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Evidence for the ISM • How do we know there is an interstellar medium (ISM)? 1) The Oort Limit http://map.gsfc.nasa.gov/m_uni/uni_101mw.html • Hydrostatic equilibrium, but for the whole Galaxy! - gravity of Galactic disk balanced by "pressure" (= individual velocities) of stars - measure velocities of stars → density of disk ρ0 0.08 M/pc3 • Total density: • Density of stars: • What's left? number density ρstars 0.06 M/pc3 mass density ρISM 0.02 M/pc3 1.3 x 10-24 g/cm3 nISM 0.8 H atoms / cm3 (but in very few places is the actual value close to this average!) Astronomy 16: The Interstellar Medium 1 Extinction – Discrete Clouds 2) Extinction • Clearly present in discrete clouds spread throughout Galaxy Dark cloud Barnard 68 (ESO / VLT ANTU) Horsehead Nebula (Nigel Sharp / NOAO / NSF; © AURA) http://www.astro.lu.se/Resources/Vintergatan/ Astronomy 16: The Interstellar Medium 2 Extinction – Diffuse Gas • Robert Trumpler (1930) : - catalog of 100 open clusters spread throughout Galaxy - cluster fitting: distance estimates for each cluster → "photometric distance" - nearby clusters: diameter depends on concentration, number of stars → "diameter distance" photometric distance equals diameter distance photometric distance more than diameter distance from Trumpler, Publications of the Astronomical Society of the Pacific, 42, 214 (1930) - plot "photometric" vs "diameter" distance: Distant clusters are fainter than they should be! → ~0.7 mag/kpc (modern value: ~2 mag/kpc) of extinction No globular clusters or background galaxies close to Galactic plane ("zone of avoidance") Astronomy 16: The Interstellar Medium 3 Reddening & Spectra 3) Reddening - stars in same MK class have different B – V ; B – V increases with overall extinction Dark cloud Barnard 68 (ESO / VLT ANTU) → ISM also makes stars redder 4) Interstellar absorption lines - in binary systems, some lines do not show Doppler shift due to binary motion Astronomy 16: The Interstellar Medium 4 Trumpler’s “Reddening” from Trumpler, Publications of the Astronomical Society of the Pacific, 42, 249, 267 5 Astronomy 16: The Interstellar Medium Extinction & Dust • Extinction is due to small dust particles in the ISM - combination of absorption and scattering Absorption: Scattering: • At a given distance, a star appears fainter than implied by its distance modulus: m M 5 log 10 d 5 A extinction in magnitudes (A > 0) “AV = 3” means star is 3 magnitudes fainter in V filter due to dust Towards Galactic center, AV 30 ! Aλ = kλ d , where kλ mag/pc is extinction coefficient at wavelength λ Astronomy 16: The Interstellar Medium 6 Optical Depth & Cross Section Recall optical depth, τλ : (stopped at this slide Tuesday) I ( L) I (0) e L In "stellar structure", we wrote: r Same situation here, but we convert ρ, to number density, n. We thus now write: nd where σλ is cross section (units m2 or cm2) of each dust grain. If dust grains were hard spheres of radius a & photons were bullets, then σλ = πa2 . But if light diffracts, σλ = Qλπa2 , where Qλ is "extinction efficiency factor" at wavelength λ. E.g. graphite grains of various radii Note: Qext = Qabs + Qscat from Draine & Lee, The Astrophysical Journal, 285, 89 (1984) Astronomy 16: The Interstellar Medium 7 Optical Depth & Cross Section Qλ is "extinction efficiency factor" at wavelength λ. or, a cartoon view… Astronomy 16: The Interstellar Medium 8 Extinction & Optical Depth mobserved mwithout 2.5 log 10 dust Fobserved Fwithout dust 2.5 log 10 e So A 1.086 and k 1.086 n But how do we measure Aλ (or equivalently τλ) ? V M V 5 log 10 d 5 AV Direct observation: V Spectrum: MV Parallax: d } AV But if star is close enough for parallax, AV is probably small! If we don't know d , we can't get AV ! Can resolve this because dust produces selective extinction - blue light gets scattered more than red light (blue skies, red sunsets) - more extinction → more reddening Astronomy 16: The Interstellar Medium 9 Reddening Extinction curve: V blue red note: inverse wavelength units! http://www.iras.ucalgary.ca/~volk/figs1.html B So longer wavelengths show less extinction. Thus