Interstellar medium and star formation
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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 l.jpg

Interstellar Medium and Star Formation

Astronomy G9001

Prof. Mordecai-Mark Mac Low

Historical overview of observations l.jpg


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

Slide3 l.jpg

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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  • 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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  • 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?

Hi lines l.jpg
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 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

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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.). 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 l.jpg
Hydro Concepts

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

Discretization l.jpg

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 doesn’t work!


Von neumann stability analysis l.jpg

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

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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 l.jpg
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?

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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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  • 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…

Boundaries l.jpg

  • “Ghost” zones allow specification of boundary values

    • Reflecting

    • Outflow

    • Periodic

    • Inflow

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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

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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

Assignments l.jpg

  • 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