NIRISS AMI Observations of Extrasolar Planets Around a Host Star
This example science program provides a walk-through of a JWST observing program using NIRISS Aperture Masking Interferometry (AMI), focusing on overarching science goals from the GTO program "NIRISS/AMI Architecture of directly-imaged extrasolar planetary systems" (PI: Rameau) for context. This article discusses how to navigate the Exposure Time Calculator to determine exposure times required to meet the science goals, and how to set up the observation templates in the Astronomer Proposal Tool GUI.
See also: JWST High-Contrast Imaging Roadmap, NIRISS Aperture Masking Interferometry, NIRISS AMI Recommended Strategies, HCI NIRISS Limiting Contrast, Step-by-Step ETC Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star, Step-by-Step APT Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star
The "NIRISS/AMI Architecture of directly-imaged extrasolar planetary systems" GTO program (PI Rameau) will target HD 218396 which has 4 known planetary companions, between 15 and 70 AU, discovered using ground based observations. The goal of this program is to search for additional planets at distances under 15 AU that are suspected to exist based on disturbances in the circumstellar disk.
This example science program follows the steps outlined in the JWST High-Contrast Imaging Roadmap, and assumes that "Stage 1", i.e., familiarizing yourself with high contrast imaging capabilities and observing modes with JWST, has been completed.
Stage 2: Comparing your parameter space to the performance limits and capabilities of the HCI observing modes.
Identify the wavelength range(s) of interest for your intended science. How does this influence (or limit) your choice of science instrument(s), mask(s) and filter(s)?
The spectral energy distribution of a young planet peaks in the thermal infrared regime between 3–5 µm, making near-infrared coverage optimal for this program.
- Determine the apparent separations, between your host and companion source(s) at the time of observation. Which instrument(s) and mask-filter combination(s) can achieve the required working angles?
See also: HCI Inner Working Angle
The goal of this program is to search for planets that are under 15 AU from their host star (0.1" to 0.4"), requiring separations between 75-500 mas to be probed. This regime is inaccessible to the JWST coronagraphs, but can be achieved through NIRISS Aperture Masking Interferometry.
- Determine the companion contrast(s) at the wavelength(s) of interest. Are your observations feasible given the contrast limits of the instrument(s)?
See also: HCI NIRISS Limiting Contrast
The companion contrast, i.e., flux ratio between the exoplanet and host star, we expect to achieve is 10-4, which is feasible with AMI.
- For coronagraphic observations, how important is the azimuthal coverage around your science target?
This program does not use coronagraphy, so we can skip this step.
Is it possible that your scientific goals can be achieved with non-coronagraphic PSF subtraction?
The angular sensitivity and contrast ratios required by this program can not be achieved through regular imaging. We thus choose to observe this source with NIRISS AMI through the F480M filter for the combination of wavelength coverage and sensitivity.
Stage 3: Selecting a PSF calibration strategy - interferometry
- Which observing technique(s) will you include in your PSF calibration strategy?
See also: NIRISS Non-Redundant Mask, NIRISS AMI Recommended Strategies
Data analysis with the non-redundant mask (NRM) requires observations of the target and a point spread function (PSF) reference star. The PSF reference star is used to calibrate out instrumental contributions to the interferometric observables of closure phases (CP) and visibility amplitudes. Closure phases are the sum of the fringe phases from three holes. A closure phase must theoretically be zero for a point source (see Lawson 2000 and Monnier 2003 to learn more about these observables).
Stage 4: Assessing target visibilities and allowed position angles
- Familiarize yourself with JWST position angles, coordinate systems, and pointing constraints.
See also: JWST Position Angles, Ranges, and Offsets, JWST Instrument Ideal Coordinate Systems, JWST Observatory Coordinate System and Field of Regard
- Determine the viewing constraints placed on your target.
See also: JWST Target Viewing Constraints
There are no specific viewing constraints when using AMI, other than general target visibility (see Step 3 below).
Using one of the JWST target visibility tools, assess your target's visibility and allowed position angles over time.
