The Interstellar MediumAstronomy 216

Spring 2006

J. R. Graham

&

C. F. McKee

University of California, Berkeley

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Overview

Study of the “diffuse” interstellar gas & dust• Physical & chemical conditions and composition

• Energy sources

• Radiation field

• Non-thermal components: cosmic rays & B

Emphasis on Galactic ISM & aspects of ISM inexternal galaxies

Connections to Galactic structure, star formation &feedback

Broad & evolving subject• An exhaustive discussion neither desirable nor possible

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Literature Core monographs

• A. Tielens, “The Physics & Chemistry of the Interstellar Medium”(Cambridge, 2005)

• L. Spitzer, Jr., “Physical Processes in the Interstellar Medium” (Wiley1978)

• D. E. Osterbrock & G. Ferland, "Astrophysics of Gaseous Nebulaeand Active Galactic Nuclei”, 3rd Edition (University Science 2005)

• F. H. Shu, “The Physics of Astrophysics” Volumes 1 & 2, (UniversityScience Books, 1991,1992)

• G. B. Rybicki & A. P. Lightman, "Radiative Processes inAstrophysics" (Wiley, 1979)

Additional texts• L. H.Aller: Physics of Thermal Gaseous Nebulae (Reidel, 1984)• J. E. Dyson, D. A. Williams: The Physics of the Interstellar Medium

(IoP, 1997)• J. Binney & M. Merrifield, "Galactic Astronomy", (Princeton University

Press, 1998)• M. A. Dopita & R. S. Sutherland, “Astrophysics of the Diffuse

Universe”, (Springer, 2003)

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Literature

Additional resources• D. J. Hollenbach & H. A. Thronson (eds.), “Interstellar

Processes” (Reidel 1987)• G. L. Verschuur, & K. I. Kellerman (eds.), "Galactic &

Extragalactic Radio Astronomy" (2nd ed., Springer, 1988)• W. W. Duley & D. A. Williams, “Academic Press” (IoP, 1984)• V. Mannings, A. P. Boss & S. R. Russell (eds.), “Protostars &

Planets IV” (Arizona, 2000) and PPV when available• G. Ferland: HAZY (documentation for CLOUDY)

Lecture Notes• R. Genzel, “Physics and Chemistry of Molecular Clouds”, in

"The Galactic Interstellar Medium" (Springer 1992)• E. van Dishoeck, “Interstellar Chemistry” (UCB Astronomy

2000)

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Astronomical Units Angular measure

• arc second = 1/206,265 radian ≈ 4.85 µ rad

• Length• Å = 10-8 cm, 1 Å = 0.1nm, 10,000 Å = 1 µm• AU = Earth-Sun distance = 1.496 x1013 cm• pc = parsec = 206,265 AU = 3.086 × 1018 cm

• Mass• M = solar mass = 1.99 × 1033 g

• Energy• eV = 1.602 × 10-12 erg

• Radiation• L = solar luminosity = 3.83 × 1033 erg s-1

• magnitude = -2.5 log10(F/F0)• Jansky = 10-23 erg s-1 cm-2 Hz-1

• Rayleigh = 106/4! photons s-1 cm-2 sr-1

• Miscellaneous• Debye = 10-18 esu cm = 3.335 × 10-30 C m• 1 km s-1 x 1 Myr = 1.02 pc

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Evaluation (TBD)

Homework (70% of final grade)• About six written assignments will be provided• Problems will explore lecture material in depth• Grading

+ All students submit draft solutions by the deadline for grading+ Two students collaborate to prepare a master solution set based

the collective submission Master solution set is edited in consultation with instructors Master set is publication quality using LaTeX/AASTeX If you don’t submit a good initial draft your peers will not help you

when its your turn to construct the master solution set

+ Your cumulative homework grade will be assigned on the basis ofyour individual first draft solution (50%) and the master solutionset you contribute to (50%)

We will schedule a 1 hour debrief session for presentation anddiscussion of the master solution set

One hour oral final (30%)• Think of this a chance to practice for your prelim.

