NEWS              

Cometa 73P/Schwassmann-Wachmann 3

Corrispondenza con Zdenek Sekanina l'esperto mondiale di Comete

07-05-2006

Dear Prof. Sekanina,

I work with an Italian group of professional and amateur astronomers (C.Guaita, R.Crippa, C.Cosmovici, R.Behrend, L.Comolli) since many years involved in cometary studies (our last publication will appear next on Earth, Moon, and Planets with this title: Comet I-Z (C/2002 C1): determination of the rotation period from observations of morphological structures.

I contact you about two very interesting results on fragment B of comet SW3, we obtained with the 0.4 m reflector of SAS Observatory-IAU A12 (Sozzago, Italy) equipped with a CCD camera.

Between April 1.8-2.8 a main outburst of almost 3 magnitudes was detected, probably linked to a couple of strong jets, clearly visibles in the CCD images we took during the following hours (as in CBET 464). A number of minor outbursts were also detected.

A periodic photometric variation of magnitude with a probable period of 0,249 days was discovered in observation performed during 7 observative nights in March and April 2006 before the burst and was confirmed  on April 10 during 8h of continuous monitoring. In the photometry performed after the outburst,  Dm increased up to 1,5 magn.

Beeing you one of the greatest expert on cometary dinamics, we would appreciate very much your opinion about a relationship between our photometric  variation and the rotation period,  and a possible link between this rotation period and the tendency of the B fragment to outburst.   

We would also appreciate your suggestions on how to demostrate that periodic variations in magnitude are linked to the rotation period..

If usefull, we could send you our main data and other CCD images.

Thank you in advance, sincerely yours

Federico Manzini   (SAS Observatory)

Roberto Crippa (FOAM13 Observatory)

09-05-2006

Dear Mr. Manziniì and Mr Crippa
I completely agree with what you say.  There must be a correlation between a brightening (outburst, flare-up) and jet development and very often (asin this case) also between an outburst and nucleus fragmentation.  I am particularly pleased to hear from you that you detected a number of minor outbursts.  Considering that the HST imaged many dozen fragments from this event some two weeks later, I felt that it was unlikely that they all would break up later from an initially single secondary fragment, but more likely that at least some of them separated at about (but not exactly) the same time as the brightest fragment.  I believe that your minor outbursts are associated with separations of the various fragments, although this may be very difficult to prove quantitatively.

As for the periodic light variations interpreted rotationally, this again has been done for other comets many times in the past, and certainly is a plausible hypothesis.  I only am a bit disappointed that you got 1/4 of
a day for the rotation period, which is too long for me.  If rotation is a factor in nucleus fragmentation, then the rotation periods of these tiny fragments should be much shorter.  Of course it is possible that different
fragments in this event had different rotation periods and your number perhaps refers to fragment B itself.  Although I do not know the dimensions of B, they cannot be more than several hundred meters across at the most, considering that the parent comet before the 1995 breakup was at most 2 km in diameter according to Boehnhardt et al.  Stretching the estimate to a maximum, say in excess of 0.5 km across, a six-hour rotation implies only velocities of about 0.1 m/s at the equator and less elsewhere.  So, maybe rotation is no so important after all, and instead these events are driven by erratic activity.

I think to demonstrate (i.e., prove) that periodic light variations are rotational is difficult, and the best argument is by default, i.e., what else could cause them.  This depends in part on a number of cycles that your data cover: the greater number, the stronger evidence.

Sincerely yours,
Zdenek Sekanina