See also: JWST General Target Visibility Tool Help
We used the general target visibility tool to assess the allowed position angles for HD 218396 (Figure 1), using coordinates from Gaia Data Release 2 (RA = 23:07:28.8327, Dec = +21:08:2.53). We ran the target visibility tool with the following command:
jwst_gtvt 23:07:28.8327 21:08:2.53 --instrument niriss
- In the case of known companions, consider whether your observations require any restrictions on the orientation of the instrument field of view (FOV)/detector being referenced
The target is not in a binary star system, so restrictions on the orientation of the FOV are not required.
Steps 5 and 6 refer to coronagraphic observations and can thus be skipped for this example science program.
Stage 5: Use the Exposure Time Calculator (ETC) to determine your exposure parameters
To determine the exposure parameters for this observation using the JWST Exposure Time Calculator (ETC), please see the article Step-by-Step ETC Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star.
Stage 6: Select a suitable point spread function (PSF) calibrator
- Well-known: Is the target a known good PSF reference star?
When picking a PSF reference star, it is important to choose a star that is single (i.e., not in a binary system). In general, we recommend observing more than one PSF reference star if the reference star has not been previously observed interferometrically to mitigate against using an undiscovered multiple-star system as a point source reference. Websites maintained by the U.S. Naval Observatory and the Jean-Marie Mariotti Center are useful for this check.
From checking these resources, we see that HD 218172 is a single star system. Please note that the GTO program uses a different reference star and HD 218172 is another reference star that fits the criteria.
Schedulability: Do the visibility windows of the science target and the PSF calibrator overlap at the time of the desired observation?
Similar to step 3 in Stage 4, we ran the general target visibility tool on the PSF reference star, HD 218172 as shown below:
jwst_gtvt 23:05:35.3371 20:14:27.69 --instrument niriss
The visibility windows of the calibrator star (Figure 2) match the windows of the source (Figure 1).
- Proximity: Is the PSF calibrator in relatively close proximity to the science target?
The PSF calibrator star (HD 218172; RA = 23:05:35.3371, Dec = 20:14:27.69) is separated from the source (HD 218396; RA = 23:07:28.8327, Dec = 21:08:2.53) by 59.8 arc minutes, and is thus in close proximity.
- Avoidance of Binary: Is the PSF calibrator a single and unresolved source?
According to our check in Step 1, this PSF calibrator is a single star that does not have a debris disk.
- Spectral Type: Does the PSF calibrator share the same spectral properties as the science target?
The PSF calibrator star has a spectral type of F8V, and the science target has a spectral type of F0V, so the spectral properties are similar.
- Brightness: Is the PSF calibrator similar in magnitude to the science target?
The PSF calibrator star has a magnitude of M = 5.83 (Vega) and the science target has a magnitude of M = 5.26 (Vega).
Stage 7: Decide on observing strategy
- Consider the total number of observations you will require for your observing program
We will have 2 observations using the NRM with conjunction with the F480M filter: one for the source (HD 218396) and one for the PSF reference star (HD 218172).
- At the observation level: consider how you will organize (group) your observations
The target and reference observations should be scheduled close in time so that the telescope is in a similar state, thermal or otherwise, for all the observations. The science target and PSF reference star should be observed using the same telescope optical configuration, so no wavefront correction should occur between any of the observations.
The requirement to observe the science target and reference star back-to-back can be specified in the Astronomer's Proposal Tool.
The remaining steps in Stage 7 of the High-Constrast Imaging Roadmap are not applicable to this program since we are observing through only one filter, we do not need a position angle offset (as is sometimes needed for coronagraphic observations if part of the scene is blocked by an occulting mask), and we are only observing one science target.
Stage 8: Implementation in the Astronomer's Proposal Tool (APT)
For details filling out the Astronomer's Proposal Tool (APT) for this example science program, please see the article Step-by-Step APT Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star.
Lawson P. R. 2000
Principles of Long Baseline Stellar Interferometry
Course notes from the 1999 Michelson Summer School, held August 15-19, 1999. Edited by Peter R. Lawson. Published by NASA, Jet Propulsion Laboratory, California
Monnier J. D. 2003, Reports on Progress in Physics, 66, 789
Optical interferometry in astronomy
Websites to check whether a star has been observed interferometrically and found to be a single star:
U.S. Naval Observatory http://www.astro.gsu.edu/wds/single/singleframe.html
Jean-Marie Mariotti Center http://www.jmmc.fr/searchcal_page.htm