More on the size of Holmes (APOD 21 Nov 2007)

Chris Peterson · Post by **Chris Peterson** » Wed Nov 21, 2007 5:28 am

Today's APOD still has the size of Holmes's coma wrong. As of Nov 21, the diameter of the dust coma is 3 million kilometers. On the date of the image, Nov 14, the diameter was 2.1 million kilometers. If you include the region of gas outside the dusty coma, those diameters are even larger. My profile measurements show the growth of the dust coma; the measurements extend to Nov 17, although I need to add data from Nov 19 to the chart (which still lies along the same growth curve).

This image provides a nice example of how processing can affect things. I happened to get an image at the same time, which can be seen here. My image isn't as deep, so the coma looks a little smaller, but it also doesn't blow out the middle. In fact, stars aren't just visible through the outer coma, they are visible right to the center. By Nov 14, activity had dropped so low that the nucleus is almost stellar in appearance- that is, it is just barely obscured by dust.

iampete · Post by **iampete** » Wed Nov 21, 2007 6:15 am

Obviously a layman's question, but . . .

How does one define the "edge" from which one determines the size of the coma?

Is the "edge" of the coma where light transmissivity falls below some value for some frequency, or is it where the particle density falls below some # per m**3 or what?

Is this also the standard definition when the diameter of gaseous planets are stated?

Thanks.

Chris Peterson · Post by **Chris Peterson** » Wed Nov 21, 2007 3:19 pm

iampete wrote:How does one define the "edge" from which one determines the size of the coma?

Is the "edge" of the coma where light transmissivity falls below some value for some frequency, or is it where the particle density falls below some # per m**3 or what?

Is this also the standard definition when the diameter of gaseous planets are stated?

Thanks.

It's a very good question. There is no standard definition for the edge of a comet's coma. If you look at the profiles, however, there is a very obvious inflection point at the outside edge, and I used that point consistently. A slight shift to either side would somewhat alter the estimated size, but would have little effect on the computed expansion rate. This works well with Holmes because it does have a distinct edge to its expanding dust front. A more nebulous comet would require a different criterion, something along the lines of your suggestion.

In the case of the gas giants, the top of the atmosphere is also pretty sharply defined, so there isn't much problem defining a size.

iamlucky13 · Post by **iamlucky13** » Wed Nov 21, 2007 10:19 pm

Thanks for the great information and putting together a great page Chris. I hope you don't mind if I post that graph to show others what you're talking about?

I had a couple other questions

Are you talking about the inflection point at about 1000 arcseconds on the newest measurement or the one at about 500 arcseconds?

What's the current apparent magnitude of Holmes?

Would you say it's possible the change from 500 m/s to 700 m/s to be due to a second "eruption" occuring more forcefully? It seems to me that if it were due to the solar wind it would be noticeably lopsided.

Chris Peterson · Post by **Chris Peterson** » Wed Nov 21, 2007 10:39 pm

iamlucky13 wrote:Are you talking about the inflection point at about 1000 arcseconds on the newest measurement or the one at about 500 arcseconds?

The outer one, at 1000 arcsec. This is a little above the noise floor of the image. Each profile was taken the same way, perpendicular to the tail/spreading pseudonucleus. Along the other axis the coma is larger. Note that the profile curves have all been normalized to the same height, which makes the actual location of the inflection a little ambiguous. I use a second derivative method on the raw profile to identify the point and remove my own judgment from the process.

What's the current apparent magnitude of Holmes?

I'm not sure. I plan on calculating this for all the images I've made, but haven't yet had a chance. Magnitude of an extended object is a fuzzy concept. There is the total integrated magnitude, which is probably still up around 2-3, and magnitude per unit area, which has decreased to the point that the comet is barely visible naked eye.

Would you say it's possible the change from 500 m/s to 700 m/s to be due to a second "eruption" occuring more forcefully? It seems to me that if it were due to the solar wind it would be noticeably lopsided.

I don't think so, because such an outburst would have produced something like a second (or renewed) pseudonucleus, which wasn't seen. Enhanced dissipation due to the solar wind wouldn't necessarily produce a highly lopsided coma, because we are between the Sun and the comet. Or more accurately, the tendency to become oblong is mostly along our line of sight. But we are seeing some elongation of the coma, and it is along the solar-antisolar axis of the comet.