153P/Ikeya-Zhang = C/2002 C1 (Ikeya-Zhang)

This comet was a naked-eye object in Spring 2002, but has faded very rapidly since as the light curves below show.

Comet Ikeya-Zhang is the return of C/1661 C1, which is unofficially referred to as Hevelius's comet. As no comets were officially named for their discoverer until 1759, it is not correct to call this object Comet Hevelius or Hevelius's Comet. A linkage has been claimed to comets or possible comets seen in 877 and 1273, but this is still rather uncertain. In particular, the position of the 877 object is only very poorly known and it may have been a nova rather than a comet. For detailed comments on the possible linkage to the 1273 comet and the problems involved with it, see the link to Graeme Waddington's article in March 2002.

The light curve of the comet and its dynamical evolution are explored in a series of articles by Mark Kidger and Graeme Waddington that appeared in the magazine "The Astronomer" between March and June 2002. You can see these articles by clicking on the following links:

	Light curve	Dynamics
March	Mark Kidger	Graeme Waddington
April	Mark Kidger	Graeme Waddington
May	Mark Kidger	Graeme Waddington
June	Mark Kidger & Graeme Waddington

C/2002 C1 (Ikeya-Zhang) has just recently satisfied the MPC's rules for being numbered (a satisfactory linkage of observations at two returns to perihelion) and is now officially designated 153P/Ikeya-Zhang. It is now regarded as being "controlled" and has a permanent numbered designation.

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The light curve of C/2002 C1 (Ikeya-Zhang)

The light curve shown below is constructed from 267 observations. The magnitudes listed as m₂ are taken in R with a 10" aperture. CCD total magnitudes are taken with apertures from 0.2' to 5.6'. m₁ is the total visual magnitude. The comet had a peak magnitude of m₁=3, a solid, if not spectacular performance and was a relatively easy naked-eye object for a time around perihlion.

Although the CCD data shows a steady and consistent decline from maximum after perihelion, the visual data shows a small bump in the light curve that started in late April. At this time the comet was close to perigee, although its heliocentric distance was increasing rapidly. This means that the true brightness of the comet was actually increasing as it got further from the Sun.

Although the comet has faded considerably it is still being observed actively.

On the right we see the light curve with the data split by types. Normally the m₂ observations taken with CCD follow the total visual magnitude fairly closely, although several magnitudes fainter. As we see here though, there are many interesting differences. Note how in early February the difference between m₁ and m₂ is about 4 magnitudes. In some places it is suggested that the difference between the two magnitudes can be approximated by a constant: this is not even approximately true. It may be true that for a particular comet the difference between m₁ and m₂ is constant, or nearly constant. We see though that in late April this difference has increased to nearly 7 magnitudes.

The reason for this is two-fold. First the comet was close to perigee and the coma increased consisderably in size. From mid-April the coma's true visual diameter increased from 250 000km to 1.5 million km and the apparent diameter was more than 15'. This means that the CCD observers with their fixed 10" aperture saw an increasingly small fraction of the total light from the coma. The second reason was the fact the degree of condensation of the coma passed from being very high around perihelion (many observers estimated DC=9) to being extremely low (DC=0) at perigee.

When the data is separated by observers (left) we see that there is generally good agreement. Two things are worth commenting though. MPC Station 620 (the Observatorio Astronómico de Mallorca) estimates both the magnitude of the inner coma (m₂) and an estimate of the total magnitude using a large aperture. With such a large and active comet it is though very difficult to get a large enough aperture to include the full coma. We see that even before perihelion, both the CCD measures by Station 620 and those by Sostero and collaborators are still a magnitude or more fainter than the total visual magnitude (m₁).

We also see that a few visual estimates in June 2002 are unusually faint. The reason for this is the nature of the comet and the extreme difficulty of observing it. After perigee the comet was extremely diffuse, with almost no central condensation making visual estimation difficult. When the comet was observed in poor conditions (poor sky transparency or light pollution), a substantial part of the extended coma could not be seen and thus the estimate made was much fainter than the true total brightness.

An interesting phenomenon is seen during the April/May outburst. In the plot to the right the data is shown as magnitude corrected for geocentric distance against the logarithm of heliocentric distance. To guide the eye two lines of equal slope are drawn through the CCD and the visual data. Note the outburst in the visual data to the upper right of centre. There is a fine series of observations at this time by Francisco Rodríguez supplemented by some estimates from Faustino García. These show that the comet increased in brightness by about 1.5 magnitudes in the outburst.

