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4-Evidence of abundances from spectral lines (Absorption Spectra (Will a…
4-Evidence of abundances from spectral lines
Star light has absorption lines
Study of these can find photospheric composition
Optical depth maths
BB radiation
Lines are generally superimposed on a bb spectrum
Absorption Spectra
Depend on temperature and pressure in the stellar atmosphere
hotter temp = greater mean thermal energy
break up of molecules, then ionization of atoms
Will a line be seen?
Hottest stars have opaque atmosphere
UV absorbed by earths atmosphere
Strength depends on 3 things
how much the element is in the right ionization state
Amount of element which is present
cross-section for absorption, strength of transition probability
Equivalent width
width at which 100% absorption occupies the same area as the observed line
relationship between EW and number of absorbing atoms is curve of growth
Maths:
Sources of broadening
Local
Dopper
thermal motion along line of site
gaussian
speeds described by maxwell-boltzmann
Natural
due to heisenburg uncertainty
lorentzian
Pressure
radiation affected by nearby particles
lorentzian
linear stark
only affects atoms/ions with perm EDM
Broadening proportional to E field strength
Strongly effects HI lines in hot stars
quadratic stark
atoms/ions perturbed by passing electrons
proportional to square of E field strength
Affects most lines in hot stars
Van der Waals
Neutral atoms with momentary dipoles perturb eachother
E cubed shift
common in cooler stars
Zeeman
Lines split by magnetic fields
splitting proportional to square of wavelength
effect varies
Non-Local
opacity broadening
rotational
important in giants and supergiants