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1. Introduction:Discovery & History of the ISM

James R. Graham

University of California, Berkeley

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M104

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IC410

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Early History 1755/1796 Kant & Laplace nebular

hypothesis 1787: Messier’s catalog of nebulae 1758-1860: W., C. & J. Herschel list

5079 objects in the “General Catalog” 1860-1900: W. Huggins & J. E. Keeler

(Lick), spectra of nebula• Andromeda (stellar) vs. Orion (line)

1904: Hartmann, stationary Ca II H &K in spectroscopic binary δ Ori• Interstellar or circumstellar?

1912: V. Hess, discovery of ionizing“die Höhenstrahlung” during balloonflights• Not from the sun

M42

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Early 20th Century

1919: E. E. Barnard (Lick), catalog of dark nebulae• Holes in stellar distribution or obscuring matter?

1926: A. Eddington predicts interstellar H2

1927-34: Clay, Bothe, Kohlörster & Alvarez: cosmic raysare high energy, charged particles• Not γ rays

1930: R. Trumpler (Lick), proof of interstellar extinction• Comparison of luminosity & angular diameter distances to open

clusters

• 1930: Plaskett & Pearce, narrow Ca II & Na I absorption isinterstellar• Struve shows stronger CaII K in more distant stars

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Barnard 68

B,V & I B, I & K

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Mid 20th Century

1934: P. W. Merrill, diffuse interstellar bands

1934: F. Zwicky & W. Baade propose supernovae as adistinct class• 1938: W. Baade identifies the Crab Nebula as a SNR

1937– 40: Swings, Rosenfeld, McKellar & Adams: firstinterstellar diatomic molecules• CH, CH+, CN

1945: van de Hulst, predicts 21-cm line of HI

1949: Hall & Hiltner, polarization of starlight correlatedwith reddening• Aligned grains & interstellar magnetic field

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Diffuse Interstellar Bands

• ~ 200 DIBs known• Most DIBs are unidentified• Some DIBs may be due to large carbon-bearing

molecules•C60

+ is a candidate for λλ 9577, 9632 bands

BD+63o1964

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Late 20th Century 1950’s: Cosmic rays consist of heavy particles (protons, α

particles, etc.)

1951: Ewen & Purcell et al. detect of 21 cm line of H0

1950’s – 60’s: 21 cm HI maps of the Galaxy• Disk contains 5 x 109 M of atomic gas (≈ 10% of disk mass)

• 1963: Weinreb, Townes et al., interstellar OH masers• 1966: Lynds & Stockton discover the HI Lyman α forest

• 1968: NH3: first polyatomic interstellar molecule

QSO 1422+23, zem=3.62(∆v = 6.6 km/s)

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Late 20th Century

1960’s: Soft X-ray background fromlocal 106 K gas and hot cluster gas

1970: Wilson, Jefferts & Penzias, 2.6mm CO J=1–0 emission

1973: Carruthers, UV lines of H2

1970’s – 80’s: Infrared• H2 IR rotation vibration lines, very small

dust particles/very large molecules

1980’s – 90’s: Sub-millimeter• Warm interfaces of molecular clouds, cold

protostellar regions

∆v (MHz)

CO 1-0 in Orion

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Impact of Space Astronomy

1973 – 80: Copernicus UV satellite• Detection of H2 for many lines of sight• Highly ionized atoms (e.g., O VI) → very hot &

tenuous component of ISM

• Depletion of refractory elements from gas into grains

1983: IRAS• First all-sky survey at 12, 25, 60 & 100 µm

• Cirrus clouds and dust properties

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IRAS All Sky View

Blue: 12 µm Green: 60 µm Red: 100 µm

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The Impact of Space Astronomy

1990 – 91: COBE satellite• Galactic distribution of C+ & N+

1995 – 98 : ISO satellite• Mid- and far-IR spectroscopy

• Nature of grains (silicates,ices) and PAHs

• H2 lines as probe of shocksand PDRs

• Excitation of gas by stars &Active Galactic Nuclei

van Dishoeck 2004

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The Impact of Space Astronomy 1998 – 2001: SWAS

(Submillimeter WaveAstronomy Satellite)• High frequency (~ 500 GHz)

