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Dust trails along asteroid 3200 Phaethon's orbit

S. Urakawa3, S. Takahashia, Y. Fujiia, M. Ishiguroa, T. Mukaia and R. Nakamurab aGraduate School of Science and Technology, Kobe University, Rokko-dai-cho l-l,Nada, Kobe 657-8501, Japan bNational Space Development Agency of Japan, Earth Observation Research Center, Roppongi 1-9-9, Minato, Tokyo,Japan

We performed observations of the zodiacal light on November 16-18, 1999 at Mauna Kea (4200 m, Hawaii) by using a cooled CCD camera to search for the dust trails along the orbit of asteroid 3200 Phaethon, which is thought to be the parent object of the Geminid meteoroid stream. No significant enhancement of the brightness larger than 15100 appears along a line of sight passing through the orbit of Phaethon.

1. INTRODUCTION

The asteroid 3200 Phaethon, discovered as 1983TB using IRAS [1] has the orbital elements: semi-major axis a=1.271 AU, eccentricity e=0.890, inclination 1=22.1°, longitude of the ascending node f2=265.6° and argument of perihelion w=321.8°. Since the Earth approaches this orbit in the middle of December, Phaethon is thought to be the parent object of the Geminid meteoroid stream. Furthermore, Phaethon is expected to be a comet in a dormant phase as the parent objects of meteoroid streams are generally thought to be comets.

A faint glow at an ecliptic latitude ¡3 of 6° and a helioecliptic longitude A — A0 of 78° has been found in our zodiacal light observations at Hale Pohaku (2800 m, Hawaii) on November 2nd, 1997 (see Figure lb in [2]). The analysis of this faint structure suggested that this feature related to the orbit of asteroid 3200 Phaethon. However, since its brightness of about lSio® is near our detection limit, we could not conclude the existence of dust trails associated with asteroid 3200 Phaethon. The confirmation of this structure was the motivation of the observations presented below.

2. OBSERVATION AND DATA ANALYSIS

We made photometric observations of a region in Cancer, where the orbit of asteroid 3200 Phaethon could be seen on November 17th, 1999. We used a wide-angle lens (Sigma 24 mm lens, F=2.8) attached to a cooled CCD camera (Mutoh CV-16) at Mauna Kea (4200 m, Hawaii). The angular resolution and the field of view are 2.50' pixel-1 and 32°x21° (768x512 pixels) respectively. A special filter is designed to fit the broadest window of visible airglow and artificial sky lines, between Hg at 435 nm and NI at 524 nm (see Figure 2 in [3]). The exposure time was set to 3 mins, and the temperature of the CCD chip was kept at —29°C. In order to check the dark and readout noise during the observations, 53 dark frames were taken throughout the night. The frames for the flat fielding were taken using the same instrument inside the integrating sphere at the National Institute of Polar Research (NIPR), Tokyo, Japan.

The observed frames allow us to estimate the optical depth t(z) of atmospheric extinction via the photometry of standard stars, by changing the zenith angle with time i.e. r(z)=0.113/cos z. From the photometry of solar analog stars, we found that 1 ADU in our system is equal to 2.845io®, where 1510®=1.28xlO~8 W m-2 sr-1 /im"1 at 500 nm.

2.1. The brightness of the predicted dust trail

The expected brightness of the dust trail is listed in Table 1. It is assumed that the dust trail can be considered as a cylindrical dust tube with a diameter 0.03 AU, existing along the orbit of Phaethon. Furthermore, the number density of meteoroids (dust grains) is constant inside the tube and the dust grains are spheres with uniform diameters. The brightness of possible dust trails at the observation time was calculated based on the scattering properties of dust grains shown given in [4], where the phase angle (Sun-dust grain-Earth angle) is 120° ~ 140°. It was found [5] that an enhancement of brightness caused by the dust trails associated with comet 55P/Tempel-Tuttle in 1998. It was detected when the Earth entered into the dust trail of the Leonid meteoroid stream, and it was estimated that the number density of dust grains with a typical diameter of 10 fim was 1.5 xlO"10 m~3.

Table 1

The expected brightness of dust trails associated with the orbit of asteroid 3200 Phaethon

Table 1

The expected brightness of dust trails associated with the orbit of asteroid 3200 Phaethon

Number density (m 3)

Dust diameter (/im)

Brightness (5io©)

1.5 x 10"10

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