Forward Modeling Tool

The Forward Modeling tool, used for planning JWST exoplanet observations, provides an interface to generate and extract theoretical transmission spectra of exoplanets from the grids published by Fortney et al. (2010) as used in Sing et al. (2016) and Goyal et al. (2018). This web-based tool is part of the STScI's Exoplanet Characterization ToolKit (ExoCTK) suite of web-based services.

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See also: PandExo: The Exoplanet ETC 

When writing JWST proposals for transiting exoplanet science, an important component of the process involves having exoplanet atmospheric models available in order to use them to predict what JWST might be able to observe. The atmospheric forward modeling tools of STScI's Exoplanent Characterization Toolkit (ExoCTK) provide a simple interface for retrieving atmospheric models from the grids of Fortney et al. (2010) (as used by Sing et al. 2016) and Goyal et al. (2018), useful for proposal preparation.

Predicting the types of atomic and molecular signatures that JWST will be able to observe in the different wavelength ranges offered by its instruments is crucial for proposal preparation, as it motivates not only the particular choice of the instrument but also the overall observational setup in terms of the required signal to noise to retrieve said features. 



Fortney's grid web-service

The Fortney Grid web-based interface has several options to retrieve the theoretical transmission spectra of exoplanets published by Fortney et al. (2010) as used in Sing et al. 2016. To generate them, the user has to input a set of parameters that define the atmospheric structure of the exoplanet.

  • The first input is the expected planetary temperature (Planet Temp (K), in Kelvins).
  • The second is the Chemistry Type, where the user can choose between chemical equilibrium models that either include or exclude titanium oxide (TiO). 
  • The third is the type of clouds or scattering (Clouds or Scattering) the user wants to implement in the modeling, which follows the prescription defined in Sing et al. (2016).

For clouds, a wavelength-independent cross section component is included to emulate grey clouds, with 1× (Weak Cloud), 10× (Medium Cloud), and 100× (Strong Cloud) the cross section of molecular hydrogen gas at 350 nm. For scattering, the modeling assumes Rayleigh scattering opacity (i.e., a variation of the cross section with the inverse of the wavelength to the fourth power) with an amplitude 10× (Weak Rayleigh), 100× (Medium Rayleigh), and 1,000× (Strong Rayleigh) the amplitude of the cross section implied by Rayleigh scattering produced by molecular hydrogen gas.

Finally, the user needs to input the Planet Mass, Radius and the Stellar Radius in order to generate a transmission spectrum for the planet of interest. A visual of the input page with all above parameters is shown in Figure 1.

Figure 1. Fortney Grid Forward Modeling input interface

Upon clicking Submit, the tool will show the resulting relative transit depth in parts-per-million (ppm) in a plot, like that shown in Figure 2, that the user can explore.
Figure 2. Fortney Grid Forward Modeling output example


To save the model, the user can click on the Save File button which will in turn download the current atmospheric model in its original units in a format ready to be ingested to tools like, e.g., PandExo.



Generic grid web-service 

The web-based interface for the "Generic" grid of Goyal et al. (2018) allows the user to rescale the transmission spectra published in Goyal et al. (2018) for each specific combination of planetary parameters. As in Fortney's grid, the user enters a set of parameters that define the atmospheric structure of the exoplanet.

  • The first set are the Scaling Parameters which define the parameters used to scale the Goyal et al. (2018) spectra: in particular, the atmospheric temperature (in Kelvins) and the planetary gravity (in m/s2).
  • The second set of parameters are the Stellar and Planetary Radii; the former has to be defined in solar units and the second in units of Jupiter radii.
  • Finally, the user also has to select the Standard Model Parameters, which define the overall handling of the atmospheric properties and include the following parameters:
    • Type of condensation, either local or rainout. Rainout condensation assumes material which condenses is depleted from all layers of the model in the condensate forming layer and above. Local condensation assumes material which condenses is depleted independently in each layer of the model atmosphere.
    • Metallicity of the planet. 
    • Carbon-to-oxygen ratio.
    • Type of scattering to be used to generate the models, ranging from 1 (which corresponds to normal scattering) to 1,100 (corresponds to enhanced scattering from the optical to IR).
    • Type of clouds to be used to generate the models, with values from 0 to 1, as seen in Figure 3.

Figure 3. Generic Grid Forward Modeling input interface

Upon clicking Submit, the tool will show the resulting transit depth as a function of wavelength in a plot, like that shown in Figure 4, that the user can explore.
Figure 4. Generic Grid Forward Modeling output example

If needed, the user can download the model to their own computer by clicking on the Save File button. These models can also be directly input to PandExo.



References

Fortney, J. J., et al. 2010, ApJ, 709, 2, 1396-1406.
Transmission Spectra of Three-Dimensional Hot Jupiter Model Atmospheres

Goyal, J. M., et al. 2018, MNRAS, 474, 4, 5158-5185.
A library of ATMO forward model transmission spectra for hot Jupiter exoplanets

Sing, D., et al. 2016, Nature, 529, 7584, 59-62.
A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion




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