Nucleus brightness and dust distribution

Forums Comets Spectrum of Comet 46P/Wirtanen Nucleus brightness and dust distribution

Robin Leadbeater

Hi Nick,

A figure I have seen for the diameter of the nucleus is 1.2km so a quick back of envelope calculation using the brightness of the moon as a reference and assuming the same albedo suggests that the naked nucleus would have a brightness of ~ mag 12 currently.  

(distance ratio)^2 ~1000. x (diameter ratio)^2 ~10^7  = 10^10  = ~ 25 mag + mag -13 = ~ mag 12


which is in the ball park of what I would have guestimated it to be from the guider image. I suspect you are right though and the nucleus is indeed hidden from view by dust. 

That green channel profile includes a lot of Swan band emission. Here is a vertical slice through the spectrum in a region devoid of emission lines.

It is much narrower with the reflected/scattered component mostly confined close to the centre in a region with FWHM ~ 8arcsec. There is some spread, presumably dust, but approximately 30% of the total flux comes from a region similar in width to that produced by a star in the spectrograph which with my setup is  typically ~5 arcsec FWHM (wider than a normal image due to astigmatism in the spectrograph optics and  guiding errors)