# Implemented transit models¶

PyTransit implements a set of transit models that all share a common interface that is described in more detail in Transit Models.

## Road Runner model¶

RoadRunner (`pytransit.RoadRunnerModel`

) is a fast and flexible transit model presented in Parviainen (accepted to MNRAS 2020).
I’ll write a proper documentation soon, but these example notebooks should help you up to speed until then

## Uniform model¶

The uniform model (`pytransit.UniformModel`

and `pytransit.UniformModelCL`

) reproduces an exoplanet transit over a uniform disc.
This model is useful when modelling secondary eclipses, or when the effects from the stellar limb
darkening can be ignored.

## Quadratic model¶

The quadratic transit model (`pytransit.QuadraticModel`

and `pytransit.QuadraticModelCL`

) reproduces an exoplanet transit over a
stellar disk with the limb darkening modelled by a quadratic limb darkening model, as presented
in Mandel & Agol (ApJ 580, 2001).

## Oblate star model¶

TBD

## Power-2 model¶

Power-2 model (`pytransit.QPower2Model`

and `pytransit.QPower2ModelCL`

) implements the transit model with a power-2 law
limb darkening profile presented by
Maxted & Gill (A&A 622, A33 2019).
The model is fast to evaluate and aims to model the limb darkening accurately for *cool stars*.

**Notes:**

- Accurate limb darkening model for cool stars.
- Fast to evaluate.

## General model¶

The general model (`pytransit.GeneralModel`

) implements the flexible transit model presented by
Giménez (A&A 450, 2006). The stellar limb
darkening follows a “general” limb darkening model, and the accuracy of limb darkening can be increased as needed.

The model is calculated using a polynomial series and both the number of polynomials npoly and the number of limb darkening coefficients nldc can be set in the initialisation. Higher npoly leads to a more accurate transit model, but also increases computation time. Increasing the number of limb darkening coefficients doesn’t significantly increase computation time, but

**Notes:**

- A flexible model that can model limb darkening accurately.
- Somewhat slower to evaluate than the specialized models.
- PyTransit implements a special “transmission spectroscopy mode” for the general model that accelerates the transit model evaluation significantly for transmission spectroscopy where the light curves are computed from a spectroscopic time series.
- The four-coefficient model presented in Mandel & Agol (ApJ 580, 2001) is not implemented in PyTransit since the Giménez model offers the same functionality with higher flexibility.

## Chromosphere model¶

Optically thin shell model (`pytransit.ChromosphereModel`

and `pytransit.ChromosphereModelCL`

) by
Schlawin et al. (ApJL 722, 2010) to model a transit
over a chromosphere.