Astrochemistry essay Flashcards
1
Q
1st paragraph (8)
A
- dissipation of interstellar turbulence in shock-waves leads to formation of molecular clouds from interstellar medium (Klessen, 2004)
- Lifetime order of 10^6 - 10^8 years, light-years in diameter
- n = 10^6 cm^-3, region of possible star formation
- low density –> low collision rate, no 3 body collision in gas phase
- problem for energy dissipation e.g. H2 - no dipole moment so cannot emit photon
- high extinction coefficient of dust grains means radiation doesn’t penetrate
- so cosmic rays cause heating, cool by thew emission of rotational energies of CO
- low temp. means reaction energy barriers are an issue
2
Q
2nd paragraph (2)
A
- simple molecules can be formed in the gas phase but found to be formed more efficiently on dust grains
- some complex molecules found to exclusively form on dust grains e.g. CH3OH
3
Q
3rd paragraph (4)
A
- in gas phase cosmic rays excite H2 to H2+ - centre of all ion-molecule reactions in interstellar medium
- H2+ and H2 to H3+ and H - proceeds rapidly so difficult to detect H2+
- H3+ readily protonates species like CO to HCO+ and O to OH+. This makes it the most important species in terms of , chemistry of a molecular cloud
- principle gas phase reactions are ion-neutral, ion-electron dissociative reactions, radiative recombination and neutral-neutral
4
Q
4th paragraph (6)
A
- 1 % mass in ISM is dust - outflows from dying stars transported by stellar winds
- formed of silicates - emitted from silicon burning stars when then turn to super-novae
- silicate core, layer of organic material, outer layer of volatile compounds e.g. methane, ammonia and water
- molecules form by successive H addition to accreted C, O, N
- accreted CO can undergo oxidation, protonation and deuteration in processes that have energy barriers due to catalytic grain activity
- Co-adsorption of C, O and N can form complex molecules such amino acids
5
Q
5th paragraph (3)
A
- collision of molecules to surface initiates chemistry
- probability it will stick to the dust grain is dependent of the collision energy, surface temperature, chemical nature
- desorption can be the starting material or a newly formed molecule
6
Q
6th paragraph (3)
A
- heating from nearby stars and shock-waves can result in desorption of molecules from ices
- supported by increase abundance of molecules know to be formed on dust grains
- results in molecules that could have formed on the surface of the dust grains or by further reaction in the hot gas
