Angular Differential Imaging Technique Research Articles

The high-albedo, low polarization disk around HD 114082 that harbors a Jupiter-sized transiting planet

Aims. We present new optical and near-infrared images of the debris disk around the F-type star HD 114082 in the Scorpius-Centaurus OB association. We obtained direct imaging observations and analyzed the TESS photometric time series data of this target with the goal of searching for planetary companions to HD 114082 and characterizing the morphology of the debris disk and the scattering properties of dust particles. Methods. HD 114082 was observed with the VLT/SPHERE instrument in different modes – the IRDIS camera in the K band (2.0–2.3 μm) together with the IFS in the Y, J, and H bands (0.95–1.66 μm) using the angular differential imaging technique as well as IRDIS in the H band (1.5–1.8 μm) and ZIMPOL in the I_PRIME band (0.71–0.87 μm) using the polarimetric differential imaging technique. To constrain the basic geometrical parameters of the disk and the scattering properties of dust grains, scattered light images were fitted with a 3D model for single scattering in an optically thin dust disk using a Markov chain Monte Carlo approach. We performed aperture photometry to derive the scattering and polarized phase functions, the polarization fraction, and the spectral scattering albedo for the dust particles in the disk. This method was also used to obtain the reflectance spectrum of the disk and, in turn, to retrieve the disk color and study the dust reflectivity in comparison to the debris disk HD 117214. We also performed the modeling of the HD 114082 light curve measured by TESS using models for planet transit and stellar activity to put constraints on the radius of the detected planet and its orbit. Last, we searched for additional planets in the system by combining archival radial velocity data, astrometry, and direct imaging. Results. The debris disk HD 114082 appears as an axisymmetric debris belt with a radius of ~0.37″ (35 au), an inclination of ~83°, and a wide inner cavity. Dust particles in HD 114082 have a maximum polarization fraction of ~17% and a higher reflectivity when compared to the debris disk HD 117214. This high reflectivity results in a spectral scattering albedo of ~0.65 for the HD 114082 disk at near-infrared wavelengths. The disk reflectance spectrum exhibits a red color at the position of the planetesimal belt and shows no obvious features, whereas that of HD 117214 might indicate the presence of CO2 ice. The analysis of TESS photometric data reveals a transiting planetary companion to HD 114082 with a radius of ~1 RJup on an orbit with a semimajor axis of 0.7 ± 0.4 au. No additional planet was detected in the system when we combined the SPHERE images with constraints from astrometry and radial velocity. We reach deep sensitivity limits down to ~5 MJup at 50 au and ~10 MJup at 30 au from the central star.

Iterative angular differential imaging (IADI): An exploration of recovering disk structures in scattered light with an iterative ADI approach

Context. Distinguishing the signal from young gas-rich circumstellar disks from the stellar signal in near-infrared (NIR) light is a difficult task. Multiple techniques have been developed over the years of which angular differential imaging (ADI) and polarimetric differential imaging (PDI) have been most successful. However, both techniques cope with drawbacks such as self-subtraction. To address these drawbacks, we explore iterative ADI (IADI) techniques to increase signal throughput in total intensity observations. Aims. The aim of this work is to explore the effectiveness of IADI in recovering the self-subtracted regions of disks by applying ADI techniques iteratively. Methods. IADI works by feeding back all positive signal of the result from standard ADI over multiple iterations. To determine the effectiveness of IADI, a model of a disk image is made and post-processed with IADI. We explored two versions of IADI, classical IADI, which uses the median of the data set to reconstruct the point spread function (PSF), and PCA-IADI, which uses principal component analysis to model the PSF. In addition, we explored masking based on polarimetric images and a signal threshold for feeding back signal. Results. Asymmetries are a very important factor in recovering the disk because these lead to less overlap of the disk in the data set. In some cases, we were able to recover a factor ~75 more flux with IADI than with ADI. The Procrustes distance is used to quantify the impact of the algorithm on the scattering phase function. Depending on the level of noise and the ratio between the stellar signal and disk signal, the phase function can be recovered a factor 6.4 in Procrustes distance better than standard ADI. Amplification and smearing of noise over the image due to many iterations did occur. By using binary masks and a dynamic threshold this feedback was mitigated, but it is still a problem in the final pipeline. Finally, observations of protoplanetary disks made with VLT/SPHERE were processed with IADI giving rise to very promising results. Conclusions. While IADI has problems with low-signal-to-noise-ratio (S/N) observations due to noise amplification and star reconstruction, higher S/N observations show promising results with respect to standard ADI.

