Impact of angular differential imaging on circumstellar disk images

Abstract

Context. Direct imaging of circumstellar disks requires high-contrast and high-resolution techniques. The angular differential imaging (ADI) technique is one of them, initially developed for point-like sources but now increasingly applied to extended objects such as disks. This new field of application raises many questions because the disk images reduced with ADI depend strongly on the amplitude of field rotation and the ADI data reduction strategy. Both of them directly affect the disk observable properties. Aims. Our aim is to characterize the applicability and biases of some ADI data reduction strategies for different disk morphologies. A particular emphasis is placed on parameters mostly used for disks such as their surface brightness distribution, their width if the disk is a ring, and local features such as gaps or asymmetries. We first present a general method for predicting and quantifying those biases. In a second step we illustrate them for some widely used ADI algorithms applied to typical debris disk morphologies: inclined rings with various inner/outer slopes and width. Last, our aim is also to propose improvements of classical ADI to limit the biases on extended objects. Methods. Simulated fake disks seen under various observing conditions were used to reduce ADI data and quantify the resulting biases. These conclusions are complemented by previous results from NaCo L' real-disk images of HR 4796A. Results. As expected, ADI induces flux losses on disks. This makes this technique appropriate only for low-to medium-inclination disks. A theoretical criterion is derived to predict the amount of flux loss for a given disk morphology, and quantitative estimates of the biases are given in some specific configurations. These biases alter the disk observable properties, such as the slopes of the disk surface brightness or the radial/azimuthal extent of the disk. Additionally, this work demonstrates that ADI can very easily create artificial features without involving astrophysical processes. For example, a particularly striking feature appears for a ring when the amplitude of field rotation is too small. The two ring ansae are surrounded by two flux-depleted regions, which makes them appear as bright blobs. This observation does not require any astrophysical process such as dust blown by radiation pressure, as previously proposed in H-band images of HR 4796A. Conclusions. The ADI techniques behave as spatial filtering algorithms and can bias disk observables. Therefore, the filtering process needs to be properly calibrated when deriving disk parameters from processed images.

First Scattered-light Images of the Gas-rich Debris Disk around 49 Ceti

“…To calibrate the PSF-subtraction throughput and artifacts, we used the forward-modeling methods described in Choquet et al (2016) and Milli et al (2012) for the NICMOS and SPHERE images, respectively. The forward-model intensities were scaled to the same fluxes as the images within ellipses slightly larger than and including the disk, and reduced chi-square values c red 2 were computed within the same area: 1 8×5 2 semiminor and semimajor axes in the NICMOS image, and 1 4×5 4 in the SPHERE image.…”

Belt(s) of debris resolved around the Sco-Cen star HIP 67497

“…This is likely due to the lower signal-to-noise ratio (S/N) of those data. We derived the flux profile (non-corrected from artifacts introduced by the ADI; Milli et al 2012) along the position angle of the ring from the H2H3, and YJ PCA images (Fig. 2).…”

New constraints on the dust surrounding HR 4796A

“…In that case, the Henyey-Greenstein g coefficient is an additional free parameter that can be used to quantify the anisotropy of scattering. Because the star subtraction is performed using ADI, the disc can be significantly self-subtracted (Milli et al 2012), and we implement the forward-modelling technique already described in Milli et al (2014) for β Pictoris to retrieve the best disc parameters. The results are shown in Table 2.…”