A Vast Protoplanetary Disk

by Paul Gilster on September 29, 2006

The disks of gas, dust and debris that surround young stars are breeding grounds for planets, a premise that every new exoplanet detection seems to confirm. But we know little about the disks themselves, and a key area of uncertainty continues to be the nature of disks around stars more massive than the Sun. What effect, for example, does their luminosity have on the disk, and how do the processes of large star formation affect planetary systems?

The European Southern Observatory’s Very Large Telescope is providing data that will shape a more refined view of these disks. At the heart of these new studies is HD 97048, a star some 600 light years away in the stellar spawning ground known as the Chameleon 1 dark cloud. HD 97048 is two and a half times as massive as the Sun, and fully forty times more luminous, making it ideal for such study.

A flared protoplanetary disk

Image: Artist’s impression of a flared proto-planetary disc, similar to what has been deduced from VISIR observations on ESO’s Very Large Telescope around the 2.5 solar mass star HD 97048. Credit: ESO.

Infrared mapping of this star’s disk with ESO’s VISIR (VLT Imager and Spectrometer for the InfraRed) instrument shows a huge, flared disk, reaching twelve times further than the orbit of Neptune. “This is,” says Pierre-Olivier Lagage (CEA Saclay, France), leader of the team that carried out the observations, “the first time such a structure, predicted by some theoretical models, is imaged around a massive star.”

Unclear of the meaning of the word ‘flared’ in this context, I checked with Dr. Lagage. His answer: “A flared disk is a disk whose thickness increases rapidly when going further away from the star, so that any point at the surface from the disk is in direct view from the star.”

Astronomers on Lagage’s team estimate the disk must contain a large amount of gas, amounting to ten times the mass of Jupiter, and perhaps 50 Earth masses worth of dust. That would make for a dust mass about a thousand times larger than what we’ve yet seen around older stars like Beta Pictoris, Fomalhaut and Vega. Lagage again: “From the structure of the disc, we infer that planetary embryos may be present in the inner part of the disc.”

In other words, the older stars mentioned contain dusty disks filled with debris thought to be the result of larger bodies colliding. We can do measurements on the dust but we have yet to detect the parent bodies that spawned it. Add up the mass of the dust and the presumed bodies that produced it and you get a mass similar to what is being seen around HD 97048. The young star’s disk, then, is simply much less evolved, a precursor for later planet formation.

Follow-up observations using the European Southern Observatory’s VLT interferometer are planned. A report on this work appears as Lagage et al., “Anatomy of a Flaring Proto-Planetary Disk Around a Young Intermediate-Mass Star” in the September issue of Science Express, with abstract available here.

{ 5 comments }

Eric James September 30, 2006 at 1:53

I’m no expert, but that never stopped me before…

It seems to me that such a system might have difficulty forming planets. I’d suspect the intense solar winds might tend to blow off the relatively light embryonic materials to a distance where only lighter bodies like plutons and comets might form. I’m thinking that the farther away from the sun the disc materials are, the longer it would take to clump significantly. In these cases, I suppose it might be so far away that these suns might primarily only have large clouds of relatively small materials around them, with few significant bodies. Does this make sense?

Is there a correlation between solar mass and the size of super-Jupiters? Is there a correlation with solar mass and planetary proximity to the solar mass?

P. S. The above concepts are purely lay conjecture. If any of it turns out to be right, I claim all rights to those concepts and the forthcoming publicity tour and the ensuing Nobel Prize. If it’s all wrong; I disown them, never wrote, read, saw or heard of them… don’t know what the heck you are talking about… stop following me!

Administrator September 30, 2006 at 8:19

I’m with you on the possible effect of solar winds on embryonic materials, but we won’t really know until more studies of massive stars are performed. Re the correlation between solar mass and the size of hot Jupiters, I don’t have the answer but let me float that question past some people who should.

Administrator September 30, 2006 at 17:11

Eric, I’ve checked with Greg Laughlin (UC-Santa Cruz), and rather than trying to summarize, let me just quote him:

“There’s no apparent correlation between the observed radii of extrasolar
planets (from transits) and the mass of the parent star. For example,
both HD 209458 and HD 149026 are somewhat more massive than the Sun,
and the size of their planets varies wildly: 1.35 Rjup vs. 0.72 Rjup.
There’s a slight hint that the size of planets might be inversely
correlated with stellar metallicity, however.

“I plotted planetary mass vs. stellar mass, and don’t see a trend. One
intersting thing that does pop out is a total absence of planets for
stars with masses between 0.4 and 0.6 Msol, but this is probably an
observational selection effect.”

Greg also mentions this site:

http://vo.obspm.fr/exoplanetes/encyclo/catalog-RV.php?mdAff=diag#tc

as a good resource for such correlation questions.

Eric James September 30, 2006 at 18:07

Paul,

Thanks for the research. That’s an interesting site (added to favorites).

I have a couple of follow-up questions that I also hope you can help me with (I don’t see the information in the site you provided).

Is there any correlation with planet mass, planet placement and stellar activity? Meaning, does the intensity of the star (and it’s accompanying solar winds) have any apparent effect on planetary formation?

The metallicity issue seems to indicate that magnetic fields might play a significant role in planet development. Is this correct?

Administrator October 1, 2006 at 12:33

“Is there any correlation with planet mass, planet placement and stellar activity? Meaning, does the intensity of the star (and it’s accompanying solar winds) have any apparent effect on planetary formation?

“The metallicity issue seems to indicate that magnetic fields might play a significant role in planet development. Is this correct?”

I hope others more qualified will jump in on this. Personally, I am not aware of any correlation yet established between planet mass, placement and stellar activity (I assume you mean flares, etc.) But I think we’re dealing with such a small sample that such correlations would be suspect in any case.

Metallicity is a different and interesting issue; let me do some research. Can anyone else help with the metallicity question?

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