TY - JOUR
T1 - Limb darkening and exoplanets
T2 - Testing stellar model atmospheres and identifying biases in transit parameters
AU - Espinoza, Néstor
AU - Jordán, Andrés
N1 - Publisher Copyright:
© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2015/4/10
Y1 - 2015/4/10
N2 - Limb darkening is fundamental in determining transit light-curve shapes, and is typically modelled by a variety of laws that parametrize the intensity profile of the star that is being transited. Confronted with a transit light curve, some authors fix the parameters of these laws, the so-called limb darkening coefficients (LDCs), while others prefer to let them float in the light-curve fitting procedure. Which of these is the best strategy, however, is still unclear, as well as how and by how much each of these can bias the retrieved transit parameters. In this work we attempt to clarify those points by first recalculating these LDCs, comparing them to measured values from Kepler transit light curves using an algorithm that takes into account uncertainties in both the geometry of the transit and the parameters of the stellar host. We show there are significant departures from predicted model values, suggesting that our understanding of limb darkening still needs to improve. Then, we show through simulations that if one uses the quadratic limb darkening law to parametrize limb darkening, fixing and fitting the LDCs can lead to significant biases - up to ~3 and ~1 per cent in Rp/R*, respectively - which are important for several confirmed and candidate exoplanets. We conclude that, in this case, the best approach is to let the LDCs be free in the fitting procedure. Strategies to avoid biases in data from present and future missions involving high precision measurements of transit parameters are described.
AB - Limb darkening is fundamental in determining transit light-curve shapes, and is typically modelled by a variety of laws that parametrize the intensity profile of the star that is being transited. Confronted with a transit light curve, some authors fix the parameters of these laws, the so-called limb darkening coefficients (LDCs), while others prefer to let them float in the light-curve fitting procedure. Which of these is the best strategy, however, is still unclear, as well as how and by how much each of these can bias the retrieved transit parameters. In this work we attempt to clarify those points by first recalculating these LDCs, comparing them to measured values from Kepler transit light curves using an algorithm that takes into account uncertainties in both the geometry of the transit and the parameters of the stellar host. We show there are significant departures from predicted model values, suggesting that our understanding of limb darkening still needs to improve. Then, we show through simulations that if one uses the quadratic limb darkening law to parametrize limb darkening, fixing and fitting the LDCs can lead to significant biases - up to ~3 and ~1 per cent in Rp/R*, respectively - which are important for several confirmed and candidate exoplanets. We conclude that, in this case, the best approach is to let the LDCs be free in the fitting procedure. Strategies to avoid biases in data from present and future missions involving high precision measurements of transit parameters are described.
KW - Stars: atmospheres
UR - http://www.scopus.com/inward/record.url?scp=84938117705&partnerID=8YFLogxK
U2 - 10.1093/mnras/stv744
DO - 10.1093/mnras/stv744
M3 - Article
AN - SCOPUS:84938117705
SN - 0035-8711
VL - 450
SP - 1879
EP - 1899
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 2
ER -