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Outflows from High Mass Protostars

Outflows from High Mass Protostars

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Outflows from High Mass Protostars

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  1. Outflows from High Mass Protostars Debra Shepherd National Radio Astronomy Observatory Cores, Disks, Jets & Outflows in Low & High Mass Star Forming Environments Observations, Theory & Simulations Banff, Alberta, Canada - July 12-16, 2004

  2. Contents Motivation – why study outflows from Young Stellar Objects Review of Outflow Energetics Observational summary: Outflows from Mid- to Early-B (Proto)stars Outflows from young O Stars Impact of Luminosity on Outflows Impact of Mass-loading and Disk Turbulence on Outflows Summary

  3. Motivation Outflow & infall dynamics affect: Energy input & turbulent support of molecular clouds Dissipation of molecular clouds Final mass of the central star Disk (and planet) evolution Outflows provide a fossil record of mass-loss history of a protostar (or protostellar cluster). Outflow orientation establishes that a velocity gradient in circumstellar material (e.g. masers, dense gase) is due to a disk. Why massive protostellar outflows may differ from lower-mass flows: OB stars usually form in clusters  expect dynamical interactions Expect massive YSO to evolve to ZAMS more rapidly: O stars: tevolve ~ few x 104 yrs, G star: tevolve ~ few x 107 yrs OB stars reach ZAMS while still embedded  increased radiation pressure may affect outflow dynamics

  4. . . (Yorke 2003) Kelvin-Helmholtz time scale (time to reach ZAMS): tKH= GM 2/RL Accretion time scale: tacc = M* /Macc For M* ~ 8 M tacc = tKH And for M* > 8 M the star reaches the ZAMS while still accreting – ionizing radiation affects outflow & infall Massive vs Low-Mass Protostars . Ae

  5. . O9 O8 O5 O6 O7 B0 Critical Macc at which all stellar UV photons are absorbed by in-falling matter plotted against spectral type (Walmsley 1995, Churchwell 2002): -4.0 -4.2 Log Mcrit (M yr -1) -4.4 -4.6 -4.8 -5.0 -5.2 . 50 40 45 35 30 Teff (1000 K) Formation Mechanisms Disk accretion: Sufficiently large, non-spherical accretion rates can overcome radiation pressure (Behrend & Maeder 2001, Yorke & Sonnhalter 2002, Tan & McKee 2002). Isolated and cluster formation possible. Disk  linked accretion & outflow may be similar to low mass protostars, e.g., x-winds (Shu et al. 2000) & disk winds (Konigl & Pudritz 2000). . ..

  6. . Formation Mechanisms Mergers destroy accretion disks around lower mass components and disrupt their outflows. Resulting massive star will likely have rotating circumstellar material but accretion is not a necessary criteria for formation. Coalescence: coalescence of stars/protostars with masses below the critical value of 8 M (Bonnell & Bate 2002). Radiation pressure no longer a problem. Requires cluster formation only.

  7. . . . Ef Pf Mf LZAMS Lacc Outflow Energetics . • Independent studies establish correlations like M ~ Lbol0.6 for: • The bipolar molecular outflow rate • The mass accretion rate • The ionized mass outflow rate • For Lbol = 0.3 to 105 Lsun  strong link between accretion & outflow for most Lbol • e.g. Cabrit & Bertout 1992, Shepherd & Churchwell 1996, Henning et al. 2000

  8. . . Pf Mf Lbol Mcore Lacc LZAMS Lbol Mout Mcore More recently Beuther et al. (2002) added new massive outflows to correlations: Outflow Energetics . Mechanical force,Pf vs Lbolcorrelation holds Mf vs Lbol correlation may be an upper limit May be a function of the entrainment efficiency? Mout ~ 0.1 Mcore0.8(derived from 1.2 mm dust emission) Dust emission increases with protostar age. (see also Sarceno et al. 1996, Chandler & Richer 2000, Richer et al. 2000) .

