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Interstellar Medium and Star Formation Astronomy G9001 Prof. Mordecai-Mark Mac Low

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 - PowerPoint PPT Presentation

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Page 1: Interstellar Medium and Star Formation

Interstellar Medium and Star Formation

Astronomy G9001

Prof. Mordecai-Mark Mac Low

Page 2: Interstellar Medium and Star Formation

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

Page 3: Interstellar Medium and Star Formation

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?

Following Li & Greenberg 2002, astro-ph/0204392

Page 4: Interstellar Medium and Star Formation

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).

Page 5: Interstellar Medium and Star Formation

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.

Page 6: Interstellar Medium and Star Formation

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).

Page 7: Interstellar Medium and Star Formation

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)

Page 8: Interstellar Medium and Star Formation

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?

Page 9: Interstellar Medium and Star Formation

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)

Page 10: Interstellar Medium and Star Formation

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

Page 11: Interstellar Medium and Star Formation

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

Page 12: Interstellar Medium and Star Formation

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)

Page 13: Interstellar Medium and Star Formation

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)

Page 14: Interstellar Medium and Star Formation

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

Page 15: Interstellar Medium and Star Formation

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

Page 16: Interstellar Medium and Star Formation

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

Page 17: Interstellar Medium and Star Formation

Structure of Course

Lectures, Discussion, Technical Exercises Class Project Grading

– Exercises (30%)– Participation (20%)– Project (50%)

Page 18: Interstellar Medium and Star Formation

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

Page 19: Interstellar Medium and Star Formation

Hydro Concepts Solving equations of continuum

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

2

0, where

, where 4

, where /

D Dv v

Dt Dt tDv

p GDt

D ep v p kT

Dt

Page 20: Interstellar Medium and Star Formation

Discretization

Consider a simple flux-conservative advection equation:

This can be discretized on a grid of points in time and space

0, or in 1D t xv v

t x

0

0

j

n

x x j x

t t n t

Following Numerical Recipes

Page 21: Interstellar Medium and Star Formation

Discretization of Derivatives

The simplest way to discretize the derivatives is just FTCS:

But, it doesn’t work!

1

1 1 2

2

n nj j

n nj j

O tt t

O xx x

t

x

Page 22: Interstellar Medium and Star Formation

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

( ) , where n n ik j xj e

11 1

2

n n n nj j j jv

t x

1 ( 1) ( 1)

2n ikj x n ikj x n ik j x n ik j xt

e e v e ex

1 , so 1 sin2

ik x ik xt v tv e e i k x

x x

Page 23: Interstellar Medium and Star Formation

Stability (cont.) This instability can be fixed using a Lax scheme:

ρjn->0.5(ρj+1

n+ ρj-1n) in the time derivative, so that

Now, if we do the same stability analysis, we find

1 111 1

1

2 2

n nj jn n n

j j j v tx

2 2 2

1,

2 2

so now ( ) cos sin

and ( ) cos sin ,

which is only 1 if 1

ik x ik x ik x ik xte e v e e

xv t

k k x i k xx

v

v

tk k x k x

x

xt

Page 24: Interstellar Medium and Star Formation

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?

1v t

x

Page 25: Interstellar Medium and Star Formation

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.

1 111 1

1

2 2

n nj jn n n

j j j v tx

11

1 1 12

2

1

2

n n n nj j j

n nj j jjn

vt x t

2 2

22

x

t x

Page 26: Interstellar Medium and Star Formation

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

Page 27: Interstellar Medium and Star Formation

ZEUS organization Operator splitting (Strang 1968):

– Separate different terms in hydro equations– Source, advection, viscous terms each

computed in substep:

Take . Then the momentum equation

can be broken up into two steps

transporsource t

S

P

v

dS

dS

d

dt

d

vS

dS

tt dt

S

d

Page 28: Interstellar Medium and Star Formation

ZEUS flowchart

Timestep determined by Courant criterion at each cycle

Page 29: Interstellar Medium and Star Formation

ZEUS grid Staggered grid

to allow easy second-order differencing of velocities

Grid naming scheme…

Page 30: Interstellar Medium and Star Formation
Page 31: Interstellar Medium and Star Formation

Boundaries

“Ghost” zones allow specification of boundary values– Reflecting– Outflow– Periodic– Inflow

Page 32: Interstellar Medium and Star Formation

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

Page 33: Interstellar Medium and Star Formation

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

Page 34: Interstellar Medium and Star Formation

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

Page 35: Interstellar Medium and Star Formation

ZEUS output

To view output use IDL to read HDF files

Page 36: Interstellar Medium and Star Formation

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