More fun with Holmes on Friday night

David Kingsley

With a rough visual fix, I the locked on to the comet with a Nexstar 8 SCT. The tracking mount made it possible to keep the comet centered in the eyepiece for the next hour or so. Thick clouds, thin clouds, and brief periods of no clouds passed by. Definitely not the best viewing conditions. But 10% of the time or so, the comet was clear and bright. I had equipped the Nexstar scope with a 8-24 mm zoom eyepiece and a filter wheel for my trip to Moorea this spring. That made it possible to quickly examine Southern Sky objects with a range of magnifications and different types of filters. The same tricks were very useful for getting the most out of the limited clear periods last night.

Visually, the near stellar nucleus and asymmetric bright fan were still there Friday night, and obviously yellow in color. The larger round bright annular shell surrounding the nucleus and fan has continued to grow. My rough reticle measurements with the Celestron microguide eyepiece give an estimated visual diameter of

1.3' Wed night
2.4' Thus night
3.5' Friday night

The expanding shell thus nearly doubled in diameter from Wed to Thurs, and appears to still be growing linearly at about 1.1 arcminutes per day. At a current distance between us and the comet of 1.63 astronomical units (152 million miles), that corresponds to an increased diameter of 48,000 miles per day, or roughly 2000 miles per hour (John Pierce made a very similar calculation based on change seen from Wed to Thurs). Of course growth in diameter comes at twice the rate of growth in radius, so the actual expansion rate must be about 1000 miles per hour from the center to the shell. That's enough to add about 6 times the diameter of the earth every day to the growing dust shell around the comet.

As the comet shell has gotten larger, its apparent color has dropped to my eye. The inner nucleus and assymetric fan are still a sulfur colored yellow. However, the bright surrounding dust shell now looks mostly greyish with faint hints of green.

The comet still looks roughly like a planetary nebula, with a central near stellar nucleus , a prominent spherical disc, and a brighter ring to the edge of that disc, suggesting the nucleus is surrounded by an expanding annular shell, rather than a solid sphere. However the physical processes are very different. The bright shell of Comet Holmes is made of emitted dust, is illuminated by reflected sunlight, and is growing at 1000 miles per hour. In contrast, the bright shell of a planetary nebula is produced by a superwind from a dying central star, glows from excitation and emission of a surrounding gas envelope, and expands about 50 times faster than the outburst from comet Holmes (25 km/sec for planetary nebula vs. 0.45 km/sec for Holmes).

Thinking more about comparisons with planetary nebula last night, I clicked a series of different filters across the the comet during occasional clear spells, including a Lumicon Deep Sky filter, a DGM optics Narrow Pass Band filter, a Lumicon OIII filter, a Star spectroscope, and one half of an Orion polarizing filter.

None of the filters enhanced the nucleus, fan or expanding annular shell. However the Lumicon Deep Sky filter and the Lumicon OIII filter did help bring out a much larger very dim outer envelope that has been just visible unfiltered every night. This outer envelope surrounds the near stellar nucleus, the asymmetric fan, and the bright annular disc talked about above. On wed, this dim outer envelope was about twice the diameter of the 1.3' annuar disc (outer envelope about 2.5 to 2.6'). Last night, I could still see a dim outer envelope surrounding the now much larger 3.5' bright disc, so the envelope must also be rapidly growing. However, it was too dim to accurately measure with the fleeting sky conditions last night.

I don't know if the apparent response of this outer envelope to the deep sky filter was simply an improvement of sky background by filtering out contaminating street light glow, or whether there is some actual excitation component to an outer gas layer surrounding comet Holmes. Some comets show emission lines from solar illumination of gas molecules in a surrounding envelope. The gas molecules emitted from a comet nucleus typically expand faster than dust particles, so I assume this dim outer envelope around Holmes is indeed gas instead of dust. A little hunting on the internet turned up an interesting historical report back from astronomers in the 1800s who examined Comet Holmes with with a spectroscope when it also flared in brightness over a hundred years ago. They noted an emission line around 515 nm in the dim part of the comet away from the nucleus. That's in a region near emission lines for cyanogen according to the Lumicon swan band comet filter description (a filter that I did not have last night). It's also in a wavelength range for emission from molecular carbon (C2) that was prominent in the Hale Bopp spectrum. I couldn't see any visible absorption or emission lines with the star spectroscope on Friday night. However that spectroscope was designed to look at stellar rather than diffuse spectra. I will be interested to see results of professional spectroscopy done on the comet Holmes outburst this time around.

For the 1893 report, see
http://tinyurl.com/2a8bwa

Finally, on Friday night, a polarizing filter again had little or no effect on the visual appearance of the comet, regardless of position angle. I tried different fixed positions of the eyepiece filter combination, as well as rapidly turning it through various orientations to try to pick up subtle visual shifts. I would still like to try this again on a cloudness night, with no distractions from passing clouds or reflections from a near full moon. However, I think any visual changes with polarizing filter are very hard to see. And as Richard Crisp has mentioned, his dramatic polarization images of the comet are processed to accentuate small differences in polarization that should be much less apparent in the eyepiece.

It has been great fun to watch this thing evolve over the last three nights. Most images I have seen are either overexposed in central regions, or show the central regions well but then miss the outer envelope. This sketch by Jeremy Perez shows most of the different features now detectable in the eyepiece, including: a bright central nucleus, an asymmetric fan from the central nucleus, a bright annular shell surrounding the nucleus and fan, and a dimer outer envelope. I think the outer envelope appears much more subtle than shown in the drawing, but this is one feature that can may get easier to see if you have a deep sky or comet filter.

http://www.perezmedia.net/beltofvenus/archives/000743.html

Skies look crappy now but I hope it will be possible to get another look tonight.

--David Kingsley