  9. CO Outflows from low and high mass stars show a mass-velocity (MV) relation in the form of a power law dM(v)/dv ~ vg with g ranging from -1 to -8; g steepens with age and energy in the flow (e.g. Rodriguez et al. 1982; Lada & Fich 1996; Shepherd et al. 1998; Richer et al. 2000; Beuther et al. 2002). A similar relation of H2 Flux-velocity also exists with g between -1.8 and -2.6 for low and high mass outflows (Salas & Cruz-Gonzalez 2002): Outflow Energetics H2Vbreak versus outflow length, l, correlation: g steepens and Vbreak increases for older flows - similar to molecular flows. Vbreak differs but overlap where g(H2 ) ~ g(CO) association between CO & H2 in molecular outflows. Vbreak ~ l 0.4 g ~ l 0.08

  10. . . Jet Knot Spacing: T Tauri stars: Knot spacing = 500-1000 AU, vjet ~ 300 km/s  timescale = 10-20 yrs HH 80-81: Knot spacing = 2000-3000 AU, vjet ~ 500-1400 km/s  timescale = 10-30 yrs Cautionary note: HH 30 knots are ejected every few years and knots appear to merge after a few years. (Stapelfeld et al. in prep). Thus, knot spacing at large distances from the central source may be more a function of the evolution of the jet structure and excitation rather than an intrinsic property of the star/disk system. Ionized wind mass loss rates: Determined from optical lines [SII] & [OI] for T Tauri stars and radio emission for massive protostars up to early B spectral type. Ionized Jet Energetics Cabrit (2002), Rodriguez et al. (1994), Marti et al. (1995), Shepherd & Kurtz (1999)

  11. Ionization fraction in jets: Collimated jets from low-mass sources appear to be mostly neutral (e.g. Bacciotti & Eisloffel 1999; Bacciotti, Eisloffel, & Ray 1999; Lavalley-Fouquet, Cabrit, & Dougados 2000). Ionization fraction xe ~ 0.01 to 0.1 within 50 AU from source. Molecular jets from low-mass Class 0 sources have sufficient momentum to drive molecular flows (e.g. Richer et al. 1989; Bachiller et al. 1991; and discussion in Cabrit 2002) The SiO jet from one early B protostar, IRAS 20126, may also have adequate momentum to power the larger-scale CO flow but uncertainties in assumed SiO abundance makes this difficult to prove (Cesaroni et al. 1999; Shepherd et al. 2000): SiO: Pwind ~ 2 x 10-1 2 x 10 -9 Msun km/s/yr [SiO/H2] CO: Pwind ~ 6 x 10-3 Msun km/s/yr Ionized Jet Energetics . .

  12. K band reflection nebula GGD 27 ILL IRAS 18162-2048, GGD 27, HH 80-81 Yamashita et al. (1989) Aspin et al. (1991) Marti, Rodriguez, Reipurth (1993, 1995) Gomez et al. (1995, 2003) Stecklum et al. (1997) Benedettini et al. (2004) B star cluster with Lbol ~ 2 x 10 4Lsun Tdyn ~ 10 6 yrs Mf ~ 570 Msun Mf ~ 6 x 10 -4 Msun yr -1 GGD 27 ILL powers jet & illuminates reflection nebula, Sp. type < B1 CO opening angle > 40o Collimated, ionized jet, no apparent UC HII region CO red & blue-shifted emission 6 cm continuum Young Early B Stars HH 80 North HH 81 HH 80 . POSTER J7: Gomez et al. K band & 8.5 mm K band & 6 cm IR nebula GGD 27 ILL B2 star  Later than B3?

  13. G192.16-3.82 Indebetouw et al. (2003) Devine et al. (1999) Shepherd et al. (1998,1999,2001) Lbol ~ 3 x 10 3Lsun Tdyn ~ 2 x 10 5 yrs M2.6mm ~ 10 Msun Mf ~ 95 Msun Mf ~ 6 x 10 -4 Msun yr -1 Pf ~ 4 x 10 -3 Msun km s -1 yr -1 . . K-band 7 mm continuum & model Young Early B Stars YSO CO(1-0) B2 ZAMS star with UC HII region 50o-90o-45o opening angle outflow Collimation consistent with wind-blown bubble Evidence for 100AU accretion disk, 1000AU rotating torus NH3 core not graviatationally bound – near end of accretion phase [SII]

