NIRCam Time-Series Imaging of HAT-P-18 b
This article illustrates a JWST observing program using NIRCam time-series imaging to search for the secondary eclipse of a transiting exoplanet. This includes determining exposure times with the Exposure Time Calculator (ETC), and setting up the observation template with the Astronomer's Proposal Tool (APT).
The goal of this program is to detect the secondary eclipse of the extrasolar planet HAT-P-18 b using NIRCam's time-series imaging mode. HAT-P-18 b is a low-gravity giant planet with a predicted equilibrium temperature of 852 K, a radius of 0.995 RJ, and a mass of 0.2 MJ (Wallack et al 2019), placing it in an interesting mass range between Neptune and Saturn with relatively low surface gravity.
lts equilibrium temperature is near the transition from CO to CH4-dominated chemistry, so departures from chemical equilibrium could be evaluated by measuring its composition at a range of atmospheric temperatures and pressures. It would be best to do this via both transmission and emission spectroscopy observations. Hartman et al. (2011) first detected the transit. Detecting the secondary eclipse is difficult with Spitzer, but straightforward with JWST using NIRCam and its F444W filter. We have constructed this observing program to search for the secondary eclipse so that emission spectroscopy can be conducted in subsequent programs. Measuring the planet-to-star flux at two NIRCam wavelengths simultaneously will also constrain the planet day-side temperature and the day-night energy transport and circulation.
- RA = 17h 05m 23.151s Dec = +33° 00′ 44.97″ (J2000)
- Teff = 4800 K dwarf
- K = 10.23 mag (Vega)
- Tc = 2454715.02254 BJD (primary transit epoch)
- Ts = 2454717.65 BJD (secondary transit epoch)
- P = 5.508023 ± 0.000006 days (period)
- Planet Teq = 852 K
- Rp = 0.995 RJ
- W = T14 = 2.71 hours (total transit duration)
If the planet and star are both blackbodies, then these parameters predict that the planet / star signal will be about 433 ppm. This should be a straightforward detection (much larger than the anticipated systematic noise level) provided that enough photo-electrons are collected.
Step 1 - Determine Required Wavelength Coverage
Main article: Time Series Observations Roadmap
The emission we aim to detect in the secondary eclipse is observable at near-infrared wavelengths, using the NIRCam F444W filter.
Step 2 - Determine Array Configuration
Several subarrays are available for NIRCam Time Series Imaging observations. Given that targets are typically bright point sources and transit observations can be many hours in duration, it may be advisable to choose the smallest possible subarray to avoid data volumes exceeding the capacity of the Solid State Recorders (SSRs). For this observation we therefore choose the smallest subarray available for Time Series Imaging observations on NIRCam, SUB64P, which measures 64 x 64 pixels (corresponding to 2.0" and 4.0" fields of view in the short- and long-wavelength channels, respectively). This subarray has a frame read time of 0.05016 seconds.
We choose the RAPID readout pattern for this observation, for which there is no averaging of reads into groups.
For all types of time series observations (TSOs) with NIRCam, the short- and long-wavelength channel detectors must be configured with the same readout pattern, subarray configuration, and exposure time parameters (i.e., number of groups, number of integrations).
Step 3 - Choose Pupil/Filter Configuration
NIRCam Time Series Imaging can be performed simultaneously in the short- and long-wavelength channels. For such observations, users should choose filters with similar sensitivities and bright source limits; for example 2 broad-band filters such as F150W and F356W, or two narrow-band filters such as F212N and F323N. Note that for NIRCam Time Series Imaging, both channels are configured for photometry, i.e. in this template the user cannot perform simultaneous photometry and grism spectroscopy in the same observation. For very bright targets, weak lenses are available to defocus the target and avoid saturation.
For this example observation, we combine the medium-band F210M filter and the wide-band F440W filter for the two channels. Due to the steep spectrum of the source, we chose a medium-band rather than wide-band filter in the short wavelength channel to avoid saturation while observing simultaneously with both channels.
Step 4 - Calculate Required Exposure Configuration using the JWST Exposure Time Calculator (ETC)
Main article: JWST Exposure Time Calculator Overview
The Step-by-Step ETC Guide for NIRCam Time-Series Imaging Science Use Case walks the user through navigating the JWST Exposure Time Calculator (ETC) to determine exposure parameters appropriate for the science goals for this program, providing a conservative average SNR estimate.
Because the implementation of the signal to noise calculation in the ETC is optimised for non-TSOs, it does not accurately compute the SNR for long transit-like observations. For transit-type observations, the duration of the exposure is typically determined by the duration of the event being captured rather than SNR requirements. We therefore recommend that the ETC is used to determine the optimal SNR in a single integration (i.e. high enough for the detection and following the appropriate instrument guidance on saturation), and from there multiplying the number of groups by the required number of integrations to cover the required time period.
Step 5 - Determine the Appropriate Target Acquisition Strategy in ETC
All NIRCam TSOs require target acquisition to place the target at the appropriate pointing location. Please refer to the above pages for a detailed description of this procedure, and the various options for execution. The ETC can be used with dedicated Target Acquisition calculation templates for calculating the optimal number of groups to reach the recommended SNR (SNR ≥ 30).
The Step-by-Step ETC guide discusses how the exposure parameters are chosen for this program's TA.
Step 6 - Complete the Astronomer Proposal Tool (APT) template
The Astronomer Proposal Tool (APT) is used to submit JWST proposals. The Step-by-Step APT Guide for NIRCam Time-Series Imaging Science Use Case provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template.
Hartman, J. D., Bakos, G. A., Sato B., et al. 2011, arXiv:1007:4850
HAT-P-18b and HAT-P-19b: Two Low-Density Saturn-Mass Planets Transiting Metal-Rich K Stars
Wallack, N. L, Knutson, H. A., Morley, C. A., et al. 2019, arXiv:1908.00014
Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses