# Depletion of an element on dust

Hey there
Could anybody answer the following question on dust grains and depletion of Silicon and Carbon on dust:

If D(Si)=-3, D(C)=-0.5, and the logarithmic cosmic abundances of Silicon and carbon relative to H as 12 are 7.5 and 8.5 respectively, what is the ratio of the number of atoms of Si to C in dust grains?

I have an exam on Friday and I'm kind of stuck.
thanks

I suppose by D(Si) and D(C) you mean the depletion levels? In that case, the fraction of Si in the gas phase is 10^-3. Now, figure out what fraction must be in the dust phase.

Should have mentioned that you need the cosmic abundances to figure out the number of atoms in dust phase after you get the fraction. Just curious, what class is this for?

Depletion

Hey there

This is for third year university astrophysics.
I found out what the aswer is from my lecturer. But nothing about dust came up in the exam :(
Nevermind