  14. W75 N B Star Cluster Torrelles et al. (2004) Shepherd et al. (2003, 2004) Combined outflows: Lbol ~ 4 x 10 4Lsun (combined) Tdyn ~ 2 x 10 5 yrs M2.6mm ~ 340 Msun Mf ~ 165 Msun Mf ~ 10 -3 Msun yr -1 Pf ~ 2 x 10 -2 Msun km s -1 yr -1 . . 2 cm 6 cm H2O masers Young Early B Stars jet VLBA H2O maser proper motions  Wide-angle flow B0.5 – B2 ZAMS stars Jet seen in ionized gas & water masers from YSO (unknown spectral type) Wide-angle outflow from B2 ZAMS star with UC HII region

  15. POSTER J38: Rosen – rotating molecular jets POSTER J16: Lebron et al Lebron et al. (in prep) Cesaroni et al. (1999,2004, in prep) Hofner et al (1999,2001, in prep) Moscadelli et al. (2000, in prep) Shepherd et al. (2000) Zhang et al. (1998, 1999) Lbol ~ 10 4Lsun Tdyn ~ 2x10 4 yrs M2.6mm ~ 50 Msun Mf ~ 50-60 Msun Mf ~ 8 x 10 -4 Msun yr -1 Pf ~ 6 x 10 -3 Msun km s -1 yr -1 B0.5 Protostar (not ZAMS but cm continuum detected) Precessing jet may create wider angle CO outflow Evidence for 2000 AU rotating torus & 10,000 AU rotating NH3 core IRAS 20126+4104 Early B Protostars Ha & CO(1-0) . . . Estimated jet full opening angle q ~ 40o (based on SiO emission) – wider than typically observed in low-mass systems (q ~ 1-2o) CO(2-1) & 1mm cont

  16. 3 or 4 early B protostars (not on Main Sequence yet) CO & SiO outflows are well collimated No cm continuum emission detected – perhaps accretion is so large that UC HII region is quenched? Thus, effects of stellar UV radiation field absent or minimized. IRAS 05358+3543 B Star Cluster Beuther et al. (2002, 2004) Sridharan et al. (2002) Combined outflows: Lbol ~ 6 x 10 3Lsun (combined) Tdyn ~ 3-4 x 10 4 yrs M1.2mm ~ 75 - 100 Msun (near each protostar) Mf ~ 20 Msun (combined) Mf ~ 6 x 10 -4 Msun yr -1 Early B Protostars . Collimated flows are produced by early B protostars.

  17. 2.2 mm image, 13CO contours trace disk. Disk seen in silhouette Outflow M 17 new massive YSO Chini et al. (2004) Lbol ~ 6 x 10 4Lsun (B0 – O9 star, 20 Msun) Mdisk > 110 Msun Rdisk ~ 20,000 AU (largest known) Mf ~ unknown Early B to Late O Protostars Outflow opening angle > 120o based on reflection nebula (molecular outflow not mapped). Spectra of central source (Ha, Ca II, and He I)  ongoing accretion. Massive accretion disks (Mdisk/M* > 5) exist around stars as early as late O type  turbulent disks, linked accretion/outflow. POSTER C22: Nuernberger on the M17 disk POSTER D15: Yamashita et al. on the M17 disk See also POSTER D2: Beltran et al. on massive disks

  18. HH 80-81 like jets may be possible in more massive stars (up to late O spectral type) IRAS 16547-4247 Brooks et al. (2003) Garay et al. (2003) Lbol ~ 6 x 10 4Lsun (B0 – O9 star assuming single central star) Mcore ~ 900 Msun Mf ~ unknown Early B to Late O Protostars 2.12 mm image 1.2 mm continuum contours 8.4 GHz contours Centimeter continuum consistent with a thermal jet. Synchrotron emission farther out in jet  strong B field collimating ionized gas. Spectra show HV molecular gas (v ~ 20 km/s) but outflow not mapped yet. Assuming v = 1000 km/s, inner synchrotron knots were ejected ~ 140 years ago. See also POSTER J5: Davis et al. - jets from massive YSOs (IRAS 18151-1208) and Poster J27: Wolf-Chase et al. - search for jets near HM YSOs

  19. POSTER C9: Forster – Infall/Outflow in massive cores Poster C16: Klein et al. – Protostar Cores in outer Galaxy W75 N G192.16 HH 80-81 IRAS 20126 IRAS 05358 (Yorke 2003) Location of early B stars with outflow in M vs t plot: Outflows from Early B (proto)stars ZAMS stars No jet with UC HII regions (W75N, G192) Jet (HH 80-81) Ae Not on ZAMS – little or no ionizing radiation from protostar, jet-like molecular outflows

