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Interstellar Medium and Star Formation. Astronomy G9001 Prof. Mordecai-Mark Mac Low. Dust Excess Mass Visual Nebulae Emission lines Continuum light Polarization Optical Absorption Lines. HI lines, & radio continuum UV Absorption lines X-ray emission Molecular line emission

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interstellar medium and star formation

Interstellar Medium and Star Formation

Astronomy G9001

Prof. Mordecai-Mark Mac Low

historical overview of observations

Excess Mass

Visual Nebulae

Emission lines

Continuum light


Optical Absorption Lines

HI lines, & radio continuum

UV Absorption lines

X-ray emission

Molecular line emission

IR emission

Gamma Rays

Historical Overview of Observations

Following Li & Greenberg 2002, astro-ph/0204392

  • Naked eye observations of dust clouds
  • Holes in the heavens (Herschel 1785) vs obscuring bodies (Ranyard 1894, Barnard 1919)
    • Partial obscuration of continuous nebulae
    • Smooth dimming of star fields
    • Shapley-Curtis debate 1920
      • Shapley saw no obscuration in globulars: but they were out of plane!
      • Does obscuration contribute to distance scale?
  • Extinction was known since 1847 (though not taken seriously in Galaxy models)
  • Reddening discovered by Trumpler (1930)
  • Wavelength dependence established obscuration as due to small particles
  • Reddening proportional to NH
    • Extremely high NH measurable in IR against background star field: NICE (Lada et al. 1994, Cambrésy et al. 2002).
excess mass
Excess Mass
  • Vertical stellar motions allow measurement of non-stellar disk mass
  • Excess density of 6 x 10-24 g cm-3 found by Oort (1932)
  • We now know that this is a combination of ISM and dark matter.
  • Similar methods still used to measure dark matter density.
visual nebulae
Visual Nebulae
  • Nebulae first thought to be stellar
  • Spectroscopy revealed emission lines from planetary nebulae, establishing their gaseous nature (Huggins 1864)
  • Reflection nebulae distinguished from emission nebulae by continuous spectrum, reddening of internal stars
  • Measurements of Doppler shifts in emission lines revealed supersonic turbulent motions in Orion emission nebula (von Weizsäcker 1951, von Hoerner 1955, Münch 1958).
  • General linear polarization of starlight by ISM discovered by Hill (1949) and Hiltner (1949).
  • Alignment of dust in magnetic field (tho mechanism remains debated)
  • Revealed large scale field of galaxy
  • Radio polarization of synchrotron shows field in external galaxies as well
  • At high extinctions (high densities), IR emission polarization fails to trace field (Goodman et al. 1995)
optical absorption lines
Optical Absorption Lines
  • Ca II H & K lines have different dynamics from stellar lines in binaries (Hartmann 1904)
    • Na I D lines behave similarly (Heger)
    • Now used to trace extent of warm neutral gas
    • Reveals extent of local bubble (Frisch & York 1983, Paresce 1984, Sfeir et al 99)
  • Lines spread over 10 km/s, although individual components only 1-2 km/s wide
    • Interpreted as clouds in relative motion
    • Reinterpretation in terms of continuous turbulence?
hi lines
HI lines
  • HI fine structure line at 21 cm (Ewen & Purcell 1951) reveals cold neutral gas (300 K)
  • Pressure balance requires 104 K intercloud medium (Field, Goldsmith, Habing 1969)
  • Large scale surveys show
    • Supershells and “worms” (Heiles 1984)
    • Vertical distribution of neutral gas (Lockman, Hobbes, & Shull 1986)
  • Distribution of column densities shows power-law spectrum suggestive of turbulence (Green 1993)
radio continuum
Radio Continuum
  • First detected by Reber (1940): Nonthermal
  • Explanation as synchrotron radiation by Ginzburg
  • Distinction between thermal (HII regions) and non-thermal (relativistic pcles in B)
  • Traces ionized gas throughout Milky Way
  • Evidence for B fields and cosmic rays in external galaxies
uv absorption lines
UV Absorption Lines
  • Copernicus finds OVI interstellar absorption lines (1032,1038 Å) towards hot stars
  • Photoionization unimportant in FUV
  • Collisional ionization from 105 K gas, but this gas cools quickly, so must be in an interface to hotter gas
  • First evidence for 106 K gas in ISM
x ray emission
X-ray emission
  • Confirms presence of hot gas in ISM
  • Diffuse soft X-ray background (1/4 keV) anticorrelates with NHI: Local Bubble (McCammon et al. 1983, Snowden et al. 1990)
  • Detection of SNRs, superbubbles
  • X-ray shadows of cold clouds show contribution from hot halo (Burrows & Mendenhall 1991, Snowden et al. 1991)
molecular line emission
Molecular line emission
  • Substantial additional mass discovered with detection of molecular lines from dense gas
  • Millimeter wavelengths for rotational, vibrational lines from heterogeneous molecules
  • NH2 and H2O first found (Cheung et al. 1968, Knowles et al. 1969) then CO (Penzias et al. 1970), used to trace H2
  • Superthermal linewidths revealed (Zuckerman & Palmer 1974) showing hypersonic random motions
  • Map of Galactic CO from roof of Pupin (Thaddeus & Dame 1985)
ir emission
IR emission
  • Only with satellite telescopes such as IRAS was IR emission from cold dust in the ISM detectable: the “infrared cirrus”
  • IR penetrates dust better than visible, so it allows observation of star formation in dense regions
gamma rays
Gamma Rays
  • Gamma ray emission from Galactic plane first detected with OSO 3 and with a balloon (Kraushaar et al. 1972, Fichtel et al. 1972)
  • Confirmed by SAS 2 and COS B at 70 Mev.
  • CR interactions with gas and photons:
    • Electron bremsstrahlung
    • Inverse Compton scattering
    • Pion production
  • Independent estimate of mass in molecular clouds
changing perceptions of the ism
Changing Perceptions of the ISM
  • Densest regions detected first
  • Modeled as uniform “clouds”
  • Actually continuous spectrum of ρ, T, P.
  • Detection of motion showed dynamics
    • Combined with early analytic turbulence models
    • Success of turbulent picture limited then
  • Analytic tractability favored static equilibrium models (or pseudo-equilibrium)
    • Focus on heating/cooling, thermal phase transitions
  • New computational methods now bringing effects of turbulence back into focus
structure of course
Structure of Course
  • Lectures, Discussion, Technical
  • Exercises
  • Class Project
  • Grading
    • Exercises (30%)
    • Participation (20%)
    • Project (50%)
project schedule
Project Schedule
  • Feb 24: Written proposal describing work to be done (1-3 pp.). I’ll provide feedback on practicality and interest.
  • Mar 10: Oral presentation of final project proposals to class.
  • Apr 7: Proof-of-concept results in written report (2-4 pp., including figures)
  • Apr 28: Oral presentation of projects to class in conference format (10-15 minute talks)
  • May 5: Project reports due
hydro concepts
Hydro Concepts
  • Solving equations of continuum hydrodynamics (derived as velocity moments of Boltzmann equation, closed by equation of state for pressure)

