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Ionized gas in massive star forming regions Guido Garay Universidad de Chile

Ionized gas in massive star forming regions Guido Garay Universidad de Chile. Great Barriers in High-Mass Star Formation. Townsville, September 15, 2010. Aim. Review the characteristics of sources of ionized gas within massive star forming regions.

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Ionized gas in massive star forming regions Guido Garay Universidad de Chile

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  1. Ionized gas in massive star forming regionsGuido GarayUniversidad de Chile Great Barriers in High-Mass Star Formation Townsville, September 15, 2010

  2. Aim Review the characteristics of sources of ionized gas within massive star forming regions. Depending on the origin of the gas, we distinguish two type of sources: HII regions and Jets Outline HII regions Origin: molecular gas surrounding a high-mass star Jets Origin: gas from putative star-disk system

  3.  HII regions Source of ionization: UV photons emitted by embedded young luminous high-mass stars.  Physical parameters  Based on their sizes, densities and emission measures, three classes of HII regions have been identified: Class Diameter Density EM Ref. (pc) (cm-3) (pc cm-6) Compact 0.1 < D < 0.5 >103 >106 Mezger et al. (1967) Ultracompact 0.02 < D < 0.1 >104 >107 Wood & Churchwell (1989) Hypercompact D < 0.02 >105 >108 Kurtz (2004) Hoare et al. (2007) Time discovery line  increase in angular resolution, observing frequency and sensitivity.

  4. Rather than discrete, there is a continuous distribution in the value of the parameters Garay & Lizano (1999) (see also Churchwell 2002) HCHII UCHII CHII HCHII UCHII CHII

  5. Rather than discrete, there is a continuous distribution in the value of the parameters HCHII Garay & Lizano (1999) (see also Churchwell 2002) … + more data from recent surveys:  DePree et al. (2004)  Sewilo et al. (2004)  Garay et al. (2006) Murphy et al. (2010) HCHII  HCHII are uncommon

  6.  There is a significant correlation between the parameters How do we explain these trends? Evolutionary sequence Classical model: spherical bubble expanding in a uniform density medium (Spitzer 1978) O7.5 B1 no: ambient density, cs: sound speed Rs: initial Stromgren radius O7.5 Lines indicate model relations for:  no=106 cm-3 and Nu=3x1048 s-1 (upper)  no=106 cm-3 and Nu=3x1045 s-1 (lower) B1

  7. This simple dynamical model   Massive stars are born in a high density ambient medium. Densities are similar to those of hot molecular cores  Hot cores are the precursors of UCHII regions.  HII regions reach pressure equilibrium with ambient medium in a time scale of a few 104 yrs. Age of compact HII regions could be much larger than this value.  Hypercompact are the youngest, smaller and denser versions of UCHII regions.  They should give us information about the process of high-mass star formation in the earliest evolutionary stages. • : single star, no dust (see Churchwell’s talk for dust considerations)

  8. Hypercompact HII regions  Characteristics of their large scale (~1 pc) surroundings Dust continuum and molecular line observations in high density tracers HCHII are found inside massive and dense cores. e.g., IRAS 16272-4837 Massive and dense cores  Very dark even at IR (IRDCs) 8 μm MSX 1.2mm dust continuum  Physical parameters: R ~ 0.4 pc M ~ 4x103 M n(H2) ~ 6x105 cm-3 Δv ~ 6 km s-1  Highly centrally condensed n  r -1.5 1 pc Massive and dense core

  9. Dynamical state:  Most in virial equilibrium  Few undergoing large scale inflow motions e.g., IRAS 16547-4247 Optically  thick lines Optically  thin lines large scale infalling motions Massive and dense core undergoing intense accretion phase  Vinf ~1 km s-1 Minf ~ 1x10-3 M yr-1 About thirty massive dense cores known with infalling motions Snell & Loren 1977, Welch et al. 1988, Garay et al. 2002, 2003, Wu & Evans 2003

  10.  Where are HCHIIs located within massive and dense cores? IRAS 13291-6249 IRAS 15520-5234 IRAS 17016-4124 1 pc 1 pc 1 pc Images: 4.8 GHz emission (HCHII region) Contours: 1.2-mm emission (Massive core)  HCHII regions typically found at the center of massive cores Whether massive stars are formed at the center or migrate there, is still an open question.

  11.  Continuum spectra Due to their high emission measure HCHIIs are expected to have turnover frequencies, νto , greater than 10 GHz. Below νto, HCHII regions frequently show power-law spectra over a wide frequency range, Sνν , with  typically ~1. =1.0±0.1 Franco et al. (2000) Range of power-law too wide to correspond to the transition from optically thick to optically thin regimes in a constant density region.

