Thursday, October 30, 2008

The geologic lifespan of Jon Bon Jovi

As a child of the 80’s, I grew up with a variety of ephemeral pop bands, who evolved from the primeval ooze, burst into an eruption of sound and video fury, only to fade into extinction. Occasionally, however, a rare artist would rise again from the depths of obscurity, a rock and roll coelacanth long after his contemporaries had lithified. After a recent sighting of one such musical living fossil, I began to wonder exactly how long this antiquated creature could reasonably be expected to survive. So in an exercise of Bon Jovichronology, I tried to deduce the musical lifetime of Bon Jovi using the principles of geochronology and some of his better known lyrics.

While the title of the song “Always” is obviously an unquantifiable hyperbole, there is a refrain from which we can attempt to derive an answer.

I’ll be there ‘till the stars don’t shine
‘till the heavens burst
And the words don’t rhyme

Thus, a Bon Jovi-eon is allegedly comparable to the lifetimes of stars, heavens, and rhyme. What are these in years?

The lifetime of the stars that shine is an astronomical question, not a geologic one. All of the stars we see right now are either
a. Short-lived massive stars,
b. Giant stars inflated by the last gasp of helium burning at the end of their lives
c. Sunlike stars that happen to be passing close to the sun right now.

Or some combination of the three. In any case, all of the stars that we can see today with the naked eye will either die out or move too far to see within millions to tens of millions of years. It is worth pointing out that the most common, longest lived M class stars are too dim to view unaided, even if they are very close. Of course, once the stars we see today disappear, they will be replaced by other, different stars. Sunlike stars are being born today, and they live for 10 billion years, so there will probably be something to be seen in the night sky for tens of billions of years, long after our sun runs out of hydrogen and swells up into a red giant, evaporating all the FM radio stations and record stores that may still exist on Earth. So this constraint is pretty open-ended.

It is hard to quantify when the heavens will burst, as heaven is a mythological/literary construct. Taken literally, this would presumably refer to Armageddon, which has a timescale of a few thousand years- orders of magnitude shorter than the stellar constraint.

So, what about the third constraint? While the timescale of rhyme is a linguistic question, we can derive a quantification from standard geochronological first principles. To do so, we will assume the following:
1. A rhyme is deemed to have decayed when the pronunciation of the words changes so that those which used to rhyme don’t anymore.
2. Like radionuclides, we will assume that the rhyme decay constant is invariant. It almost certainly isn’t, since audio recordings and dictionaries will probably effect it, but that doesn’t mean that we can’t pretend.
3. Rhyme decay is irreversible.
Using these assumptions, we can take an English language work of known age, and ratio the remaining rhymes to the original rhymes to determine the decay constant, lambda. The relevant equation is lamba = -ln(N/N0)/t, where N is the remaining rhymes, N0 is the original rhymes, and t is elapsed time. Lamba is related to the half life, as follows: T1/2 = ln2/lamba. To calculate the English rhyme decay constant, I will use a subset of Shakespeare’s sonnets, published in 1609. Looking at every tenth sonnet, we see that out of a total of 210 rhymes, 12 have decayed, and 198 are intact. –ln(198/210)/400 gives us lamba=0.000147/yr, or a halflife of 4712 years. This is slightly shorter than the halflife of carbon-14.

A approximation is that after 10 halflives, 99.9% of a nuclide has decayed, and the original has generally dropped below detection limits. Applying this to Bon Jovi suggests that after 47,120 years, essentially all of the words that currently rhyme won’t.

For comparison, this is approximately the elapsed time since humans first arrived in Australia. Europe was still inhabited by the Neanderthal, and it would be 20,000 more years before Cro-Magnon people painted their caves. A similar period of time would have to elapse after that before this descendant of subsequent African emigrants invented rock and roll.

Of course, for the cynics among you, there is an alternative explanation.
Technically, shine and rhyme don’t rhyme right now. The heavens burst whenever it rains, and to the sea level observer, the stars stop shining just before sunrise.
So a jaded reader could interpret this as a one night stand song. This interpretation is irrelevant for two reasons.
Firstly, the known timescale of Bon Jovi’s success is 3-4 orders of magnitude longer than a single night.
More importantly, though, is the observation that during the 80’s, jaded cynics weren’t listening to Bon Jovi; they were playing records by the British black tee-shirt crowd instead. I should know. I was one of them.

related post: Postdoc song

Wednesday, October 29, 2008

Dick Zimmer for Senate

Dick Zimmer is socially moderate, fiscally astute, and all kinds of awesome. Let me explain, for those of you who are unfamiliar with central Jersey politics.

