Spectroscopy of the ISM Flashcards
observations in the UV, optical and IR, absorption lines, emission lines, interstellar ices, PAHs
What are the two types of absorption study?
- absorption studies from UV an IR wavelengths are crucial for determining the composition of interstellar material
- we can observe light from an embedded protostar, i.e. a star that is within the cloud
- OR background starlight, light from a star on the other side of the cloud
Composition of Interstellar Dust
- comparison of interstellar dust spectra with laboratory spectra allows us to identify the constituents of interstellar dust
- these include:
- -PAHs
- -SamC (small amorphous carbon grains)
- -LamC (large amorphous carbon grains)
- -aSil (amorphous silicates
How do the different components of interstellar dust contribute to spectra?
- PAHs are molecules that lead to sharp defined peaks in the spectra
- small and large amorphous carbon grains and amorphous silicates all produce smoother continuous emission curves
Observations in the UV
- there is a hump in the UV region of the absorption spectrum at 217.5nm
- this suggests that dust grains are preferentially absorbing at this wavelength
- this has been attributed to a graphitic component of the dust
Unreddened Star
- nothing between us and the star
- the only extinction is from geometric dilution of flux
Observations in the Optical
-there are clearly defined troughs in the optical wavelengths which become more pronounced with increasing extinctions
they are seen in every direction across all lines of sight
-this pattern is described as the DIB, diffuse interstellar band
-the most promising candidates for explaining this are carbon-rich, large, complex molecules
Observations in the IR
- these are longer wavelengths than UV or optical and allow us to see the vibrational molecular transitions
- there is a very deep absorption feature around 9.7 micrometers due to silicates
- there is another deep absorption feature at around 3.0 micrometers attribute to ice (water and CO), this is only seen in dense clouds
- there are also smaller absorption features attributed to CO2, CH4, NH3 etc.
Resolution of Spectrometer
R = Δλ/λ
-where Δλ is the channel width of the spectrometer
Interstellar Ices
- molecules in interstellar gas can collide with cold (<100K) dust grains and stick
- the grains become coated in an icy water-rich mantle
- at very low temperatures (<20K, e.g. molecular clouds) CO is also in solid form
- icy mantles on the dust grains are sites of active surface chemistry thought to be the origin of chemical complexity in space
- this icy coating also makes the grains sticky helping them to grow in size (important for planet formation)
What are PAHs?
- polycyclic aromatic hydrocarbons
- aromatic means that the molecules are composed of rings of chains
- they come in many different shapes and sizes
Significance of PAHs in Spectroscopy
- IR emission lines found in regions of relatively high extinction were unidentified for a long time
- they were eventually recognised in laboratory work
- the observed bands are not attributed to any single carrier but rather to families of PAHs
- in these families, PAHs share the same bands and the combined effect of excitation of these bands produces the peaks in the spectra
Laboratory Work
- vaccuum chamber filled with different combinations of known species
- take spectrum
- compare with observation
What are the carriers of DIBs?
- since the diffuse interstellar bands are more strongly observed with greater extinction o must have something to do with the material along the line of sight
- laboratory experiments have found close alignment with two of the bands and the emission spectrum of C60+
- this family of molecules, buckminsterfullerenes (C60 and C70) have been identified in space so could potentially be DIB carriers
- C60+ is formed in space when a high energy proton ionise C60
