Jets from high-mass young stellar objects

Transcription

1 Jets from high-mass young stellar objects Guido Garay Universidad de Chile Outflows, Winds and Jets: From young stars to supermassive black holes Charlottesville, March 6, 2012

2 Aim Review the current status of our knowledge about the phenomenon of highly collimated ionized flows in high-mass YSOs. Outline Signposts of outflow phenomena and their emission mechanisms. Characteristics of jets associated with luminous YSO s.

3 Signposts of outflow phenomena and their emission mechanisms. 1. Primary phenomena: Jets Highly collimated, high velocity flows that emanate from young stellar objects. Thought to be the base of large scale outflow events (secondary phenomena) like molecular bipolar outflows and HH systems. Two main types: Ionized jets Molecular jets

4 1.1 Thermal ionized jets Emission mechanism: Free-free emission from (partially) ionized material. Source of ionization: UV photons from shocks produced by the impact of neutral collimated wind on the surrounding high density material. How do we find them?: Detectable as weak radio continuum sources at cm wavelengths. Observational signatures: Distinctive flux density and size dependence with frequency.

5 Flux density and size dependence with frequency. Flux density. Power law radio continuum spectra with indices near 0.6 S ν ν with =(4β-6.2) /(2β-1) for n e r -β e.g. S ν ν 0.6 for β=2 Reynolds (1986) Elongated morphologies. Angular size along the jet: θ ν ν γ with = -2.1/(2β-1) e.g. θ ν ν -0.7 for β=2 Reynolds (1986)

6 Cepheus A HW2 jet 3.6 cm S L = 700 AU Curiel et al. (2006) Rodriguez et al. (1994) Observed flux density and size dependence with frequency Biconical thermal jet

7 1.2 Non-thermal ionized jets Emission mechanism: Synchrotron emission from relativistic electrons. Electron acceleration: Fermi process in strong shocks produced where the fast collimated wind impact on the surrounding high density material. How do we find them?: Detectable as weak radio continuum sources at cm wavelengths. Observational signatures: Negative spectral indices ( -0.3) Elongated morphologies.

8 W3(H 2 O) jet 8.4 GHz S AU ---- : Model of non-uniform synchroton source 1.6 ( r) B( r) 9.0 Reid et al Wilner et al n er 2 r 500 AU r 500 AU 0.8 mgauss cm 3 see Chen s talk

9 1.3 Molecular jets Highly collimated, high velocity molecular structures. Emission mechanism: Line emission from highly excited (T K 300 K) molecular gas. How do we find them?: Best tracers: Detectable through high angular resolution observations of high excitation lines. CO and SiO. Abundance of SiO is increased by several orders of magnitude in the gas phase due to shocks.

10 HH 211 molecular jet Chain of knots. SiO 5-4 SiO 3-2 SiO AU The highest excitation and highest velocity knots are closely linked to the driving source. Lee et al. (2007) Hirano et al. (2005)

11 Molecular jets are usually enclosed within low velocity, low collimation flows. Chen et al. (2012) IRAS Molecular jet (EHV gas) Santiago-Garcia et al. (2009) The EHV range depends on the mass of the driving source. Low mass : V -V o ~ 25 km s -1 High mass: V -V o ~ 150 km s -1

12 Origin: Accelerated ambient material? e.g., prompt entrainment at internal working surfaces in the jet (HH211). Bullets? Not yet clear!

13 2. Secondary phenomena. Signposts of the interaction of collimated wind with ambient cloud, by means of shock waves, away from the driving source. 2.1 Herbig-Haro objects Nature: Large scale (~ several pc) working surfaces within giant outflows. Emission mechanism: Low-excitation shocked gas How do we find them?: Detectable at optical and near IR wavelengths.

14 Signatures At low extinctions: optical lines such as H, [O II], [N II], [S II]. At moderate extinctions: near-ir lines such as H 2 and [FeII]. H [SII] 1.5 pc VLT H m HST HH 111 Chain of H 2 emission knots Brooks et al HH objects trace ejection events that took place > 10 5 yrs ago.

