MyCn18

geckzilla · Post by **geckzilla** » Sat Sep 28, 2013 12:48 am

Something bothers me about this nebula that a few searches failed to satisfy. The star is off center. I wondered if it could possibly be a foreground star which just so happens to line up close enough to the center to be mistaken for the parent star. Surely I am not the first person to have this idea but I can't find any mention of it anywhere, just articles which use the star to study the surrounding nebula without any mention about the questionable position of the star. The real star could be hidden behind that dense cloud in the middle, much like NGC 6537. Is there even a way to make sure the visible star really is the progenitor?

stephen63 · Post by **stephen63** » Sat Sep 28, 2013 3:47 am

2MASS 13393507-6722518
This is the star in question. I couldn't find anything on it.

alter-ego · Post by **alter-ego** » Sat Sep 28, 2013 4:32 am

The offset central star in the Etched Hourglass nebula seems to still pose a mystery. It's clear the hourglass is tilted, so projection could play a role in the apparent position of the star in question. The following paper references the progenitor star as an AGB, with information about flux rate, and also models the hourglass tilt to be ≈30° to 35°.
http://trs-new.jpl.nasa.gov/dspace/bits ... 9-2127.pdf.
This paper (~1999), with reference to another publication, seems to imply the progenitor star is known (?).

The following paper (1999) states the off-centered star is a mystery, but mentions it discusses several mechanisms that may cause the offset.
http://iopscience.iop.org/1538-3881/118/1/468

Maybe these discussions might help understand the certainty, or uncertainty, of the progenitor star. The answer is not obvious, and maybe not known with certainty.

geckzilla · Post by **geckzilla** » Sat Sep 28, 2013 6:20 am

Thanks, those were exactly what I was looking for. Of interest to me is section 3.3 of the Sahai paper.

The central star is probably surrounded by hot dust in the waist region because the expected K-band photospheric flux of 0.016 Jy is significantly smaller than the observed value of 0.033 Jy

I need more than my layman's knowledge to understand the section fully but it is interesting to me that it is all based on the simple assumption that the star must be associated with the nebula. Either the idea that it is not is either so easily dismissed that it wasn't included in the paper or it was not taken into consideration. Hmmm.

geckzilla · Post by **geckzilla** » Fri Oct 04, 2013 5:57 pm

I came up with another idea. If, spectroscopically, the star is definitely within the nebula, it might not just be a foreground star but it might actually be orbiting my hypothetical star behind the cloud. So all it would take to determine this would be to measure its motion to see if it's consistent with orbiting such a star. ...I wouldn't know how to accomplish this, though.

rstevenson · Post by **rstevenson** » Sat Oct 05, 2013 2:23 am

geckzilla wrote:... So all it would take to determine this would be to measure its motion to see if it's consistent with orbiting such a star. ...I wouldn't know how to accomplish this, though.

By coincidence, just before seeing your post here, I had read the article posted in this thread, which explains quite well the complicated and extended process that goes into determining a star's orbit.

Rob

neufer · Post by **neufer** » Sat Oct 05, 2013 5:50 pm

Strabismus

Beyond · Post by **Beyond** » Sat Oct 05, 2013 8:56 pm

Or, Heterotropia, if he was seeing them both at the same time.

geckzilla · Post by **geckzilla** » Wed Jan 28, 2015 1:31 am

Ok. I finally got it. MyCn 18's star is not off-center. Its outflows were sculpted by an unseen dust torus (it is too small to see) but the dust torus is not a perfectly round dust torus. For instance, take this nebula which exhibits a lop-sided dust torus:

We can't see the central star to know if it would appear off-center when viewing down the "barrel" of the outflows, but at least we know it's possible.

Another important concept is to understand that outflows can stop suddenly, leading to things which look like this:

Can they start back up again? I'm not sure. I don't have any examples I'm sure of but I think MyCn18 might be one. Here's a 3d model I made in Blender of some outflows viewed from the side and then viewed at an angle. On the left is a "perfect" nebula in which the outflow expands through the aperture of a perfect torus of dust. On the right shows the central outflows are expanding out more freely on one side and more restricted by the dust torus on the other side. I know it's not very precise and I haven't done a lick of math but damn it, it makes a lot more sense to me than to say that the star is the off-center one.

BMAONE23 · Post by **BMAONE23** » Wed Jan 28, 2015 4:16 am

Missed this thread back in 2013. The first impression I got was that the central star was well centered within the exterior limits of the nebula and the central portion was offset from the star.

geckzilla · Post by **geckzilla** » Wed Jan 28, 2015 4:24 am

A lot of people don't even see a 3d structure when looking at it for the first time. They see a kind of flat figure-8 thing. It's difficult to tell if the outer portions are really centered perfectly on the star, though. It could be centered or could be off a little. The edges are fuzzy on the left side even for the larger (and presumably older) structures.

Starship Asterisk*

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