  20. O6 (proto)star C34S outflow detected along axis of UC HII region expansion – outflow affecting the ionized gas? No accretion disk. Star located in a 10,000 AU dust-free cavity 3.6 cm HII region expansion Young O Stars C34S(J=3-2) Red-shifted But – new observations suggest a more complicated picture G5.89-0.39 Cesaroni et al. (1991) Acord et al. (1997, 1998) Faison et al. (1998) Feldt et al. (1999) Lbol ~ 3 x 10 5Lsun Tdyn ~ 3 x 10 3 yrs (very young) Mf ~ 80 Msun Mf ~ 3 x 10 -2 Msun yr -1 Blue-shifted .

  21. Young O Stars POSTER J12: Klaassen et al. POSTER C51: Sollins on G5.89 Feldt et al. (2003): ~ O5 star detected, no disk-like structure but small excess 3.5 mm emission  circumstellar material. G5.89-0.39 New Results Sollins et al. (2004): SiO(5-4) outflow opening angle ~ 90o. Outflow axis does not match axis found in C34S or CO/HCO+. Dust continuum extension along SiO outflow axis. Multiple flows? H, K, & L’ NIR image Watson et al. (2002): CO outflow larger than SiO. Outflow roughly ^ to UC HII region expansion. Lbol ~ 3 x 10 5Lsun Tdyn = 7.5 x 10 3 yrs Mf > 77 Msun Mf > 10 -3 Msun yr -1 .

  22. POSTER C2: Beuther – sub-mm lines & continuum in Orion O Protostars Orion H2 ‘fingers’  Explosive event forming fragmented stellar wind bubble (McCaughrean & Mac Low 1997) Or a precessing flow(Rodriguez-Franco et al. 1999) Subaru J, H, K Images

  23. ? POSTER J21: Satoko – SiO in Orion H2 and HV blue-shifted CO Orion Snell et al. (1984) Plambeck, Wright, Carlstrom (1990) Dougados et al. (1993) Menten & Reid (1995) Chernin & Wright (1996) Greenhill et al. (1998, in prep) Doeleman et al. (2004) And many others Lbol >10 4Lsun Tdyn ~ 1.5 x 10 3 yrs (very young) Mf ~ 8 Msun Mf ~ 5 x 10 -3 Msun yr -1 O Protostars . SiO masers Assuming a single source powers the outflow: Outflow opening angle ~ 90o – 120o. Low collimation even at highest CO velocities. Elongated emission seen in 7mm continuum within 25 AU of protostar (source I)  disk or outflow? SiO masers trace outflow opening angle and slow equitorial flow.

  24. Blue: 3.6mm green: 4.5mm orange: 5.8mm red: 8mm POSTER S8: Smith et al. – Spitzer images of DR21 DR21 outflow powered by mid-IR cluster of OB stars, most have no circumstellar material. A newly discovered O star DR21:IRAC-4 appears to have a hot, accreting envelope: Lacc > L* DR21 Roelfsema et al. (1989) Lbol ~ 3 x 10 5Lsun Garden et al. (1991) Tdyn > 5 x 10 4 yrs Davis & Smith (1996) Mf ~ 3000 Msun Smith et al. (2004, in prep) Mf < 6 x 10 -2 Msun yr -1 O Protostars .

  25. Characteristics of OB outflows (and accretion disks). A few references not already mentioned (there are MANY more): A few recent significant contributions • Billmeier, Jayawardhana, Marengo, Mardones, Alves (POSTER D3) – Mid-IR imaging of Massive Young Stars • Forster (POSTER C9) – Infall & Outflow in Massive Cores • Kim, Churchwell, Friedel, Sewilo (POSTER J11) – Molecular Outflows from Massive Stars • Varricatt, Davis, Ramsay-Howat, Todd (POSTER J23) – A Near Infrared Imaging Survey of High Mass Young Stellar Candidates • Beuther, Schilke, Sridharan, Menten, Walmsley, Wyrowski (2002) – Massive Molecular Outflows • Beuther, Schilke, Gueth (2004) – Massive Molecular Outflows at High Spatial Resolution • Henning, Schreyer, Launhardt, Burkert (2000) – Massive YSOs with Molecular Outflows • Molinari, Testi, Rodriguez, Zhang (2002) – The Formation of Massive Stars. I. High-Resolution Millimeter and Radio Studies of High-Mass Protostellar Candidates. • Ridge et al. (2002) – Massive Molecular Outflows • Shepherd, Testi, Stark (2003) – Clustered Star Formation in W75N • Su, Zhang, Lim (2004) – Bipolar Molecular Outflows from High-Mass Protostars • Zhang, Hunter, Brand, Sriharan, Molinari, Kramer, Cesaroni (2001) – Search for CO Outflows toward a Sample of 69 High-Mass Protostellar Candidates: Frequency of Occurrence

