International Astronomical Union Circular No. 7468
Fri, 28 Jul 2000 15:36:35 -0400

COMET C/1999 S4 (LINEAR)

J. Licandro, G. Tessicini, and I. Perez, Centro Galileo Galilei; and S. Hidalgo, Instituto de Astrofisica de Canarias, report that the inner coma of C/1999 S4 appears extremely elongated in the tailward direction on J, H, and K_s images obtained with the 3.6-m Telescopio Nazionale Galileo (+ ARNICA camera) at La Palma on July 26.9 and 27.9 UT. There is no clear central condensation, but the photometric peak appears to move from the anti-tailward border of the coma toward p.a. 81 deg on July 26.9 and 84 deg on July 27.9, with a velocity of 7".4/day (26 m/s). The brightness of the central coma decreased from J = 8.83 +/- 0.04 on July 26.9 to 9.46 +/- 0.07 on July 27.9, as measured in a 30"-diameter aperture around the peak. Color maps do not show any major structure, the mean colors being J-K = +0.61 +/- 0.05, and H-K = +0.12 +/- 0.06 on July 26.9. These data suggest that a major event has occurred in the nucleus of the comet.

A. V. Filippenko and R. Chornock, University of California at Berkeley, report that inspection of a CCD spectrum (range 320-1000 nm) of this comet obtained on July 28 UT with the Shane 3-m reflector at Lick Observatory reveals no clear emission lines superposed on the solar reflection spectrum, unlike the case in spectra they obtained with the same instrument on July 6 and June 27. There is no evidence for the normally strong CN emission near 380 nm. The comet's morphology is peculiar, lacking an obvious head that is brighter than the adjacent tail, although the leading edge of the head/tail combination is very sharply defined.

S. Nakano, Sumoto, Japan, writes that A. Asami (Bisei Spaceguard Center) was unable to find a peak in the comet's central-condensation brightness to determine an astrometric position from a CCD image taken in fair conditions on July 28.490 UT with a 0.25-m f/5 reflector.