Introduction

The Lagrangian points L4 and L5 are the triangular solutions of the restricted three-body problem. The L4 is located in the leading side and the L5 in the trailing side of an orbit. In the solar system, it is widely known that certain asteroids called the "Trojan family" exist around the Sun-Jupiter Lagrangian points. It is also known that (5261) Eureka is a Mars Trojan [3], Kuchner et al. [4] tried to find the dust clouds around the Sun-Jupiter L5 point by analysing the COBE-DIRBE infrared data, but they obtained a negative result.

Concerning the Earth-Moon Lagrangian points, in the 1960s Kordylewski [1] reported the existence of the clouds based on naked eye observations. Roach [2] has shown that the satellite OSO-6 could have caught the enhancement of brightness' near the L4 and ¿5, and their size and brightness were deduced. However, their existence could not be confirmed by the observations (e.g. Roosen [5], Roosen and Wolff [6], Bruman [7]). The theoretical approaches have been performed to examine the enhancement of the number density of the dust grains near the Earth-Moon Lagrangian point. Roser [8] has shown that the conditions for supply and loss of dust grains to the libration point regions are not favorable for libration clouds, and he suggested, as a very high upper bound, a surface brightness of 25io® at L4.

Recent CCD Photometry techniques are much superior than those of photo-multipliers used before, and consequently provides a possibility to detect a slight enhancement in the brightness of the zodiacal light (see e.g. Ishiguro et al. [9]). This modern tool would allow us to learn whether these faint clouds exist or not. We will report the results of our search for the libration clouds in the Earth-Moon system, using a cooled CCD camera at Mauna Kea (4200m), Hawaii.

2. OBSERVATIONS AND DATA ANALYSIS

The libration clouds may exist at the triangle points of the Earth and the Moon in the Moon's orbital plane. This restriction prevents us from observing the liberation clouds freely. That is, the detection should be done after the Moon's setting(i4) and before the Moon's rising (¿5), when the zenith angle is larger than 30°. The sky under such conditions would be hazy and the zodiacal light near the libration points would be brighter compared with at the zenith. Consequently, it is not easy to distinguish the faint light of libration clouds from the background light consisting of airglow and zodiacal light. Furthermore, we have to take care that the Lagrangian point should be far away from the Milky Way and and bright planets/stars during the observation.

We made the observation of the sky region where the liberation clouds are expected, at Mauna Kea (4200m), Hawaii on November 17 and 18, 1999. This high altitude observation site is preferable to reduce the contamination of the scattered light by the atmosphere of the Earth. In addition, the location of Mauna Kea is suitable for the observation of libration clouds because the ecliptic plane in Autumn passes through the zenith, such that the libration clouds could be seen at higher altitude. In addition, the position of the L4 point was away from the Milky Way in November.

The bright planets, Jupiter and Saturn, were located about 15° away from the Lagrangian point on November 17 and 18, 1999. It was expected that their scattering light would somewhat pollute the frame. However, since a good chance of detection is so rare (i.e. only two or three times per year) we performed the observation in November at Mauna Kea. In order to avoid the contamination of bright planets, the ¿4 point could not be sited at the center of the frame. A cooled CCD camera (Mutoh CV-16), at the temperature of —30°C during the observation, was used with a wide-field lens (Sigma 24mm, F=2.8). Its angular resolution using 2x2 binning, is 2.50' pixel-1 and the field of view is 32c,x21°. A broad band filter designed to transmit light between 439-524nm, where no bright emission lines of the airglow exist, was used. The exposure time was set to be 3 minutes.

Flat frames, taken by using an integrating sphere of the National Institute of Polar Research, were adopted in the data reduction process. We removed stars from the frames and tried to estimate the sky brightness which consists of zodiacal light, airglow from upper atmosphere, integrated starlight of unresolved stars and scattered light from the Earth's atmosphere, by using similar methods to those in Ishiguro et al. [9]. We applied the Fourier filtering technique to enhance the structures of sky brightness with a scale smaller than 10°. In order to reduce the noise arising from the fluctuation of detected photons and dark current, the brightness in the resulting frame was averaged over 24x24 pixels (l°xl°).

Our data reduction process is suitable for enhancing clouds with a scale of about 1°-10°. If a cloud had a scale of about 6° as reported by Roach [2], and a brightness above the detection limit of about 15io®, we would be sure to find the clouds in the resulting frame.

3. RESULTS

Figure 1 is one of the Fourier filterd frames taken on the 17th November 1999, where the i4 position is included. The X and Y axes correspond to the frame of the CCD array,

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