NIRCam Grism Time-Series Observations of GJ 436b

Example Science Program #30

This page outlines an example science program for JWST/NIRCam grism time series observations (TSO) of exoplanet GJ 436b, including descriptions of how to apply the ETC and the APT tools.

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For this science use case, we illustrate how to measure the 2.4–5.0 µm emission spectrum of the planet GJ 436 b using NIRCam grism time-series observations (TSO).

Science Motivation

GJ 436 is an M2.5 dwarf star located 10 pc away, so it is bright in the near infrared (NIR), with K = 6.1 mag (Vega). It hosts the planet GJ 436 b, which has a mass and radius similar to Uranus and Neptune (22 ME and 4.2 RE) and a zero-albedo equilibrium temperature of 700 K. If in chemical equilibrium, its atmosphere would have relatively high CH4 and relatively low CO and CO2 molecular mixing ratios. However, Stevenson et al. (2010) interpreted the Spitzer 3.6 and 4.5 µm photometric secondary eclipse data to find the reverse to be true, suggesting significant non-equilibrium atmospheric chemistry.

We now describe NIRCam GTO grism time series observations over 2.4–5.0 µm that will be made to investigate this puzzle further. This spectral region covers features of H2O, CH4, CO2, and CO and will be much more diagnostic than the existing Spitzer photometry. Covering this entire wavelength range will require observations of two secondary eclipses, one with the F322W2 filter + Long-wavelength (LW) grism and one with the F444W filter + LW grism. All time-series grism observations are conducted with Module A using GRISMR, which is dispersed along the rows of the detector. The transit time of GJ 436 b is T14 = 0.76 hr. We assume that the secondary eclipse is equally long and we plan to obtain scientific data for a total of 2T14 each visit, with equal time spent during and before / after the secondary eclipse.

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 Step-by-Step PandExo Guide provides a more robust spectroscopic SNR estimate, using the same calculation engine as ETC (Pandeia).

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 (conservative, average SNR estimate)

Step-by-Step PandExo Guide (robust, spectroscopic SNR estimates)

Step-by-Step APT Guide


Stevenson, K.~B., Harrington, J., Nymeyer, S., et al. 2010, Nature, 464, 1161
Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b



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