Another 17m bright LINEAR object discovered in the constellation of Auriga on Sep. 27 revealed itself as a comet at closer inspection. During the following nights comet C/1999 S4 (LINEAR) showed a 10" coma and a 0.4' tail at PA=220°.
Orbital calculations showed that this comet was then far from the sun and would reach perihelion on July 26, 2000 at 0.767 AU. Three days in advance it would pass Earth at only 0.373 AU, making it an object for the unaided eye at an extrapolated maximum brightness of 3.5 - 4.0m (IAUC 7267).
Until teh beginning of November a total of 45 observations from 10 members of the German comet section were received. Adding 395 international observations, mainlyby experienced observers, allows to make rather certain statements about the evolution of this comet. Following the evolution of the comet is described in chronologically order, based on the analysis and including professional results:
Prior to the conjunction with the Sun (April 2000) the brightness of the comet rose steadily, but with n < 3 until March (from 14.3m to 13.2m). The coma diameter increased from 0.7' (90.000 km) to 1.3' (180.000 km). Since the beginning of 2000 a short dust tail was observed photographically, and since February a false nucleus was visually discernable. The DC-value varied between DC 3 and DC 4. Photometric observations indicated a decreasing dust production rate during a three week period in December 1999 (IAUC 7342).
The first observations after conjunction in the morning sky were awaited with great anticipation. It was K. Kadota who succeeded on May 4 with a 18cm-reflector, equipped with a CCD. He estimated the brightness as 13.0m with a coma of 0.4' (IAUC 7418). N. Biver, observing from Hawaii, estimated the brightness as 11.3m on May 25 and as 11.0m on May 29. Michael Jäger could photograph the comet on May 27. Because twilight, moon, haze and the low altitude influenced the observation he could only give a rough estimate of 11.0m for the brightness; with the coma spotting a significant false nucleus and a 3' tail. On June 4 he estimated the brightness as 10.5m; the photograph showed an elliptical coma (with the false nucleus on the top) and a faint tail of 5'.
Thus, its brightness at the end of May was consistent with the derived brightness parameters (m0=8.5m / n=2.4) prior to conjunction. The apparent diameter of the well-condensed coma had increased to 1.8', which means however, that the absolute coma diameter remained constant. A 5' long dust tail was detectable photographically. From June 5 to June 20 a phase of increased activity followed (probably a result of multiple separations of small fragments), clearly recognized by a significant "wave" in the brightness diagram. Based on this analysis the comet was up to 1.2m brighter during this phase of enhanced activity than expected, triggering new hopes. Photometric observations at Lowell-Observatory yielded the following production rates of gas and dust (molecules/s, Af) in the nights June 10 to 12: OH: 2.57×1028 (40-times increase to the value derived in December 1999), CN: 3.47×1025, dust: 676. Significant variability was observed during the June observations, with the gas-production rates on June 11 being 30-50 percent larger than on June 10, and then on June 12 dropping to 10 percent less than on June 10 (IAUC 7455). During this period the apparent coma diameter increased from 2.5' to 5'. The absolute coma diameter increased as well, from 230.000 km to 340.000 km and reached its maximum shortly after June 20. The DC-value increased from DC 3 to DC 4-5.
Between June 13 and June 30 the apparent brightness kept almost constant around 8.2m, only to increase significantly thereafter. This was the result of the fact that since June 20, the heliocentric magnitude was no longer in-, but rather decreasing due to a subdued activity. Not until around July 7, when the predicted brightness was reached again, did the activity stabilize itself, thereafter remaining constant until July 24. In parallel the absolute coma diameter decreased until July 7 to 220.000 km, resulting in a much slower increase of the apparent coma diameter (to only 7') than expected. The coma itself condensed further and reached a value of DC 5 on July 7. From June 20 on the tail could be seen visually, steadily increasing its length in the following weeks.
Observations with the HST showed a dramatic increase in activity on July 5, with the flux in a 0".15 square aperture increasing by a factor of about 1.5 in just under 4 hours (from July 5.776 to 5.940 UT). During HST observations one day later (July 6.717-6.889), the activity levels were decreasing and were about 3 times lower for the final observation, compared to the peak value from the previous day. The flux in the last HST image (on July 7.961) was about 7 times lower than the peak value measured on July 5. On July 7, at least one 'fragment' was seen 0".85 (460 km, projected) from the nucleus in the tailward direction, and a sharp tailward spike of emission was observed, reminiscent of the morphology observed during the outburst in C/1996 B2 (Hyakutake) in late-March 1996. Production rates were: CO: 5×1026 (July 5), S2: 1.4×1024 (July 6) and H2O: 1.2×1029 (July 6). One firm conclusion is that CO is strongly depleted in C/1999 S4, relative to the observed abundances in C/1996 B2 and C/1995 O1 (IAUC 7461). IAUC 7449 of July 7 pointed to the fact that this comet exhibited significant non-gravitational parameters.
