Section 5

Question 1

What is needed to determine the composition of interstellar material?

Answer 1

Absorption studies from UV to IR wavelengths

Question 2

In observations in UV, what do stars with extinction show and what is it due to?

Answer 2

a hump in the SED around 217.5nm due to graphitic component

Question 3

In the interstellar extinction curve, what causes higher Rv (colour excess) ?

Answer 3

Large dust grains and higher density (grain growth which leads to planet formation)

Question 4

What causes ‘humps’ on extinction curve?

Answer 4

When there is extinction due to reddening from gas and dust in line of sight obscuring background stars and embedded stars

Produces infrared background source

Question 5

What is seen on extinction curve if there is no material along line of sight?

Answer 5

No features in the UV as there is no extinction (unreddened star)

Question 6

What do absorption studies show?

Answer 6

The amount of material along line of sight, extinction

Question 7

What produces lower values of Rv?

Answer 7

When there is more extinction in UV relative to visible

Question 8

When observing in optical, when are absorption features (dips) more prominent?

Answer 8

When there is more extinction (more material sitting between the star and observer)

Question 9

What do broader features along sight lines of observations in optical correlate with?

Answer 9

Diffuse interstellar bands, different to atomic or ionic elements/characteristics in graph
Molecule in this region remains a mystery but carbon-rich molecule is most promising

Question 10

In IR observations what does the absorption band at 3um (dense clouds) show in SED and how is this visible?

Answer 10

That there is water and CO ices
Visible in IR as molecules can stretch and bend bonds due to vibrational modes

Question 11

In IR observations what does the absorption band at 10um show in SED?

Answer 11

Silicate component
There is an alignment with silicate bonds in dust grains which cause flux to decrease

Question 12

What do emission lines in IR spectra show?

Answer 12

light emitted by cloud in mid-infrared
spectra due to large organic molecules

Question 13

Why are emission lines visible in IR?

Answer 13

Due to vibrational modes of molecule caused by molecules absorbing UV light and getting excited

Question 14

How can the composition of dust grains be determined?

Answer 14

From taking high resolution spectra as grains emit in IR and cause sharp emission features as dust grains do not act as a black body

Question 15

Which band is visible in both absorption and emission lines?

Answer 15

silicate (10um)

Question 16

What happens when molecules in interstellar gas collide with cold (<100K) dust grains?

Answer 16

They stick and grains become coated in an icy water-rich mantle

Question 17

What acts as a precursor to biological molecules?

Answer 17

Icy mantles on dust grains (sites of active surface chemistry)

Question 18

How is molecular hydrogen formed?

Answer 18

On dust grains with icy coating, making the grains sticky and helping them grow in size and stabilise the hydrogen bond

(also leads to planet formation)

Question 19

What causes IR emission in regions of high extinction?

Answer 19

Polycyclic aromatic hydrocarbons (PAHs)
confirmed using laboratory spectra (hydrocarbons excited on gas chambers and spectra measured)

Question 20

Why do PAHs produce vibrational modes visible in IR observational bands?

Answer 20

Due to the sharing of bonds as they do not attribute to single carrier but instead families of PAHs

Question 21

Why are PAHs considered to be carriers in diffuse interstellar bands?

Answer 21

Due to sharp infrared features in SEDs

Buckminsterfullerene identified as carrier

Question 22

What does spectroscopy show?

Answer 22

It identifies the composition of interstellar dust and gas