Section 5 Flashcards
What is needed to determine the composition of interstellar material?
Absorption studies from UV to IR wavelengths
In observations in UV, what do stars with extinction show and what is it due to?
a hump in the SED around 217.5nm due to graphitic component
In the interstellar extinction curve, what causes higher Rv (colour excess) ?
Large dust grains and higher density (grain growth which leads to planet formation)
What causes ‘humps’ on extinction curve?
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
What is seen on extinction curve if there is no material along line of sight?
No features in the UV as there is no extinction (unreddened star)
What do absorption studies show?
The amount of material along line of sight, extinction
What produces lower values of Rv?
When there is more extinction in UV relative to visible
When observing in optical, when are absorption features (dips) more prominent?
When there is more extinction (more material sitting between the star and observer)
What do broader features along sight lines of observations in optical correlate with?
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
In IR observations what does the absorption band at 3um (dense clouds) show in SED and how is this visible?
That there is water and CO ices
Visible in IR as molecules can stretch and bend bonds due to vibrational modes
In IR observations what does the absorption band at 10um show in SED?
Silicate component
There is an alignment with silicate bonds in dust grains which cause flux to decrease
What do emission lines in IR spectra show?
light emitted by cloud in mid-infrared
spectra due to large organic molecules
Why are emission lines visible in IR?
Due to vibrational modes of molecule caused by molecules absorbing UV light and getting excited
How can the composition of dust grains be determined?
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
Which band is visible in both absorption and emission lines?
silicate (10um)
What happens when molecules in interstellar gas collide with cold (<100K) dust grains?
They stick and grains become coated in an icy water-rich mantle
What acts as a precursor to biological molecules?
Icy mantles on dust grains (sites of active surface chemistry)
How is molecular hydrogen formed?
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)
What causes IR emission in regions of high extinction?
Polycyclic aromatic hydrocarbons (PAHs)
confirmed using laboratory spectra (hydrocarbons excited on gas chambers and spectra measured)
Why do PAHs produce vibrational modes visible in IR observational bands?
Due to the sharing of bonds as they do not attribute to single carrier but instead families of PAHs
Why are PAHs considered to be carriers in diffuse interstellar bands?
Due to sharp infrared features in SEDs
Buckminsterfullerene identified as carrier
What does spectroscopy show?
It identifies the composition of interstellar dust and gas
