Interstellar Medium and Star Formation Astronomy G9001 Prof. Mordecai-Mark Mac Low Historical Overview of Observations    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 IR emission Gamma Rays Following Li & Greenberg 2002, astro-ph/0204392 Dust 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? 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). 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     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). 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) 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 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 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 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 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 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 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 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     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 Lectures, Discussion, Technical  Exercises  Class Project  Grading  – Exercises (30%) – Participation (20%) – Project (50%) 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  Solving equations of continuum hydrodynamics (derived as velocity moments of Boltzmann equation, closed by equation of state for pressure) D   v  0, Dt Dv   p  , Dt De      p v , Dt    D  where   v Dt t where  2  4 G  where p   kT /  Following Numerical Recipes Discretization Consider a simple flux-conservative advection equation:     v    0, or in 1D  vx t t x  This can be discretized on a grid of points in time and space x j  x0  jx  tn  t0  nt Discretization of Derivatives  The simplest way to discretize the derivatives is just FTCS:   t   x   nj 1   nj  O  t  t  nj 1   nj 1 2x t  O  x 2  But, it doesn’t work! x Von Neumann stability analysis  The difference equation is  nj 1   nj t   nj 1   nj 1   v    2x    Suppose we assume  If |ξ(k)| > 1, then ξn grows with n exponentially!  nj   neik ( jx ) , where    e n 1 ikj x   e n ikj x t n ik ( j 1) x n ik ( j 1) x  v  e  e   2x Dividing by ξneikjΔx, and rearranging t ik x vt  ik x  1  v e e , so   1  i sin k x   2x x  |ξ(k)| > 1 for some k, so this scheme is unstable Stability (cont.)  This instability can be fixed using a Lax scheme: ρjn->0.5(ρj+1n+ ρj-1n) in the time derivative, so that n n      1 n j 1 j 1 n 1 n  j    j 1   j 1   vt   2 2x    Now, if we do the same stability analysis, we find 1 ik x t  ik x   e  e v eik x  e  ik x  ,   2 2x vt so now  ( k )  cos k x  i sin k x x vt 2 2 and  ( k )  cos k x  sin 2 k x, x v t which is only  1 if 1 x Courant condition v t  1is fundamental x to explicit finite difference schemes.  The requirement that  Signals moving with velocity v should not traverse more than one cell Δx in time Δt.  Why is Lax scheme stable? Numerical Viscosity  Suppose we take the Lax scheme n n   j 1   j 1  1 n n 1 n  j    j 1   j 1   vt   2 2x   and rewrite it in the form of FTCS + remainder n n n n n       j 1   j 1 1  j 1  2  j   j 1   v         t 2x t   2  n 1 j n j This is just the finite difference representation of a 2 2   diffusion term    x  like a viscosity. 2   x  t2  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 ZEUS organization  Operator splitting (Strang 1968): – Separate different terms in hydro equations – Source, advection, viscous terms each computed in substep: Take S   v . Then the momentum equation dS     vS   P   dt can be broken up into two steps dS dS  dS      dt dt  source dt transport ZEUS flowchart  Timestep determined by Courant criterion at each cycle ZEUS grid Staggered grid to allow easy second-order differencing of velocities  Grid naming scheme…  Boundaries  “Ghost” zones allow specification of boundary values – – – – Reflecting Outflow Periodic Inflow 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 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 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  To view output use IDL to read HDF files 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