  26. Expect high luminosity of massive protostars to play an important role in • Dynamical Evolution • Outflow (& disk) thermal structure and morphology • Review by Konigl (1999) summarizes the issues: • Enhanced field-matter coupling near disk surface due to UV radiation may cause higher accretion rates and mass outflow rates (e.g. Pudritz 1985). • Disk photo-evaporation could create a low-velocity disk outflow (e.g. Hollenbach et al. 1994; Yorke & Welz 1996). • Radiation pressure higher for dusty gas (e.g. Wolfire & Cassinelli 1987), may contribute to flow acceleration and also lead to the ‘opening up’ of the outflow streamlines (e.g. Konigl & Kartje 1994). • Expect a strong, radiatively driven stellar wind (although momentum factor of 10-100 too low to power observed molecular outflow). • Consider an example of how radiatively-driven stellar winds could potentially affect outflow dynamics (Yorke & Sonnhalter 2002): • Frequency-dependent opacity calculations – 30Msun and 60Msun molecular cores which collapse to produce a protostar (M* ~ 30Msun& 34Msun, respectively) with disk and radiatively-induced outflow. Impact of Luminosity on Outflow Structure

  27. F30: M* = 31 Msun in 2.4x104 yrs F60: M* = 34 Msun in 4.5x104 yrs Radiatively-Induced Outflows 104 yrs 17 Msun 2.1x104 yrs 31 Msun Density: gray scale & white contours Velocity: arrows Carbon grain temperature: solid black contours Silicate grain temperature: dotted contours Numbers: age & M* 104 yrs 13 Msun 3x104 yrs 34 Msun 1.5x104 yrs 24 Msun 2x104 yrs 28 Msun 2.2x104 yrs 31 Msun 3.5x104 yrs 34 Msun 1.9x104 yrs 29 Msun 2.4x104 yrs 31 Msun 2.5x104 yrs 33 Msun 4.5x104 yrs 34 Msun Stellar radiation softens at outflow-shock boundry  cloud collapses along flow axis to produce well-collimated jet. Radiatively-induced outflow encased by shock fronts, relatively poorly collimated flow. Yorke & Sonnhalter 2002

  28. X-winds: Mass loading impact on outflows . Shu et al. (1994) – ‘X-point’ (outflow launching point) ~ 5 R* in a typical T Tauri star. Hartmann & Kenyon (1996) – FU Ori stars in outburst have no hot UV continuum: magnetospheric accretion columns have been crushed onto stellar surface. Even late B stars have high enough accretion rate to crush accretion columns.  Massive star outflows can not be due to X-winds from truncated disks. X-wind from rapidly rotating protostar (Shu et al. 1998) or pure disk-wind (Konigl & Pudritz 2001).

  29. . • Ouyed & Pudritz (1999) – Magnetized Disk-Wind Simulations: For Mw ~ 10-8 Msun yr -1, disturbances appear to grow producing instabilities & shocks – outflows become episodic. As Mw increase 3 orders of magnitude, jet behavior goes from episodic to steady ejection. Ideal MHD assumed: all solutions re-collimate. • Anderson, Li, Krasnopolsky & Blandford (POSTER D17) – How mass loading from the disk affects structure and dynamics of the wind. Mw = 3 x 10 -5 to 3 x 10 -10 Msun yr -1Degree of collimation increases with mass loading up to ~10 -5 Msun yr -1. • Re-collimation of the wind expected for ideal MHD where trecomb >> tdynamical and plasma and B fields are frozen-in. But, ideal MHD assumptions break down as: • Plasma T and r increase • Turbulence in the disk or wind increases • B field decreases • Not clear what the implications are, will this affect outflow? Mass loading impact on outflows . . Expected for outflows & disks associated with luminous YSOs.

