The Interstellar Medium Astronomy 216 Spring J. R. Graham & C. F. McKee University of California, Berkeley

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1 The Interstellar Medium Astronomy 216 Spring 2006 J. R. Graham & C. F. McKee University of California, Berkeley

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 in external galaxies Connections to Galactic structure, star formation & feedback Broad & evolving subject An exhaustive discussion neither desirable nor possible AY 216 2

3 Literature Core monographs A. Tielens, The Physics & Chemistry of the Interstellar Medium (Cambridge, 2005) L. Spitzer, Jr., Physical Processes in the Interstellar Medium (Wiley 1978) D. E. Osterbrock & G. Ferland, "Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, 3rd Edition (University Science 2005) F. H. Shu, The Physics of Astrophysics Volumes 1 & 2, (University Science Books, 1991,1992) G. B. Rybicki & A. P. Lightman, "Radiative Processes in Astrophysics" (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 3

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 4

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 = x10 13 cm pc = parsec = 206,265 AU = cm Mass M = solar mass = g Energy ev = erg Radiation L = solar luminosity = erg s -1 magnitude = -2.5 log 10 (F/F 0 ) Jansky = erg s -1 cm -2 Hz -1 Rayleigh = 10 6 /4! photons s -1 cm -2 sr -1 Miscellaneous Debye = esu cm = C m 1 km s -1 x 1 Myr = 1.02 pc AY 216 5

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 of your individual first draft solution (50%) and the master solution set you contribute to (50%) We will schedule a 1 hour debrief session for presentation and discussion of the master solution set One hour oral final (30%) Think of this a chance to practice for your prelim. AY 216 6

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8 1. Introduction: Discovery & History of the ISM James R. Graham University of California, Berkeley

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13 Early History 1755/1796 Kant & Laplace nebular hypothesis 1787: Messier s catalog of nebulae : W., C. & J. Herschel list 5079 objects in the General Catalog : 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 balloon flights Not from the sun M42 AY

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 H : Clay, Bothe, Kohlörster & Alvarez: cosmic rays are 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 is interstellar Struve shows stronger CaII K in more distant stars AY

15 Barnard 68 B,V & I B, I & K AY

16 Mid 20th Century 1934: P. W. Merrill, diffuse interstellar bands 1934: F. Zwicky & W. Baade propose supernovae as a distinct class 1938: W. Baade identifies the Crab Nebula as a SNR : Swings, Rosenfeld, McKellar & Adams: first interstellar diatomic molecules CH, CH +, CN 1945: van de Hulst, predicts 21-cm line of HI 1949: Hall & Hiltner, polarization of starlight correlated with reddening Aligned grains & interstellar magnetic field AY

17 Diffuse Interstellar Bands ~ 200 DIBs known Most DIBs are unidentified Some DIBs may be due to large carbon-bearing molecules C 60 + is a candidate for λλ 9577, 9632 bands BD+63 o 1964 AY

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 H s 60 s: 21 cm HI maps of the Galaxy Disk contains 5 x 10 9 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: NH 3 : first polyatomic interstellar molecule QSO , z em =3.62 ( v = 6.6 km/s) AY

19 Late 20th Century 1960 s: Soft X-ray background from local 10 6 K gas and hot cluster gas 1970: Wilson, Jefferts & Penzias, 2.6 mm CO J=1 0 emission 1973: Carruthers, UV lines of H s 80 s: Infrared H 2 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 CO 1-0 in Orion v (MHz) AY

20 Impact of Space Astronomy : Copernicus UV satellite Detection of H 2 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

21 IRAS All Sky View Blue: 12 µm Green: 60 µm Red: 100 µm AY

22 The Impact of Space Astronomy : COBE satellite Galactic distribution of C + & N : ISO satellite Mid- and far-ir spectroscopy Nature of grains (silicates, ices) and PAHs H 2 lines as probe of shocks and PDRs Excitation of gas by stars & Active Galactic Nuclei AY van Dishoeck 2004

23 The Impact of Space Astronomy : SWAS (Submillimeter Wave Astronomy Satellite) High frequency (~ 500 GHz) Pointed observations of H 2 O, H 18 2 O, O 2, CI, & 13 CO Chemistry, composition & collapse of molecular clouds : FUSE (Far-UV Space Explorer) 2004: SPITZER 2007: SOFIA 2012: JWST NeVII AY 216 Kruk et al Herald et al. 2005

