C.W. Hergenrother recovered comet 73P/Schwassmann-Wachmann (P = 5.4 years) on Oct. 22, 2005 in Leo. The comet showed a strongly condensed 6" coma of 19.0 mag and an 8" tail in p.a. 300°. Investigations hint to component C, which will pass Earth on May 12 at a distance of only 0.079 AU, expected to be visible in binoculars during these days (IAUC 8623).
On Jan. 6, 2006 the first fragment of this comet was recovered. While the total brightness of the primary component was measured to be 16.3 mag, the fragment's, at a relative distance of 23' (p.a. 270°) from the primary, measured only 18.9 mag. Z. Sekanina suggests that this might be the fragment B which was observed in 1995-96 (IAUC 8659). CCD images on Jan. 23 showed this fragment to possess a 40" tail at p.a. 290° (IAUC 8663).
The coverage via CCD by amateurs of this comet began in November 2005, when its brightness was about 17.5 mag. Visual observers succeeded in the first days of January 2006 (appx. 14.5-15.0 mag). Around Feb. 8 the brightness of the coma, measuring almost 0.5', was estimated visually to be 13.8-14.0 mag. Because the comet was much brighter during this first weeks, it was expected that component C should peak between 4 mag and 7 mag. However, a possibility existed that the comet could disintegrate completely during the current perihelion passage. A prediction of the evolution of fragment B was even more uncertain. It was expected to remain 2-3 mag fainter than the primary component.
Eventually, the number of fragments discovered was so huge that the nomenclature had to be modified. In the future fragments may be designated by two letters. All in all 69 components / fragments got an official designation (up to 73P-BS) - astonishingly including a dozen positioned ahead of the primary component. On Apr. 7 all known fragments stretched over 12° on the sky (CBET 473)! Not surprisingly, it became difficult to follow the evolution of all fragments. Following some general informations about the evolution of components C, B and G is given. Thereafter you may find an overview about the componentes H to Y and the fragments R, AP, AQ, AS and BC, which were observed visually.
Simulating the fragment pair K and S by Z. Sekanina showed that the comet is in a state of continuing disintegration. According to his model fragments K and S separated in mid-February, both having dimensions of boulders with accordingly short live times (CBET 453).
The primary component C evolved rather unspectacular during this apparition, as can be demonstrated by analyzing 58 observations by 8 members of the German Comet Section and 340 international observations (until end of September 2006). The brightness evolution was different prior and after perihelion and can be well described by the formulae
pre-perihelion: m = 11.2m + 5×log D + 7.5×log r post-perihelion: m = 11.2m + 5×log D + 13.2×log r
yielding a maximum brightness of 5.8 mag just at the date of the closest approach to Earth on May 12. Studying the diagram, the estimates seem to show a slight decrease in brightness of about 0.3 mag during those days, which very probably is due to the full Moon.
The apparent coma diameter was about 0.5' in January, increasing to only 2' until mid-March. Starting on Apr. 10, when it measured 5', the expected fast increase due to the extreme Earth approach began. This increase was noticed first by observers using binoculars, who were able to see the very diffuse outer coma to its whole extent. Around Apr. 20 the coma had already increased to 8' and at the change of the months April/May to 14'. The maximum apparent coma diameter of 16' was reached shortly before perigee (around 7 May) - probably also influenced by the full Moon. Thereafter it decreased even faster - very likely exaggerated by the deteriorating observing conditions. On May 15 the apparent coma diameter was estimated as 6', and at the end of the month it had decreased to 4'. The absolute coma diameter increased much less: measuring 35.000 km in January it increased slowly to 70.000 km in mid-April. The maximum absolute coma diameter of 85.000 km was reached in the last week of April. After that it decreased - in part certainly affected by the interfering Moon and the unfavourable observing situation (full Moon, then low altitudes) - quite rapidly, reaching about 40.000 km in mid-May and 30.000 at the end of this month. After perihelion the apparent coma diameter decreased further from 4'' at the beginning of June to 2' at the start of July and 1.5' at the end of August. The absolute coma diameter after perihelion was rather constant at about 40.000 km.
