OGLE 2004-BLG-254: a K3III Galactic bulge giant spatially resolved by a single microlens

A. Cassan, J. -P. Beaulieu, P. Fouque, S. Brillant, M. Dominik, J. Greenhill, D. Heyrovsky, K. Horne, U. G. Jorgensen, D. Kubas, H. C. Stempels, C. Vinter, M. D. Albrow, D. Bennett, J. A. R. Caldwell, J. J. Calitz, K. Cook, C. Coutures, D. Dominis, J. DonatowiczK. Hill, M. Hoffman, S. Kane, J. -B. Marquette, R. Martin, P. Meintjes, J. Menzies, V. R. Miller, K. R. Pollard, K. C. Sahu, J. Wambsganss, A. Williams, A. Udalski, M. K. Szymanski, M. Kubiak, G. Pietrzynski, I. Soszynski, K. Zebrun, O. Szewczyk, L. Wyrzykowski

Research output: Contribution to journalArticlepeer-review

26 Citations (Scopus)


Aims. We present an analysis of OGLE 2004-BLG-254, a high-magnification (A(o) similar or equal to 60) and relatively short duration (t(E) similar or equal to 13.2 days) microlensing event in which the source star, a Bulge K-giant, has been spatially resolved by a point-like lens. We seek to determine the lens and source distance, and provide a measurement of the linear limb-darkening coefficients of the source star in the I and R bands. We discuss the derived values of the latter and compare them to the classical theoretical laws, and furthermore examine the cases of already published microlensed GK-giants limb-darkening measurements.

Methods. We have obtained dense photometric coverage of the event light curve with OGLE and PLANET telescopes, as well as a high signal-to-noise ratio spectrum taken while the source was still magnified by A similar to 20, using the UVES/VLT spectrograph. We have performed a modelling of the light curve, including finite source and parallax effects, and have combined spectroscopic and photometric analysis to infer the source distance. A Galactic model for the mass and velocity distribution of the stars has been used to estimate the lens distance.

Results. From the spectrum analysis and calibrated color-magnitude of the event target, we found that the source was a K3 III Bulge giant, situated at the far end of the Bulge. From modelling the light curve, we have derived an angular size of the Einstein ring theta(E) similar or equal to 114 mu as, and a relative lens-source proper motion mu = theta(E)/t(E) similar or equal to 3.1 mas/yr. We could also measure the angular size of the source, theta(*) similar or equal to 4.5 mu as, whereas given the short duration of the event, no significant constraint could be obtained from parallax effects. A Galactic model based on the modelling of the light curve then provides us with an estimate of the lens distance, mass and velocity as D-L similar or equal to 9.6 kpc, M similar or equal to 0.11 M-circle dot and v similar or equal to 145 km s(-1) (at the lens distance) respectively. Our dense coverage of this event allows us to measure limb darkening of the source star in the I and R bands. We also compare previous measurements of linear limb-darkening coefficients involving GK-giant stars with predictions from ATLAS atmosphere models. We discuss the case of K-giants and find a disagreement between limb-darkening measurements and model predictions, which may be caused by the inadequacy of the linear limb-darkening law.

Original languageEnglish
Pages (from-to)277-288
Number of pages12
JournalAstronomy & Astrophysics
Publication statusPublished - Dec 2006


  • gravitational lensing
  • techniques : high angular resolution
  • stars : atmospheres
  • stars : individual : OGLE 2004-BLG-254
  • MACHO 98-SMC-1
  • LENS
  • STAR


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