIRI images of HD 141569A, we will scrutinize the disk morphology to look for direct and indirect evidence of planets. The system shows large cavities that suggest dynamical clearing by planets (e.g., Wyatt 2005;Konishi et al 2016;Perrot et al 2016), and asymmetries in the dust density distribution possibly caused by a massive collision between planetesimals (Singh et al 2021). The longer wavelengths of JWST offer a more favorable contrast regime to probe for the presence of giant planets than existing shorter wavelength observations, as discussed in §1.2.…”