Total Brightness and Coma diameter of component C
The coma itself was moderately to significantly condensed. While the degree of condensation of the coma was estimated at DC 4 in February, it increased to DC 5-6 at the end of March. This value remained for several weeks. After May 10 it decreased, reaching DC 4 again at the end of May. At the end of August the degree of condensation was estimated around DC 3. Beside a pronounced false nucleus no structure could be detected until mid-May. Images taken on May 17 then showed a kind of "Dumbbell" shaped structure of the coma: a bright part on the sunward (spanning 150°) and tailward side (spanning 120°) with the central part nearly missing. This coma morphology was even more prominent on the morning of May 18 with a significant brightness increase of the false nucleus. It is unclear if this unusual morphology (mainly the bright sun-facing part) is a geometric result due to the crossing of the cometary orbital plane by Earth on June 1.
Starting in the first days of March the southwest to west pointing tail was visually discernible. In the second half of April its length increased considerably, reaching its maximum of 1.3° during the first week of May (probably also influenced by the beginning interference of the Moon). Estimates from the end of May are inconsistent: while one part of the estimates report lengths of about 0.5° the other report lengths of about 1°. The absolute tail length remained quite constant at 400.000 km. Most of the time the tail showed a surprisingly high surface brightness.
Component B: Much more interesting than that of the main component was the evolution of component B: at 14.5 mag at the start of March it brightened slowly to 12.5 mag at the end of this month. CCD images taken between Apr. 1.8 UT and Apr. 2.8 UT then showed a sudden brightness increase of 3 mag of the central condensation.
According to 47 observations by 8 members of the German Comet Section and 230 international observations it showed at least two major outbursts, both making it even brighter than the main component for several days. The greatest outburst with an amplitude of more than 2.0 mag occurred on Apr. 2/3 April, when the apparent brightness of this component increased from about 11.5 mag to about 9.5 mag. Until this date the brightness evolution can be well described by the formula
m = 12.4m + 5×log D + 11.5×log r
After this outburst (around Apr. 5) the apparent brightness evolved according to the formula
m = 11.5m + 5×log D + 11.5×log r,
i.e. this major outburst led to an increase of the absolute magnitude of 1 mag. Due to the second outburst, beginning on May 7, component B peaked at 5.1 mag on May 11 - almost half a magnitude brighter than expected using the above formula.
The apparent coma diameter measured only 0.5' at the end of February, but increased rapidly during the following weeks (Apr. 1: 3', Apr. 15: 6', May 1: 13'). The maximum apparent coma diameter of 15' was reached around May 8 (pre-perigee probably also due to the increasing interference by the Moon thereafter). In mid-May it still measured 12', but decreased to 9' on May 20 and to 4' on May 31. The absolute coma diameter measured only 15.000 km at the beginning of the apparition, but it increased to 40.000 km on Apr. 1 and to 60.000 km on Apr. 15. The maximum absolute coma diameter of 80.000 km was reached at the end of April. On May 5 it had already shrunk to 60.000 km and measured 35.000 km at the end of May (probably exaggerated by the Moon interference and unfavourable observing conditions).
Total Brightness and Coma diameter of component B
Each outburst began with the false nucleus becoming very pronounced. During the next days it then faded until it was a difficult object even in a 30cm-telescope. In parallel a very elongated (cigar-shaped) central condensation developed, within which subfragments were discernible on the tailward side of the false nucleus. At times, a cone-shaped inner coma expanded from the false nucleus. This morphology as well as the apparition of subfragments gave reason to the assumption of the disintegration of component B within a short time. Instead this morphology change took place several times. Due to this very variable morphology the DC value varied considerably. Apart from the outbursts it was around DC 3-4, while it was at DC 7-8 during the outbursts. Three sudden DC-increases can be discerned in a diagram using weighted sliding 3-day-means: on Apr. 3 (up to DC 7-8), Apr. 26 (DC 5-6) and May 8 (DC 7).
