Astrochemistry of ISM

Speaker: Siyi Feng

Siyi Feng

A big picture

The life cycle of the ISM

This figure is from Siyi Feng's teaching .

Molecular in ISM

How to choose the tracer

• H${}_2$ has no dippolement moment, so it is not easy to detect. However, it is the most abundant molecule in the universe. Therefore, we need some proxy like CO, i.e., CO is the tracer of H${}_2$.
• SiO gas is a tracer of shock. Because SiO is solid when the temperature is not very high. If we detect SiO (the spectrum comes from the gas), it means that there is a shock. Or there will be no SiO in the gas phase.

How to fit the spectrum in radio band

Models

• LTE model
• Non-LTE model: Large Velocity Gradian (LVG) approximation

From continuum

To get density, temperature. Use black-body radiation or grey body raditation to fit the continuum.

Substract the continuum

1. Extract the emission line regime
2. Fit the continuum
3. Substract the continuum

Identify the emission line

• By eye, still not automatic
• Infer the molecular emission line datebase

The $\mathit{\text{MIAO}}$ project

Proto-stellar shocks

From Kristensen et al.(2012)