Friday, August 18, 2006

Friday's star formation

I missed some talks in the morning, but sleep is important, too. I came in time for the last three talks of S237 on "Triggered Star Formation in a Turbulent ISM" and all three were interesting.

C. Norman was talking about his theoretical work on disk simulations and what struck me a little was that people have enough confidence in the simulations being a good approximation for the real world, to do physics with "observations" on the simulated world and derive properties and laws from there. Of course, this is tempting since one has complete control over the simulation and can get much better "data" than in the real world. There are tons of arguments and tests that reassure and convince people that this really works and, indeed, why should a program that implements a well-understood physical process fail, except for (also well-understood) limitations like resolution and other approximations. But still, if one could observe it, one would not need simulated data, so since we cannot, there is also a lack in testing the simulations to the real world.

Using the density distribution function of the ISM (log-normal except at very low densities) and a scaling relation for the critical density where SF sets in, Norman showed an alternative to the Schmitt-law (also called Kennicutt-law) that has a more shallow slope and flattens out at high gas surface densities. The observed slope then has to be understood by a change in SF-efficiency, which offset the model with respect to each other. Since the observed correlation is very tight, this would mean that also SFE correlates tighly with gas density.

The next speaker, M. Krumholtz, started from simple arguments to understand why SF is so inefficient in the sense that the SFR would be 50 higher (both in the Milky-Way and in extreme cases like Arp220) if all the molecular clouds that are present would collapse and form stars. I cannot reproduce the whole line of argument now, but he also used the density distribution (depending on mass and virial parameter) in the turbulent medium and integrated over the region above the critical value to get SFRs which fell into the observed regime. This could be tested, if the census of molecular clouds in nearby galaxies would stretch to smaller masses, by simply comparing the predicted SFR as derived from the molecular gas content with the observed one.

The last talk and summary of this symposium was given by B. Elmegreen and he again stressed the importance of turbulence for SF, instead of the older picture of monolithic collapse of a molecular cloud.

In the afternoon, there will be a "poster session", which basically means that everyone who has a poster, stands by it to answer the questions of the people coming by. The problem is that, although not all have posters, many do and one has to find the balance to also look at the other posters and talk to the author, but that won't work if they just do the same thing. Surprisingly, it always works out somehow anyway and I am happy that they scheduled this also for S235 for which no poster session was planned initially.


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