Re: Extracting u1 and u2 values
Posted by karenacollins on Mar 14, 2018; 3:11am
URL: http://astroimagej.170.s1.nabble.com/Extracting-u1-and-u2-values-tp976p977.html
Hi Rachel,
The u1 and u2 values are fitted to the light curve data, unless those values are "Locked" to specific values. The AIJ transit fitter does *not* use the Claret & Bloemen (2011) theoretical values for u1 and u2, only the light curve data, when those values are not locked. For typical ground-based observations, the light curve precision is not generally good enough for fitting of those values. In that case, I would recommend going to the astroutils website and enter the stellar parameters for the host star (or select a known planet designation depending on your needs). The website will then extract theoretical values for u1 and u2 from the Claret & Bloemen (2011) tables. Those values should then be copied into the u1 and u2 "Prior Center" settings and "Lock" should be enabled next to those values. Here is an example:
The fitter then holds the u1 and u2 values fixed at the Claret & Bloemen (2011) values, and does not fit them, but does fit all of the other "unlocked" transit model parameters. An intermediate choice is to lock one value to the Claret & Bloemen (2011) value and let the other one fit to the light curve data.
Karen
URL: http://astroimagej.170.s1.nabble.com/Extracting-u1-and-u2-values-tp976p977.html
Hi Rachel,
The u1 and u2 values are fitted to the light curve data, unless those values are "Locked" to specific values. The AIJ transit fitter does *not* use the Claret & Bloemen (2011) theoretical values for u1 and u2, only the light curve data, when those values are not locked. For typical ground-based observations, the light curve precision is not generally good enough for fitting of those values. In that case, I would recommend going to the astroutils website and enter the stellar parameters for the host star (or select a known planet designation depending on your needs). The website will then extract theoretical values for u1 and u2 from the Claret & Bloemen (2011) tables. Those values should then be copied into the u1 and u2 "Prior Center" settings and "Lock" should be enabled next to those values. Here is an example:
The fitter then holds the u1 and u2 values fixed at the Claret & Bloemen (2011) values, and does not fit them, but does fit all of the other "unlocked" transit model parameters. An intermediate choice is to lock one value to the Claret & Bloemen (2011) value and let the other one fit to the light curve data.
Karen
On 3/13/2018 3:27 PM, roconnor [via AstroImageJ] wrote:
Hello,
I have ben working my way through the AstroImageJ expanded paper and had a question about how the fixed values of u1 and u2 are extracted. The paper states that they are extracted from Claret & Bloemen (2011) using the website tool http://astroutils.astronomy.ohio-state.edu/exofast/limbdark.shtml. I was wondering how, exactly, those values are extracted-- specifically, does the program use the website tool as it is processing the data? Does it utilize the "select planet" option, even if the planet being observed is not one of the planets included in the database?
Thanks for your clarification!
Rachel
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