Until July 23 the apparent brightness increased continuously, reaching its maximum on this date at 6.1m. In addition, the DC-value (DC 6), the tail length (visually: 1.5° = 1.7 million km, photographic: 3°) and the apparent coma diameter (9-10') reached their maximum as well on the same day. The last value means that the absolute coma diameter, however, had continued to shrink, down to 150.000 km. The coma itself and the tail showed a morphology quite similar to the one of comet C/1996 B2 (Hyakutake). Beside the gas tail a short dust tail was seen visually.
Photometric Observations at Lowell Observatory on July 13 yielded the following production rates: OH: 1.74×1028, H2O: 2.63×1028, CN: 2.82×1025, dust: Af=355 (IAUC 7455). Using Chandra and EUVE X-rays of this comet could be detected on July 14 (IAUC 7464). Observations in the millimeter region showed HCN on 21 and 24, with an outgassing rate of a few percent that of comet C/1995 O1 (Hale-Bopp) (IAUC 7467).
As a total surprise M. Kidger was the first to witness the total disruption of the nucleus on the Canaries: The central condensation was highly condensed and showed the typical 'teardrop' form on July 23.9 and 24.9 UT, although its brightness decreased by a factor of about 3 between the two nights and the gas tail had disappeared on the second night. On July 25.9 UT the central condensation was seen to be strongly elongated (length about 15") in p.a. 80°. The condensation's brightness faded further and its length increased to about 30" and 45"-50" (p.a. 80°) on July 26.9 and 27.9, respectively. On July 27.9, there was no evidence of any local brightness peak that would indicate the presence of subnuclei. The expansion velocity of the condensation was about 40 m/s, indicating that it was particulate material and not gas. The gas tail reformed as an extension of the major axis of the central condensation (IAUC 7467). Another group confirmed that there was no clear central condensation on July 26.9 and 27.9 UT, but the photometric peak appears to move from the anti-tailward border of the coma with a velocity of 7".4/day (26 m/s). According to observations at the Lick Observatory on July 28 with the 3-m reflector the head was not any brighter than the adjacent tail, although the leading edge of the head/tail combination was very sharply defined. A. Asami was unable to find a peak in the comet's central-condensation brightness to determine an astrometric position on July 28.490 UT (IAUC 7468/7470). Z. Sekanina suggested in IAUC 7471, that comet C/1999 S4 was a trailing fragment of a more massive comet that has been moving in the same orbit, arriving at perihelion long (centuries?). Trailing fragments of known comet pairs have a tendency to sudden disintegration. If much of the comet's mass did indeed dissipate into a cloud of dust in the recent event, the total mass involved could be estimated by further monitoring the tail. Experience with the past initially bright comets that later became headless and disappeared shows that a narrow, bandlike tail - a developing synchronic formation - should survive the head by several weeks or even longer. A very preliminary analysis suggests that the event may have begun as early as July 23.6 UT and involved submillimeter-sized and larger dust (repulsive accelerations up to 0.024 of the solar attraction).
Observations by M. Kidger at the 2.5m-telescope on La Palma on Aug. 1.9 UT yielded no fragments brighter than R = 22.0m. The continued presence of a well-defined leading edge or point to the coma suggested that an unresolved fragment of the nucleus in this position continued to release dust. However, this structure had faded considerably since the disruption occurred. Measurements of the coma brightness distribution indicated that the maximum of light was about 75" from the leading border of the coma with the tail traceable for at least 20' (IAUC 7472/74). Photometric observations by D. Schleicher and L. Woodney on July 29.2 UT yielded the following production rates: OH: 2.5×1027, CN: 2.5×1024, dust: Af=32. The values are 8-10 times less than those measured on July 13 (7475)!
H. Weaver and R. West reported the following results of large groups of observers with the HST and VLT: images taken during Aug. 5.167-5.396 UT with the HST and with the Very Large Telescope (VLT) during Aug. 6.978-6.999 revealed about a dozen active fragments, most of them located within about 20" of the western tip of the dust tail. The correspondence between fragments in the HST and VLT images is generally very good, but the brightest fragment in the HST image is not seen in the VLT image, indicating rapid variability in the activity levels. The dynamic nature of the fragments was further highlighted by a dramatic change in the appearance of the fragments in VLT images taken during Aug. 9.976-9.996, when they were barely detectable. Although the latter images were taken under mediocre observational conditions, that alone seems unlikely to account for the observed changes. A very preliminary estimate for the R magnitude of one well-isolated fragment is about 24. A completely inactive fragment with a diameter of 100 meters observed under these conditions (r = 0.79 AU, Delta = 0.69 AU) would have R about 25.9m (IAUC 7676).
The following two brightness formulae can be derived, not taking into account the brightness wave of the period June 5 to July 7:
Total Brightness and Coma diameter
Andreas Kammerer
