Circumstellar Disk Models for Use with Roman Space Telescope Coronagraph Instrument Data Simulations


The Nancy Grace Roman Space Telescope Coronagraph Instrument will be the first high-performance stellar coronagraph using active wavefront control for deep starlight suppression in space, enabling the detection of faint interplanetary dust structures present in the habitable zone of nearby (within 10 pc) Sun-like stars. Simulations at 0.03 AU resolution of dust disks at 10 pc have been produced by the Science Investigation Team, with resampled 21 mas pixels, and convolved with publicly available field-dependent PSFs of the Roman Space Telescope Coronagraph Instrument Hybrid Lyot Coronagraph (HLC) mask and Wide Field of View Shaped Pupil Coronagraph (SPCWIDE) mask designs. The resulting simulation files are appropriate for insertion into simulated observing scenarios for the Roman Space Telescope Coronagraph Instrument, and cover two possible types of disks: narrow disk rings similar to the circumstellar debris disk HR 4796A, and disk annuli with radial dust density power-laws that approximate the density distribution of the Solar System zodiacal cloud (Kennedy et al. 2015, ApJS, 216, 23).

Both types of disks are then scaled to a surface brightness in V of 22.5 magnitudes per square arcsecond at the peak surface brightness of the given disk in a face-on (inclination=0 degrees) configuration. The models have a range of inner radii of the disks. The vertical density distribution of the disk is approximated by a Gaussian with a vertical full width at half-maximum of 0.2*R AU, where R is the radial distance from the star.

The dust in the disks is assumed to have a scattering phase function equivalent to the dust in Saturn's G ring (Hedman & Stark 2015, ApJ, 811, 67).

The exact mass of dust in these models is not set, since the precise optical properties of exo-zodiacal dust disks are currently highly uncertain. However, the scaling is appropriate for the surface brightness of the zodiacal cloud at 1 AU.

Convolved images were constructed using the field-dependent PSFs appropriate for each type of mask, and are publicly available at IPAC at Roman Space Telescope Coronagraph Instrument Off-axis PSFs for HLC narrow FOV imaging mode . The HLC convolutions were validated against Observing Scenario 6 (OS6).

Point sources with flux ratios of 10-8 and 3x10-9, respectively, were convolved at 3.5 and 4.5 λ/D, and their core photometry was compared to the noiseless OS6 images (both with and without model uncertainty factors; MUFs), and verified to be consistent within a few percent of OS6 simulations when scaled by the reported integrated stellar flux (stellar flux value used: 1.15x108 photons/s for a V=5 Vega mag G0 star).

NOTE: If you use these models, please cite Mennesson et al. 2018, Proc. SPIE, 10698.

Additional Information About the Models

Inclination angle ranges from a face-on disk and inclination of 0 degrees to a nearly edge-on disk with an inclination of 89 degrees, with intermediate inclinations of 30, 45, and 60 degrees. Radius in arcseconds for the ring models includes disk radii of 1,2,3,4,5,6,8,10, and 12 AU. The "zodi" models are annuli with an outer radius of 14 AU and the model radius in arcseconds/10 in the filenames refers to the inner radius of the disk, and includes inner radii of 1,2,3,4,5, 6 and 8 AU. Two files are included to simulate the field stops for both modes that correspond to the extent of the highest contrast regions of each mode. For the HLC, it was assumed that this extends from 3 λ/D to 9 λ/D (λ=575 nm), and for the SPCWIDE it was assumed to extend from 6.5 λ/D to 20 λ/D (λ=825 nm).

Scaling Models

The scaling used for these models as a function of inclination angle implies that each model has the same total mass of dust for a given radius model. One can roughly infer how many zodis of surface brightness are present by multiplying by the ratio of the radius to 1 AU (or 0.1") squared. For example, the ring_inc0_r0.5.fits full resolution model would correspond to a surface brightness at the disk radius equivalent to 25x the surface brightness of Zodiacal dust at the same radius. Models can thus be linearly scaled up or down to the desired "zodi" level.

Users interested in making use of self-consistent dust disk models are encouraged to use the radiative transfer models that include IR SEDs and images from the archival WFIRST Preparatory Science Project: The Circumstellar Environments of Exoplanet Host Stars (PI: C. Chen, 2010). While these models are more limited in geometry and are based on OS6, they can still provide a useful foundation to explore how surface brightness relates to disk mass.

The simulated ring model data can be downloaded here. The simulated zodi model data can be downloaded here. Detailed information on the data products, along with further explanation of the simulation workflow, can be found in this README file. The instrument parameters used in these simulations are from Cycle 6.

Other Circumstellar Simulations

The Macintosh Science Investigation team has created circumstellar disk models for use with Roman Space Telescope Coronagraph Instrument data simulations. See the public dataset and associated publication.