NIRCam Time-Series Imaging of HAT-P-18 b

Example Science Program #29

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).

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Science motivation

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 0.197 MJ (Hartman et al. 2011), placing it in an interesting mass range between Neptune and Saturn with relatively low surface gravity.

HAT-P-18 b shows H2O absorption in its HST WFC3 G141 transmission spectrum (Tsiaras et al. 2017). Its 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, and 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.

Here are the system's star and Planet parameters adopted here (see Hartman et al. 2011):

  • RA = 17h 05m 23.151s Dec = +33° 00′ 44.97″ (J2000)
  • Teff = 4800 K dwarf
  • K = 10.23 mag (Vega)
  • Tc = 2454715.02174 BJD
  • Ts = 2454717.65 BJD
  • P = 5.508023 ± 0.000006 days
  • Planet Teq = 852 K
  • Rp = 0.995 RJ
  • W = T14 = 2.71 hours

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.

The Step-by-Step ETC Guide 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.

The ETC includes an error term for residual flat field errors which affects long exposures, and significantly underestimates the SNR for exoplanet transit spectroscopy where we take relative measurements. For exposures longer than ~10,000s, ETC calculations have a "noise floor" above which an increase in exposure time no longer results in an increase in SNR that scales with the square root of the exposure time. Since we are making relative measurements on the same pixels for exoplanet transit spectroscopy, our precision is not affected by the "noise floor" imposed by the residual flat field errors. Using the JWST ETC to compute a full exposure with 18067 integrations is not yet possible for time series observations (TSOs). Therefore, as detailed in the we must use in the Step-by-Step ETC Guide, we must use the Poisson limit to compute the SNR for the full exposure, from the SNR of a single integration.

The Astronomer Proposal Tool (APT) is used to submit JWST proposals. The Step-by-Step APT Guide provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template. 

Continue the tutorial:
Step-by-Step ETC Guide (uses Poisson limit)

Step-by-Step APT Guide

Concluding comments

This may not be the most efficient way to search for the secondary eclipse of HAT-P-18; a time-series spectroscopy program could both search for the eclipse and obtain emission spectra if successful. However, we present it as a simple use case for NIRCam time-series imaging. Other use cases for time-series imaging could include multi-object variability or astroseismology studies in stellar clusters, variability and rotation studies of spatially-resolved objects (e.g., solar system bodies), and studies of stochastic phenomena. These would take good advantage of NIRCam's high spatial resolution and wide-field capabilities.


Hartman et al. (2011, ApJ, 726, 52): ADS

Tsiaras, et al. (2017, arXiv:1704.05413): ADS



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