Arecibo Radar Observations of C/2001A2(B) (LINEAR)

Details Arecibo Radar Observations of C/2001A2(B) (LINEAR) Arecibo Radar Observations of C/2001A2(B) (LINEAR)

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001dps....33.4302n&link_type=abstract

American Astronomical Society, DPS Meeting #33, #43.02; Bulletin of the American Astronomical Society, Vol. 33, p.1120

Astronomy and Astrophysics

Astronomy

1

Scientific paper

On 2001 July 7-9 we observed comet C/2001A2(B) (LINEAR) with the Arecibo Planetary radar. We obtained radar echoes with a velocity dispersion of about +40 to -80 m/s (+300 to -600 Hz Doppler shift at at 2380 MHz), presumably from a particle cloud surrounding the nucleus. The total radar cross section was 4+/-2 km2 in the expected (opposite-circular, or "OC") polarization, indicating that a substantial particle cloud was responsible for the echo. The echo was visible in the unexpected (SC) polarization as well at about 20% of the OC strength, indicating that the particle size was not small compared to the 12.6 cm wavelength of the radar. The comet's nucleus would be expected to be visible as a narrow feature centered on the ephemeris velocity of the comet. All of the spectra would be consistent with a 10 Hz wide, 5-sigma peak at the ephemeris velocity, which would correspond to a 2 km object with a 6 hour rotation period, assuming a reasonable cometary radar albedo of 4%. However that feature is not clearly distinguishable from the much broader coma echo. In part of the July 7 data, a relatively narrow ( ~ 150 Hz) peak is seen near the ephemeris velocity. If this peak were the nucleus, and assuming a fairly rapid (3 h) rotation rate, the nucleus would have to be 16 km in diameter to have that bandwidth. If the nucleus were that large, it should have presented a much stronger echo, unless its radar albedo was 0.01 or less. Thus, either the nucleus was small (D < 3 km), it was masked by the particle cloud, or the radar albedo is extremely low (less than 0.01). The latter two possibilities seem less likely than simply a small nucleus. The data seem to rule out anything larger than a 20-Hz wide, 6-sigma peak, so we give an upper limit of 3 km diameter for the nucleus. Imaging was attempted on 2001 July 10, but as yet no echoes have been seen in the data.

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