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 across detector columns.  We plan to obtain scientific data for a total of 2T14 each visit (T14 = total transit duration), with equal time spent during and before / after the secondary eclipse.

The list of star and planet parameters we adopt for this program is as follows (Simbad, Exoplanet catalog, Torres et al 2008):

  • RA = 11h42m11s.09  Dec = +26d42m23s.66 (J2000)
  • Spectral type = M2.5
  • Teff = 3350 K
  • K = 6.07 (Vega)
  • Tc= 2454221.61588 (primary transit epoch)
  • P = 2.64394± 9.85041e-05 (period)
  • MP = 23.2 ME
  • Planet Teq= 852 K
  • Rp= 4.2 RE
  • W = T14 = 1.02 hrs (total transit duration)

Note that these properties can also be retrieved via the ExoMAST portal, which contain information from both the Exoplanet Catalog and NexSci.



Step 1 - Determine required wavelength coverage

Main article: TSO Roadmap

The spectroscopic transitions we aim to detect in the secondary eclipse emit between 2.4–5.0 µm, requiring near-infrared coverage.



Step 2 - Select an instrument observing mode

Main articles:  NIRCam Grism Time Series
See also: NIRCam Time-Series Observation Recommended StrategiesNIRCam FiltersNIRCam Grisms

Since we wish to observe spectroscopic transitions between 2.4–5.0 µm, we choose NIRCam Grism Time Series over NIRISS Single Object Slitless Spectroscopy (which does not cover this wavelength range). Since this mode uses slitless spectroscopy, we will not have to be concerned with pointing-related flux variations that affect slit spectroscopy (e.g., NIRSpec BOTS Operations).

The grism osbervations in the long-wavelength channel can be used with a number of different broadband filters in the 2.4–5.0 µm range: F277W, F322W2, F356W, and F444W. For these observations, we perform the same length of observation with the F322W2 and with the F444W filters, covering 2 secondary eclipse events.

There is no choice of grating element with this template.



Step 3 - Determine subarray configuration

Main article: NIRCam Detector Subarrays

As our target is bright and isolated, we choose to read out the smallest subarray, SUBGRISM64. This subarray measures 64 rows × 2048 columns, with a single-frame readout time of 0.34 s (assuming 4 outputs). NIRCam subarrays are usually read out with Noutput = 1, but the Noutput = 4 option is available for time series observations for even faster read times.



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 Grism Time-Series of GJ 436b walks the user through navigating the JWST Exposure Time Calculator (ETC) to determine exposure parameters appropriate for the science goals for this program. Users are recommended to use the JWST ETC for initial estimation of the signal-to-noise-ratio in a single integration. 



Step 5 - Use PandExo for more detailed modeling of spectroscopic exoplanet transits

Interested users are encouraged to use PandExo (Batalha et al. 2017) for detailed modeling of exoplanet transits and to optimize exposure configurations for their observing program.



Step 6 - Determine the appropriate target acquisition strategy in ETC

Main articles: NIRCam Grism Time-Series Target AcquisitionJWST ETC NIRCam Target Acquisition

Target Acquisition is required for spectroscopic time series observations with NIRCam, and are performed using a dedicated 32 × 32 pixel subarray using the F335M (medium-band) filter. In this particular case, the exposure time calculations will show that we cannot perform TA on the science target itself without saturation. Analysis has shown that at low levels of saturation, TA can still return accurate centroiding results; this is detailed in the above article.

The Step-by-Step ETC guide discusses how the exposure parameters are chosen for this program's TA.



Step 7 - Complete the Astronomer Proposal Tool (APT) template

Main articles: JWST Astronomers Proposal Tool OverviewNIRCam Grism Time-Series APT Template

The Astronomer Proposal Tool (APT) is used to submit JWST proposals. The Step-by-Step APT Guide for NIRCam Grism Time-Series of GJ 436b provides instructions for filling out the APT observation templates. The exposure parameters determined by the ETC are specified in the APT observation template. 



References

Batalha, Natasha E., Mandell, Avi, Pontoppidan, K., et al. 2017, PASP, 129, 064501

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

Torres, G., Winn, J. N., Holman, M. J. 2008, ApJ 677, 2 (arXiv)
Improved parameters for extrasolar transiting planets

PandExo homepage

Simbad properties for GJ 436

Exoplanet Catalog - entry for GJ 436 b




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