Hello everybody and welcome to yet another hour of Galaxies and Cosmology and

before I start with the lecture slight changes in the schedule in the sense

that for tomorrow we had planned a normal exercise session, but the exercise that I

had planned to do there is one that we can only do a tad bit later. And it makes more

sense anyway to also already talk about proposals and the proposal exercise tomorrow. So what

we'll be doing tomorrow is talk about how to write proposals. Given that last Tuesday

you had talk about paper writing, it makes sense to put this in one block. This will

be done by Tommy Dauzer. It should take about an hour plus something. Next week there is

no lecture scheduled, so the next real lecture, not the exercise, will be Monday two weeks

from now. Okay?

So last Monday we were talking about molecules in the Milky Way and other spiral galaxies

and we came to the conclusion that standard chemistry does not work for these, for the

formation of molecules because the interaction time scale between two atoms or ions in order

to produce a molecule is too small so you can't get rid of the excess energy. We found

however that if you have simple molecules, for example H2, by using the fact that if

an ion approaches a molecule, you polarize the molecule and as a result produce a temporary

dipole moment, it is possible to generate most molecules which then leads to very complex

formation schemes for molecules as this one here, which is just an excerpt from a very

complex, far more complex reaction network. The problem however for all of these was that

we cannot form the simplest of all molecules which is the H2 and the reason for this is

that two body recombination and polarization and so on is not possible for H2 just because

it has no permanent dipole moment and therefore it can't relax into a state that is stable.

And the only solution for forming this is you need some kind of a solid state surface

in which you can dump all of the excess energy. So let's diffuse this around. And so this

means what we have to do next is we have to look at the properties of the coldest part

of the universe at the properties of dust. There is a lot of evidence for the presence

of dust in lines of sight through the interstellar medium and the most important evidence there

is in the so-called depletion of heavy elements or solid state material into the solid state

of the line of sight. What I show here is a figure that has on the y-axis the logarithm

of the ratio of the abundance of certain elements with respect to hydrogen compared to solar

abundance. So this is the logarithm of the abundance of the line of sight. And this is

the logarithm of the abundance of the line of sight with respect to solar abundance.

So this is the log of N might be any element so oxygen, zinc, magnesium and so on divided

by hydrogen so that's the number ratio for this element seen along a line of sight. And

this is the sum of the properties. The assumption here is that the sun is roughly representative

in its elemental or chemical composition to that of the interstellar medium or to that

of the galaxy as a whole. That's not entirely correct but it's correct enough. On the y-axis

the properties of these elements namely what's plotted here is the temperature at which a

given element solidifies. And what you see here in this figure is for two lines of sight

that certain elements like oxygen for example along that line of sight has the same as the

solar abundance also zinc here for example. But if you look at elements out here copper,

magnesium, chromium, iron and so on you see that along this line of sight to the object

the elemental abundance is a factor 10 to a factor 100 less than what you would assume

from solar composition. Now if you look along a line of sight to a star you're only picking

up, this is measured by absorption lines, so you're only picking up absorption in gas.

So this doesn't tell you that this material isn't there. What it tells you is that these

elements are not there in the gas phase because that's what you're probing with the measurement.

Along another line of sight which is just characterized by a different Doppler velocity

you see that also things like carbon and oxygen are depleted as well. And that is true for

almost all lines of sight. So you see that elements that form solids at temperatures