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The Interstellar Medium- Overview IV Astronomy 216 Spring 2005 The Interstellar Medium- Overview IV Astronomy 216 Spring 2005 James Graham University of California, Berkeley The Molecular Milky Way AY 216 2 Local Star Forming Regions l Much of our knowledge of star formation comes from a few nearby regions • Taurus-Auriga & Perseus + Low mass (solar type stars) + 150 pc • Orion + Massive stars (OB stars) in Orion + 500 pc l How representative are these regions of the Galaxy as a whole? AY 216 3 Ophiuchus Wilking et al. 1987 AJ 94 106 CO Andre PP IV AY 216 4 Orion + CS cores > 200 M§ & Stellar clusters (Lada 1992 ApJL 393 25) AY 216 5 Conditions for Collapse l Supersonic motions are observed in molecular clouds • Approach thermal line widths on small scales l If cloud cores make stars, then they must be gravitationally bound • For thermal support GM/R ~ c2 = kT/µm c is the sound speed and µ ≈ 2.3 is the mean molecular R ≈ 0.1 (M / M§) (10 K / T) pc AY 216 6 Magnetic Fields l 850 µm polarimetry toward B1 shows polarized continuum Matthews & Wilson 2002 emission • Aligned grains and hence a component of the magnetic field in the plane of the sky • No correlation between the polarization angles measured ApJ in optical polarimetry. 574 822 l The polarized emission from the interior is consistent with with OH Zeeman data if the total uniform field strength is ~ 30 µG AY 216 7 Molecular Cloud Cores Myers et al. 1991 ApJ 376 561 AY 216 8 Fragmentation & Collapse: I l Jeans instability—the dispersion relation for a perturbation dr ~ ei(wt - kx) is 2 2 2 2 2 2 w = c (k - kJ ), kJ = 4"Gr0/c 2 2 • k - kJ < 0 makes w imaginary • Exponential growth when k < kJ • Growth rate, -iw, increases monotonically with decreasing k + Longest wavelength perturbations (largest mass) grow fastest + Fastest collapse of the largest scales suggests fragmentation unlikely (Larson 1985 MNRAS 214 379) AY 216 9 Fragmentation & Collapse: II l The Virial Theorem—dot Euler equation with r and integrate over V r r 0 3P dV r dm P r dS = Ú - Ú ( ⋅ —f ) - Ú ext ⋅ V V S 3 GM 2 0 = 3c 2 M - - 4pR3P 5 R ext Pext † V AY 216 10 Virial Theorem l There is a minimum radius where the cloud can no longer support itself against gravity 3c 2 M 3 GM 2 P = - 4pR3 20 pR4 P P~1/R3 † R Rmax l Beyond the critical point self-gravity decreases R without any help from P l Equilibria states which require P to decrease with decreasing R are unstable AY 216 11 Stable and Unstable Virial Equlibria l For a given P there are two equilibria P A B R Rmax l Squeeze the cloud at • Requires more pressure to confine it at its new radius • Re-expand (stable) l Squeeze the cloud at A • Requires less pressure to confine it AY 216 • Contract (unstable) 12 Non-Thermal Pressure l Magnetic fields are difficult to measure in cloud cores • OH absorption measurements are Stokes I Crutcher difficult because of the small chance 1667 MHz of a background radio source 1665 MHz l Single measurement of |B| cos q ≈ - 1993 19 ± 4 µG towards B1 ApJ 3 -3 • OH probes nH ≈ 10 cm Stokes V 2 5 -3 407 175 l B /8" ~ 3 x 10 K cm • Comparable to the thermal pressure of a 10 K core with n ~ 104 cm-3 AY 216 13 Magnetized Cloud l Assume a uniformly magnetized cloud 3 GM 2 1 0 = 3c 2 M - + R3B2 - 4pR3P 5 R 3 Magnetic flux is F = pR2B = const. for a good conductor • Gravitational and magnetic terms ~ 1/R i.e., they remain in constant proportion † AY 216 14 Protostellar Accretion 3 l dM/dt ~ c /G l Mo = 0.01 M§ Ro= 3.5 R§ -5 -1 l dM/dt = 10 M§ yr for 105 yr • Accretion shut off at 1 M§ 1 R • Gas photosphere cools § at constant R for ~ 1 day 100 R • Loiters for ~ 3000 yr on 5 R 1500 R § 2 the D main-sequence § § • Followed by Hayashi contraction AY 216 15 T Tauri Stars & Herbig Ae/Be Stars l T Tauri stars have long PMS evolution HAeBes • 10-100 Myr l Herbig Ae/Be stars are 2-10 M§ • tPMS < 10 Myr l For M > 5 M§ there is no PMS phase • Birthline indicates approximate location where YSOs become visible T Tauri AY 216 16 The Four Stages of Star Formation 1 2 4 Shu et al. (1987 3 ARAA 25 23) AY 216 17 SED Classification Class I Infall with simultaneous bipolar outflow Class II Visible T Tauri stars with disks & winds Class III Circumstellar material accreted or dissipated, leaving a pre-main-sequence star, possibly with planets AY 216 18 Inside-Out Collapse in B335 Choi et al. 1995 ApJ 448 742 AY 216 19 The HH 211 