If we look though at the CCD data (m₂) we see that the observations show exactly the opposite trend: as m₁ brightens, m₂ fades, all the points lie below the line rather than being spread around it and the difference between the two magnitudes increases substantially. This is surprising because we would expect an outburst to be marked by an increase of gas production by the nucleus and a significant increase in brightness in the 10" aperture.

The apparant coma diameter as estimated by visual observers.

The Degree of Condensation (DC) of the coma estimated by visual observers.

Comet Ikeya-Zhang and the comet of Hevelius

One of the mysteries associated with Hevelius's discovery and observation of the 1661 comet is why he did not observe it when theoretically bright and favourably placed in the evening sky in late December 1660 or January 1661. Similarly, it is hard to understand why Hevelius's comet should have been as much as 3 magnitudes brighter than Comet Ikeya-Zhang when these are consecutive returns of the same comet. Being so much brighter the 1661 comet should have been a spectacular object in the evening sky before perihelion.

The catalogue of cometary absolute magnitudes of David Hughes shows Hevelius's comet with an absolute magnitude of 4.0 - similar to Halley's Comet and much brighter than the average of 6.5. Cometa Ikeya-Zhang, in contrast, had an absolute magnitude of around 7.0 before perihelion: are they really the same object?

The key words here are "before perihelion". If we take the comet's light curve slope as being constant at n=10.7 (from the plot above), we can look at the variation of the absolute magnitude with time. This is the plot shown above. The median of the 80 observations up to T+20 days is m₀=7.14 however, from T+20 days we see how the absolute magnitude gets progressively brighter until reaching a more Hevelius-like m₀=5.0.

This certainly is a major perihelion assymmetry. However, to reproduce Hevelius's data we would need to assume that, in the 1661 return, the assymmetry started earlier and was of somewhat larger amplitude than that observed in 2002.

We can see this in the plot to the right. Here we plot the increase in brightness over the extrapolated ephemeris with time. The rise is rather slow, but reaches 2 magnitudes by mid-June. John Bortle's prediction based on an assumed perihelion assymmetry in 1661 was for Comet Ikeya-Zhang to continue increasing in brightness after perihelion for about 2 weeks until reaching magnitude 2. We can see that the rise was both slower and later than had been predicted.

Hevelius's data in 1661 are though consistent with Comet Ikeya-Zhang in other ways. Hevelius recorded how the comet grew large and diffuse before fading away and disappearing. This is exactly the behaviour that Comet Ikeya-Zhang has shown. The visual data from 2002 shows a huge increase in the size of the visible coma and a rapid decrease in the degree of condensation.

It seems then that the behaviour of Hevelius's comet and Comet Ikeya-Zhang have been very similar overall, although there are some major differences that are, as yet, unexplained.

CCD observations in a 10 arcsecond aperture by:

• Ramón Naves & Montse Campàs - MPC 213
• Rolando Ligustri - MPC 235
• Albert Sánchez - MPC 442
• Miguel Camarasa - MPC 445
• Salvador Sánchez & Juan Rodríguez - MPC 620
• Giovanni Sostero & Luca Donato
• Toni Climent - MPC J97
• Luis & Salvador Lahuerta (Valencia, Spain) - MPC J98

CCD aperture photometry in apertures of 0'.2, 1'.0, 1'.1, 1'.5, 1'.6, 1'.7, 1'.8, 2'.3, 2'.4, 2'.5, 2'.6, 2'.7, 2'.8, 2'.9, 3'.0, 3'.1, 3'.2, 3'.5 and 3'.6 by:

• R.Ligustri
• M.Zorzenon
• P.Beltrame
• V.Savani
• F.Scarpa
• M.Zorzenon
• G.Degano

Visual data by:

• Miguel Camarasa (Spain)
• Faustino García (Asturias, Spain)
• Francisco Rodríguez (Gran Canaria, Spain)
• Francisco Ocaña (Cuenca, Spain)
• Federico Fernández (Tenerife, Spain)
• Valentín Díaz (Elche, Spain)
• Francisco Reyes (Murcia, Spain)
• Guy Hurst (Basingstoke, United Kingdom)
• Rafael Benevides (Spain)
• Giovanni Sostero (Italy)

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Última actualización 17/01/2003
Por M.R.Kidger