5. Spectroscopy of the ISM Flashcards
In the interstellar extinction curve, what is affecting the Rv value?
Size of dust grains
Density
How does Rv link to density and grain size?
Higher Rv = more dense and larger grains
What are the two ways to do absorption studies?
- Interstellar cloud between us and background star (background spectrum) and measure spectrum as it passes through cloud, includes absorption
- Forming star, warming up surrounding dust and gas (background IR light source)
Why are molecules good at absorbing IR compared to atoms?
Bonds that can stretch and bend (IR modes)
Link between absorption studies and colour excess?
Absorption from UV through to IR correlates with E(B-V)
[higher level of extinction = high absorption level in the dust]
What is the UV hump at 217.5 nm due to?
Graphitic component
What are diffuse interstellar bands (DIBs)?
Broad absorption features at optical, and can’t be explained by ionic or atomic species
Probably from macromolecular complex molecules
What does IR absorption at 9.7 microns show?
Silicate component
What does IR absorption at 3.0 microns show?
Water and CO ices (dense clouds only)
Why do we have sharp emission features in large-carbon rich molecules?
Complex vibrational modes that emit IR wavelengths
How do large carbon rich molecules produce an IR emission spectrum?
Absorb UV light from star
Energy redistributed through molecule (excited)
So vibrating with different vibrational modes corresponding to what we see in an IR spectrum
What can emit IR spectra?
Large carbon-rich macromolecule
Interstellar dust
What causes sharp dust emission features at 10 microns?
Silicates
(see in emission and absorption)
What does the emission spectrum of interstellar dust show about our previous assumptions?
That they are not perfect black bodies
Relevance of ice on dust grains?
Molecules in interstellar gas can collide with cold (<100K) dust grains and stick, as well as CO (<20K)
What role do the icy mantles on dust grains have?
Sites of active surface chemistry, thought to be the origin of chemical complexity in space
What makes grains grow to larger sizes?
Icy coating makes grains sticky
What are PAHs?
Polycyclic
How does molecular hydrogen form?
Solely on dust grain surface (does not form in the gas phase)
How were sharp features in mid-IR emission spectra identified to be PAHs?
Made molecules in lab, put in gas chamber, excited them and measured spectra
Why can we not say observed PAH bands are due to any single carrier?
PAHs come in many shapes and forms and share bonds, molecules add up to form the shape of the spectrum
Could PAHs be the culprit of diffuse interstellar bands (DIBs)?
Yes - it has been confirmed that C60+ is the carrier of 2 DIBs
What does spectroscopy do?
Identify composition of dust and gas
What do observations in the UV, optical and IR reveal?
UV: absorption features that allow extinction measurement (as dust is main source of opacity)
Optical: also extinction measurement, also absorption bands in diffuse ISM (DIBs)
IR: Chemical complexity of gas and dust in ISM - absorption bands of molecules (gas and ice) and silicates
