Resolution of a cool giant during a caustic passage in OGLE 2004-BLG-482

Resolution of a cool giant during a caustic passage in OGLE 2004-BLG-482

First posted: 16-Aug-2004
Archived: 11-May-2005

PLANET succeeded in resolving the atmosphere of a cool giant, undergoing microlensing event OGLE 2004-BLG-482, during a caustic passage.

PLANET observations with the Canopus 1.0m telescope near Hobart (Tasmania, Australia) on the two nights starting 15-Aug and 16-Aug showed a rise to a magnification of about 2 mag above baseline and indicated that a peak will occur within the following 24-48 hours at a very high magnification, which might exceed A0 ∼ 100, as pointed out in PLANET anomaly alert #15 (issued 16-Aug, 16:05 UT). As announced in PLANET anomaly update #15-1 (issued 17-Aug, 13:30 UT), further PLANET observations with the Danish 1.54 m telescope at ESO LaSilla (Chile) and the Canopus 1.0m telescope near Hobart (Tasmania, Australia) indicated that the source had entered a caustic and was about to leave it within the following 12-24 hours. In PLANET anomaly update #15-2 (issued 18-Aug, 9:30 UT), we reported that the most recent PLANET data confirmed our earlier interpretation. The event was found close to its peak at a magnification of about 3 mag above baseline and the trailing limb of the source was predicted to exit the caustic at about HJD = 2453436.95 (19-Aug, 10:50 UT). PLANET anomaly update #15-3 (issued 20-Aug, 7:30 UT) reported that the caustic passage lasted from HJD = 2453234.57 (17-Aug, 1:45 UT) until HJD = 2453237.01 (19-Aug, 12:15 UT), so that it took about 2.44 days from the caustic entry of the leading limb of the source until the caustic exit of its trailing limb.

In addition to our broad-band photometric measurements, PLANET has been able to obtain high-resolution spectra with the UVES spectrograph at the VLT Kueyen telescope located at ESO Paranal (Chile) during the nights of 17-Aug and 18-Aug. Making use of our previously awarded target-of-opportunity status, in total 16 spectra have been taken in the night of 17-Aug during 6 consecutive hours alternating between the two instrumental setups 580 and 860, thereby covering a spectral range of 4500-10800 Å at λ/(Δλ) ∼ 40,000. During the night of 18-Aug, we got 4 spectra taken from 4:35 UT when the lens was at a fractional radius of 0.80, where strong signatures due to differential magnification of different parts of the source can be expected as the lens transits its limb. The latter observations were only possible thanks to the visiting astronomers C. De Breuck and B. Rocca Volmerange kindly accepting that our target-of-opportunity observations would take place in order to catch this unique opportunity. Complementary post-caustic spectral observations have been carried out during 4 hours on the night of 20-Aug.

The target is most likely a M5-6III giant located in the Sagittarius dwarf galaxy. A radial velocity measurement and accurate spectral typing will tell us for sure soon. Combined with the photometric data, our spectroscopic measurements provide excellent opportunities for probing the chemical composition of the stellar atmosphere, whereas the measurement of the caustic passage duration will reveal the relative proper motion between lens and source star.

Light curve showing data from the PLANET sites Boyden, Tasmania, Perth, and LaSilla Danish, as well as from OGLE.

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