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Deriving the Physical Structure of High-mass Star Forming Regions

Deriving the Physical Structure of High-mass Star Forming Regions. Yancy L. Shirley. Collaborators: Neal Evans, Kaisa Young, Dan Jaffe, Claudia Knez, & Jingwen Wu. May 2003. SF in the Milky Way. 10 11 stars in the Milky Way Evidence for SF throughout history of the galaxy (Gilmore 2001)

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Deriving the Physical Structure of High-mass Star Forming Regions

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  1. Deriving the Physical Structure of High-mass Star Forming Regions Yancy L. Shirley Collaborators: Neal Evans, Kaisa Young, Dan Jaffe, Claudia Knez, & Jingwen Wu May 2003

  2. SF in the Milky Way • 1011 stars in the Milky Way • Evidence for SF throughout history of the galaxy (Gilmore 2001) • SF occurs in molecular gas • Molecular cloud complexes: M < 107 M0(Elmegreen 1986) • Isolated Bok globules M > 1 M0(Bok & Reilly 1947) • SF traces spiral structure(Schweizer 1976) M51 Central Region NASA

  3. SF Occurs in Molecular Clouds Lupus • Total molecular gas = 1 – 3 x 109 Mo • SF occurring throughout MW disk (Combes 1991) • SF occurs in isolated & clustered modes • SF occurs within dense molecular cores BHR-71 Pleiades VLT

  4. Orion Dense Cores CO J=2-1 VST, IOA U Tokyo Lis, et al. 1998

  5. High-mass Dense Cores Optical RCW 38 • Embedded clusters visible in Near-IR W42 Near-IR Blum, Conti, & Damineli 2000 J. Alves & C. Lada 2003

  6. High-mass Cores : Complexity S106 Near- IR Subaru

  7. Star with M > 100 Mo appear to exist (Kudritzki et al. 1992): How do massive stars (M > few M0) form? Basic formation mechanism debated: Accretion (McKee & Tan 2002) How do you form a star with M > 10 Msun before radiation pressure stops accretion? Coalescence (Bonnell et al. 1998) Requires high stellar density: n > 104 stars pc-3 Predicts high binary fraction among high-mass stars Theories predict dense core structure & evolution: n(r,t) & v(r,t) Observational complications: Farther away than low-mass regions = low resolution Dense cores may be forming cluster of stars = SED dominated by most massive star = SED classification confused! Very broad linewidths consistent with turbulent gas Potential evolutionary indicators from presence of : H2O, CH3OH masers Hot core  Hyper-compact HII  UCHII regions  HII  Star ? High-Mass Star Formation

  8. Hot Cores & UCHII Regions • Hot Cores & UCHII Regions observed in same high-mass regions : W49A VLA 7mm Cont. BIMA DePree et al. 1997 Wilner et al. 1999

  9. Outline • What is lacking is a fundamental understanding of the basic properties of the ensemble of high-mass star forming cores • Texas survey of high-mass star forming cores: • Plume et al. 1992 & 1997 CS line survey • Dust Continuum 350 mm Survey • Mueller, Shirley, Evans, & Jacobson 2002, ApJS • Constrain n( r ), T ( r ) • High-mass cores associated with H20 maser emission • Arectri catalog of H2O maser sources • Plume et al. 1992 & 1997CS survey towards (0,0) position • CS J = 5 -4 Mapping Survey • Shirley, Evans, Young, Knez, Jaffe 2003, ApJS • Dense gas properties

  10. CS Dense Core Survey • CS J=7-6 detected 104 / 179 cores with H2O masers • Plume et al. 1992 • H2O masers trace very dense gas • n > 1010 cm-3 for the 22 GHz 616-523 transition • Low J CO Surveys generally trace lower density gas. • H2O maser positions are known accurately to within a few arcseconds. HII regions and luminous IR sources may not be spatially coincident with dense gas. • Multi-transition study and initial mapping • Plume et al. 1997 • 71 cores detected in CS and C34S J = 2-1, 3-2, 5-4, and 7-6. • 21 of the brightest cores mapped in CS 5-4 • <R> = 1.0 pc, <Mvir> = 3800 Mo • LVG modeling of multiple CS transitions

