# Are there examples of the effect of shock waves on chemical evolution in of interstellar medium or circumstellar medium?

Are there any examples of calculations or observations of suspected effects of shock waves on chemical evolution in interstellar medium or circumstellar medium?

• What shock waves? – Carl Witthoft Feb 13 '20 at 16:25
• @CarlWitthoft the one sentence question clearly asks about "shock waves...of interstellar medium or circumstellar medium". In what way is your comment constructive? cf. 1, 2, 3 – uhoh Feb 14 '20 at 1:24
• Ouch! Anyway, I think the correction partition of the sentence is "shock waves" and "chemical evolution of interstellar ..." – Kornpob Bhirombhakdi Feb 14 '20 at 1:41
• @uhoh I"m asking what the source & type of shock wave: gravitational, radiative particles, etc. – Carl Witthoft Feb 14 '20 at 13:18
• @user-2147482075 I've made a small edit to adjust the wording because someone has voted to close the question. I've also added a bounty to generate more activity here. – uhoh Feb 23 '20 at 3:37

Shock waves in the interstellar medium (ISM) can be caused by a spiral density wave, supernova shock, or turbulent flow collision. These waves sweep, compress and heat the ISM, thus modifying which chemical reactions can occur. These physical conditions can harbor the chemical reactions to create complex molecules such as NH$$_3$$, CH$$_3$$OH, H$$_2$$O and more.

Based on a chemical/dynamical model, Bergin et al. (2004) investigates the formation of molecular clouds behind shock waves. In the $$10^6$$ years after the shock wave has passed, the temperature of the ISM is rather high (10 000 K) and molecules are photodissociated. As the ISM cools down, H$$_2$$ can start to form, followed by CO (which needs to be shielded from photodissociation by the H$$_2$$). Within 20 million years after the passage of a shock wave, atomic gas has been turned into molecular material.

The chemistry that occurs due to a supernova shock could even explain where some molecules found in comets come from. During a shock, water vapor is formed through reactions with hydrogen and oxygen. Bergin et al. (1998) shows that H$$_2$$O and CO$$_2$$ formed in the shock can deposit on grains.

• I see what's happened. You'd made an edit to address my comment but didn't reply to let me know, then I assumed it was unchanged (which it wasn't). I would have awarded the full bounty if I'd realized it. Anyway, thanks for the update! – uhoh Mar 2 '20 at 8:34

This is a good question, and probably there is no certain answer right now. Also, the impacts of shock waves depend on what scenarios we are talking about.

For an example in the case of supernovae, especially in Type II (H-rich), the shock wave forms double structures: forward and reverse shocks. Area in between the shocks can sometimes form a small region that supports cooling and dust forming, therefore, enhances the chemical evolution. However, if we think about the shocks blowing circumstellar materials out into the void, the shocks slow down chemical evolution.

• I've started a bounty to generate more activity. If you can elaborate a little bit on this and cite a few sources (papers, books, Wikipedia...) that would be great! – uhoh Feb 23 '20 at 3:36