Thursday, October 09, 2008

Nebular nonsense or neat knowledge?

Last week we talked about Gounelle & Meibrot's astrophysical nay-saying with regards to the origin of short-lived nuclides. As I pointed out, one of the long standing problems in this field is that cosmochemists and astronomers don’t often talk to each other. Just before I left the ANU for work in the real world, the planetary science institute started holding occasional lunchtime seminars to address this issue. The seem to still be cruising along. I’ve been attending when I’m in town, and that’s the origin of the reading list for these papers I’ve been blogging.
The seminars themselves have been a mixed bag. They are really good when folks from one discipline or the other nicely fill in blanks in the other group’s knowledge set. But there have also been some where older, less patient faculty have wasted everyone else’s time by talking past each other instead of letting the rest of the group actually learn.
Luckily, here I can type uninterrupted to you guys, and simple direct any interjectory comments into the spambot. Hester & Desch look at star formation in the same astronomical setting as Gounelle & Meibrot did, but with an observational, rather than a numerical approach. H&D is actually an earlier paper, and is referenced in G&M, but in ways that I don’t actually understand.
Basically, H&D look at the distribution of dust, accretion disks, new stars, and “blister zones” where the radiation from big stars has blasted a hole in the surrounding dust. One thing I don’t understand is why they refer to ionized hydrogen as “H II”, since hydrogen only has one electron to lose- is this obscure, counter-intuitive astrospeak?
Anyway, there are two main points that H&D make. The first is that as the blister zone expands around a big, hot, young star, it creates a shock front which can induce star formation. So instead of all stars forming at once, there is a sort of stellar domino effect.
The other big point was that the radiation effect from the nearby big stars on the accretion disk dynamics was often more important than the radiation from the star at the center of the disk. This, of course, has the implication that modeling accretion in an isolated heliocentric system might give incorrect results, if the main radiative forcing is actually external.
What I don’t get is how G&M arrived at the opposite conclusion, when they state that:
“It is thus assumed that the massive star, which evolved into a SN, and the protoplanetary disk were coeval and formed in the same stellar cluster (e.g., Hester & Desch 2005).
Yet the whole point of the H&D paper is that the star formation is NOT coeval, as small star formation is induced by the radiative pressure from big stars on the cloud. G&M don’t seem to even acknowledge, much less address this point. Instead they use rapidly decreasing linear star formation models, in which the youngest small stars are already several million years old by the time the first supernova goes off.
Anyway, I’m a geologist, not an astronomer, so maybe someone knowledgeable- like the blogospheric queen of galactic star formation, can point out what I’ve missed. But even if this entire post reads like scientific gibberish, I still recommend having a look at the Hester & Desch paper. It has lots of pretty Hubble Space Telescope pictures in it.

J. J. Hester & S. J. Desch 2005 Understanding our origins: Star formation in H II Region Environments. Chondrites and the protoplanetary disk ASP conference series, Vol 341. A. N. Krot, E. R. D. Scott, & B. Reipurth, eds.


dkary said...

Just to answer one of your questions, yes, HII is obscure astro-speak (we're good at that).
Traditionally, astronomers describe the ionization state of gasses using roman numerals. A "I" is neutral. A "II" is singly ionized. A "III" is doubly ionized. Etc.

Of course, this can lead to lots of different forms of confusion. For example, you could have someone talking about an HII region (like the Orion nebula) inside an H_2 cloud, and in conversation this can be a little obscure. As a result, H_2 is usually called molecular hydrogen.

Chuck said...

Why isn't neutral zero?