extinction not only changes magnitude, it changes color index also! "color excess" EB V ( B V ) ( B V )0 observed color index intrinsic color index, equal to MB – MV From shape of extinction curve, can show that (roughly!): AV RV 3 .1 E B V Astronomy 16: The Interstellar Medium 10 Color Excess & Dust/Gas Ratio Example: O6III star is observed with V = +12.4 & B = +13.8 From HR diagram, we know that O6III stars have (B – V)0 = -0.30 and MV = -5.5 What is distance to star? Relation between dust & gas: • Star's color excess gives amount of extinction • Star's spectrum shows ISM absorption lines of H, from which equivalent width gives column density, NH = ∫ n dl from Diplas & Savage, • EB-V vs NH gives straight line: The Astrophysical Journal, 427, 274 (1994) N H 5.8 10 21 EB V cm -2 mag -1 Comparison to dust in this room? Astronomy 16: The Interstellar Medium 11 Dust Properties & Formation • Size of interstellar dust grains: 50 Å – 0.25 μm (cf. sand: 50-2000 μm, silt 2-50 μm, toner ~10 μm) • Tiny part of ISM – 1 dust particle every 106 m3 ! - by mass, ISM is 99% gas, 1% dust • Temp: absorbs photons, reradiates as 20-40 K blackbody http://www.ipac.caltech.edu/Outreach/Gallery/IRAS/allsky.html • Composition: silicates, graphite, water ice • Formation: need high pressure, temperature steadily falling - condensation in winds of cool giants & of AGBs - expanding/cooling ejecta of novae & supernovae • Critical role in astrochemistry : site of molecule formation - e.g. H2 molecule can never form by 2 H atoms colliding: tcollision 10-13 sec, tbond formation 10-9 sec → so atoms will usually just rebound But H atoms can stick to dust grain & bond, then escape Astronomy 16: The Interstellar Medium 12 Grain Shape & Polarization • Reddened light is polarized - grains preferentially absorb one pol. and leave other - need something to break symmetry → dust grains are elongated, not round! from Worm & Blum, The Astrophysical Journal, 529, L57 (2000) • But only works if all grains aligned in same direction - global Galactic magnetic field causes alignment from Han & Wielebinski, Chinese Journal of Astronomy & Astrophysics, 2, 293 (2002) Astronomy 16: The Interstellar Medium 13 The Gaseous ISM • Dust is important, but remember that 99% is gas! • Abundances: 85% H, 10% He, 5% rest (by number) from Wilms et al, The Astrophysical Journal, 542:914 (2000) • Gaseous ISM exists in (at least) five phases - molecular medium (MM) - cold neutral medium (CNM) - warm neutral medium (WNM) - warm ionized medium (WIM) - hot ionized medium (HIM) (aka "coronal gas") Astronomy 16: The Interstellar Medium 14 Molecular Medium (MM) • Grouped into "clouds" – ill-defined variety of structures • M ~ 1 – 106 M; GMCs have M > 104 M • size ~ 1-100 pc ; T ~ 10 K ; nH ~ 102 – 106 cm-3 • Opaque! E.g. nH = 104 cm-3 and L ~ 1 pc. What is AV ? • 1% of ISM volume (f = 0.01), 50% of ISM mass! • Almost entirely molecular hydrogen (H2), but H2 has few emission lines at low T & so is hard to see • Best tracer: carbon monoxide - only 0.01% by number, but has rotational transition at λ = 2.6 mm (ν = 115.27 GHz) Not absorbed by dust – can see through whole Galaxy! from Dame et al, The Astrophysical Journal, 547, 792 (2001) • 100s of molecules now detected: C2H5OH, C24H12, glycine… • Only known site of star formation Astronomy 16: The Interstellar Medium 15 The Molecular Ring • CO observations show inner Galaxy dominated by molecular ring at R ~ 4 kpc • Many supernovae, H II regions, open clusters here also from Clemens et al, The Astrophysical Journal, 327, 139 (1988) Astronomy 16: The Interstellar Medium 16 Neutral Medium • Cold Neutral Medium (CNM) - atomic hydrogen, nH ~ 20 cm-3, T ~ 100K, f ~ 0.02 • Warm Neutral Medium (WNM) - atomic hydrogen, nH ~ 0.3 cm-3, T ~ 6000K, f ~ 0.5 http://instruct1.cit.cornell.edu/courses/astro101/lec08.htm • Seen through "spin-flip" or "hyperfine" transition of H I - λ = 21.1 cm, ν = 1420.4 MHz (discovered at Harvard, 1951) - not absorbed by dust; most useful tracer of ISM (spontaneous transition from high to low occurs once every 11 million years!) http://www.ras.ucalgary.ca/CGPS/gallery CNM Astronomy 16: The Interstellar Medium 17 Warm Ionized Medium • Ionization potential of H in ground state = 13.6 eV - photon w. E > 13.6 eV (UV: λ < 911 Å) can ionize H - H will recombine → Hα seen at 656.3 nm (why not Lyα?) NRAO/VLA/GBT http://www.ipac.caltech.edu/2mass/ http://www.amtsgym-sdbg.dk/as/orion-2002/ • Discrete component: "H II regions" (f ~ 0.03) - ionized bubbles produced by UV photons around hot stars - seen in Hα, in IR (hot dust), in radio ("free-free" emission) • Orion Nebula (Messier 42) - top left: Hα - top right: infrared - bottom left: radio N.B.: extinction seen in optical, but not in IR/radio Astronomy 16: The Interstellar Medium 18 H II Regions Astronomy 16: The Interstellar Medium 19 Strömgren Spheres • Theorist's H II region: "Strömgren Sphere" - "photoionization equilibrium" between ionizations & recombinations 4 3 2 S R nH B 3 - LHS = total no. of ionizations per second - RHS = total no. of recombinations per second - S* = no. of ionizing photons emitted per second (can be derived from Planck equation) e.g. O5 star: S* 5 x 1049 photons/sec B1 star: S* 3 x 1045 photons/sec - R = radius of H II region (cm) - nH = density of gas being ionized (cm-3) - αB = "recombination coefficient" 2.5 x 10-13 cm3/sec • UV has short mean free path: H II regions have sharp edges - 100% ionized inside, 0% ionized outside • Oxygen and nitrogen ions in H II regions act as thermostat: T ~ 8000-10000 K regardless of central star Astronomy 16: The Interstellar Medium 20 Hypothetical & Real HII Regions Strömgren Says “Spheres,” with Radii…: O Star will destroy it’s birthplace rather thoroughly. Nature says… NGC 3603 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. The Rosette Nebula Astronomy 16: The Interstellar Medium 21 Diffuse WIM • Diffuse component of WIM recently identified (f ~ 0.20 ?) - aka "Diffuse Ionized Gas" (DIG) or "Reynolds Layer" - faint Hα from recombinations over entire sky (hard to map) - T ~ 8000 K, ne = nH+~ 0.1 cm-3 - H II regions confined to thin disk of height ~100-200 pc, but DIG is in disk of height ~ 1000 pc - ionization source unknown: escaped photons from O stars? http://www.skymaps.info/ Astronomy 16: The Interstellar Medium 22 Hot Ionized Medium John Vickery and Jim Matthes/ Adam Block/NOAO/AURA/NSF • "Coronal gas" - n ~ 0.003 cm-3 ; T ~ (5-10) x 106 K ; f ~ 0.40? - first seen in O VI absorption lines towards stars - also seen in X-ray/UV emission (but absorbed by gas) - hot interiors of supernova remnants? • Left: optical image of edge-on spiral galaxy NGC 4631 X-ray: NASA/CXC/UMass/D.Wang et al., UV: NASA/GSFC/UIT) • Right: X-rays (blue), UV from stars & H II regions (orange) Astronomy 16: The Interstellar Medium 23 The Multi-phase ISM • 1960s: "two phase ISM" (Field, Goldsmith & Habing 1969) - cold (neutral) clouds, embedded in warm (10% ionized) intercloud medium; two phases in pressure balance P nkT P / k 1000 K cm 3 - occasional hot cavities produced by SNe, but not part of big picture • 1970s: "3 phase ISM" (Cox & Smith 1974; McKee & Ostriker 1977) - hot cavities left by old SNRs merge & interconnect → HIM is persistent & pervasive phase of ISM - CNM=clouds; WNM/WIM=cloud envelopes; HIM=cavities - pressure balance: P / k ~ 2500 3000 K cm 3 - probably not completely correct, but useful complete picture from McKee & Ostriker, The Astrophysical Journal, 218, 148 (1977) Astronomy 16: The Interstellar Medium 24 Recycling in the ISM • Over billions of years, gas moves through all phases! cooling SNRs recombination starSee the reading, instead… light dust SNRs starlight Adadpted from Dopita & Sutherland, "Astrophysics of the Diffuse Universe" (Springer, 2003) Astronomy 16: The Interstellar Medium 25 Clustered Supernovae • Basic three-phase picture assumes SNe are randomly located - but in reality SN progenitors found in "OB associations" - clustered SNe: >100 stars, all going SNe within ~ 1 Myr! → "supershell" : similar evolution to SNR, but 100x energy → can escape from Galaxy's gravity to form "chimney" 21cm H I (WNM) HIM ~ 1 kpc ! cooling? from McClure-Griffiths et al, The Astrophysical Journal, 594, 833 (2003) Astronomy 16: The Interstellar Medium 26