  11. CO: Molecular Cloud Tracer CO J=3-2 Emission Hubble Telescope CSO NASA, Hubble Heritage Team

  12. CS & HCN Trace Dense Cores CO 1-0 CS 2-1 HCN 1-0 Helfer & Blitz 1997

  13. CS LVG Models • Initially assumed n( r ) and T( r ) = CONSTANT • 40 sources detected in all 4 CS transitions • <log n> = 5.93 (0.23) • <log N> = 14.42 (0.49) • 2-density component model with a filling factor for the dense component • nhigh ~ 108 cm-3 • nlow ~ 104 cm-3 • Typically, very high column densities of low density gas required (<log Nlow> = 16.16) with f ~ 0.2 Plume et al. 1997

  14. 350 mm Survey Mueller, Shirley, Evans, & Jacobson 2002 • 5 nights at the CSO 10.4-m telescope • 51 high-mass (Lbol > 100 Lsun) cores associated with H2O masers (Plume et al. 1992 sample) • 850 pc < D < 14 kpc • All cores also observed in CS5-4 survey (Shirley et al. 2003) • SHARC 350 mm scan maps (4.0 x 2.7 arcmin) • qmb ~ 14 arcsec at 350 mm • 100 arcsec chop throw

  15. 350 mm Images Mueller et al. 2002 W33A G9.62+0.10 M8E 10,000 AU 150,000 AU 50,000 AU W28A2 G23.95+0.16 W43

  16. Submm Continuum Emission • Submillimeter continuum emission is optically thin. The specific intensity along a line-of-sight is given by:

  17. Why must we model ? • Rayleigh-Jeans approximation fails in outer envelope of low-mass cores • hn/k = 44 K at 350 mm • Heating from ISRF is very important in outer envelopes of cores • Radiative transfer is optically thick at short l • Observed brightness distribution is convolved with complicated beam pattern, scanning, and chopping

  18. Radiative Transfer Procedure nd(r) L kn Sn(l) I(b) Radiative Transfer Simulate Obs. Td(r) Nearly orthogonal constraints: SEDMass x Opacity I(b)n(r) Gas to Dust Physical Model n(r) Iterate Observations

  19. Dust Opacity OH = Ossenkopf & Henning 1994 coagulated dust grains

  20. Calculated Temperature Profiles Mueller et al. 2002

  21. Radiative Transfer Models 50,000 AU Mueller et al. 2002

  22. Best-fitted Power Law • Single power-law density profiles fit observations • n( r ) = nf (r / rf) –p • p = - dln n/ dln r • Distribution of power law indices • <p> = 1.8 (0.4) • Similar to distribution of low-mass cores modeled by Shirley et al. (2002) & Young et al (2003) Mueller et al. 2002

  23. Evolutionary Indicators ? Mueller et al. 2002

  24. “Standard” Indicators Mueller et al. 2002

  25. 350 mm Survey Summary • Density and Temperature structure of outer envelope characterized • <p> = 1.8 (0.4) • <n(1000 AU)> is order of magnitude higher than nearby low-mass star-forming cores • Beuther et al. 1.2mm mapping 69 cores: <p> = 1.6 (0.5) • Single power law models fit our sample • CAVEAT:may be contribution from compact components (UCHIIs or disks) within central beam • W3(OH) UCHII may contribute as much as 25% of the central flux assuming optically thick free-free scaled from 3mm flux (Wilner, Welch, & Forster 1995) • <Rdec> = 0.16 (0.10) pc • <Tiso> = 29 (9) K isothermal temperature • Definitive trends lacking for evolutionary indicators • Except perhaps Tbol vs. Lbol/Lsmm • Lbol ranges from 103 to 106 Lsun • SEDs not well contrained in many cases due to lack of Far-IR photometry

  26. CS J = 5 - 4 Survey Shirley et al. 2002 • 63 high-mass star forming cores associated with H2O masers mapped at CSO 10.4m • <D> = 5.3 (3.7) kpc with 28 UCHII regions included • 57 peak positions observed in C34S J=5-4, 9 in 13CS J=5-4 • Over-sampled On-The-Fly mapsin CS J=5-4 • qmb ~ 25 arcsec at 245 GHz • Median peak integrated intensity S/N = 40 • 10 arcsec binned maps • Provide consistent sample from which to determine the properties of the deeply embedded phase of high-mass star formation

