Interstellar Medium (ISM)

Solar wind and IPM (Interplanetary Medium)

• carries particles and magnetic fields outward
• defines the Heliosphere  (region of space dominated by the Sun)
• each star has a similar region of domination around it
ISM
• region between the stars
• consists of particles and magnetic fields, from stars and "primordial"
• "particles" can be either gas or dust

There are three aspects of the Interstellar Medium in this photograph.  The blue nebula at left surrounding the stars of the Pleiades is a reflection nebula, which scatters blue light from the stars.  The red nebula at right (California nebula) is an emission nebula, shining in the red light of hydrogen-alpha.  In the upper left and slanting diagonally across the middle of the photograph, the background stars appear to be less numerous due to interstellar absorption from dust, which obscures the more distant stars.

• Gas (atomic nuclei, neutral atoms, molecules)
• obeys gas dynamics (e.g. ideal gas law), emission processes.
• Factor of 10¹² larger number of gas particles than dust particles.
• Seen by emission or absorption (mostly of spectral lines).
• Dust (grains, aggregates of molecules)
• only small in number (avg. 1 particle in 10⁶ m³),
• but BIG -- of order 1 micron in size.
• Make up about 1% of ISM by mass.
• Dominates opacity (interstellar absorption and other effects).
• Seen by reflection or absorption (continuum)

Dark Nebulae and Extinction

Extinction (= Absorption + Scattering) means reduction of overall brightness of objects seen through dust.  Distance modulus equation is modified when we take dust into account:

m - M = 5 log d - 5 + A

where A is amount of absorption in magnitudes.  For A = 1, star is reduced in brightness (flux) by a factor of 2.5.  If A were uniform throughout galaxy, then A = kd (proportional to distance) but A is not uniform.
Dust in a galaxy

• Light is not scattered and absorbed uniformly--blue light is scattered more than red.
• Same effect for Earth's atmosphere
• Affects color index, e.g. B - V, due to color excess
CE = CI (observed) - CI (intrinsic)
• In most regions of galaxy, the visual absorption A_V ~ 3 CE, so can use CE measurements to determine amount of dust (to star)
• Can correct the distance estimated from Spectroscopic Parallax.
• Can show that A_l= 1.086 t_l.

• Fractional polarizationFP = (I - I_perp)/(I  + I_perp)
• Gives info about grain elongation
• Grains align along magnetic field, so can get info on ISM magnetic field direction
• Magnetic fields lie along spiral arms

When you see dust from the side, it appears BLUE and polarized.

This polarization does not necessarily depend on grains being elongated.

Pleiades

• Possible explanations for grains:
• Elongated, dirty-ice grains
• Grains of graphite
• Particles with small cores and icy mantles
• Large, complex molecules called polycyclic aromatic hydrocarbons (PAHs):Model of the shape of a dust grain

• Silicate particles
• Sizes can be guessed from interstellar extinction curve (0.2 micron => graphite)
• Structure, make-up can be guessed from the infrared spectrum (both ices and silicates)
• Where do they come from?
• silicates and refractory material formed in atmospheres of cool stars (red supergiants)
• ices may come from dense molecular clouds -- next topic

Again, 99% of ISM by mass is gas, and there are 10¹²gas "particles" for every dust grain.

Interstellar Absorption Lines

• Unlike dust, gas is largely transparent--except in narrow spectral lines.
• See absorption lines in star-light that are not due to star -- Ca I, Ca II, Ti I, Ti II, Na I, and molecules CN and CH
• What should such lines look like?
• sharp and narrow (because cool and low thermal doppler broadening)
• can be shifted due to bulk doppler shifts--multiple clouds.

• Emission Nebulae are RED and have both optical lines and radio continuum.
• Hydrogen lines are due to recombination of ionized hydrogen (H II).
• Ionization occurs due to UV photons ( > 91.2 nm, Lyman continuum) from hot (T = 20,000 K) O and B stars.
• Gas fluoresces by converting UV photons to visible (electrons cascade down to lower energy levels)
• If gas is uniform, an O or B star have roughly spherical volume of H II (Stromgren Sphere).
• Radio continuum comes from the free electrons

Supernovae

Image, Latest Image (shock wave slams into ring)

Planetary Nebulas
Image 1
Image 2

Interstellar Radio Lines

21-cm line

• Lowest energy state of hydrogen
• Electron in one of two spin states "up" or "down"
• Define "up" to be parallel with proton spin--then this is higher energy state--would rather be antiparallel
• Energy difference between "up" and "down" corresponds to radio photon of 21 cm wavelength
• Spontaneous flip occurs only once every few million years!
• Collisions in ISM occur once every 400 years, so most flips are due to collisions (no radiation)
• Rare photon emission, but there are enormous numbers of H atoms along each line of sight, so line is strong!

Recombination lines

• Electrons recombining with hydrogen in H II regions can attach in very high excitation levels, then transition to slightly lower ones, i.e. n = 105 to n = 104.  These low-energy transitions give off low-energy photons in the      radio region of the spectrum, giving another way to study H II regions.

Molecular Lines

• CO rotational line, with J = 0 to 1 transition at 115 GHz, and the 1 to 2 transition at 230 GHz.  These correspond to mm wavelengths--radio again.
• Molecules that have been detected: CO, CN, OH, H₂CO, CH₃OH, H₂O, NH₃, HCN, HC₃N, HNCO
• Most molecules, however, are good old H₂ (molecular hydrogen).
• Chance of these forming in ISM seem very small (one collision every 400 years) but
• dust helps
• provides place to collect and hold atoms while molecules form over millions (perhaps billions) of years
• shadows fragile molecules from dissociating radiation
• Widespread existence of molecules has implications for extraterrestrial life.

Inside a molecular cloud -- OMC-1:  Image
Star formation in M16: link
Egg Nebula:  picture