5. Spectroscopy of the ISM

Question 1

In the interstellar extinction curve, what is affecting the Rv value?

Answer 1

Size of dust grains

Density

Question 2

How does Rv link to density and grain size?

Answer 2

Higher Rv = more dense and larger grains

Question 3

What are the two ways to do absorption studies?

Answer 3

1. Interstellar cloud between us and background star (background spectrum) and measure spectrum as it passes through cloud, includes absorption
2. Forming star, warming up surrounding dust and gas (background IR light source)

Question 4

Why are molecules good at absorbing IR compared to atoms?

Answer 4

Bonds that can stretch and bend (IR modes)

Question 5

Link between absorption studies and colour excess?

Answer 5

Absorption from UV through to IR correlates with E(B-V)

[higher level of extinction = high absorption level in the dust]

Question 6

What is the UV hump at 217.5 nm due to?

Answer 6

Graphitic component

Question 7

What are diffuse interstellar bands (DIBs)?

Answer 7

Broad absorption features at optical, and can’t be explained by ionic or atomic species

Probably from macromolecular complex molecules

Question 8

What does IR absorption at 9.7 microns show?

Answer 8

Silicate component

Question 9

What does IR absorption at 3.0 microns show?

Answer 9

Water and CO ices (dense clouds only)

Question 10

Why do we have sharp emission features in large-carbon rich molecules?

Answer 10

Complex vibrational modes that emit IR wavelengths

Question 11

How do large carbon rich molecules produce an IR emission spectrum?

Answer 11

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

Question 12

What can emit IR spectra?

Answer 12

Large carbon-rich macromolecule

Interstellar dust

Question 13

What causes sharp dust emission features at 10 microns?

Answer 13

Silicates

(see in emission and absorption)

Question 14

What does the emission spectrum of interstellar dust show about our previous assumptions?

Answer 14

That they are not perfect black bodies

Question 15

Relevance of ice on dust grains?

Answer 15

Molecules in interstellar gas can collide with cold (<100K) dust grains and stick, as well as CO (<20K)

Question 16

What role do the icy mantles on dust grains have?

Answer 16

Sites of active surface chemistry, thought to be the origin of chemical complexity in space

Question 17

What makes grains grow to larger sizes?

Answer 17

Icy coating makes grains sticky

Question 18

What are PAHs?

Answer 18

Polycyclic

Question 19

How does molecular hydrogen form?

Answer 19

Solely on dust grain surface (does not form in the gas phase)

Question 20

How were sharp features in mid-IR emission spectra identified to be PAHs?

Answer 20

Made molecules in lab, put in gas chamber, excited them and measured spectra

Question 21

Why can we not say observed PAH bands are due to any single carrier?

Answer 21

PAHs come in many shapes and forms and share bonds, molecules add up to form the shape of the spectrum

Question 22

Could PAHs be the culprit of diffuse interstellar bands (DIBs)?

Answer 22

Yes - it has been confirmed that C60+ is the carrier of 2 DIBs

Question 23

What does spectroscopy do?

Answer 23

Identify composition of dust and gas

Question 24

What do observations in the UV, optical and IR reveal?

Answer 24

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