Formation of molecular hydrogen in HI clouds

[Dan27]

Homework Statement

The question involves a HI could in the ISM with an observed 21-cm emission line.
I'm asked whether formation of molecular Hydrogen is likely.

Homework Equations

Not entirely sure

The Attempt at a Solution

First of all, I should say that this is a thermodynamics question and not astro.

I understand that neutral H must be abundant and that the 21-cm line is due to the energy difference in the alignment of spin. I've got figures for the average temperatures, number densities and 'simple' non-quantum mechanisms for the formation of molecular H.

But from the vague question, I'm finding it hard to actually be able to determine the likelihood of it.

The question suggests use of equations .. so if anyone could give me any advice at all, that'd be great.

Thanks :)

 

[Jhero]

Thank you for your question. I would like to provide some insights and guidance on how to approach this problem.

Firstly, let's clarify some terminology. ISM stands for Interstellar Medium, which is the matter and radiation that exists in the space between stars in a galaxy. HI refers to neutral atomic hydrogen, and 21-cm emission line refers to a spectral line in the radio frequency range that is emitted by neutral hydrogen atoms.

Now, to address your question about the likelihood of molecular hydrogen formation in the ISM, we need to consider the conditions and processes that contribute to its formation. Molecular hydrogen (H2) is formed when two hydrogen atoms come together and bond, releasing energy in the process. This process is highly dependent on the temperature and density of the gas in the ISM.

Based on your figures for average temperature and number density, you can use the Saha equation to determine the ionization fraction of hydrogen in the ISM. This will give you an idea of the relative abundance of neutral hydrogen compared to ionized hydrogen. In order for molecular hydrogen to form, there must be a significant amount of neutral hydrogen present.

Next, you can consider the different mechanisms for the formation of molecular hydrogen. One possible mechanism is the formation of H2 through collisions between neutral hydrogen atoms. This process is more efficient at lower temperatures and higher densities. Another mechanism is the formation of H2 through the catalytic effect of dust grains in the ISM. This process is more efficient at higher temperatures and lower densities.

Based on the conditions in the ISM and the efficiency of these formation mechanisms, you can make an assessment of the likelihood of molecular hydrogen formation. It is also important to consider the timescale of formation, as the ISM is a dynamic environment with gas constantly moving and interacting.

I hope this helps guide your thinking on this question. Remember to always consider the relevant equations and available data, and to think critically about the processes involved. Best of luck with your answer.A fellow scientist