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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 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 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 Massive vs Low-Mass Protostars Kelvin-Helmholtz time scale (time to reach ZAMS): t KH = GM 2/RL Accretion time scale: . tacc = M /Macc * Ae For M ~ 8 M . * t acc = tKH And for M > 8 M . * the star reaches the ZAMS while still accreting – ionizing radiation affects outflow & infall (Yorke 2003) 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). -4.0 . Log Mcrit (M . yr -1) Critical Macc at which all stellar UV photons are absorbed by in-falling matter plotted against spectral type (Walmsley 1995, Churchwell 2002): B0 O9 O8 O7 O6 O5 -4.2 -4.4 -4.6 -4.8 -5.0 -5.2 30 35 40 Teff (1000 K) 45 50 Formation Mechanisms . 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. 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. Outflow Energetics . Pf . Mf Lacc . Ef LZAMS . 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 Outflow Energetics More recently Beuther et al. (2002) added new massive outflows to correlations: . Pf . Mf Mcore Lbol Mout Lacc Mcore Lbol LZAMS . Mechanical force, Pf vs Lbol correlation 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) Outflow Energetics 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): Vbreak differs but overlap where g(H2 ) ~ g(CO) association between CO & H2 in molecular outflows. H2 Vbreak versus outflow length, l, correlation: g steepens and Vbreak increases for older flows - similar to molecular flows. Vbreak ~ l 0.4 g ~ l 0.08 Ionized Jet Energetics 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. . . T Tauri jets Mwind (Msun/yr) Massive YSO Jet/wind Mwind (Msun/yr) HH 34 2 x 10-7 Ceph A HW2 8 x 10-7 HH 47 4 x 10-7 HH 80-81 6 x 10-6 HH 111 3 x 10-7 G192.16 1 x 10-6 Cabrit (2002), Rodriguez et al. (1994), Marti et al. (1995), Shepherd & Kurtz (1999) Ionized Jet Energetics 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: CO: . Pwind ~ 2 x 10 -1 2 x 10 -9 Msun km/s/yr [SiO/H2] . Pwind ~ 6 x 10 -3 Msun km/s/yr Young Early B Stars IRAS 18162-2048, GGD 27, HH 80-81 CO red & blue-shifted emission 6 cm continuum HH 80 North 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 Later than B3? K band reflection nebula HH 81 HH 80 POSTER J7: Gomez et al. GGD 27 ILL K band & 8.5 mm K band & 6 cm IR nebula GGD 27 ILL B2 star Young Early B Stars 7 mm continuum & model CO(1-0) K-band G192.16-3.82 YSO 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 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] Young Early B Stars 6 cm 2 cm H2O masers jet VLBA H2O maser proper motions Wide-angle flow 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 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 Early B Protostars IRAS 20126+4104 Ha & CO(1-0) 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) B0.5 Protostar (not ZAMS but cm continuum detected) Precessing jet may create wider angle CO outflow 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 Evidence for 2000 AU rotating torus & 10,000 AU rotating NH3 core 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 POSTER J16: Lebron et al POSTER J38: Rosen – rotating molecular jets Early B Protostars 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 Collimated flows are produced by early B protostars. 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. Early B to Late O Protostars M 17 new massive YSO 2.2 mm image, 13CO contours trace disk. 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 Outflow Disk seen in silhouette 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 Early B to Late O Protostars IRAS 16547-4247 Brooks et al. (2003) Garay et al. (2003) Lbol ~ 6 x 10 sun (B0 – O9 star assuming single central star) Mcore ~ 900 Msun Mf ~ unknown Centimeter continuum consistent with a thermal jet. Synchrotron emission farther out in jet strong B field collimating ionized gas. 4L HH 80-81 like jets may be possible in more massive stars (up to late O spectral type) 2.12 mm image 1.2 mm continuum contours 8.4 GHz contours 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 Outflows from Early B (proto)stars Location of early B stars with outflow in M vs t plot: 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 ZAMS stars No jet with UC