ISM & AstrochemistryLecture 5

Dark Clouds

Steady state reached after about 1 million years

Most hydrocarbons peak at about 105 yr

Problem: Gas-grain interaction

Sticking time ~ 3 109/n(cm-3) yr

Results in icy mantles; no gas-phase except H, D and He.

Results from a pseudo-time dependent model with T=10K, n(H2)=106 cm-3

Fractional abundances varying over time Molecular D/H ratios

•At late times the abundance of H2D+ is similar to HD2+ : this prediction was

confirmed by Vastel et al. (2004)

•H2D+ ~ HD2+ ~ H3

+, as seen by Caselli et al. (2003) towards the prestellar core L1544.

• D3+ becomes the most abundant deuterated molecule.

•The atomic D/H ratio rises to ~0.8: important for surface chemistry

Density & temperature distribution in L1544 dark cloud

Radial fractionation in L1544

Star-Forming Hot CoresDensity: 106 - 108 cm-3

Temperature: 100-300 K

Masses ~ 103-104 solar masses

Very small UV field

Small saturated molecules: NH3, H2O, H2S, CH4

Large saturated molecules: CH3OH, C2H5OH, CH3OCH3

Large deuterium fractionation

Complex Molecule Formation

Ice Chemistry at 10K

-store material for 1M yr

-Ices absorb high energy particles (photons, cosmic rays)

- hydrogen addition more efficient

- star formation leads to heating and evaporation of ices

- complex molecules end up in the gas phase, detectable by radio telescopes

IRAS 16293-2422

OCS 9-813CS 5-4

N2D+ 3-2 D2CO 5-4

Complex molecules around low mass protostars (hot corinos)

Recent observations at the JCMT and IRAM show evidence of complex molecules in a low-mass protostar for the first time

Detected species include methanol, acetaldehyde, methyl formate, ethyl cyanide, as well as evidence for large abundances of D-bearing molecules such as D2CO

Chemical modelling of IRAS 16293

• Depth dependence of temperature and density

• Deuterium fractionation

• Accretion – ion-grain recombination

• Limited grain surface chemistry – CO → H2CO → CH3OH

• Follow frozen molecule and grain-produced molecules

• Desorption by cosmic rays and by thermal heating

• Follow gas and mantle compositions

Chemical modelling of IRAS 16293

Pre-stellar core at 105 yr:

Mantles contain a layered history of the cloud composition.

Water and methane dominate the inner layers of the mantle, formed by the hydrogenation of O and C.

CO and CH3OH and H2CO, both formed by the hydrogenation of CO, dominate the outer layers. T = 10K

Chemical modelling of IRAS 16293

Pre-stellar core at 105 yrs:

Because the outer layers form from highly-depleted gas, molecular D/H ratios can be large e.g.

HDCO/H2CO ~ 0.1

D2CO/H2CO ~ 0.005

Not sufficient to explain the observations.

Chemical modelling of IRAS 16293

Assume that gas is heated over time by central protostar (Viti & Williams)

Gas undergoes a ‘desorption wave’ as grains warm to observed temperature

Grains undergo differential desorption as most weakly bound material desorbs first

Chemical modelling of IRAS 16293

Following warming of grains evaporate mantle materials can form other complex species

Time-scales are dependent on the arrival of the ‘desorption wave’

Column densities are strong functions of time

.. and therefore of the physical model adopted

Summary

• Information on rate coefficients is better than ever

• Pure gas-phase chemistry is well understood

• Not withstanding this, gas-phase chemistry has difficulties

• Gas-grain interaction is critical to understanding molecules in star-forming regions

• Sensitive to many uncertain physical parameters – grain composition, size and morphology; binding energies, mobilities, surface chemistry, cloud density, temperature, structure

• But, increasingly sensitive observations of both gas and grain species at increasing resolution