  27. CS Rotational Transitions • Heavy linear molecule with many rotational transitions observable from the ground • J = 5 - 4 transition good probe of dense gas: • mb = 1.98 Debye • nc(10K) = 8.8 x 106 cm-3 • neff(10K) = 2.2 x 106 cm-3

  28. CS J=5-4 Survey S158 Shirley et al. 2003 G19.61-0.23 M8E S231 W44 S76E

  29. CS J=5-4 vs. Dust Continuum • CS J=5-4 is an excellent tracer of dense gas in high-mass star forming regions Shirley et al. 2003

  30. Deconvolved Size vs. p • Convolution of a Gaussian beam pattern with a power law intensity profile yields a deconvolved source size that varies with p Shirley et al. 2003

  31. Optical Depth Effect on Linewidth • C32S is typically optically thick, therefore must use rare isotope (C34S) in linewidth sensitive calculations Shirley et al. 2003

  32. Linewidth-Size • Weak correlation with best fit: Dv ~ r0.3 • C34S linewidth 4x larger than predicted linewidth fromCasselli & Myers (1995) indicating high turbulence: <Dv(C34S)> = 5.0 (2.0) km/s Shirley et al. 2003

  33. Size, Mass, & Pressure • Median core size: R = 0.32 pc • Alternatively Rn = 0.40 pc • Median projected aspect ratio: (a/b) = 1.2 • Median virial mass: Mvir = 920 M0 corresponding to S = 0.6 g cm-2 • Corrections for p and Dv broadening necessary • Mean mass per OB association ~ 440 M0(Matzner 2002) • Median pressure <P/k> = 1.5 x 108 K cm-3 Shirley et al. 2003

  34. Virial Mass vs. Dust Mass • The virial mass is consistently higher by a factor of 2 to 3 than the mass determined from dust continuum modeling. • Uncertainty in dust opacity may account for difference Shirley et al. 2003

  35. Cumulative Mass Spectrum • Slope of mass spectrum similar to IMF and distribution of OB associations G ~ -1.1 (0.1)(Massey 1995) Shirley et al. 2003

  36. Luminosity and Mass Shirley et al. 2003

  37. CS J=5-4 Survey Summary • CS J=5-4 is an excellent tracer of dense gas in high-mass star forming cores • Aspect ratios consistent with spherical symmetry • Median size of 0.32 pc and median virial mass of 920 Msun • Virial mass a factor of 2 to 3 larger than dust-determined mass • Cumulative mass spectrum G ~ -0.9 similar to IMF of OB associations • High median pressure of 1.5 x 108 K cm-3 ameliorates the lifetime problem for confinement of UCHII regions • L/M is 100x higher than estimates from CO and has a smaller dispersion • L/M 2x higher for cores with UCHII and/or HII regions • Lbol strongly correlates with Mvir. Combined with low dispersion of L/M perhaps indicates that mass of most massive star is related to the mass of the core

  38. High Mass Pre-protocluster Core? • Have yet to identify initial configuration of high-mass star forming core! • No unbiased surveys for such an object made yet • Based on dense gas surveys, what would a 4500 M0, cold core (T ~ 10K) look like? • Does this phase exist? Evans et al. 2002

  39. Conclusions & Future Work • Initial characterization of n( r ) indicates a power law density structure of outer envelope • CS J=5-4 traces dense gas properties associated with star formation • CS J=7-6 + HCN & H13CN J=3-2 Mapping Survey(Texas Thesis projects of Jingwen Wu & Claudia Knez) • Radiative transfer modeling of dense gas & v( r ) • Combination of bolometer camera + interferometric dust continuum imaging with radiative transfer modeling is a powerful diagnostic of the density & temperature • How much emission is coming from a compact component within central beam? • SMA & ALMA submm continuum needed! • SOFIA & SIRTF needed to improve SED

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