Visually the tail of component B was first noted at the beginning of April. During the following weeks it increased in length constantly, reaching 0.8° (250.000 km) around May 8 (shortly before the Moon began to interfere), and 1.2° (400.000 km) at the end of May (based only on a few observations). The orientation was the same as that of the main component.
According to John Bortle component B was a bright star-like object with a very faint coma on Apr. 3. The dust production rate increased threefold between Mar. 26 and Apr. 5. CCD images on Apr. 2/3 and 3/4 showed two bright jets in p.a. 90° and 305°, respectively (IAUC 8701). The brightness of the inner coma region peaked on Apr. 6. On Apr. 8 and 9 the inner coma region was elliptical with a length of 8" along p.a. 220°. On Apr. 10 the dust production rate had decreased threefold. On the evening of Apr. 11 the coma looked lance-like without the pronounced false nucleus and a shallow brightness profile - very reminescent of comet C/1999 S4 shortly before its disintegration. The 1.5m-telescope above Mt.Lemmon showed a 12x15' coma (along p.a. 230°) and a 30' tail in p.a 230°; no secundary condensations could be detected (CBET 473).
According to observations with large telescopes and the Hubble Space Telescope the pronounced brightness increase of component B at the beginning of April was due to the breaking off of a larger fragment. This was first recognised on images taken on Apr. 15. Interestingly it did not show as starlike, but as a diffuse condensation tailwards of the false nucleus. During the early hours of Apr. 24 a significant outburst occurred on the main nucleus of component B, which caused a significant increase in the overall brightness until Apr. 27 (Comet's Mailing List). The next outburst occurred on May 2. An additional outburst happened on May 8. Images taken on May 18 show a chain of subfragments behind the false nucleus. On May 18 a further subfragment appeared, showing the same brightness as the main nucleus. Until May 21 the brightness of the subfragment decreased significantly, as well as the overall brightness of component B.
Component G was discovered on Feb. 20 and 22, 2006 by R.A. Tucker and E.J. Christensen. Very probably it is not identical with components E and F, observed in 2001. Component G was of magnitude 17.2, showing a 15" coma and a fan-like 20" tail in p.a. 250-310°. On Mar. 5 its brightness was measured as 17.3 (IAUC 8679/92). CCD images on Mar. 25 showed a 2x5" central condensation (elongated in the direction of the tail = 260°). On Apr. 2 a sub-nucleus could be detected at a distance of 8". According to Z. Sekanina the separation should have taken place around Mar. 6. Because of a substantial non-gravitational decceleration the sub-nucleus should be short-lived (IAUC 8701, CBET 464). On Apr. 5 an outburst took place, which increased the brightness to 12 mag. According to E.J. Christensen on Apr. 7 the much brighter component showed a 1.5' coma and a 2' tail in p.a. 230° in the 1.5m-telescope (CBET 473).
Component G could be detected visually for only a short period, as reported via 5 observations by 3 members of the German Comet Section and 25 international observations. According to these component G reached 12.0 mag due to the outburst. During the following three weeks the brightness remained at about this level, but decreased rapidly in the first days of May, when the visual surveillance ended. The apparent coma diameter increased from 0.5' on Apr. 6 to 1.5' at the end of April - a value that remained until the end of the visual apparition. The absolute coma diameter increased from 7.000 km to 15.000 km around Apr. 20, only to decrease equally rapidly to 7.000 km until May 3. At the date of the outburst the coma was strongly condensed (DC 8), but became more and more diffuse thereafter, decreasing to DC 0 in the first days of May. On CCD images the component showed a well developed tail.
Total Brightness and Coma diameter of component G
Images taken with large telescopes on Apr. 27 showed more than 15 fragments around component G (Comet's Mailing List). On May 6 the brightness of the now very diffuse component was estimated at only 18.0 mag, with the false nucleus having a brightness of only 19.0 mag; a faint tail towards p.a. 220° was discernible. On May 9 it was only an extremely diffuse 4.5' object with 5 condensations; a faint 4' tail extended from the brightest condensation (Comet's Mailing List, MPEC 2006-J54). However, Michael Jäger and Gerald Rhemann still succeeded in recording remnants of component G on June 1.