  12. Possible explanations for the power-law: • HCHIIs possess density gradients. • For a region in which the electron density goes as n  r -β • Flux density depends with ν as Sν ν, with =(4β-6.2) /(2β-1) • Angular size depends with νas θννγ, with  = -2.1/(2β-1) e.g. HCHII G28.20-0.04: Sν ν1.1  n  r -2.8 (  ν-0.5) Shell model: β = 2.8 Ri = 0.0063 pc Ro = 0.055 pc ni = 6x105 cm-3 Is the expected size dependence with ν actually observed?  e.g., Avalos et al. (2005)

  13.  HCHIIs are hierarchically clumped structures. • Ensemble of clumps with a distribution • of optical depths produce: •  Power-law spectral index covering • a wide frequency range • No dependence of angular size with frequency Ignace & Churchwell (2004)

  14. Caveat: Contribution from dust and free-free emission at frequencies of ~50 GHz can be of the same order, affecting the spectral index interpretation. e.g., G75.78+0.34-H2O a = + 1.4 Franco et al. (2000)  n  r -4  Cte. density HCHII region + hot dust disk Kurtz (2010)

  15.  Radio recombination lines HCHII regions often have broader line widths than UCHII regions. vCHII≈30 km s-1 vHCHII> 40 km s-1 Origin of line broadening? Possible mechanisms:  Large-scale organized motions: rotation expansion infall  Pressure broadening v  n7.4 ne

  16. High angular resolution observations indicate that ordered motions are present. e.g. G28.20-0.04 N H53a H92a H92a v 74 km s-1 Sewilo et al. (2008) Velocity gradient of 103 km s-1 pc-1.  Rotating torus with a velocity of 5 km s-1 at 0.005pc Sewilo et al. (2004)

  17.  G5.89-0.39 Acord et al. (1998) VLA observations in two epochs with Δt = 5 years,  Expansion motions with a velocity of about 35 km s-1. In most cases the bulk motions are not able to explain the observed linewidths.

  18.  Pressure broadening seems the most important contributor. e.g. W51e2 Line v ne (km s-1) (cm-3) H66 50.9 2.2e6 H53 32.5 4.7e6 H30 26.8 --- H66 H53 H30 For G28.20-0.04 N: Line v ne (km s-1) (cm-3) H92 74 .4 3e5 H76 57.6 9e5 H53 39.7 7e6 In addition to the high densities, the RRL observations indicate the presence of density gradients ( at the higher frequency seeing deeper into the region).

  19.  Models  Accretion flow (Keto 2002) Ingredients:Gravity + Ionization Two characteristic radii Ri : radius of ionization equilibrium Rg : gravitational radius (escape radius for ionized gas) If Ri < Rg ionized gas can not expand gravitationally trapped HII region However, Rg < RHCHII 

  20.  Accretion disk (Keto 2007) Ingredients: Gravity + Ionization + Rotation of primordial cloud Third characteristic radius radius at which centrifugal and gravitational forces balance Actual situation depends on the relative values of the three R’s. e.g., if Ri > Rd, Rg  radial wind driven by the thermal pressure of the ionized gas.

  21. Photoevaporating disks Ingredients: Keplerian circumstellar disk + luminous YSO MWC 349A Rg = 0.0007 pc , Rd = 0.0015 pc Hollenbach et al. (1994) Lizano et al (1996) Density gradient of photoevaporated wind produces spectral index of ~ 0.8. Lugo et al. (2004)

  22. In the case of low-mass protostars, observations clearly show a Disk-Jet symbiosis. Disk: Protostar grows by accreting from disk. Jet: Carries away angular momentum and mechanical energy from disk into the surroundings, allowing accretion to proceed. If this symbiosis also holds for high-mass protostars, then  Accretion disk + jet Ingredients: Gravity + Ionization + Rotation + Magnetic field

  23.  Jets Jets ≡ Highly collimated ionized flows that emanate from YSOs. Emission mechanism: thermal free-free emission. Detectable as weak radio sources. Source of ionization: UV photons from shocks produced by the impact of neutral collimated wind on the surrounding high density material. Jets are almost always found in the case of low-mass protostars. Thought to be the “base” of the large scale outflow phenomena like the bipolar outflows and HH systems.

