Interstellar Medium and Star Formation

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Interstellar Medium and Star Formation

• Astronomy G9001
• Prof. Mordecai-Mark Mac Low

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Historical Overview of Observations

• HI lines, radio continuum
• UV Absorption lines
• X-ray emission
• Molecular line emission
• IR emission
• Gamma Rays

• Dust
• Excess Mass
• Visual Nebulae
• Emission lines
• Continuum light
• Polarization
• Optical Absorption Lines

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Dust
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?

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Reddening

• 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).

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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.

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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).

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Polarization

• 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)

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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?

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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)

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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

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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

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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)

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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)

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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

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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

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Changing Perceptions of the ISM

• Densest regions detected first
• Modeled as uniform clouds
• Actually continuous spectrum of ?, 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

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Structure of Course

• Lectures, Discussion, Technical
• Exercises
• Class Project
• Grading
• Exercises (30)
• Participation (20)
• Project (50)

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Project Schedule

• Feb 24 Written proposal describing work to be
  done (1-3 pp.). Ill 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

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Hydro Concepts

• Solving equations of continuum hydrodynamics
  (derived as velocity moments of Boltzmann
  equation, closed by equation of state for
  pressure)

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Discretization
Following Numerical Recipes

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

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Discretization of Derivatives

• The simplest way to discretize the derivatives is
  just FTCS
• But, it doesnt work!

x
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Von Neumann stability analysis

• The difference equation is
• Suppose we assume
• If ?(k) gt 1, then ?n grows with n
  exponentially!
• Dividing by ?neikj?x, and rearranging
• ?(k) gt 1 for some k, so this scheme is unstable

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Stability (cont.)

• This instability can be fixed using a Lax scheme
  ?jn-gt0.5(?j1n ?j-1n) in the time derivative, so
  that
• Now, if we do the same stability analysis, we find

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Courant condition

• The requirement that is fundamental
  to explicit finite difference schemes.
• Signals moving with velocity v should not
  traverse more than one cell ?x in time ?t.
• Why is Lax scheme stable?

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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.

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ZEUS

• 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

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ZEUS organization

• Operator splitting (Strang 1968)
• Separate different terms in hydro equations
• Source, advection, viscous terms each computed in
  substep

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ZEUS flowchart

• Timestep determined by Courant criterion at each
  cycle

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ZEUS grid

• Staggered grid to allow easy second-order
  differencing of velocities
• Grid naming scheme

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Boundaries

• Ghost zones allow specification of boundary
  values
• Reflecting
• Outflow
• Periodic
• Inflow

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Version Control

• Homegrown preprocessor EDITOR
• Clone of 70s 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

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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

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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

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ZEUS output

• To view output use IDL to read HDF files

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Assignments

• 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