Fragment H: discovered on Mar. 4 by R. Kowalski, situated 875" off fragment G (T=June 8.29 TT). Fragment H was of magnitude 20.0, displaying a diffuse 4" coma, slightly elongated in p.a. 275° without a central condensation. On Mar. 23 it had brightened to 18.9 mag; showing a 9" coma and a 40" tail in p.a. 255° (IAUC 8685/92). According to P. Birtwhistle on Apr. 2 the fragment was of magnitude 20 and star-like, whereas on Apr. 9 it was very diffuse (Comet's Mailing List).
Fragment J: discovered on Mar. 5 by R.E. Hill, together with two more fragments, lined up between B and G. Fragment J followed G in a distance of 170" (June 8.14). It showed a diffuse, slightly condensed 8" coma of magnitude 19.8 and a 10" tail in p.a. 275°. On Mar. 23 it was of magnitude 19.9, showing a 9" coma and a 20" tail in p.a. 230° (IAUC 8685/92).
Fragment K: discovered by R.E. Hill on Mar. 5, positioned 611" from G (June 8.24). It was of magnitude 21.7, showing a diffuse 4" coma without central condensation or tail. On Mar. 25 it was of magnitude 20.9-21.3 (IAUC 8685/93). According to E.J. Christensen the complex K/S showed a 15x10" coma on Apr. 2, elongated E-W (Comet's Mailing List).
Fragment L: discovered by R.E. Hill on Mar. 5, positioned about 1145" from G (June 8.35). It displayed a 5", slightly condensed coma of magnitude 19.8 and a 7" tail in p.a. 275°. On Mar. 23 it was of magnitude 21.0, displaying a very diffuse 5" coma and a 15" tail in p.a. 245° (IAUC 8685/92). On Apr. 2 the 8" coma was of magnitude 20, according to P. Birtwhistle (Comet's Mailing List).
Fragment M: discovered on Mar. 23 by E.J. Christensen and P. Birtwhistle near fragment H. Fragment M (June 8.28) was of magnitude 20.8, displaying a 4" coma, but no tail (IAUC 8692). On Apr. 2 the uncondensed 8" coma was of magnitude 19.3, showing a 12" tail in p.a. 230° according to P. Birtwhistle (Comet's Mailing List). According to E.J. Christensen the fragment showed a significantly condensed 12" coma and a 20" tail in p.a. 230°. On Apr. 7./8. he could detect two condensations of equal brightness in a distance of 4" (p.a. 210°) within the coma (Comet's Mailing List).
Fragment N: discovered on Mar. 23 by E.J. Christensen and P. Birtwhistle near fragment H. Fragment N (June 8.30) was of magnitude 21.4, displaying a star-like head and a 70" tail in p.a. 255°; it was situated about 40" north of the fragments' line (IAUC 8692). According to P. Birtwhistle on Apr. 2 the highly condensed 12" coma was of magnitude 18.2, showing a 30" tail in p.a. 230° (Comet's Mailing List). The brightness increase peaked on Apr. 5 with an amplitude of 3 mag. On Apr. 9 the fragment had faded considerably (Comet's Mailing List).
Fragment P: discovered on Mar. 24/25 by E.J.Christensen. Fragment P (June 7.88) was of magnitude 19.8-21.7 (IAUC 8693). According to P. Birtwhistle on Apr. 2 the 10" coma was of magnitude 20, displaying a 15" tail (Comet's Mailing List) in p.a. 230°.
Fragment Q: discovered on Mar. 24/25 by E.J.Christensen. Fragment Q (June 7.74) was of magnitude 20.7-21.3 (IAUC 8693). According to E.J. Christensen on Apr. 2 the fragment showed a diffuse 6" coma, but no tail (Comet's Mailing List).
