EXOFAST Inputs


This page describes the inputs required to run our applet, which fits transit and RV data using AMOEBA.

BJDTDB | Flux | Error | D0 | ... | DN - A list of times, fluxes, and errors and (optionally) an arbitrary number of detrending variables for each data point. These must be in whitespace delimited columns. For a description of BJDTDB, see our time applet.

Select Planet - This is a drop down menu populated by all of the transiting planets in the exoplanets.org database, which is updated once per day from their site. If a planet is selected, the values from their site are used to begin the fit and any user input "priors" not be used to begin the fit, but will be used to calculate a chi^2 penalty, as outlined below.

Prior - This is the parameter value that the amoeba fit will start at. These starting values for RV parameters are ignored during the RV fit.

Prior Width - This is the uncertainty in the prior. A penalty term will be added to the chi^2 equal to ((value - prior)/prior_width)^2. If you do not want the prior to influence the fit, leave it at "inf". This assumes the errors are Gaussian and uncorrelated.

TC - The time of central transit (may be an integer number of periods off). If only fitting a transit and none is specified, the mean of the input times will be used. If your light curve covers a large fraction of out of transit time (or multiple transits) this default is likely to fail and will have to be refined by hand.

Rp/R* - The radius of the planet, in stellar radii (roughly the square root of the depth of transit).

i - The orbital inclination, in degrees.

a/R* - The semi-major axis of the orbit, in stellar radii.

F0 - The baseline flux of the transit light curve.

u1 - The linear limb darkening coefficient.

u2 - The quadratic limb darkening coefficient.

logg* - The stellar surface gravity. If fitting just the RV, this must be specified, including width (unless it is a known planet and "Spec Priors?" is set).

Teff - The stellar effective temperature. This must be specified, including width (unless it is a known planet and "Spec Priors?" is set). If this is wrong, none of the stellar properties, anything derived from the stellar properties, should be trusted.

[Fe/H] - The stellar metalicity. This must be specified, including width (unless it is a known planet and "Spec Priors?" is set). If this is wrong, none of the stellar properties, anything derived from the stellar properties, should be trusted.

Why do I need logg, Teff, and [Fe/H] at all?
logg, Teff, and [Fe/H] are used to derive the stellar mass and radius via the Torres relations. If there is no transit data, we use these both to derive additional information (e.g., msini). If we have transit data (alone or with RV data), we can derive the stellar mass and radius from logg by itself. However, since that constraint is poor, we add a penalty to the chi^2 equal to ((mass_derived - mass_torres)/err_torres)^2 + ((radius_derived - radius_torres)/err_torres)^2. The stellar parameters are required to fit RV+transit data simultaneously. If you really don't care about these, do the fits seperately, enter a large width on the priors, and then simply ignore the stellar properties and anything derived from them.

P - If RV data is not used, you must specify a precise period. If the transit data do not span multiple periods, you must specify a prior width, too. Otherwise, the starting value will be determined from the RV data.

e - The eccentricity of the orbit. If fitting the transit only, it is strongly recommended that you fix the orbit to be circular by checking the "Circular?" box. The photometry alone provide very little constraint on these parameters, but they appreciably affect the derived physical parameters.

ω* - The argument of periastron of the orbit. If fitting the transit only, it is strongly recommended that you fix the orbit to be circular by checking the "Circular?" box. The photometry alone provide very little constraint on these parameters, but they appreciably affect the derived physical parameters.

K - The RV semi-amplitude, in m/s.

γ - The RV systemic velocity (zero point), in m/s.

Slope - The slope in the RV (m/s/day).

Circular? - Check this box to force the orbit to be circular.

RV Slope? - Check this box to fit a slope to the RV (m/s/day).

Spec Priors? - Check this box to use the logg, Teff, [Fe/H] and their uncertainties from exoplanets.org as the priors in the fit. Note that many uncertainties neglect large systematic uncertainties. We impose a minimum uncertainty of 0.05 in logg, 80 K in Teff, and 0.08 in [Fe/H], when automatically taken from exoplanets.org.

Long Cadence? - Check this box for Long Cadence Kepler data to generate the model by resampling the light curve 10 times uniformly spaced over 29.5 minutes for each data point and averaging.

Debug? - Check this box to redirect stderr to the screen. This may give you some insight into any problems that are occurring. If you think you've found a bug, please copy the output of the program with "Debug?" checked when you email me.

Minimum Period - The minimum period for the Lomb-Scargle periodogram. Use this to explore other periods that are only slightly disfavored. This is not influenced by and does not influence the period prior.

Maximum Period - The maximum period for the Lomb-Scargle periodogram. Use this to explore other periods that are only slightly disfavored. This is not influenced by and does not influence the period prior. The default is the range of input times.

N Min - The number of minimums in the Lomb-Scargle periodogram to fit with a full, Keplerian orbit. The actual number of peaks will be the lesser of this number or the number of local minimums within the range Pmin to Pmax. The fit will always move forward with the fit inside this range that has the lowest chi^2/dof for the Keplerian fit. The default may need to be increased for highly eccentric planets.

Test Input? - If a fit doesn't work, it will help to refine your input values. Check this box to avoid fitting, and just plot the input parameters over your data.


Copyright © Jason Eastman (jason.eastman@cfa.harvard.edu) All Rights Reserved. Questions, comments, or bug reports encouraged.