13' The interstellar medium: dust

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13. The interstellar medium dust 13.5
Interstellar polarization 14. Galactic cosmic
rays 15. The galactic magnetic field
The Crab nebula, M1, a supernova remnant in Taurus
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• Interstellar polarization
• Polarized light has electric field confined to
  one plane
• transverse to propagation
• Stars emit light which is unpolarized
• Partial polarization is possible after starlight
  has
• passed through a dust cloud of aligned
  elongated
• dust grains
• Degree of polarization can be expressed in
  magnitudes
• using a polarizing filter on a polarimeter

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• The observations
• Polarization is limited to stars near galactic
  plane, b ? 5º
• Mostly the observed polarizations are small
  ?mp?0.03,
• but occasionally as high as 0.15 mag.
• All highly polarized stars are also highly
  reddened by IS
• dust
• But, some reddened stars are not polarized at
  all
• Ratio of polarization to extinction is

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• The explanation
• Polarization requires alignment of rotating dust
  grains
• in the weak galactic magnetic field (actually
  it is the
• rotation axes which are aligned)
• Polarization requires the grains to be elongated,
  not
• spherical
• Polarization is strong when we see distant stars
  through
• a transverse magnetic field (l 140º and
  320º), but
• weak when we look along the field lines (l
  30º
• and 260º)
• Direction of the field is approximately along
  spiral arms

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Observations of interstellar polarization as
function of galactic coordinates. The plane and
amount of polarization is shown by the short
lines for each star
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• Galactic cosmic rays
• Cosmic rays are high energy particles, mainly
  protons
• (90 by number) or a-particles (4He nuclei)
  (9 ).
• Remainder are nuclei of heavier elements,
  especially
• 12C, 16O, 14N, 20Ne, 24Mg, 28Si and 56Fe.
• Cosmic ray energies are in the range 109 to 1020
  eV
• lt109 eV, CR merge with solar wind and
  deflected by
• Earths mag. field at 1020 eV very few or
  no CR
• exist. Far fewer high energy CR than low flux
  ? E-2.7

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The energy spectrum of galactic cosmic rays. Note
the smooth and featureless spectrum. Note also
the very low flux of high energy particles.
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• CR travel in the Galaxy at speeds gt0.99 c
• CR fill the whole galactic disk and arrive on
  Earth
• travelling in all directions
• CR are confined to the Galaxy by a weak
  galactic
• magnetic field
• Bgal 3 10-6 G
• CR particles bent into curved path of radius r
  E/ceB
• by a mag. field.
• At E 1010 eV, r 0.7 AU CR tightly confined
• E 1020 eV, r 36 kpc - size of Galaxy, no
  confinement

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• Composition of cosmic rays
• Composition of CR shows some similarities with
  that
• of Sun
• But CR have much higher abundance of light
  elements
• lithium, beryllium and boron (Li, Be and B)
  than in Sun
• (e.g. Li/H 1011 in Sun 4 106 in CR)
• Compared with stars, CR have higher abundance of
• elements heavier than O, and they are
  deficient in
• elements H, He.

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Abundances of elements in CR show Li, Be and B
much enhanced
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• Interaction of cosmic rays with the ISM
• Heavier cosmic ray particles (e.g. C, N, O
  nuclei) crash
• into IS gas clouds, mainly HI, and the high
  energy
• collisions cause fragments of these nuclei to
  be broken
• off. Some of these fragments are nuclei of the
  elements
• Li, Be and B.
• Such nuclear reactions are known as spallation
  reactions
• Spallation causes the abundance of Li, Be, B to
  slowly
• build up in CR over their lifetime.
• Composition of CR thus slowly but continuously
• changing with time over millions of years

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EGRET (1991) satellite all-sky gamma-ray survey
showing the Galaxy in gamma-rays. The gamma rays
are emitted when cosmic rays interact with the
interstellar medium.
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• Age of cosmic rays
• The typical path length of CR particles through
  ISM
• can be determined from the observed amount of
• Li, Be and B in CR, based on there being 106
  H
• atoms m-3 in ISM
• Path length through ISM found is 2 106 light
  years
• Velocity of CR is V c
• Hence mean age of CR particles is 2 106
  years
• Size of Galaxy is 105 light years, so CR must
  travel
• in curved paths (this is indirect evidence for
  a mag. field)
• Oldest CR are age 4 106 years (twice mean
  age)

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Motion of a charged particle in a magnetic
field. The path is a helix oriented along the
field lines.
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• Original composition of cosmic rays
• The original (t 0) composition of CR can be
  predicted
• by extrapolating their slowly changing
  composition
• backwards through 4 106 years
• This t 0 composition is dominated by 12C, 16O
  with a
• little 14N, 20Ne, 24Mg, 28Si, 56Fe. This is
  the composition
• of CR at their source

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CR abundances at their source (supernovae?) are
predicted to be rich in alpha particles and also
C and O nuclei. The arriving cosmic rays contain
small amounts of Li, Be and B.
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• Source of cosmic rays
• Presumed source of CR is supernova explosions
• There are probably 2 or 3 supernovae/century in
  a
• typical spiral galaxy, including the Milky Way
• CR lose their energy by colliding with ISM in a
  few
• million years. Hence supply of new CR must be
• continuous
• Energy density of CR in Galaxy 106 eV/m3
• Total energy of all CR in whole galactic disk
  1048 J
• Energy replacement rate 1034 J/s

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The Crab nebula The Crab Nebula is the remnant
of a star that exploded in 1054 AD. It
was observed by Chinese astronomers
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The Vela super- nova remnant The Vela
supernova remnant, 10,000 years after the
explosion
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Thye origin of cosmic rays may be from the
acceleration of atoms in the ISM by shock waves
from nearby supernova explosions
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• Energy released in supernova explosions
• Each supernova releases energy of about 1044 J
• This energy is initially in form of kinetic
  energy of
• ejected material, photons and neutrinos
• Mean energy released by 3 supernovae/century
• (3 1044/3 109) J/s 1035 J/s
• (as 1 century 3 109 s)
• The energy released by supernovae is about 10
  greater
• than that required to account for the energy
  of CR
• CR may be accelerated to high energy in shock
  fronts in
• ISM near the supernova site

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• The galactic magnetic field
• Evidence for a galactic magnetic field
• Faraday rotation of plane of polarization of
  radio
• waves
• IS dust grain alignment causing polarization of
  some
• stars reddened by IS dust
• Zeeman splitting of 21-cm line of HI
• Cosmic ray confinement in Galaxy

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• All methods give consistent estimates of the
  field at
• B 3 106 gauss
• (cf. B? 0.3 G)
• Magnetic field appears to be oriented along the
• Galaxys spiral arms

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End of lecture 11