astronomy 216 spring 2006 - uc berkeley astronomy...

The Interstellar MediumAstronomy 216

Spring 2006

J. R. Graham

&

C. F. McKee

University of California, Berkeley

AY 216 2

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

AY 216 3

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)

AY 216 4

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)

AY 216 5

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

€

AY 216 6

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.

AY 216 7

1. Introduction:Discovery & History of the ISM

James R. Graham

University of California, Berkeley

AY 216 9

M104

AY 216 10

AY 216 11

IC410

AY 216 12

AY 216 13

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

AY 216 14

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

AY 216 15

Barnard 68

B,V & I B, I & K

AY 216 16

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

AY 216 17

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

AY 216 18

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)

AY 216 19

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

AY 216 20

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

AY 216 21

IRAS All Sky View

Blue: 12 µm Green: 60 µm Red: 100 µm

AY 216 22

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

AY 216 23

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

AY 216 24

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

AY 216 25

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

AY 216 26

Galactic Coordinates

AY 216 27

Equatorial Coordinates

AY 216 28

Local Star Formation Regions

Aquila Rift

CoronaAustralias

ρ Oph

Lupus

Coalsack

MuscaChamaeleon

η Cha Cluster

TW Hya Assn

UpperSco

UpperCen Lup

Lower Cen Crux

AY 216 29

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

AY 216 30

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)

AY 216 31

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

AY 216 32

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

AY 216 33

Schematic of the Local ISM Sun is at the center

• Brightest stars (giants& supergiants)

• Denser portions of theISM (orange)

500 pc

AY 216 34

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

AY 216 35

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

AY 216 36

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

AY 216 37

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

AY 216 38

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

AY 216 39

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

AY 216 40

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

AY 216 41

Energy Densities in Local ISM

€

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

AY 216 42

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

AY 216 43

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

AY 216 44

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?

AY 216 45