Publikace ÚTF

OGLE 2008–BLG–290: an accurate measurement of the limb

Group of people, Heyrovský D.

Context. Not only is gravitational microlensing a successful tool for discovering distant exoplanets, but it also enables characterization of the lens
and source stars involved in the lensing event.
Aims. In high-magnification events, the lens caustic may cross over the source disk, which allows determination of the angular size of the source
and measurement of its limb darkening.
Methods. When such extended-source effects appear close to maximum magnification, the resulting light curve differs from the characteristic
Paczy´nski point-source curve. The exact shape of the light curve close to the peak depends on the limb darkening of the source. Dense photometric
coverage permits measurement of the respective limb-darkening coefficients.
Results. In the case of the microlensing event OGLE 2008-BLG-290, the K giant source star reached a peak magnification at about 100. Thirteen
different telescopes have covered this event in eight different photometric bands. Subsequent light-curve analysis yielded measurements of linear
limb-darkening coefficients of the source in six photometric bands. The best-measured coefficients lead to an estimate of the source effective temperature
of about 4700+100
−200 K. However, the photometric estimate from colour-magnitude diagrams favours a cooler temperature of 4200 ± 100 K.
Conclusions. Because the limb-darkening measurements, at least in the CTIO/SMARTS2 Vs- and Is-bands, are among the most accurate obtained,
the above disagreement needs to be understood. A solution is proposed, which may apply to previous events where such a discrepancy also
Key words.

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