Dick Zimmer was the congressional representative for NJ’s 12th district* from 1990 to 1996. In 1996, he ran for the Senate seat vacated by Bill Bradley, who was retiring. Zimmer lost to democratic representative Bob Torricelli, and the congressional seat he vacated to run for senate was take by Mike Pappas.

Mike Pappas was a disgrace to the Garden State, who spent his time in congress singing love songs to Kenneth Starr. He was defeated in 1998 by Rush Holt, who is still the representative for district 12.

In 2000, Dick Zimmer challenged Holt for his old seat. In this race of two great candidates, Zimmer ended up losing by less than a thousand votes. At that point, he left politics, and spent his time teaching at Princeton University and working in the private sector as a lawyer.

In the same year, Frank Lautenberg retired from the senate at the ripe old age of 76.

In 2002, Torricelli ran for re-election to the Senate. Six weeks before the election, he was implicated in illegally receiving campaign funds from North Korea. Rather than risk losing the election, the Democratic Party pulled Torricelli’s name off the ballot, and drafted Lautenberg out of retirement to substitute in. Lautenberg won the election. This year, he is running for his second term out of retirement, or his fifth overall.

Having witnessed the rise and fall of the Ming dynasty in his youth, Lautenberg is a fan of big, intrusive government. Twenty-year-olds in Virginia or Montana can thank him for making beer illegal, as he championed the practice of withholding federal funds in order to bribe state legislatures into social engineering.

In contrast, Dick Zimmer has a socially moderate, fiscally conservative record. He supports limited, but effective government. With a 1013 dollar deficit, we need somebody who isn’t a profligate spender. Dick Zimmer won’t use your tax dollars to force people halfway across the country to live a certain way. And he won’t take is marching orders from Harry Reid. He’s a great legislator, and we’d be lucky to get him back in office.

It’s OK if you’re voting D for President and house this year. We all stray from time to time. But an R in the senate column will benefit New Jersey and the nation.

Of course, some of y’all may prefer a technoblogospheric nepotistic argument instead. In which case, I recommend you vote for Carl’s dad.

* In New Jersey, congressional districts are named after Turnpike exits.

Sunday, October 26, 2008

My congressman *is* a rocket scientist

I still support Rush Holt for congress, just like I did two years ago. Senate is forthcoming.

Friday, October 24, 2008

Geomorphology/hydrogeology question

Does anyone out there know what the world's largest intermittent river systems are? I'm not looking for wise-ass answers like "Rio Grande". I'm just wondering how common it is to have coherent drainages thousands of km in length that only occasionally carry water.

Thursday, October 23, 2008

Liquid Nitrogen for Grownups

Everyone who has ever seen a mad scientist outreach knows that liquid nitrogen is fun. You can freeze people’s balls with it, shrink balloons, cast gasoline candles, and make great clouds of fog. It was even the molecule of the day last week. But there is a little known secret side to this substance. Believe it or not, it is actually a commonly used substance for real labs doing actual work. And while we may occasionally make ice cream with it, that isn’t why our labs buy it by the silo full. So below I’ll go through a couple of the actual scientific uses of LN2.

1. IR detectors. Infra red spectroscopy studies the wavelength of IR radiation given off or absorbed by various materials, depending on their composition, structure, or impurities. However, at room temperature, everything gives off a fair bit of IR radiation. So an IR detector in a microscope will detect the ‘glow’ of the surrounding instrument- as well as the detector components themselves- unless it is cooled down. Liquid nitrogen is the cheapest way to do this.

2. Gas separation. Water and carbon dioxide are two commonly studied fluid phases in geology. Studying them often involves separating them from air or other combustion gasses. Dry ice/ethanol slurries are cold enough to condense water, and liquid nitrogen condenses CO2. so you stick various parts of your gas line in buckets of those substances, and they freeze out for later analysis. These are called cold traps.

