The biggest surprise of the year 2007 presented Comet 17P/Holmes, showing an outburst of extreme amplitude during the last week of October. Based on the evolution of the last perihel passages the brightness was predicted at 16 ± 1 mag, which was confirmed during spring/summer of 2007. CCD-observations on Oct. 20.9 UT showed the comet at magnitude 17.8, on Oct. 23.0 UT the magnitude was 16.8-17.3 (CBET 1111).
But on Oct. 24.067 UT (1:35 UT) J.A. Henriquez Santana (Teneriffa) found the comet in his 20cm-telescope as an absolutely stellar object of magnitude 8.4! Observations with a 50cm-telescope on Oct. 24.21 UT (5:00 UT) showed a stellar-like object of magnitude 7.4. During the following 6 hours the brightness increased at a rate of 0.5 mag/h, as is shown by the following visual estimates: Oct. 24.34 UT (8:10 UT): 4.6 mag/stellar, 24.50 UT (12:00 UT): 4.0 mag/yellowish coma: 10", 24.55 UT (13:10 UT): 3.5 mag/in a 6.6cm-Refractor absolutely stellar, Oct. 24.63 UT (15:10 UT): 3.0 mag (IAUC 8886). Bernd Brinkmann photographed the comet on Oct. 24.847 UT (20:19 UT): the 2.8 mag comet presented a yellow-orange disc of 20" diameter without any internal structure (resembling Saturn) and a faint 40-60" sharp-edged outer coma. Near-nucleus structures close to the nucleus (namely a fan pointing south) were first seen during this night. CCD-observations on Oct. 25.0 UT showed an 8" central condensation, displaced by 10" in southwestern direction from the center of the 50" coma. A plume extended 15" in southwestern direction. At Oct. 25.13 UT the diameter of the bright inner coma was estimated as 69", the diameter of the outer coma as 3'. Observations by John Bortle on Oct. 26.076 UT showed a 5.8' large outer coma and a 2.9' bright disk, in which the false nucleus together with a 0.5' long fan-shaped extension in southwestern direction was seen. On Oct. 26.905 UT Bernhard Brinkmann estimated a coma diameter of 7.3'. Observations between Oct. 24.9 UT and 26.5 UT showed a dust-prominent coma with a fan, extending to the southwest from the nucleus. A near-nucleus dust ring expanded from the nucleus with a speed of 400 to 600 km/s. Observations with a 61cm-telescope on Oct. 26.4 UT showed a round coma of diameter 3.5', extremely evenly illuminated centered exactly on the false nucleus. The innermost central region had a diameter of 6" from which a drop tear-shaped region extended up to 20" in southwestern direction. Observations on Oct. 27.99 UT showed a bright, well defined coma (slightly unsharper edge to the southwest) of diameter 7' with a 1.8' central region, offset to the southwest. On the evening of Oct. 29 John Bortle estimated the diameter of the bright coma as 8', the one of the outer coma as 14' and calculated an expansion rate of 1.3'/day (CBET 111 / Comet's Mailing List).
Comet Holmes was discovered accidently by Edwin Holmes on Nov. 6, 1892 during a large outburst, located near the Andromeda Galaxy as a 4 mag object of diameter 5'. Until the end of November the coma increased to 30', but the apparent brightness remained almost unchanged. Briefly a 0.5° long tail was observed photographically as well as a nebulous object 0.5° behind the coma. During December the comet faded, reaching 10 mag in mid-January. But then on Jan. 16, 1893 a second outburst occurred, reaching 5 mag or brighter. After that the brightness decreased rapidly, reaching 10 mag at the beginning of February and 12 mag during the first days of March. During the following perihelia in 1899 and 1906 the comet reached only magnitudes of 13 and 15, respectively. A close Jupiter passage changed the orbit considerably, so that the comet was thought to be lost. After intensive calculations done by Brian Marsden it was recovered in 1964.
A series of observations made at the Pic du Midi Observatory between Oct. 24 and Nov. 4 showed the evolution of dust stripes in the inner coma. These appeared first at some distance from the nucleus and extended outward in straight lines. They most likely originate from larger fragments, which are nevertheless too small to be accessible by terrestrial telescopes, drifting away from the comet at about 50 to 100 m/s and slowly dispersing, so that the dust stripes should disappear soon. Based on these observations the outburst should have begun on the evening of the Oct. 23 (UT) with material (primarily dust) being released during the next two days, topped by an additional outburst on Oct. 24.40 UT which increased the brightness even more and is responsible for the round coma which is still expanding at a speed of 570 m/s. The brightness of the false nucleus decreased steadily after the outburst, but has stabilized during the first days of November and developed a dust lane, probably heralding an additional outburst (CBET 1123).
