Debris disks modeling and characterization: a case study approach to identifying debris disk morphology and asymmetry
Date
2024
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
According to the Catalog of Circumstellar Disks, 323 circumstellar disks have been resolved. What these observations show is that each disk presents its own challenge in terms of modeling and characterization, speaking to the diversity of disk structures. An explanation given for most of the features is that an embedded planet induces secular perturbation. In that case, the various features in a disk can be signpost for planets. By observing debris disk features, it may be possible to find and characterize the planets. ☐ To discover and characterize debris disks, one must obtain high-quality photometry in order to accurately fit photosphere models for the host stars. Since the K-band always falls on the Rayleigh-Jeans side of a star’s spectral energy distribution, its reliability is particularly important for uncovering mid-infrared excess blackbody models to infrared photometry from WISE, Spitzer, and Herschel. Our models allow us to identify disks that subtend large angles on the sky; these disks are excellent candidates for follow-up with ground-based angular differential imaging, Space Telescope Imaging Spectrograph (STIS) on HST, and NIRCam on JWST. Finally, we find that the disks surrounding γ Oph and γ Tri may have warm dust that could be resolvable with JWST’s Mid-Infrared Instrument (MIRI) coronagraph at 23μm. Multiwavelength observations provide benchmarks for collisional cascade models and may reveal planet-disk interactions. ☐ Debris disk morphology is an important aspect of understanding the dynamic processes that shape debris disks. Asymmetry and other distortions can be a sign that a disk is interacting with the Interstellar Medium (ISM) or being stirred internally by planets. Previous direct imaging studies of HD 15115 from STIS and Spectro-Polarimetric High-Contrast Exoplanet Reserch (SPHERE) have shown a brightness asymmetry that may be a sign of one of the aforementioned interactions. In addition, ALMA observations have indicated the possibility of a second, warmer planetesimal belt. For this study, we analyzed direct imaging data obtained by the Gemini Planet Imager (GPI), a ground-based adaptive optics instrument, in K1 band, and in both spectroscopic (spec) and polarimetric (pol) mode. GPI allows us to explore the HD 15115 debris disk within 1” from the star, complementing the analysis done with the other observatories, which observed the disk outside of 1”. We used Markov Chain Monte Carlo to explore the parameter space for the MCFOST radiative transfer model and forward modeled the speckle noise by using the python software pyklip. The residual we obtained showed a good chance that a second debris disk exists, which further confirms the findings from ALMA observations. In addition, our analysis of the spine fitting and disk surface brightness show an asymmetry between the east and west sides of the disk, in agreement with the other observations emission from dust. While Two Micron All-Sky Survey (2MASS) photometry is the near-infrared benchmark for most stars that host debris disks discovered by Spitzer and Wide-Field Infrared Survey Explorer, the brightest stars (Ks < 5) saturated the 2MASS detectors. To accurately model debris disks that orbit the nearest, brightest stars, we construct new photosphere models by replacing saturated 2MASS data with JHK photometry from the NASA Catalog of Infrared Observations, archival RCIC photometry, and saturation-corrected WISE W1 photometry. We estimate the dust temperature and minimum distance from the star by fitting blackbody and modified done on HD 15115. ☐ Protoplanetary disks have also been observed with asymmetry and non-axisymmetric structures. However, due to the gas-rich nature of the disk, the non-axisymmetric structures are shaped by turbulent gas viscosity. The result of such physical processes affects the shape of the spirals, gaps, and misaligned inclinations of the disk components. TW Hydrae is one such disk: HST observations have revealed nonaxisymmetric shadows projected onto the disk surface. While initially, it seemed like the disk might have only one shadow, further observations have revealed that there are two shadows. We hypothesized this is due to two inclined inner rings at different orientations, each revolving with different periods. To model the shadows, we used empirical radial dust distribution and the dust radiative transfer model MCFOST to create the synthetic images. Results indicate that while the two inclined inner rings can fully explain the rotation of the shadows, the changing depth of the shadows requires the ring inclinations to evolve, which is not possible unless the rings are being torqued by an unseen object. We posit that a contributing factor towards the shadows is spiral waves and clumps.
Description
Keywords
Debris disk, Direct imaging data, Protoplanetary disks, Non-axisymmetric structures