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Fraser
2005-Aug-03, 07:00 PM
SUMMARY: With new instruments, astronomers are filling in all the pieces that help to explain how planets form out of extended disks of gas and dust around newborn stars. This process seems to happen quickly, often just a few million years is all it takes to go from dust to planets. But astronomers have found one proto-planetary disk that refuses to grow up. It's 25 million years old, and still hasn't made the transition to form planets. Lee Hartmann is with the Harvard-Smithsonian Center for Astrophysics, and the lead author on the paper announcing the find.

View full article (http://www.universetoday.com/am/publish/audio_oldest_planetary_disk.html)

What do you think about this story? Post your comments below.

lswinford
2005-Aug-04, 08:36 PM
Maybe we have it backwards. Perhaps one of the binary stars is captured and the original star system had planets but the gravity dynamics with the second sun shattered the planets and churned them into a ring--those parts that had the angular momentum to continue to orbit the stellar center of gravity.

GOURDHEAD
2005-Aug-04, 09:20 PM
It's 25 million years old, and still hasn't made the transition to form planets. I assume that the age of the disk is assumed to be that of the star system which it encircles. How accurately can the age of the stars be known? What are the stochastic constraints on such determinations? Can the age of disks be determined independly from that of the star system?

cran
2005-Aug-05, 12:28 AM
Perhaps a study of the chemistry of the disc might point to an answer...if it's poor in silicates and metals, and only has the lighter volatiles to build from, that might slow the whole process down... :unsure:

How accurately can the age of the stars be known? What are the stochastic constraints on such determinations?
I'll admit ignorance here, but I'm satisfied that, with all of the studies done on stars, the formation, chemistry, and evolution (and therefore, age estimates) are as confident as can be about something we cannot pick up and hold in our hands - I'll accept that our sun, for instance, is only about half way through its main sequence...

Can the age of disks be determined independly from that of the star system?
Sure, if we can get our hands on a piece of it...other than that... :unsure:

Maybe we have it backwards. Perhaps one of the binary stars is captured and the original star system had planets but the gravity dynamics with the second sun shattered the planets and churned them into a ring--those parts that had the angular momentum to continue to orbit the stellar center of gravity.
lswinford, you may have something there...further study of the stellar dynamics should indicate whether its a 'captured' binary or whether they co-evolved...higher resolution studies of the disc should indicate whether gravitational interference or resonance is preventing planetary formation...but I would expect the disc to be more chaotic (ie, extended streamers of material, larger remnants and vortices) if a planetary system had been torn apart, rather than (as seems to be implied) an homogenous disc in the ecliptic.

Guest
2005-Aug-05, 12:54 PM
Can someone review for me in quick, layman's terms, how scientists can estimate the age of a dust disk and/or star?

russel
2005-Aug-05, 09:06 PM
The questions and concerns expressed about how stellar ages are estimated are completely valid, and in my opinion, should have been better conveyed in the press release. I know this because I wrote the original discovery paper, with my colleague Dr. Lynne Hillenbrand, on this peculiar adolescent age star ("A Long-lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star", published in The Astrophysical Journal earlier this year). It's in that paper that the age estimates are provided. Here is a less biased view of this interesting discovery.

Either St 34 (the name of this star) has an age greater than ~25 million years and is one of, if not the oldest star known with a planetary disk, or the age estimate is wrong. Lee Hartmann argues for the former, based on a summary of the results presented in our paper; I'll present the case for the latter.

The age is estimated to be greater than 25 million years because the element lithium is not detected in its spectrum. Lithium is too fragile an element to survive the intense heat of most stars. Thus, soon after a star forms, its lithium is quickly destroyed (add a proton, and then fissions to 2 Helium atoms). So if you don't see any lithium, the star must be older than this destruction timescale. Unfortunately, estimating this destruction timescale is very difficult. For a star to destroy all of its lithium, it must transport it to its center where the temperatures exceed 2 million degrees. As you might imagine, the intensive computer calculations and simulations require to model all of the convective and energy transport processes is exceptionally difficult, and may be in error. Indeed, here is a quote from Isabelle Baraffe, the theorist who produced the stellar evolution models that I compared to to estimate an age of 25 million years for St 34:

"The Li depletion is very sensitive to the depth of the convective
envelope, which may be affected by changes in the input physics
of the models or by processes such as rotation. In this
case, it would be rather easy to change our Li predictions
and the age inferred by a factor 2 or so"
- Isabelle Baraffe, 27 Oct 2004

An age of 10 million years is certainly plausible; a disk at that age would not be unique. The theoretical uncertainties and the observational inconsistencies with lithium depletion predictions (summarized in our paper) are too great to conclude that St 34 is as old as 25 million years. There is also some compelling evidence that St 34 is physically associated with a young cluster of stars of age ~2 million years; its lack of lithium is the real puzzle.

In the press release, Lee Hartmann states that "Finding this disk is as unexpected as locating a 200-year-old person." But I bet if Lee actually met someone that old he'd be skeptical of their age. The case is the same here.

Russel White

GOURDHEAD
2005-Aug-06, 12:05 AM
Thanks Russell, I was wondering how certain one could be about stellar ages and intuitively I suspected that even 25 million years was within the tolerances on accuracy.

cran
2005-Aug-06, 02:48 AM
Yes, let me add my thanks also, russell; it's a rare treat to get information straight from the source...

That still leaves the question of whether it's possible to estimate an age for a planetary disc independently of the estimate for the age of the star...my gut feeling is 'not yet', but I'd like it confirmed or clarified.

russel
2005-Aug-09, 08:36 PM
Hi,
Just to follow-up on the disk versus stellar age question. Unfortunately it's not possible to estimate the disk age independently. Perhaps as suggested if samples (e.g. meteorites) could be returned we could estimate the last time they were molten, but that's not likely to happen any time soon. The disk is generally assumed to form at the same time as, or soon after the star, but exactly when this happens and exactly when we can begin calling it "a disk" are unclear. Russel

cran
2005-Aug-10, 01:14 AM
Thanks again, Russel :D

Awright, you geniuses...we are looking for a way to independently estimate 'disk ages' - preferably from here...ideas? ongoing research? a novel approach? anyone? :unsure:

Meanwhile, I&#39;ve got other &#39;fish to fry&#39;...(fossil ones - very crunchy...not much flavour...) <_<

C ya&#33;