The northern instrument of the english planet finder project, SuperWASP-N, was serendipitously imaging the position of comet throughout October 2007, and that the comet is not visible in unfiltered CCD images to a limiting magnitude of approximately 15 at or prior to Oct. 23.27 UT. The comet is visible in the next image taken on Oct. 23.99 UT and brightens steadily, becoming saturated at approximately Oct. 24.10 UT. From photometry performed using apertures 70" in radius, a nearly 3-fold increase in the comet's brightness is found (from V approximately 9.7 to 8.6) in the 2.6 hours of unsaturated data. The brightening of the comet during this period is consistent with a power law with an exponent of 2, which would be expected from the linear expansion of an optically thick dust coma. Assuming a constant rate of expansion, the time of the comet's intiial outburst is found to be approximately Oct. 23.8 UT. (IAUC 8897).
For the analysis 317 observations by 17 members of the German Comet Section were taken into account as well as 1315 international observations (until the first days of April 2008). After the extraordinarily steep brightness increase during the first hours it slowed down, reaching a maximum of 2.4 mag on Oct. 30, 2007. Until Nov. 25 the coma slowly faded, reaching 3.2 mag, which was kept until Dec. 15. Thereafter a secondary fading followed, reaching 3.6 mag on Dec. 20, which was kept until Jan. 10, 2008. Since then it continuously fades, reaching 5.5 mag at the end of March.
Moving weighted 3-days-means of the total brightness
Due to the expanding coma the comet grew more and more diffuse as time went by. In February 2008 the surface brightness had become so low, that the number of observations dropped considerably. For most observers the comet became invisible during the first half of March.
Total Brightness and Coma Diameter
In July/August 2007 CCD-observations showed a 0.3' (25.000 km) coma. Due to the extreme brightness of the false nucleus after the onset of the outburst this coma was outshone. The outburst produced a bright dust coma with sharp edges, making measures easy, which must be differentiated from the faint outer gas coma during the first 15 days. The quite faint outer coma expanded to 40' (2.75 Mio. km) on Nov. 5, when it disappeared visually. The diagram shows that the bright dust coma expanded from almost nil to 75' (5.9 Mio. km) on Dec. 20 and to 90' (8 Mio. km) on Jan. 5, 2008. Based on this values the rate of expansion for the faint outer coma comes out as 3.6' or 250.000 km per day and that for the initial bright coma as 1.3' or 85.000 km per day. Thereafter the coma diameter seems to stabilize or even shrink. This however ist not real but caused by the fact that the surface brightness of ever-increasing parts of the coma dropped below that of the sky background, making them invisible. With time the solar wind caused the coma to become a parabola with a sharp NE-edge and a very diffuse southwestern part, which some people noticed as a tail (however, a real tail was pointing away from us during the first weeks).
The coma was absolutely stellar (DC 9) during the first hours of the outburst, looking like a Planetary Nebula of high surface brightness (also defined as DC 9) during the following days. Thereafter the brightness of all coma components (false nucleus, central region, bright coma, faint outer coma) decreased and all components became more diffuse. Accordingly the degree of condensation decreased very steadily to DC 2-3 on Nov. 25, thereafter much slowler, reaching DC 1 at the end of 2007 and DC 0 at the end of March 2008. However, until mid-December the northern edge of the initial bright coma was still discernible visually.
Evolution of the degree of condensation (DC)
Photographically a tail was first observed on Oct. 30. A deep photograph by Michael Jäger on Oct. 31 shows a fan-shaped (p.a. 170° ... 290°), 1° long very faint main tail with a dozen faint secondary tails. A photograph on Nov. 1 shows a 2° long main tail pointing southwest and several short tails in western directions. These features, however, disappeared during the following weeks. Reports of visual tail sightings (up to 1°) - between Nov. 10 and the end of 2007 - were very rare. Thereby it is not clear, if the observers actually refered to the photographically observed short-term tail or the long axis of the the paraboloid coma.
Based on the morphology and brightness evolution of the coma, Z. Sekanina has proposed the following scenario: during spring/summer 2007 the comet showed an activity parameter of n=16. Shortly before the outburst the heliocentric magnitude was 15.3 mag (30 times brighter than a purely reflecting nucleus of diameter 3.3 km). The outburst began on Oct. 23.7 ± 0.2 UT. During the first hours the release of dust accelareted or the diameter of the dust decreased, resulting in an exponential increase of the coma's cross section. The brightness plateau was reached 24 hours later with a heliocentric magnitude of 1.4 mag. Thus the amplitude of the outburst was in the order of 14 magnitudes (factor 400.000). The plateau indicates 2µm dust particels with an effective cross section area of 57±10 mill. km2 and a total mass of 1011 kg. The outburst could have been caused by the disintegration of a cometary companion, which fragmented from the nucleus. In this sense the faint outer coma would be the result of the fragmentation process and the parallel dust stripes are the result of the disintegration of the greater fragments of the companion. Contrary to comet Holmes comet 41P/Tuttle-Giacobini-Zinner shows short-term outbursts, indicating a dust deficiency (CBET 1118). John Bortle pointed to another theory by Sekanina, that outbursts could be caused by the sudden collapse of a honeycomb-structured nucleus, thereby releasing huge amounts of dust and gas. However, according to Dr. Clay, the very homogenous coma speaks against this theory. John Bortle pointed to the fact, that comet Holmes was one of the very brightest comets at distances greater than 1 AU at maximum brightness (Comet's Mailing List).
Andreas Kammerer
