Dust and the Helix Nebula (2009 Dec 31)

[APOD Robot]

Dust and the Helix Nebula

Explanation: Dust makes this cosmic eye look red. The eerie Spitzer Space Telescope image shows infrared radiation from the well-studied Helix Nebula (NGC 7293) a mere 700 light-years away in the constellation Aquarius. The two light-year diameter shroud of dust and gas around a central white dwarf has long been considered an excellent example of a planetary nebula, representing the final stages in the evolution of a sun-like star. But the Spitzer data show the nebula's central star itself is immersed in a surprisingly bright infrared glow. Models suggest the glow is produced by a dust debris disk. Even though the nebular material was ejected from the star many thousands of years ago, the close-in dust could be generated by collisions in a reservoir of objects analogous to our own solar system's Kuiper Belt or cometary Oort cloud. Formed in the distant planetary system, the comet-like bodies would have otherwise survived even the dramatic late stages of the star's evolution.

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[neufer]

Dust and the Helix Nebula

The close-in dust could be generated by collisions in a reservoir of objects analogous to our own solar system's Kuiper Belt or cometary Oort cloud. Formed in the distant planetary system, the comet-like bodies would have otherwise survived even the dramatic late stages of the star's evolution.

• Oort-la-Kuiper, solar winds come sweepin' down the plain
  And the wavin' sleet can sure smell sweet
  When the wind comes right behind the rain.
  
  Oort-la-Kuiper, Ev'ry night my honey lamb and I
  Sit alone transfixed watching the Helix
  Makin' lazy circles in the sky.
  
  We know we belong to the ice
  But the ice that we have will suffice!
  And when we say
  Yeeow! Ayipioeeay!
  We're only sayin'
  You're doin' fine, Oort-la-Kuiper!
  Oort-la-Kuiper O.K.
  
  

[bystander]

Obviously, Will never met you.

[kovil]

Bravo !!
A Two Thumbs Up !! on the description accompanying the photo !

I appreciate the distance (700 LY est.) and the diameter of the object (2 LY dia. est.). Thanks very much for those, they always add substance to the photo.

Now here's where I'd like to steer the direction of the discussion:
". . .the nebula's central star itself is immersed in a surprisingly bright infrared glow."

If it's "surprising", then your models are incorrect to begin with. This should be your first clue.
2nd; what else can cause infrared glow ? And why are those mechanisms discarded from study ?

In regards to this science paper:
http://arxiv.org/PS_cache/astro-ph/pdf/ ... 2296v1.pdf
I do not agree with the authors' assumption of following the argument of 'dust disks' as the cause of infrared glow.
"The excess emission from the star most likely originates from a dust disk with blackbody temperatures of 90–130 K. Assuming a simple optically thin debris disk model, the dust is distributed in a ring between 35 and 150 AU from the central star, possibly arising from collisions of Kuiper-Belt-like Objects or the break-up of comets from an Oort-like cloud that have survived from the post-main-sequence evolution of the central star."

Here's another assumption they made:
"If the blackbody radiator is heated by the central star WD 2226−210, of which the effective temperature is 110,000 K and radius is 0.024 R⊙, it would have to be 41–91 AU from the star."
[my bolding of the text]

Why does the 'excess infrared' need be 'heated by the star' for its genesis ?
There are other things that can cause infrared radiation, why ingnore those mechanisms ?

No disrespect to this paper's authors, but they would do well to think outside the box on their next papers. !
Kovil

[Chris Peterson]

Now here's where I'd like to steer the direction of the discussion:
". . .the nebula's central star itself is immersed in a surprisingly bright infrared glow."

If it's "surprising", then your models are incorrect to begin with. This should be your first clue.
2nd; what else can cause infrared glow ? And why are those mechanisms discarded from study ?

It was a surprisingly colorful dawn this morning. Does that mean some model has a problem? They expressed it this way because most white dwarfs don't have dust rings. They are quite clear that while such rings are unusual, they are not contradictory to current theory and they have been observed around other white dwarfs. They don't discuss other mechanisms because basically, there aren't any.

I do not agree with the authors' assumption of following the argument of 'dust disks' as the cause of infrared glow...

Here's another assumption they made:
"If the blackbody radiator is heated by the central star WD 2226−210, of which the effective temperature is 110,000 K and radius is 0.024 R⊙, it would have to be 41–91 AU from the star."
[my bolding of the text]

Why does the 'excess infrared' need be 'heated by the star' for its genesis ?

While they leave their options open by stating that the source "most likely" is heated dust, they don't propose an alternate explanation for a simple reason: there really isn't one. It is virtually certain that heated dust is the source of the IR radiation, something that has been seen in hundreds or thousands of other stars (just not many white dwarfs). The mechanism is very well understood in terms of physics that has been solid for 200 years. Since the source is continuum radiation and follows the statistics of blackbody radiation, there is really little reason to look for other explanations.

There are other things that can cause infrared radiation, why ingnore those mechanisms ?

There really isn't another plausible explanation.

No disrespect to this paper's authors, but they would do well to think outside the box on their next papers.

The paper wasn't really about the dust as such, since this is mainly just an observation. What it was about was some of the characteristics of the odd progenitor star for this nebula, and mechanisms by which it could have a dust ring after the nebula was ejected. It does a good job suggesting how this could be, and provides an avenue for future observations that would further support its conclusions.

The paper's purpose is to provide a plausible explanation for an observation, not to explore exotic and highly unlikely possibilities (if there even are any in this case).