OGLE?2008–BLG–290: an accurate measurement of the limb darkening of a galactic bulge K Giant spatially resolved by microlensing

摘要

Context. Not only is gravitational microlensing asuccessful tool for discovering distant exoplanets, but it also enablescharacterization of the lens and source stars involved in the lensingevent. Aims. In high-magnification events, the lens causticmay cross over the source disk, which allows determination of theangular size of the source and measurement of its limb darkening. Methods. When such extended-source effects appearclose to maximum magnification, the resulting light curve differs fromthe characteristic Paczynski point-source curve. The exact shape of thelight curve close to the peak depends on the limb darkening of thesource. Dense photometric coverage permits measurement of therespective limb-darkening coefficients. Results. In the case of the microlensing eventOGLE2008-BLG-290, the K giant source star reached a peakmagnification at about 100. Thirteen different telescopes have coveredthis event in eight different photometric bands. Subsequent light-curveanalysis yielded measurements of linear limb-darkening coefficients ofthe source in six photometric bands. The best-measured coefficientslead to an estimate of the source effective temperature of about4700+100-200K.However, the photometric estimate from colour-magnitude diagramsfavours a cooler temperature ofK. Conclusions. Because the limb-darkeningmeasurements, at least in the CTIO/SMARTS2 - and -bands, are among the mostaccurate obtained, the above disagreement needs to be understood. Asolution is proposed, which may apply to previous events where such adiscrepancy also appeared. Key words: gravitational lensing: micro -techniques: high angular resolution - stars: atmospheres - stars:individual: OGLE2008-BLG-290