3. Gas purification. Sometimes, instead of condensing a gas for analysis, you simply want to remove it. For example, the excimer laser that I used to keep running used a gas mixture of fluorine, argon, and helium. Invariably, water would contaminate the gas reservoir, reacting with the fluorine to form HF and oxygen. The oxygen is no big deal, but the HF prevents the formation of excited dimmers that actually produce the laser light. Since HF is polar, it condenses much more easily than the other gasses, so we use liquid nitrogen to freeze it out. In that case, though, the gas is in a continuous flow, and you can’t let it get too cold or the argon might condense as well. For all you former students reading, this is why I get so pissed off when you guys let that cold trap run dry- if the coil warms up, then the entire 3 months worth of HF gets back into the gas stream and fucks up the electrodes when you activate the trigger.

4. Some solid state charged particle detectors, like the EDS system on the microprobe, also like to run at low temperatures. But I don’t know the exact reason for that.

5. Dry gas. Some of the liquid nitrogen in storage will inevitably boil off in storage as a result of imperfect insulation. This gas is moderately pure, but very dry, and thus is handy for things like venting vacuum systems, flushing furnaces, dusting shit off, and other random tasks that require low pressure, low oxygen, and/or dry gas.

How about y’all?

Tuesday, October 21, 2008

For whom should I vote?

My initial predilection for McCain has been approximately offset by his terrible campaign and VP choice. Senate is an easy R, and house is an easy D this year (will post endorsements this week), but not sure on Pres.

Anyone willing to try to influence a moderate, fiscally conservative Republican voter is welcome to post in comments.

I should point out, however, that either candidate would probably be the best president of the last 20 years, to it's a good kind of confusion.

Monday, October 20, 2008

LOL dunes

One of the things I least like about the climate change discourse in our society is the shrillness of the doom and gloom that surrounds the arguments for urgent action. For one thing, the environmental over does doom and gloom, to the point where our doom-o-meters get saturated and we no longer care. But more importantly, it shows an intellectual laziness that perturbs me. I mean really, emphasizing the negative aspects of the collapse of civilization is just so easy. It requires no consideration, no creativity. I think it would be far more impressive to detail the comforting aspects, the positive slants, and even the humor inherent in agricultural society crumbling into dust.
Take desertification, for example. Surely there is some cheap joke that can be devised about sand dunes rolling into former pastureland. After all, a longitudinal dune isn’t going to stop at a cheeseburger- it will consume the whole ranch. And what better way to illustrate this than a LOL dune:

This longitudinal dune is prograding north from the Simpson Desert in far western Queensland, and is currently in the process of burying the stockyard shown above. It is a bit too slow to eat any beef, however. The station stopped running cattle in 2003, and the land was sold off as a nature reserve. These days, only camels, kangaroos, and reptiles live out here.

Saturday, October 18, 2008

Carbon sequestration- the basics

Our civilization is currently putting dangerous amounts of carbon dioxide into the Earth’s atmosphere. People are finally starting to realize this, and are looking for ways to reduce the impact as painlessly as possible. One way to do this is to bury some of carbon dioxide that we produce, so that it can’t reach the atmosphere or shallow ocean. While this is a valid and sensible thing to try, the process is often poorly understood or misrepresented. So I’ll run through some of the basics.

Firstly, carbon dioxide is poisonous. We all exhale this gas, so those sorts of levels obviously aren’t going to do us too much harm, but percent levels will, and very high levels can displace air, leading to asphyxiation. In 1986, carbon dioxide dissolved in an African lake uncontrollably exolved, and the resulting cloud of CO2 gas killed 1800 people. The annual human production of carbon dioxide is about four million times larger that the flux of CO2 into that lake, so sequestering CO2 poorly could be extremely dangerous to a very large number of people.

CO2 is also acidic when dissolved in water, and can be quite reactive in certain geological environments. So you can’t just stash the stuff any old place, or it might escape and kill somebody.

Secondly, once CO2 is mixed in with other gasses, it requires energy to extract and purify it. So sequestering CO2 from a reaction that produces pure gas will be more efficient that removing CO2 from combustion exhaust, or from the atmosphere.

Thirdly, CO2 is 3.6 times heavier than the carbon was before it was combusted. So transport costs for CO2 will generally be even greater than they were for fuel.

So, what does this tell us?

If we want to sequester carbon efficiently, the best way to do so is sequestering pure CO2 from a factory that is located very close to the sequestration site.