  24. Are young massive stars associated with ionized jets? Garay & Lizano (1999) reported a handful of ionized thermal radio jets associated with massive YSOs, all of which have luminosities < 2x104 L. Source Lumin.  S References (L) (GHz) (mJy) Cepheus A HW2 1.0x104 8 10 0.6 Rodríguez et al. 94 IRAS 20126+4104 1.3x104 8 0.2 -- Hofner et al. 99 W75N(B) VLA1 1.5x104 8 4 0.7 Torrelles et al. 97 IRAS 18162-2048 1.7x104 5 5 0.2 Martí et al. 95

  25.  Cepheus A HW2 jet S0.7 -0.6  L = 700 AU M v = 6x10-4 Myr-1 km s-1 Rodriguez et al. 1994 Observed flux density and size dependence with frequency biconical thermal jet

  26. The number of detections has increased during the last decade and detections have been made towards progressively more luminous YSOs. Source Lumin.  S References (L) (GHz) (mJy) G35.2-0.7 N 1.6x104 9 0.4 >1.3 Gibb et al. 03 IRAS18089-1732 3.2x104 9 1.1 0.58 Zapata et al. 06 CRL2136-RS4 5.0x104 9 0.56 1.2 Menten & Tak 04 IRAS 16547-4247 6.2x104 9 6 0.5 Garay et al. 03 G345.494+1.468 7.0x104 9 9 0.85 Guzman et al. 10 G331.512-0.103 2.0x105 9 166 1.1 Bronfman et al. 08

  27. ATCA survey of radio continuum emission toward luminous massive proto-stellar objects 4.8 GHz Thermal jet 0.3 pc 0.07 pc Lobes Triple radiocontinuum sourcetoward IRAS 16547-4247(L = 6104 L) Garay et al (2003) Triple radiocontinuum source toward IRAS 16562-3959 (L = 7104 L) Guzman et al. (2010)  Jets are found associated with luminous YSOs.

  28. Molecular outflows (large scale) are relatively frequent towards high-mass YSOs, however jets are still rare… Possible explanations:  Different formation mechanism?  Obscured by bright CHII region?  Short timescale for jet phase? Bipolar outflows in high mass protostar have dynamic ages of 105 yrs >longer than the K-H time of the jet/disk stage of 104 yrs.  jet may turn off and the large scale outflow will still persist as a fossil for a relatively long time.

  29. Characteristics of jets associated with high-mass YSOs Velocity : 1000-3000km s-1 Size : 0.01 pc Momentum rate: 10-2 - 10-1 Mkm s-1 yr-1 103 times more luminous and energetic than low-mass jets ! High-mass jets Momentum rate Low-mass jets Rodriguez et al. 2007 Jet luminosity  Jets associated with luminous YSOs are powerful

  30. Summary  HII regions  HCHIIs are probes of the earliest phase of evolution of regions of ionized gas excited by young high-mass stars. However, still far from understanding them: density gradients : n  r -2.5 ?  Sν ν1.0 ensemble of clumps ? Disk rotation ?  Broad linewidths Outflows ? Pressure broadening ?  Morphologies ?  Are the high-mass stars exciting HCHII regions formed at the center of massive and dense cores or they migrate there?

  31.  Ionized jets  Jets are found associated with high-mass YSOs (up to luminosities of 2x105L).  They are 103 times more energetic and luminous than low-mass jets.  They are rare. Open questions: Disk wind?  Which is the nature of the driving mechanism? X-wind?  Which are their lifetimes?  Do jets rotate?

  32. To address these questions we need to probe HCHIIs and ionized jets with high spatial resolutions (< 2 AU) and high sensitivities. ALMA

  33.  Variability of radio emission e.g., NGC 7538 IRS1 Lobes show a clear decrease in radio flux density of 20-30% over 12 years. • Possible explanations: • Ionizing photon flux from star is decreasing. • Inyection of fresh gas in core steals ionizing photons for the • lobes Franco-Hernandez & Rodriguez (2004)

  34. Ultracompact HII regions  Spectral energy distribution SEDs of UCHII regions  At ν< 30 GHz ( > 1cm): free-free emission from ionized gas 1 1 ν-0.1 ν2 1 • At ν> 300 GHz ( < 1 mm): thermal dust emission from warm cocoon Kurtz et al. 1994 Modified Planck function

  35.  Morphologies UCHII regions exhibit a variety of morphologies + spherical, irregular, and unresolved morphologies (Churchwell 2002) Cometary 14% Morphologies depend on the characteristics of the exciting star and of the environment, as well as on their interaction. Shell 28% Bipolar 8% Core-halo 16%

  36.  Observational consideration How many massive protostars we expect to see in our Galaxy? Massive stars spend short time in the pre-main sequence: Kelvin-Helmholtz time Rate of massive star formation in the Galaxy:   Massive protostars are very rare

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