24 Galactic Perspectives The Galaxy, like other spirals, has matter between the stars ISM is ~ 1% of the total mass ~ 10 9 M out of ~ M The ISM has a profound effect upon 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 (> 10 6 K: x-ray) Nature & importance has only become evident in the last 50 years NGC 891 AY

25 Optical Observations of the Milky Way Dark bands in all-sky optical maps trace interstellar material stars Historic Lund Observatory image ( ) depicts 7000 brightest stars Dark clouds dominate a large sector of the Galaxy to the left of the center (Aquila rift) l = o Next inner-most spiral arm, the Sagittarius-Carina Lund Observatory AY Tangent point evident 25

26 Galactic Coordinates AY

27 Equatorial Coordinates AY

28 Local Star Formation Regions Aquila Rift ρ Oph Upper Sco Upper Cen Lup TW Hya Assn Corona Australias Lupus Coalsack Musca η Cha Cluster Lower Cen Crux Chamaeleon AY

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

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

31 Far-IR Spectrum of the ISM COBE spectrum of the Galactic plane, mm Cool dust peaks at 140 µm T dust 20 K C + 2 P 3/2-2 P 1/ µm Diffuse WIM C ionized by UV photons hv > ev Å Lyman edge Å C edge C + is common near FUV sources Hot, young stars Includes a large fraction of the Galactic disk AY

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), containing low-density, hot gas (~ K) + Supernovae & winds from stars in OB associations Interior of the Local Bubble n ~ 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 Galactic Center From Henbest & Couper, Guide to the Galaxy Cambridge pc AY

33 Schematic of the Local ISM Sun is at the center Brightest stars (giants & supergiants) Denser portions of the ISM (orange) 500 pc AY

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 K, n cm -3 Even dense regions are ultra-high vacuum Lab UHV is Torr (n cm -3 ) Compare with n cm -3 at STP Far from thermodynamic equilibrium Complex processes & interesting physics AY

35 Cosmic Abundances Element H He C N O Na N/N H * Element Mg Ca Fe AY Al Si S N/N H Mass fraction of H, He and metals X = 0.738, Y = 0.249, Z = Mass per H = 1.37 AMU Typical reference abundances are Solar - presumably ISM 4.5 x 10 9 yr ago (see Table) HII regions- gas phase only B-star atmospheres - samples recent ISM Considerable disagreement, e.g C: * Solar photospheric abundances, Asplund et al. astro-ph/

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 (H 0 or H I) H-one Molecular hydrogen (H 2 ) H-two Regions are H II, H I, or H 2 Transition zones H II H I H 2 are thin AY

37 Ionized Gas: HII Regions H II regions surrounding early-type (OB) stars Photoionized T 10 4 K n e cm -3 Bright nebulae associated with regions of star formation & molecular clouds Warm Ionized Medium T 8000 K n e cm -3 Hot Ionized Medium: tenuous gas pervading the ISM Ionization by electron impact T 4.5 x 10 5 K n cm -3 AY

38 Atomic Gas: H I Regions Lyman α forest Intergalactic clouds N HI > 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

39 Molecular Clouds (H 2 Regions) Cold dark clouds (M M ) T 10 K n cm -3 Giant molecular clouds (M M ) T 20 K n cm -3 Molecular material exhibits complex structure including cores and clumps with n cm -3 Molecular clouds are the sites of star formation AY

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 or carbonaceous 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

41 Energy Densities in Local ISM u thermal = 3 2 p = 0.39 p / k 3000cm -3 K evcm-3 u hydro = 1 ρ v 2 = 0.54 ( n 2 H cm -3 )( σ 1d 5kms -1 ) 2 evcm -3 u magnetic = B 2 8π = 0.22( B 3µG) 2 evcm -3 u starlight = 0.5 evcm -3 u cosmic rays = 0.8 evcm -3 u 3K CBR = 0.25 evcm -3 All six energy densities are of comparable order of magnitude AY

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 its phases AY

43 Motivation Star formation is the fundamental process which determines the observational properties of galaxies History of star formation yield the Hubble sequence + Smooth E s + Spiral disk galaxies + Irregular galaxies AY

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

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