Observing exoplanets with the JWST NIRCam grisms

The near-infrared camera (NIRCam) on the James Webb Space Telescope (JWST) will incorporate 2 identical grisms in each of its 2 long wavelength channels. These transmission gratings have been added to assist with the coarse phasing of the JWST telescope, but they will also be used for slitless wide-field scientific observations over selectable regions of the λ = 2.4 − 5.0 μm wavelength range at spectroscopic resolution R ≡ λ/δλ ≃ 2000. We describe the grism design details and their expected performance in NIRCam. The grisms will provide point-source continuum sensitivity of approximately AB = 23 mag in 10,000 s exposures with S/N = 5 when binned to R = 1000. This is approximately a factor of 3 worse than expected for the JWST NIRSpec instrument, but the NIRCam grisms provide better spatial resolution, better spectrophotometric precision, and complete field coverage. The grisms will be especially useful for high precision spectrophotometric observations of transiting exoplanets. We expect that R = 500 spectra of the primary transits and secondary eclipses of Jupiter-sized exoplanets can be acquired at moderate or high signal-to-noise for stars as faint as M = 10 − 12 mag in 1000 s of integration time, and even bright stars (V = 5 mag) should be observable without saturation. We also discuss briefly how these observations will open up new areas of exoplanet science and suggest other unique scientific applications of the grisms.