1. Inside a typical astrochemical model
Serena Viti
IFSI-CNR (Rome)

2. Preview Why bother with astrochemical models A tour inside one
Its ‘beauties’ and ‘horrors’
How to use it
Improvements

3. Why astrochemistry? The universe is chemically rich
There is a large degree of diversity in a variety of interstellar and circumstellar regions
There is a large degree of diversity in the same region as a function of time

4. Why do we need astrochemical models?
To represent the chemical and physical behaviour of astronomical objects
To interpret astronomical data
To test astronomical and chemical ideas
To estimate chemical abundances of the ISM, taking into account most of the chemical and dynamical processes affecting the gas and dust.

5. Needs of an astrochemical model
Flexibility:
Time dependent/Time independent
Depth dependent/Single Point
Gas-grain/Gas chemistry
Static/Dynamic
Modularity
Easy to implement new experimental and theoretical input data

6. Inside a chemical model: examples of input parameters
Initial elemental abundances
Initial temperature
Initial density
Size
Radiation field
Cosmic ray ionization rate
Dynamical ‘switches’
Chemical species and rate file
Degree of depletion

7. What does a model do?
Calculates density profiles as a fn of time and space
Calculates temperature structure as a fn of time and space
Calculates chemical rates for all relevant reactions
Calculates fractional abudances (gas-phase and solid by taking into account feeze out and desorption)

8. Inside a chemical model: examples of output parameters
Chemical abundances as a fn of time and space
How the density varies with time and space
Thermal balance
Visual Extinction
Radiation field as a fn of time and space

9. s p a c e S p a c e
Time

12. What needs to be done Form the dense clumps (gradient of densities)
This involves: depletion, surface reactions
Account for the UV field from the source (time and space dependent)
Account for X-ray emission

13. Example of output
HCO+ abundance as a fn of time at Av ~ 3 mags

14. Its ‘beauties’ Very flexible because “ab initio”
Free to include as many species and reactions as we want/need
Time-dependent
Fast

15. Its ‘horrors’ (too many to list all)
Degree of freedom may be as many as the no of reactions
Many rate coefficients are not experimental and/or have a large error bar
Rates for surface reactions missing
Surface reactions missing! …

16. What we still need: Experimentally: Branching ratios
Sticking probabilites
Mobilities of adsorbed species
Hydrogenation on grain surfaces
Desorption

17. Observations (high spectral and spatial resolution)
Laboratory Data 
Time and depth dependent Chemical Models
Observations (high spectral and spatial resolution)

18. Conclusions
Close interaction between models, observations and chemical data
Full coupling of chemistry with hydrodynamics and radiative transfer
Investigate a larger parameter space