One potential site for storing CO2 is old oil and gas fields. This is sensible, at least at first pass, since the presence of hydrocarbons in a reservoir requires some sort of trapping mechanism. Mineral sequestration is something I mentioned a while back, but it is still in the research phase at this point.

So, if you build a chemical plant (say a plastic factory) at an oil wellhead, injecting the CO2 back into that reservoir is pretty close to the ideal environment for sequestration, as long as the volume of CO2 produced is less than the volume of oil extracted. Since oil has a greater carbon density than CO2, you need to be leaving some of the carbon in some other form (like plastic) for this to work.

Similarly, if you identify as suitable large, safe reservoir for sequestration, building a cement plant or smelter nearby will give fairly pure CO2 which can then be sequestered without too much trouble.

Another sensible carbon sequestration plan would be to build a gas based power plant at the well head for a natural gas field, and then sequester the CO2 extracted from the exhaust. Methane and CO2 both contain one carbon per mole, and they are both gasses, so the CO2 volume produced should be similar to the volume of methane burned. Because CO2 is more compressible, you might even gain some space, but this could be negated if subsidence over the gas field reduces the available volume for sequestration. In this situation, you still need to separate the exhaust gasses, but the transport and sequestration problems are minimized.

So what about CO2 from coal-fired power stations? Well, that CO2 is a combustion product, so separation is required. Unlike gas and oil, coal extraction does not leave a potentially useful reservoir behind- it just makes a big hole in the ground. So safe, large storage areas will need to be found, and the CO2 will then have to be transported to the sequestration site.

Thus, it is likely that sequestering CO2 from coal combustion will be more expensive than sequestering it from other industrial processes, or from natural gas-based electricity production.

This is an important point.

Often, carbon sequestration is only mentioned in the context of “clean coal”. But if we want to sequester CO2 efficiently, we are better off doing it for non-coal CO2 sources first, as they are likely to be easier and more efficient.

As a result, it would be a very serious mistake to allow coal companies to control carbon sequestration technologies, especially if they are funded by the public. Because limiting sequestration to only that CO2 produced from coal combustion makes the process less efficient, and therefore more expensive.

The space carnival has the biggest tent this election

Here on Earth, the medial political grouping on the northern continent of the western hemisphere is preparing for an election. The the election culture of this particular nation, it is common to refer to having a 'big tent' when wooing potential voters. Of course, tents are conspicuously absent on the campaign trail, and the biggest tents are actually found at circuses and carnivals such as this.

Because this is a space carnival, our tents should be very large indeed.

Our first carnival tent encompasses the planet Earth, that six million, million, billion ton ball of rock and molten iron that we all know and love. Evidently, not everyone on this planet is in a big tent sort of mood, as NASA has canned Conference Travel in 2009. Despite this small minded view, at least some folks are looking for the big picture. plans for big telescopes have been described in a round up of future giant telescope news, which has also been described from a historical perspective, as Newton was on to this trick ages ago.
As large rocks such as the Earth inevitably end up becoming targets for smaller, jealous rocks, there has been some discussion of death by meteorite. And the fall location of the meteorite that landed last week was plotted up, just in case Anyone likes collecting meteorites.
We also have an excellent description of the Earth’s Wobble, or Precession. And for those who like to push the tent's edges, there's a
suborbital rocket experiment carrying a payload from Kentucky

Our second tent encompasses the Earth's Hill sphere, and everything in orbit around our home rock.

Although this picture only shows our largest natural satellite, there is news from the biggest artificial one as well. Educators have released Teacher Tools for the High Frontier: International Space Station. And there has been much made of the story of an Astronaut's son, who recently boarded the ISS as a Russian tourist.
On the less commercial, more nationalistic side of things, the idea that China faked its space walk is debunked here. And the ESA is readying a satellite to Study the Earth's Gravitational Field. Finally, the US Vision for Space Exploration has released a publicity video touting systems that hopefully will be built in our lifetimes.

Moving out, we can put the inner solar system under a tent that is a mere billion km wide.

We start On Mars.... Phoenix has won an award as a World-Changing Innovations of the Year. And futuristic probes like ExoFlys are also discussed.
Asteroids are especially interesting if they arrive on your doorstep, and
Pallas is highlighted at the Planetary Society's blog.