Searching for Hα-emitting sources in the gaps of five transitional disks

Context. (Pre-)Transitional disks show gaps and cavities that can be related to ongoing planet formation. According to theory, young embedded planets can accrete material from the circumplanetary and circumstellar disks and can be detected using accretion tracers, such as the Hα emission line. Aims. We aim to detect accreting protoplanets within the cavities of five (pre-)transitional disks through adaptive-optics(AO)-assisted spectral angular differential imaging in the optical regime. Methods. We performed simultaneous AO observations in the Hα line and the adjacent continuum using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) with the Zurich Imaging Polarimeter (ZIMPOL) at the Very Large Telescope (VLT). We combined spectral and angular differential imaging techniques to increase the contrast in the innermost regions close to the star and search for the signature of young accreting protoplanets. Results. The reduced images show no clear Hα point source around any of the targets. We report the presence of faint Hα emission around TW Hya and HD163296: while the former is most probably an artifact related to a spike, the nature of the latter remains unclear. The spectral and angular differential images yield contrasts of 6–8 magnitudes at ~100 mas from the central stars, except in the case of LkCa15, with values of ~3 mag. We used the contrast curves to estimate average upper limits to the Hα line luminosity of LHα ~ 5 × 10-6 L⊙ at separations ≥200 mas for TW Hya, RXJ1615, and T Cha, while for HD163296 and LkCa15 we derive values of ~3 × 10−5 L⊙. We estimated upper limits to the accretion luminosity of potential protoplanets, obtaining that planetary models provide an average value of Lacc ~ 10−4 L⊙ at 200 mas, which is about two orders of magnitude higher than the Lacc estimated from the extrapolation of the LHα - Lacc stellar relationship. Conclusions. When considering all the objects observed with SPHERE/ZIMPOL in the Hα line, 5 in this work and 13 from the literature, we can explain the lack of protoplanet detections by a combination of factors, such as a majority of low-mass, low-accreting planets; potential episodic accretion; significant extinction from the circumstellar and circumplanetary disks; and the fact that the contrast is less favorable at separations of smaller than 100 mas, where giant planets are more likely to form.

A Search for Companions via Direct Imaging in the DSHARP Planet-forming Disks

Abstract The “Disk Substructures at High Angular Resolution Project” (DSHARP) has revealed an abundance and ubiquity of rings and gaps over a large sample of young planet-forming disks, which are hypothesized to be induced by the presence of forming planets. In this context, we present the first attempt to directly image these young companions for 10 of the DSHARP disks, by using the NaCo/VLT high-contrast observations in L′ band instrument and angular differential imaging techniques. We report the detection of a point-like source candidate at 11 (174.9 au) for RU Lup and at 042 (55 au) for Elias 24. In the case of RU Lup, the proper motion of the candidate is consistent with a stationary background contaminant, based on the astrometry derived from our observations and available archival data. For Elias 24 the point-like source candidate is located in one of the disk gaps at 55 au. Assuming that it is a planetary companion, our analysis suggests a mass ranging from 0.5M J up to 5M J, depending on the presence of a circumplanetary disk and its contribution to the luminosity of the system. However, no clear confirmation is obtained at this stage, and follow-up observations are mandatory to verify that the proposed source is physical, comoving with the stellar host, and associated with a young massive planet sculpting the gap observed at 55 au. For all the remaining systems, the lack of detections suggests the presence of planetary companions with masses lower than 5M J, based on our derived mass detection limits. This is consistent with predictions of both hydrodynamical simulations and kinematical signatures on the disk and allows us to set upper limits on the presence of massive planets in these young disks.

Medium-resolution integral-field spectroscopy for high-contrast exoplanet imaging

Context. Angular differential imaging (ADI) and spectral differential imaging (SDI) are well-established high-contrast imaging techniques, but their application is challenging for companions at small angular separations from their host stars. Aims. The aim of this paper is to investigate to what extent adaptive-optics assisted, medium-resolution (R ~ 5000) integral field spectrographs (IFS) can be used to directly detect the absorption of molecular species in the spectra of planets and substellar companions when these are not present in the spectrum of the star. Methods. We analysed archival data of the β Pictoris system taken with the SINFONI integral field spectrograph located at ESO’s Very Large Telescope, originally taken to image β Pictoris b using ADI techniques. At each spatial position in the field, a scaled instance of the stellar spectrum is subtracted from the data after which the residuals are cross-correlated with model spectra. The cross-correlation co-adds the individual absorption lines of the planet emission spectrum constructively, while this is not the case for (residual) telluric and stellar features. Results. Cross-correlation with CO and H2O models results in significant detections of β Pictoris b with signal-to-noise ratios (S/Ns) of 13.7 and 16.4 respectively. Correlation with a T = 1700 K BT-Settl model provides a detection with an S/N of 22.8. This in contrast to application of ADI, which barely reveals the planet. While the adaptive optics system only achieved modest Strehl ratios of 19–27% leading to a raw contrast of 1:240 at the planet position, cross-correlation achieves a 3σ contrast limit of 2.7 × 10−5 in this 2.5 hr data set, a factor ~40 below the raw noise level at an angular distance of 0.36′′ from the star. Conclusions. Adaptive-optics assisted, medium-resolution IFS, such as SINFONI on the VLT and OSIRIS on the Keck Telescope, can be used for high-contrast imaging utilizing cross-correlation techniques for planets that are close to their star and embedded in speckle noise. We refer to this method as molecule mapping, and advocate its application to observations with future medium resolution instruments, in particular ERIS on the VLT, HARMONI on the ELT and NIRSpec, and MIRI on the JWST.