Pointed observations of H2O,H2

18O, O2, CI, & 13CO• Chemistry, composition &

collapse of molecular clouds

2000 – 01: FUSE (Far-UVSpace Explorer)

2004: SPITZER 2007: SOFIA 2012: JWST

Kruk et al. 1999Herald et al. 2005

NeVII

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Galactic Perspectives

The Galaxy, like other spirals, hasmatter between the stars

ISM is ~ 1% of the total mass• ~ 109 M out of ~ 1011 M

The ISM has a profound effectupon the evolution of the galaxies• It is the site of star formation

The ISM is inconspicuous optically• Much of the ISM is either cold (< 100

K: IR) or hot (> 106 K: x-ray)

• Nature & importance has only becomeevident in the last 50 years

NGC 891

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Optical Observations of the Milky Way

Dark bands in all-sky optical maps trace interstellar material stars• Historic Lund Observatory image (1953-1955) depicts 7000 brightest stars

Dark clouds dominate a large sector of the Galaxy to the left of the center(Aquila rift)• l = 10-50o

• Next inner-most spiral arm, the Sagittarius-Carina+ Tangent point evident

Lund Observatory

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Galactic Coordinates

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Equatorial Coordinates

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Local Star Formation Regions

Aquila Rift

CoronaAustralias

ρ Oph

Lupus

Coalsack

MuscaChamaeleon

η Cha Cluster

TW Hya Assn

UpperSco

UpperCen Lup

Lower Cen Crux

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The Galaxy in the Near-IR

A different picture of the sky emerges in the IR• COBE maps the sky between 1.3 µm and 4 mm• The near-IR (J, K & L) shows mostly stars & reduced ISM absorption• The disk-like nature of our Galaxy with its bulge is evident

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The Galaxy in the Far-Infrared

COBE 100, 140 & 240 µm• No ordinary stars, only a few with circumstellar dust shells are weakly detected

• The bulk majority of emission is from clouds of cool dust (≤ 20 K)

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Far-IR Spectrum of the ISM COBE spectrum of the

Galactic plane, 0.1-4 mm Cool dust peaks at ≈ 140 µm

• Tdust ≈ 20 K

C+ 2P3/2 - 2P1/2 157.74 µm• Diffuse WIM• C ionized by UV photons• hv > 11.25 eV

+ 911.76 Å Lyman edge+ 1101.72 Å C edge

C+ is common near FUVsources• Hot, young stars• Includes a large fraction of

the Galactic disk

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Schematic of the Local ISM Sun (center), Galactic Center at top

• Stars: OB (blue), AFG (yellow) & KM (orange)• Rings of denser gas (yellow)

+ 3-d shells of "warm"gas (~5000 K), containinglow-density, hot gas (~105-6 K)

+ Supernovae & winds from stars in OBassociations

Interior of the Local Bubble• n ~ 0.05 - 0.07 cm-3

Local Bubble is not spherical• The long axis ⊥ to the Galactic plane• Density gradients or colliding SNR

Above Arcturus is the “Wall” of denser gas• Like Loop 1, the wall appears to be driven by

the Sco-Cen association Local Fluff is a denser region (~ 0.1 cm-3)

recently encountered by the Sun• The Sun is headed to the left at ~ 20 km/s

plowing through the Local Fluff

From Henbest & Couper, “Guide to theGalaxy” Cambridge 1994

Galactic Center

120 pc

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Schematic of the Local ISM Sun is at the center

• Brightest stars (giants& supergiants)

• Denser portions of theISM (orange)

500 pc

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General Properties of the ISM

Mostly confined to the disk• Some gas in the halo, as in ellipticals

Large ranges in temperature & density• T ≈ 10 … 106 K, n ≈ 10-3 … 106 cm-3

• Even dense regions are “ultra-high vacuum”• Lab UHV is 10-10 Torr (n ≈ 4 × 106 cm-3)• Compare with n ≈ 3 × 1019 cm-3 at STP