  30. Gravitational instabilities induce spiral density waves; expected to be prevalent if Mdisk > 0.3 M*, (Laughlin & Bodenheimer 1994). Toomre Q stability parameter (Yorke, Bodenheimer & Laughlin 1995): Q = csW / p GS = 56 (M*/Msun)1/2 (Rd /AU) -3/2 (Td /100K)1/2 (S /103 g cm -3) Where cs = local sound speed, W = epicycle frequency of disk, S = disk surface density, Rd = disk radius & Td = disk temperature. For Q < 1 disk susceptible to local gravitational instability and axisymmertric fragmentation. Q = 1-2 disk susceptible to gravito-turbulence (Gammie 2001): Could be a significant angular momentum transport. Early B (proto)stars appear to have Md /M* > 0.3 and can be as high as 1-10. Example: Md ~ 3 Msun , M* ~ 8 MsunAssume Td = 100 K & Rd = 70 AU, then Q ~ 0.5 disk locally unstable, higher angular momentum transport through disk? Will the outflow be less efficient for a given Macc ? Disk turbulence - impact on outflows . . TALK: Matzner – Low-mass star formation: initial conditions and disk instabilities. Irradiation quenches fragmentation due to local instability because disk temperature is raised above parent cloud temperature. POSTER D18: Bodo et al. – Spiral density wave generation by vortices in accretion disks

  31. 2 1 0 vw Log f --- vkep -1 Lbol = Lacc Lbol = ZAMS -2 0 -1 5 2 1 6 4 3 Log Lbol (Lsun ) POSTER: Coffey et al – low mass jet can carry away adequate angular momemtum. Observations (Richer et al. 2000): Outflow Force Fco vw Mw ------------------ = ------------- = f ---- where f = ------ Accretion Force Macc vkep vkep Macc vw f ----- ~ 0.3 (1/1) for X-winds vkep ~ 0.03 (10/1) for Disk winds Decrease in Mw / Macc between Lbol = 1 and 104 Lsun? Errors are too large now to say. Disk turbulence - impact on outflows . . . . .

  32. Deflected Infall? Moutflow ~ few x M* for T Tauri stars Moutflow >> M* for OB protostars. Churchwell (1999) points out that entrainment & swept-up mass do not appear to be able to account for large observed outflow masses. Circulation models: Moutflow > M* easy (Richer et al. 2000). Most infalling material diverted magnetically at large radii into slow-moving outflow along the polar direction, infall proceeds along the equitorial plane (Fiege & Henriksen 1996: Lery, Henriksen, Fiege 1999; Aburihan et al 2001).

  33. Summary • Mid- to early-B protostars and late O protostars have accretion disks & outflows that can be well-collimated. • Once UCHII region forms, associated ionized outflows have strong wide-angle winds. Jet component not detected? Must be verified. • - This would be unlike low-mass flows where there is evidence for a 2-component wind (jet+wide-angle) in older sources (e.g. Arce & Goodman 2002; Solf 2000). • Mid to early O stars: Outflows appear to be poorly collimated. Explosive events  coalescence a possibility in some cases. • Expect changes in outflow dynamics due to increased: • - Luminosity (accretion & stellar) • - Mass-loading onto the wind • - Disk turbulence

  34. Massive Star Formation & Outflow Reviews Cesaroni 2004, in press “Outflow, Infall, and Rotation in High-Mass Star Forming Regions” Churchwell 1998, in The Origin of Stars & Planetary Systems, NATO Science Series 540, p 515 “Massive Star Formation” Churchwell 2002, ASP conf series, 267, 3 “The Formation and Early Evolution of Massive Stars” Garay & Lizano, 1999, PASP, 111, 1049 “Massive Stars: Their Environment & Formation” Konigl, 1999, New Astronomy Reviews, 43, 67 “Theory of Bipolar Outflows from High-Mass Young Stellar Objects” Lizano, 2002, Nature, 416, 29L “Astronomy: How Big Stars are Made” Richer, Shepherd, Cabrit, Bachiller, Churchwell, 2000, in Protostars & Planets IV, p 867“Molecular Outflows from Young Stellar Objects” Shepherd, 2003, ASP conf series, 287, 333“The Energetics of Outflow and Infall from Low to High Mass YSOs”