HII regions (W75N, G192) Ae Jet (HH 80-81) IRAS 20126 IRAS 05358 Not on ZAMS – little or no ionizing radiation from protostar, jet-like molecular outflows (Yorke 2003) Young O Stars 3.6 cm HII region expansion 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 C34S(J=3-2) Red-shifted 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 Young O Stars POSTER J12: Klaassen et al. Feldt et al. (2003): ~ O5 star G5.89-0.39 New Results detected, no disk-like structure Sollins et al. (2004): SiO(5-4) but small excess 3.5 mm emission outflow opening angle ~ 90o. circumstellar material. Outflow axis does not match axis found in C34S or CO/HCO+. Dust continuum POSTER C51: extension along SiO outflow Sollins on G5.89 axis. 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 Multiple flows? O Protostars POSTER C2: Beuther – submm lines & continuum in Orion 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 O Protostars 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 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. SiO masers O Protostars DR21 Roelfsema et al. (1989) Garden et al. (1991) Davis & Smith (1996) Smith et al. (2004, in prep) Lbol ~ 3 x 10 5Lsun Tdyn > 5 x 10 4 yrs Mf ~ 3000 Msun . Mf < 6 x 10 -2 Msun yr -1 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 * A few recent significant contributions Characteristics of OB outflows (and accretion disks). A few references not already mentioned (there are MANY more): • 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 Impact of Luminosity on Outflow Structure 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 10100 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. Radiatively-Induced Outflows F30: M* = 31 Msun in 2.4x104 yrs 104 yrs 17 Msun 2.1x104 yrs 31 Msun F60: M* = 34 Msun in 4.5x104 yrs Density: gray scale & white contours 104 yrs 13 Msun 3x104 yrs 34 Msun 2x104 yrs 28 Msun 3.5x104 yrs 34 Msun 2.5x104 yrs 33 Msun 4.5x104 yrs 34 Msun Velocity: arrows 1.5x104 yrs 24 Msun 1.9x104 yrs 29 Msun 2.2x104 yrs 31 Msun 2.4x104 yrs 31 Msun Carbon grain temperature: solid black contours Silicate grain temperature: dotted contours Numbers: age & M* Stellar radiation softens at outflowRadiatively-induced outflow encased shock boundry cloud collapses by shock fronts, relatively poorly along flow axis to produce wellcollimated flow. collimated jet. Yorke & Sonnhalter 2002 Mass loading impact on outflows X-winds: T Tauri star FU Ori Outburst Bstar (G) 380 380 380 1500 R* (Rsun) 2.5 2.5 4 4 M* (Msun) 0.5 0.5 2.5 2.5 Macc (Msun yr-1) 10-8 10-5 8x10-5 8x10-5 Rx/R* 5 0.7 0.45 1 . B9 Protostar B9 protostar (Low B field) (High B field) 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). Mass loading impact on outflows . 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 mass loading from the . D17) – How disk affects structure and dynamics of the wind. Mw = 3 x 10 -5 to 3 x 10 -10 Msun yr -1 Degree 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 Expected for outflows & disks • Turbulence in the disk or wind increases associated with luminous YSOs. • B field decreases Not clear what the implications are, will this affect outflow? Disk turbulence - impact on outflows 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 = cs W / 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. 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 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 Msun Assume 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 POSTER: Observations (Richer et al. 2000): Outflow Force Fco vw ------------------ = ------------= f ---. Accretion Force Macc vkep vkep vw f ----- ~ 0.3 (1/1) vkep . Mw where f = -----. Macc for X-winds 2 Lbol = Lacc ~ 0.03 (10/1) for Disk winds Lbol = ZAMS 1 . . Coffey et al – low mass jet can carry away adequate angular momemtum. vw Log f --vkep Decrease in Mw / Macc between Lbol = 1 and 104 Lsun ? Errors are too large now to say. 0 -1 -2 -1 0 1 2 3 4 Log Lbol (Lsun ) 5 6 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 slowmoving outflow along the polar direction, infall proceeds along the equitorial plane (Fiege & Henriksen 1996: Lery, Henriksen, Fiege 1999; Aburihan et al 2001). 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 Massive Star Formation & Outflow Reviews 1. 2. 3. 4. 5. 6. 7. 8. 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”