Fragment R: discovered on Mar. 24/25 by E.J.Christensen. Fragment R (June 8.20) was of magnitude 20.5-20.8 (IAUC 8693). According to P. Birtwhistle on Apr. 2 the very pronounced 20" coma was of magnitude 16.7, elongated towards p.a. 230° (Comet's Mailing List). According to E.J. Christensen on Apr. 2 the fragment had brightened dramatically, displaying an elongated (p.a. 230°) condensed 45" coma (Comet's Mailing List). On Apr. 7 it had brightened further (15.1 mag), displaying a 40" coma and a 1.5' tail in p.a. 230° (CBET 473, Comet's Mailing List). The brightness was estimated to be of magnitude 16.0 to 18.0 between Apr. 8 and 21, 17.5 on Apr. 21 and 23 and 19.0 on Apr. 25. Observations with large telescopes on Apr. 20/21 showed three condensations within the coma. On Apr. 27 an brightness increase to 14.0 mag was noticed, lasting at least until May 2. On May 6 the brightness was measured as 16.0 mag, the component showing a very diffuse coma (still with a faint false nucleus) and a tail of 5' in p.a. 217° (Comet's Mailing List, MPEC 2006-H26, -H37, -H48, -H61, -J10, -J31).
Fragment S: discovered on Mar. 24/25 by E.J.Christensen near K, with which it overlapped. Fragment S (June 8.24) was of magnitude 20.9-21.5 (IAUC 8693).
Fragment T: discovered on Mar. 24/25 by E.J.Christensen. Fragment T (June 8.60) was of magnitude 20.7-21.7 (MPEC 2006-G10).
Fragment U: discovered on Mar. 24/25 by E.J.Christensen. Fragment U (June 9.02) was of magnitude 20.3 (MPEC 2006-G10, CBET 473).
Fragment V: discovered on Mar. 24/25 by E.J.Christensen. Fragment V (June 9.08) was of magnitude 21.2-21.8 (MPEC 2006-G10, CBET 473).
Fragment W: discovered on Mar. 24/25 by E.J.Christensen. Fragment W (June 8.51) was of magnitude 21.2 (MPEC 2006-G10, CBET 473).
Fragment X: discovered on Mar. 24/25 by E.J.Christensen. Fragment X (June 8.58) was of magnitude 21.3 (MPEC 2006-G10, CBET 473).
Fragment Y: discovered on Mar. 24/25 by E.J.Christensen. Fragment Y (June 8.81) was of magnitude 18.8 (MPEC 2006-G10, CBET 473).
Fragment AP: discovered on images taken on Apr. 18 at Lincoln Observatory as a 17.5 mag object. The fragment (June 6.69) increased briefly in brightness on Apr. 19/20. On Apr. 21 it was estimated as 16.5 mag, with the coma appearing elongated in the direction of the tail. Until Apr. 26 the brightness had decreased to only 17.5-18.0 mag. On Apr. 26 the fragment showed several condensations. On May 2 it showed another short-lived outburst peaking at 15.5-16.0 mag, but had faded to 19.0 mag on May 5 (MPEC 2006-H26, -H37, -H48, -H61, -J31, -J54, -L18, Comet's Mailing List).
Fragment AQ: discovered right behind the nucleus of fragment B on images dating back to Apr. 14 as a 14.5 mag object. Around May 2 the brightness of the fragment (June 7.93) peaked at 14.0 mag. Until mid-May it had decreased to about 16.0 mag (MPEC 2006-H37, -H61, -J10, -J31, -J54, -K18).
Fragment AS: discovered on images dating back to Apr. 20 as a 16.0 mag object. The brightness of the fragment (Juni 6.34) peaked at 15.0 mag on Apr. 21, but had decreased to 16.5 mag on Apr. 26. On May 1 the brightness of the central condensation was determined to be 19.0 mag (MPEC 2006-H48, -H61, -K55).
Fragment BC: discovered on images dating back to Apr. 25 as a 19.0 mag object. The brightness of the fragment (June 6.68) increased to about 17.0 mag on Apr. 30 and peaked at 14.5 mag on May 3. Until May 5 it had already decreased to 16.0 mag (MPEC 2006-H61, -J10, -J31, -J54).
Andreas Kammerer
