Abstract
Aims: We present a detailed analysis of OGLE 2004-BLG-482, a relatively high-magnification single-lens microlensing event which exhibits clear extended-source effects. These events are relatively rare, but they potentially contain unique information on the stellar atmosphere properties of their source star, as shown in this study. Methods: Our dense photometric coverage of the overall light curve and a proper microlensing modelling allow us to derive measurements of the OGLE 2004-BLG-482 source star's linear limb-darkening coefficients in three bands, including standard Johnson-Cousins I and R, as well as in a broad clear filter. In particular, we discuss in detail the problems of multi-band and multi-site modelling on the expected precision of our results. We also obtained high-resolution UVES spectra as part of a ToO programme at ESO VLT from which we derive the source star's precise fundamental parameters. Results: From the high-resolution UVES spectra, we find that OGLE 2004-BLG-482's source star is a red giant of MK type a bit later than M3, with Teff = 3667 +/- 150 K, log g = 2.1 +/- 1.0 and an assumed solar metallicity. This is confirmed by an OGLE calibrated colour-magnitude diagram. We then obtain from a detailed microlensing modelling of the light curve linear limb-darkening coefficients that we compare to model-atmosphere predictions available in the literature, and find a very good agreement for the I and R bands. In addition, we perform a similar analysis using an alternative description of limb darkening based on a principal component analysis of ATLAS limb-darkening profiles, and also find a very good agreement between measurements and model predictions.
Highlights
Photometric and spectroscopic observations of stars yield their spectral types and other information useful for studying their atmospheres, but much of the information on the structure of the atmosphere and related physical processes is lost in the disc-integrated flux
The parameters u0, t0, tE and ρ∗ are common to all data sets, whereas Mbi and gi are associated to the data set “i”, and the linear limb-darkening coefficients (LLDC) may be chosen to be common per observing filter or per individual telescope
The first question we address now is how the individual linear limb-darkening coefficients (LLDC) are affected by including or removing some of our data sets
Summary
Photometric and spectroscopic observations of stars yield their spectral types and other information useful for studying their atmospheres, but much of the information on the structure of the atmosphere and related physical processes is lost in the disc-integrated flux. For the event MACHO 1997BLG-28, Albrow et al (1999c) derived I and V coefficients for a K2 giant (typing from spectroscopic observations) crossing a caustic cusp, and found a good agreement with stellar models predictions In such a complex event, many side effects could have affected the light curve, which somehow decrease the strength of the conclusions. From the binary-lens event OGLE 2002-BLG-069 (Cassan et al 2004; Kubas et al 2005), it was possible to obtain limb-darkening measurements for a G5 bulge giant source star in the I and R bands, and to directly test predictions from PHOENIX stellar model atmospheres by comparing the change of the Hα equivalent width during a caustic crossing (Cassan et al 2004; Thurl et al 2006) using high-resolution UVES/VLT spectra.
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