Following Numerical Recipes

  • Consider a simple flux-conservative advection equation:
  • This can be discretized on a grid of points in time and space
discretization of derivatives


Discretization of Derivatives
  • The simplest way to discretize the derivatives is just FTCS:
  • But, it doesn’t work!


von neumann stability analysis
The difference equation is

Suppose we assume

If |ξ(k)| > 1, then ξn grows with n exponentially!

Dividing by ξneikjΔx, and rearranging

|ξ(k)| > 1 for some k, so this scheme is unstable

Von Neumann stability analysis
stability cont
Stability (cont.)
  • This instability can be fixed using a Lax scheme: ρjn->0.5(ρj+1n+ ρj-1n) in the time derivative, so that
  • Now, if we do the same stability analysis, we find
courant condition
Courant condition
  • The requirement that is fundamental to explicit finite difference schemes.
  • Signals moving with velocity v should not traverse more than one cell Δx in time Δt.
  • Why is Lax scheme stable?
numerical viscosity
Numerical Viscosity
  • Suppose we take the Lax scheme

and rewrite it in the form of FTCS + remainder

This is just the finite difference representation of a

diffusion term like a viscosity.

  • Program to solve hydro (and MHD) equations (Stone & Norman 1992, ApJSupp)
  • Details of numerical methods next time:
    • Second-order discretization
    • Eulerian moving grid
    • Artificial viscosity to resolve shocks
    • Conservative advection formulation
zeus organization
ZEUS organization
  • Operator splitting (Strang 1968):
    • Separate different terms in hydro equations
    • Source, advection, viscous terms each computed in substep:
zeus flowchart
ZEUS flowchart
  • Timestep determined by Courant criterion at each cycle
zeus grid
ZEUS grid
  • Staggered grid to allow easy second-order differencing of velocities
  • Grid naming scheme…
  • “Ghost” zones allow specification of boundary values
    • Reflecting
    • Outflow
    • Periodic
    • Inflow
version control
Version Control
  • Homegrown preprocessor EDITOR
    • Clone of 70’s commercial HISTORN
    • Similar to cpp with extra functions
    • Modifies code two ways
      • Define values for macros and set variables
      • Include or delete lines
  • A few commands
    • *dk - deck, define a section of code
    • *cd - common deck, common block for later use
    • *ixx - include the following at line xx
    • *dxx[,yy]- delete from lines xx to yy, and substitute following code
    • *if def,VAR to *endif - only include code if VAR defined
file structure
File Structure
  • Baroque, to allow “automatic” installation
  • From the top:
    • zcomp, sets system variables for local system
    • zeus34.s compilation script for ZEUS, EDITOR
    • zeus34, source code with EDITOR commands
    • zeus34.n, numbered version (next time)
    • Setup block (next time) generates
      • inzeus, runtime parameters
      • zeus34.mac, sets compilation switches (macros)
      • chgz34, makes changes to code
zeus installation
ZEUS installation
  • Copy ~mordecai/z3_template
  • Run zcomp, wait for prompt. (First time takes longer)
  • View parameters, accept defaults, wait for compile to finish
  • Make an execution directory (mkdir exe)
  • Copy xzeus34, inzeus into exe
  • Run xzeus34. Progress can be tracked by typing n
zeus output
ZEUS output
  • To view output use IDL to read HDF files
  • For next class read for discussion:
    • Ferrière, 2002, Rev Mod Phys, 73, 1031-1066
  • Begin reading
    • Stone & Norman, 1992, ApJ Supp, 80, 753-790 (I will cover more from this paper next time)
  • Complete Exercise 1
    • Install ZEUS, begin reading manual, readme files
    • Begin learning IDL
    • Review FORTRAN77 if not familiar