A SYMMETRIC INNER CAVITY IN THE HD 141569A CIRCUMSTELLAR DISK

ABSTRACT Some circumstellar disks, called transitional or hybrid disks, present characteristics of both protoplanetary disks (significant amount of gas) and debris disks (evolved structures around young main-sequence stars, composed of second generation dust, from collisions between planetesimals). Therefore, they are ideal astrophysical laboratories to witness the last stages of planet formation. The circumstellar disk around HD 141569A was intensively observed and resolved in the past from space, but also from the ground. However, the recent implementation of high contrast imaging systems has opened up new opportunities to re-analyze this object. We analyzed Gemini archival data from the Near-infrared Coronagraphic Imager obtained in 2011 in the H band, using several angular differential imaging techniques (classical ADI, LOCI, KLIP). These images reveal the complex structures of this disk with an unprecedented resolution. We also include archival Hubble Space Telescope images as an independent data set to confirm these findings. Using an analysis of the inner edge of the disk, we show that the inner disk is almost axisymmetrical. The measurement of an offset toward the east observed by previous authors is likely due to the fact that the eastern part of this disk is wider and more complex in substructure. Our precise reanalysis of the eastern side shows several structures, including a splitting of the disk and a small finger detached from the inner edge to the southeast. Finally, we find that the arc at 250 AU is unlikely to be a spiral, at least not at the inclination derived from the first ring, but instead could be interpreted as a third belt at a different inclination. If the very symmetrical inner disk edge is carved by a companion, the data presented here put additional constraints on its position. The observed very complex structures will be confirmed by the new generation of coronagraphic instrument (GPI, SPHERE). However, a full understanding of this system will require gas observations at millimetric wavelengths.

Dissecting the Moth: discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging

The debris disk known as "The Moth" is named after its unusually asymmetric surface brightness distribution. It is located around the ~90 Myr old G8V star HD 61005 at 34.5 pc and has previously been imaged by the HST at 1.1 and 0.6 microns. Polarimetric observations suggested that the circumstellar material consists of two distinct components, a nearly edge-on disk or ring, and a swept-back feature, the result of interaction with the interstellar medium. We resolve both components at unprecedented resolution with VLT/NACO H-band imaging. Using optimized angular differential imaging techniques to remove the light of the star, we reveal the disk component as a distinct narrow ring at inclination i=84.3 \pm 1.0{\deg}. We determine a semi-major axis of a=61.25 \pm 0.85 AU and an eccentricity of e=0.045 \pm 0.015, assuming that periastron is located along the apparent disk major axis. Therefore, the ring center is offset from the star by at least 2.75 \pm 0.85 AU. The offset, together with a relatively steep inner rim, could indicate a planetary companion that perturbs the remnant planetesimal belt. From our imaging data we set upper mass limits for companions that exclude any object above the deuterium-burning limit for separations down to 0.3". The ring shows a strong brightness asymmetry along both the major and minor axis. A brighter front side could indicate forward-scattering grains, while the brightness difference between the NE and SW components can be only partly explained by the ring center offset, suggesting additional density enhancements on one side of the ring. The swept-back component appears as two streamers originating near the NE and SW edges of the debris ring.

Confidence Level and Sensitivity Limits in High‐Contrast Imaging

In long adaptive optics corrected exposures, exoplanet detections are currently limited by speckle noise originating from the telescope and instrument optics, and it is expected that such noise will also limit future high-contrast imaging instruments for both ground and space-based telescopes. Previous theoretical analysis have shown that the time intensity variations of a single speckle follows a modified Rician. It is first demonstrated here that for a circular pupil this temporal intensity distribution also represents the speckle spatial intensity distribution at a fix separation from the point spread function center; this fact is demonstrated using numerical simulations for coronagraphic and non-coronagraphic data. The real statistical distribution of the noise needs to be taken into account explicitly when selecting a detection threshold appropriate for some desired confidence level. In this paper, a technique is described to obtain the pixel intensity distribution of an image and its corresponding confidence level as a function of the detection threshold. Using numerical simulations, it is shown that in the presence of speckles noise, a detection threshold up to three times higher is required to obtain a confidence level equivalent to that at 5sigma for Gaussian noise. The technique is then tested using TRIDENT CFHT and angular differential imaging NIRI Gemini adaptive optics data. It is found that the angular differential imaging technique produces quasi-Gaussian residuals, a remarkable result compared to classical adaptive optic imaging. A power-law is finally derived to predict the 1-3*10^-7 confidence level detection threshold when averaging a partially correlated non-Gaussian noise.