Outflow • 1.3 mm continuum • Circumstellar disk • 12CO 2 – 1 • Molecular outflow • Top: |v| < 10 km/s • Bottom: |v| > 10 km/s • H2 2.12 µm • Shocks • Gueth & Guilloteau 1999 A&A 343 571 AY 216 20 Evolving Circumstellar Environment l Debris disk studies suggest that the quantity of circumstellar material declines rapidly with age: M µ t -2 AY 216 21 OB stars in Orion M42 l 450 pc l t < 1-2 Myr l 3500 stars within 2.5 pc radius • 0.1 - 50 M§ • 900 M§ l A few O stars • Lots of low mass stars too (Hillenbrand 1997 AJ 113 1733) AY 216 22 High Mass Stars l Evolutionary time scales for OB stars are << for low-mass stars • OB stars modify their environment soon after a stellar core has formed since their Kelvin-Helmholtz time scale 2 tKH = E/L ~ GM /LR < 104 yrs for an O star l Core H-burning begins before accretion of matter from the surrounding protostellar envelope stops l When the protostar reaches the ZAMS, it begins producing appreciable UV flux and (possibly) a strong wind modifying the surrounding conditions, structure, & chemistry l The new OB star ionizes its surroundings, giving rise to a small region of ionized gas, usually referred as the ultracompact HII (UCHII) region phase • UCHII regions have diameters < 0.05 pc • Compact HII regions have sizes in the range 0.05-0.5 pc AY 216 23 Properties of Massive Stars l Massive stars = O & B stars l Massive stars are OB stars which ionize HII regions Spectral Mass MV Teff log10 -1 Type M§ K (Q / g s ) O5 60 -5.6 48,000 49.7 O8 23 -5.2 33,500 48.6 B0 18 -4.4 30,000 47.7 AY 216 24 Dense Gas & Massive Stars l The hallmark of newly formed OB stars are NH compact and ultra- 3 compact HII regions UCHII l Intimately associated with warm and dense regions of molecular gas • G9.62+0.19 + UC HII region + NH3 hot core (dashed) + 1.3 cm continuum emission (solid) + H2O (p) & OH (o) masers (Kurtz PPIV) AY 216 25 Morphology of Compact HII Regions l High angular resolution Bipolar radio continuum Shell observations show a variety of morphologies • UC HII regions in radio continuum • Shell source G45.07+0.13 (Turner & Matthews 1984) • Bipolar source NGC 7538 (Campbell 1984). Cometary • Cometary source G34.3+0.15 (Gaume et al. 1994). AY 216 26 Statistics of UCHII Regions l Why do we see so many UC HII regions? l IRAS point source catalog & a far-IR color criterion selects UC HII • Wood & Churchwell (1989) counted 1650 Galactic UC HII • If these objects have physical parameters similar to a much smaller number of radio-observed UC HII then + R ~ 0.05 pc 48 -1 + NU ~ 4 x 10 s 5 -3 + n0 ~ 10 cm • Dynamical ages are typically ~ 5000 yr -1 l Implies ~1650/5000 = 0.3 O stars yr l Considerably larger than that estimated from other means -1 • Typical estimates give 0.82 M§ yr (10 < M/M§ < 60) (Güsten & Mezger 1983 or Downes 1987) -1 • An IMF-weighted of <MOB> 23 M§ implies ~ 0.04 O stars yr • 10 times smaller than that derived from UC HII regions AY 216 27 Champagne Flows l “Champagne flow” or blister models assume that the massive star is born in a medium with large density gradients • The star evacuates a cavity of ionized gas • At the boundary of the cloud the ionized cavity breaks out creating a flow of gas away from the cloud due to the pressure gradient between respectively the cloud and the ambient ISM AY 216 28 Bow Shocks l Cometary HII regions may be stellar wind bow shocks of an ionizing star moving supersonically through a molecular cloud (Van Buren et al. 1990; MacLow et al. 1991) • Bow shocks explain details of the morphology + Limb brightening + Emission pinches back down to the symmetry axis u Not easily explained within the champagne models AY 216 29 30 Hollenbach et al. (1994 ApJ 428 654) isks D * 2 c GM = g r hotoevaporating † P AY 216 A High-Mass Protostar with a Massive Rotating Disk l The high-mass Class 0 protostar NGC 7538S has a massive rotating disk and outflow 13 l Resolved with the BIMA in the 3.4 mm continuum and in H CN • Nearly edge-on and has a size of 30,000 AU • Outflow perpendicular to the rotating disk is mapped in SiO and HCO+ 13 l H CN velocity gradient is consistent with Keplerian rotation orbiting a ~ 40 M§ central object 4 l The mass of the continuum "disk" is 100 M § and has a luminosity of 10 L§ 13 l H CN gives 400 M§ for the rotating disk and of 1000 M§ for the extended envelope + H13CN contours/3.4 mm continuum HCO outflow AY 216 31.
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