• Far from thermodynamic equilibrium• Complex processes & interesting physics

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Cosmic Abundances

2.8 × 10-5Fe1.5 × 10-6Na

2.0 × 10-6Ca4.6 × 10-4O

1.4 × 10-5S6.0 × 10-5N

3.2 × 10-5Si2.5 × 10-4C

2.3 × 10-6Al0.085He

3.4 × 10-5Mg1H

N/NHElementN/NH *Element

• Mass fraction of H, He and “metals”• X = 0.738, Y = 0.249, Z = 0.012• Mass per H = 1.37 AMU

• Typical reference abundances are• Solar - presumably ISM 4.5 x 109 yr ago (see Table)• HII regions- gas phase only• B-star atmospheres - samples recent ISM

• Considerable disagreement, e.g C:

* Solarphotosphericabundances,Asplund et al.astro-ph/0410214

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ISM Classified by Ionization State

• Composition resembles Solar System• H is the most abundant element (≥ 90% of

atoms)

• ISM characterized by state of hydrogen• Ionized atomic hydrogen (H+ or H II) “H-two”

• Neutral atomic hydrogen (H0 or H I) “H-one”

• Molecular hydrogen (H2) “H-two”

Regions are H II, H I, or H2

• Transition zones H II H I H2 are thin

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Ionized Gas: HII Regions

H II regions surrounding early-type (OB) stars• Photoionized• T ≈ 104 K• ne ≈ 0.1 … 104 cm-3

• Bright nebulae associated with regions of starformation & molecular clouds

• Warm Ionized Medium• T ≈ 8000 K• ‹ne› ≈ 0.025 cm-3

• Hot Ionized Medium: tenuous gas pervadingthe ISM• Ionization by electron impact• T ≈ 4.5 x 105 K• n ≈ 0.0035 cm-3

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Atomic Gas: H I Regions

• Lyman α forest• Intergalactic clouds NHI > 1013 cm-2

Cool “clouds” (CNM)• T ≈ 100 K

• n ≈ 40 cm-3

• Warm neutral gas (WNM)• T ≈ 7500 K

• n ≈ 0.5 cm-3

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Molecular Clouds (H2 Regions)

Cold dark clouds (M ≈ 10 - 1000 M)• T ≥ 10 K• n ≈ 102 - 104 cm-3

Giant molecular clouds (M ≈ 103 - 106 M)• T ≥ 20 K• n ≈ 102 - 104 cm-3

• Molecular material exhibits complex structureincluding cores and clumps with n ≈ 105 - 109 cm-3

• Molecular clouds are the sites of star formation

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Other Ingredients of ISM

Heavy elements• He (≈ 10 % by number)• C, N, O (≈ “cosmic” abundances)• Si, Ca, Fe (depleted onto grains)

• Dust grains (≈ 0.1 µm size, silicates orcarbonaceous material ≈ 1% by mass of ISM)

• Photons• CMB• Star light—average interstellar radiation field• X-rays—from hot gas & the extragalactic background

• Magnetic fields & cosmic rays

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Energy Densities in Local ISM

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uthermal = 32 p = 0.39 p / k

3000cm -3KeVcm-3

uhydro = 12 ρ ‹v

2› = 0.54 nH cm-3( ) σ1d 5kms-1( )2eVcm-3

umagnetic = B2 8π = 0.22 B 3µG( )2 eVcm-3

ustarlight = 0.5 eVcm-3

ucosmic rays = 0.8 eVcm-3

u3KCBR = 0.25 eVcm-3 All six energy densities are of

comparable order of magnitude

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The ISM as a Complex System

Understanding the ISM means• Understanding the physical processes which drive mass,

momentum and energy exchange between stars and itsphases

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Motivation

Star formation is the fundamental processwhich determines the observational propertiesof galaxies• History of star formation yield the Hubble sequence

+ Smooth E’s

+ Spiral disk galaxies

+ Irregular galaxies

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Basic Questions

How are baryons transformed into stars?• Subject to

+ Gravity (well understood)

+ Radiation pressure (small)

+ Magnetic fields

+ Turbulent stresses

Fundamental questions• Why does star formation occur mostly in spiral arms

• What triggers star formation

• What determines the star formation rate in different Hubble types?

• What determines the initial mass function of stars?

• Why do stars form in multiples?

• How do planets form?

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