ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw The observations: Polarization is limited to stars near galactic plane, | b|  5º Mostly the observed polarizations are small Δm p  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:

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Observations of interstellar polarization as function of galactic coordinates. The plane and amount of polarization is shown by the short lines for each star

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Galactic cosmic rays Cosmic rays are high energy particles, mainly protons (90 % by number) or α-particles ( 4 He nuclei) (9 %). Remainder are nuclei of heavier elements, especially 12 C, 16 O, 14 N, 20 Ne, 24 Mg, 28 Si and 56 Fe. Cosmic ray energies are in the range 10 9 to eV; <10 9 eV, CR merge with solar wind and deflected by Earth’s mag. field; at ≥10 20 eV very few or no CR exist. Far fewer high energy CR than low; flux  E -2.7

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw The energy spectrum of galactic cosmic rays. Note the smooth and featureless spectrum. Note also the very low flux of high energy particles.

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw CR travel in the Galaxy at speeds >0.99 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 B gal ~ 3 × G CR particles bent into curved path of radius r = E/ceB by a mag. field. At E = eV, r ~0.7 AU – CR tightly confined; E = eV, r ~ 36 kpc - size of Galaxy, no confinement

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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 ~ 10 –11 in Sun; ~ 4 × 10 –6 in CR) Compared with stars, CR have higher abundance of elements heavier than O, and they are deficient in elements H, He.

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Abundances of elements in CR show Li, Be and B much enhanced

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Interaction of cosmic rays with the ISM Heavier cosmic ray particles (e.g. C, N, O nuclei) crash into IS gas clouds, mainly H I, 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

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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.

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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 ~ 10 6 H atoms m -3 in ISM Path length through ISM found is ~ 2 × 10 6 light years Velocity of CR is V ~ c Hence mean age of CR particles is ~ 2 × 10 6 years Size of Galaxy is ~ 10 5 light years, so CR must travel in curved paths (this is indirect evidence for a mag. field) Oldest CR are age ~ 4 × 10 6 years (twice mean age)

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Motion of a charged particle in a magnetic field. The path is a helix oriented along the field lines.

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Original composition of cosmic rays The original (t = 0) composition of CR can be predicted by extrapolating their slowly changing composition backwards through 4 × 10 6 years This t = 0 composition is dominated by 12 C, 16 O with a little 14 N, 20 Ne, 24 Mg, 28 Si, 56 Fe. This is the composition of CR at their source

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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.

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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 ~ 10 6 eV/m 3 Total energy of all CR in whole galactic disk ~10 48 J Energy replacement rate ~10 34 J/s

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw The Crab nebula The Crab Nebula is the remnant of a star that exploded in 1054 AD. It was observed by Chinese astronomers

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw The Vela super- nova remnant The Vela supernova remnant, 10,000 years after the explosion

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Thye origin of cosmic rays may be from the acceleration of atoms in the ISM by shock waves from nearby supernova explosions

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw Energy released in supernova explosions Each supernova releases energy of about J This energy is initially in form of kinetic energy of ejected material, photons and neutrinos Mean energy released by 3 supernovae/century (3 × /3 × 10 9 ) J/s ~10 35 J/s (as 1 century ~ 3 × 10 9 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

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw 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 H I Cosmic ray confinement in Galaxy

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw All methods give consistent estimates of the field at B ~ 3 × 10 –6 gauss (cf. B  ~ 0.3 G) Magnetic field appears to be oriented along the Galaxy’s spiral arms

ASTR112 The Galaxy Lecture 11 Prof. John Hearnshaw End of lecture 11