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First Lunar Occultation Recorded at the
Telescopio Nazionale Galileo

A lunar occultation of the infrared source IRAS18124-2056 was recorded in the evening of July 25, 1999 at the 3.5m Telescopio Nazionale Galileo on the Canary Islands, equipped with the Arcetri Observatory near-infrared camera ARNICA. The occultation has been recorded by F. Ghinassi, and the data have been analyzed by A. Richichi. Arcetri astronomers have a long experience with this kind of measurements, and it is possible to visit the Lunar Occultation homepage in Arcetri to learn more about them. Lunar Occultations permit to achieve a very high level of angular resolution and are used to study stellar diameters, binary stars and stellar/circumstellar features. The ARNICA camera has been used before to record Lunar Occultations at the 1.5m TIRGO telescope, thanks to special software developed by C. Baffa. The use of an array detector, such as the NICMOS3 detector used by ARNICA, for Lunar Occultations permits to achieve a much better sensitivity than with InSb photometers, which have been traditionally employed for this kind of measurements. A first example was given in AJ 112, 2786 (Richichi et al. 1996).

Here, we announce the first measurement of this kind at the TNG, and hopefully more will follow. IRAS18124-2056 is an infrared source without known optical counterparts. It is probably a giant late-type star heavily obscured by interstellar extinction in the direction of the Galactic Center. From photometry obtained at TIRGO on Sep 3, 1999, we have determined K=4.3 and J-K=1.9 for this source. A movie is available (100Kb), showing a sequence of about 4.9 seconds in a 14 x 13 pixels subarray, corresponding to about 6 x 6 arcseconds on the sky. The data were obtained at 2.2 microns with 7 milliseconds integration time and a frame rate of 125Hz. Due also to the unfavourable airmass (1.7), the source suffers from a rather large image spot and the occultation took place while it was near one of the edges of the selected subarray. Nevertheless, the occultation is well detected already in a simple un-flatfielded, raw integration of the frames, as can be seen in this lightcurve (8 kB). Treatment of the data by means of flat-fielding, background-subtraction and aperture extraction of the flux, leads to a more refined lightcurve (7 kB). In this latter, it can be seen that fringes are well detected although more work will be needed to clean the data from spurious frequencies and bad pixels.

A preliminary analysis analysis of the data shows that the source is unresolved, with an upper limit of 0.004 arcseconds on the angular diameter (which is in agreement with empirical estimations based on the flux and color).

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Last updated on 3/09/99 at 9:48:03 PM by A. Richichi