The Outer solar system tent is substantially larger- 60 AU across will net Neptune, but we'll need something ten thousand times that size to capture the Oort cloud.

In the icy regions of the outer solar system, a comet discovered from an icy region of the Earth is celebrated.The presence of water volcanoes on Enceladus has some folks doubting the laws of physics. There was a brilliant 5 day liveblog of the DPS meeting held last week. Day 2 was on Rings, Titan, Comets, and orbits. Day three consisted of More Titan, and exoplanets. While day five was icy and not-so-icy moons. Finally, there is a report on new data investigating Holmes, the exploding comet.

Expanding our tent to a few hundred thousand light years keeps the Milky Way out of the rain.

Lack of rain is good for observers of variable stars, one of whom recently had an almost perfect night. Some other stars in our galaxy host planets, and the main detection methods for finding them are summarized here. Of course, telescopes are rarely point and shoot- it is common for observers to have challenges and puzzles. Anyone familiar with the moons of Jupiter knows a bit about tidal heating, so it is only natural for this effect to be considered for the habitability of exoplanets. And just in case some of those planets are inhabited with technologically advanced internet addicts, Bebo has beamed a friend invite into space. Going back to the DPS liveblog, day one involed Mars, exoplanets, defining planets and Enceladus.

The ultimate big tent experience involves covering the entire universe with rain resistant canvass.

Not only does this involve an enormous amount of fabric, but it also raises interesting philosophical questions about what you peg the guy lines to. Fortunately, we have an explanation of why dark matter is more diffuse than ordinary matter to make this subject a little less mysterious. And cosmology has evidently also inspired jazz. Obviously, accessing the distant reaches of the universe is neigh impossible with known science, but that won't stop people's imaginations from blasting out there at “Warp 10, Scotty!”.

Friday, October 17, 2008

Absentee ballot question for expats

I haven't received my ballot yet. Have you guys? If so, where are you and when did it show up? And where is it coming from? Is anyone else still waiting?

Sunday, October 12, 2008

Anyone like collecting meteorites?

On Friday, Emily at the Planetary Society's blog posted satellite images of the fireball from the ex-asteroid that crashed into the Axis of Evil last week. Luckily, that image had grid lines on it, so I imported it into Google Earth and plotted up the location.

While a large proportion of the impactor vaporized, I would be very surprised if a few kilos didn't reach the ground. There might even be a ton or two. And it turns out that the site is close to a road, in very open country. Of course, given the low angle of impact, the debris may be significantly farther east. But that's OK- that puts it even closer to the road. Anyone out there have a Sudanese drivers license and a Landcruiser in NE Africa? That road doesn't look real good, but it may be further west than the yellow line, based on the (low res) photos.

Friday, October 10, 2008

Charitable linkage

Firstly, this week's space carnival is live from Kentucky. More importantly, the talented Maria Brumm is representing geology in this year's Donor's Choose Challenge. Show that our hearts of stone are more generous than those of the physics, mathematical, and biomedical communities. Head over to her place, and please give generously.

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.

Wednesday, October 08, 2008

Head of Skate

A few weeks back, actor Matt Damon compared the nomination of Governor Palin to a bad Disney movie. Evidently, there is now a trailer.

Monday, October 06, 2008

Mercury fly-by

If all went well, the Messenger spacecraft is 50 minutes past closest approach of its second flyby, and is currently 20,000 km above the fastest planet and receding rapidly. The spacecraft has turned away from Earth to perform its mission, so it will be another 19 hours before contact with the spacecraft is regained. I wonder if anyone on the mission will manage to get any sleep during that time. Here is the penultimate approach picture taken before the spacecraft turned away:

Camels in the outback

When I first came to Australia, I had heard all about kangaroos and emus and platypuses and all these queer and wondrous organisms that inhabit this continent. But it wasn’t until I actually took my first road trip into the wide open spaces that I learned about Australian camels. We didn’t actually see any that trip, but they’re still out there- over a million of them, by some estimates- and this last trip I finally managed to spot a decent mob- a breeding group with a cantankerous old bull, as many cows as he can handle, and all sorts of little youngsters.
Camels, of course, are not native. As the Wikipedia article explains, they were introduced in the late 19th and early 20th centuries, after a number of disastrous expeditions proved that the interior of the continent was inhospitable for other beasts of burden. By the 1930’s, the automobile had made camels obsolete as a mode of transport, so most of them were abandoned- left all alone in a continent whose arid conditions suited them perfectly. Their current range exceeds that shown in this Environment Australia fact sheet (pdf).
Australia has a long and dismal history of invasive species, but by the standard of the cane toad or the rabbit, camels are relatively benign. Conservation theory is further complicated by the fact that dromedary camels are extinct in their native range. All 14 million old world dromedaries are domesticated (There are a few wild Bactrian camels in east central Asia). If you want to see wild dromedaries, Australia is the only place where you can do so. As a result, all sorts of crazy research on their physiology and behavior is done here (e.g. During the rutting season, Bulls become hypothermic overnight so that the exertion of rutting the next day doesn’t overheat them).
One theory as to why they aren’t a disaster is that they fill the niche of browsing megafauna that was left vacant when Diprotodon and other large animals were exterminated when the continent was first inhabited. So all the creatures that would have been outcompeted by camels are already dead.

However, they have no natural predators (although cattlemen shoot them on sight), and the herd is growing at an estimated 10% each year, so aerial hunting and other control measures have recently been started. While some wild camels are recaptured to sell back to the old world, the economically viable take is not nearly enough to hold the population in check. Which is a bit of a waste, really, because camel meat is delicious. The low fat content makes it easy to cook badly, as it easily overcooks and dries out, but a well done camel curry is to die for.

Wednesday, October 01, 2008

How astrophysicists spoil cosmochemical parties

ResearchBlogging.orgAs I mentioned a while back, the presence of short-lived radioactive nuclides in early solar system material means that a nucleosynthetic event must have occurred shortly (< a few million years) before these materials formed. The simplest explanation for this is that a supernova occurred, in which these elements were produced. This supernova then injected these materials into a nearby molecular cloud, and the force of the shockwave triggered the collapse of this cloud to form our solar system. It all makes a good story, and the simplicity of the model makes interpretation of cosmochemical data more straightforward than a complicated model would. The problem is, a gang of unruly astrophysicists eventually found out about it and attacked it with calculations. Gounelle and Meibom were particularly savage.

Their paper is basically argued like this:
1. injection and collapse from a single supernova is unlikely.
2. contamination of a circumstellar disk in unlikely (this is the hypothesis they discuss in detail).
3. Therefore, the excess short-lived nuclides must have been inherited from the stellar medium, or formed from spallation or other non-stellar processes.

The argument against single-supernova injection and collapse is fairly simple. Basically, the computer says "No".

Specifically, they argue that models show that fast supernova ejecta will disperse the cloud instead of collapsing it, while low speed ejecta will not penetrate the cloud, so injection does not occur.

They then turn to the supernova ‘salting’ of a circumstellar disk.
The problem here is that prior to blowing up, stars that are big enough to end in supernovae are very luminous and hot, and thus emit a huge amount of UV. This UV will evaporate any disks nearby on timescales shorter than the life of the massive star. So by the time the star blows up, there is no disk left to inject materials into. They calculate that the probability of a disk surviving long enough, but still being close enough to receive sufficient material, is very small.

Having thus shown these two most popular theories to be unlikely, they conclude that spallation is the best hypothesis for 26Al and other short lived nuclides, while the longer lived ones (halflives > a few million years) live long enough to have been present in the interstellar medium before star formation.

I generally don’t like elimination-based arguments for poorly constrained problems like this one. In order for not 1 and not 2 to imply something, you have to also exclude the possibility of a door number 3. Additionally, they made the mirror image mistake of cosmochemists- despite their expertise in star formation, they don’t acknowledge that meteoritic concentrations of 10Be (which has to be spallation related) and 26Al are not correlated, at least in CAIs. This suggests that the origin of 26Al is not spallation.

But while I disagree with their argument, the bigger picture is still important- meteoriticists can’t just sit in a lab measuring obscure isotopic ratios without verifying the astronomical plausibility of our chemically-derived models. Like it or not, it is in our best interests to actually talk to these people, even if their skill sets, publishing habits, and scientific culture is different and strange. Because despite the vast differences in methodology between high-precision isotope geochemistry and whatever it is that astronomers actually do, we are in fact attacking the same problem: How do stellar systems form.


The Astrophysical Journal