6° ;

Figure 1. The area, including L4, taken at llh51m UT on 17th Nov. 1999. The L4 point with R.A. 02h27m and Dec. 37°25' is at the origin of the arbitrary coordinate system. The bright regions indicate the areas with brightnesses larger than 1.05100. The bright region on the left side along X = —7° ~ —8° is not a real cloud, but the ghost caused by the Fourier filtering process. A cut of the brightness along the lines marked I-V is shown in Figure 2.

and the ¿4 point is at the origin of the co-ordinates. The directions of ecliptic latitude (¡3) and longitude (A) are also indicated in Figure 1. The Fourier filtering was done along the X axis in the frame. The ¿4 point is not located at the center of the frame because we had to purge the bright areas contaminated by Jupiter and Saturn from the image. Both planets were placed at about 15° away from the L4 point. The contamination from bright planets is removed by the Fourier filtering.

The bright area in Figure 1 denotes the sky region with a brightness greater than l.OSio®. The fluctuations of the brightness along the lines at X = 0°,±6° and Y = O0,—6° are illustrated in Figure 2. The ler noise level of brightness in Figure 2 is about 0.5Sjo© for each case. The remaining cloud structure on the left side of the image in Figure 1 along the X = —7° ~ —8° is not a real cloud because it is produced at the edge of the frame in the Fourier filtering process.

No significant enhancement of brightness appears to show the existence of a real cloud near the L4 point. Roach [2] reported that the libration cloud had the size of about 6° and drifts around the libration point within the oval region with the size of about 6° along the ecliptic and of about 2° perpendicular to the ecliptic. The area imaged in Figure 1 does not contain all of the oval region proposed by Roach [2], Therefore, we examined other frames taken in the neighboring sky regions by applying the same procedure for analysis and found no significant cloud structure near the L4 point.

We can estimate the upper limit of the brightness of the expected cloud. The noise fluctuation in Figure 1 is 3cr = 1.5Sio® so the cloud brightness might be below 1.5Sio®. Our results suggest that the libration clouds might be the transient objects, if they really

A CCD search for the Earth-Moon titration clouds around L4

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