15 2.2 Radio knots or lobes Nature: Emission mechanism: Working surfaces close (scales of ~ 0.1 pc) to the collimated jet. Free-free emission or Non-thermal emission from shock excited gas Which ones dominates? Depends on electron density, n e, and magnetic field, B, within the lobe. If : n e > n e,th free-free dominates n e < n e,th synchrotron dominates crit 4 B ne,r Emin 3 e, 210 cm n th crit crit mgauss 10 cm 10 ergs Density of relativistic electrons Henriksen et al. (1991) Garay et al. (1996)

16 Luminous YSOs in which the radio emission from lobes exhibits negative spectral indices: IRAS Source Luminosity (L ) Reference HH x10 4 Marti et al. (1993) Cepheus A 1.0x10 4 Garay et al. (1996) IRAS x10 4 Garay et al. (2003) G MM1 5.0x10 4 Trinidad (2011) 4.8 GHz 0.3 pc Thermal jet

17 IRAS HH Thermal jet Carrasco et al. (2010) measured polarization in the central region. Highly collimated jet L = 5.3 pc (11 ) Marti et al Degree of polarization: 10-30% Polarization vectors jet axis B in the direction of the jet B 0.2 mgauss see next talk

18 2.3 Molecular bipolar outflows Nature: Ambient molecular gas entrained or swept up by primary jets and winds. Emission mechanism: Thermal emission or maser emission from shock excited gas How do we find them?: Maps of molecular line emission at mm and sub-mm wavelengths. Observational signatures: Strong emission in the wings of the ambient cloud line profile.

19 Characteristics Velocities: Few to ten km s -1 Several tens of km s -1 Hundred km s -1 (LV outflows) (HV outflows) (EHV outflows) Geometry: Poorly collimated Moderately collimated Highly collimated LV outflows HV outflows EHV outflows Wide range of opening angles interpreted as an evolutionary effect: Outflow-envelope interactions Luminosity increase Early B protostar HC HII UC HII yrs ~10 4 yrs 10 5 yrs ZAMS Class 0 Class I Class II time Arce & Sargent (2006) 10 2 yrs yrs 10 4 yrs B5-B3 B1-O8 Early O Beuther & Shepherd (2005)

20 Momentum rate There is strong correlation between molecular outflow parameters and luminosity of driving source: Cabrit & Bertout 1992 Bontemps et al Shepherd & Churchwell 1996 Beuther et al Luminosity Similar flow-formation process for stars of all luminosities.

21 Ionized jets associated with high-mass YSO s Garay & Lizano (1999) reported a handful of ionized thermal radio jets associated with massive YSOs, all of which have luminosities < 2x10 4 L. Source Lumin. S References (L ) (GHz) (mjy) Cepheus A HW2 1.0x Rodríguez et al. 94 IRAS x Hofner et al. 99 W75N(B) VLA1 1.5x Torrelles et al. 97 IRAS x Martí et al. 95

22 Number of detections has increased during the last decade and detections been made towards progressively more luminous YSOs: Source Lumin. S References (L ) (GHz) (mjy) G N 1.6x >1.3 Gibb et al IRAS x Zapata et al CRL2136-RS4 5.0x Menten & Tak 2004 IRAS x Garay et al IRAS x Guzman et al W75N-VLA3 1.4x Carrasco et al G x Bronfman et al Jets are found associated with luminous YSOs.

23 3.1 Characteristics of jets associated with high-mass YSOs High mass loss rates and momentum rates IRAS (L = L ) 4.8 GHz 0.3 pc Thermal jet Garay et al. (2003) Lobes Derived parameters: Ṁ jet = 8x10-6 M yr -1 Ṁv = 8x10-3 M yr -1 km s -1

24 IRAS (L = L ) Thermal jet 0.07 pc Guzman et al. (2010) Lobes Derived parameters: Ṁ jet = 1.4x10-6 M yr -1 Ṁv = 7x10-4 M yr -1 km s -1 n jet = 3x10 5 cm -3 at 1000 AU

25 High velocities Proper motions: Cepheus A HW2 HH Difference map Curiel et al Jet velocities of ~ 500 km s -1 Knots moving at 0.1 per year Marti et al. (1998) Radio recombination lines : v(fwzp) = 1100 km s-1 Jimenez-Serra et al. (2011)

26 Momentum rate Jets associated with luminous YSOs are powerful Velocities : km s -1 Sizes : AU Mass loss rates : M yr -1 Momentum rates : M km s -1 yr times more luminous and energetic than low-mass jets! High-mass jets Low-mass jets Rodriguez et al Jet luminosity

27 3.2 Are HMYSO s with jets associated with molecular bipolar outflows? Observations show that all high-mass YSO s associated with jets are also associated with large scale, high velocity collimated molecular outflows. IRAS IRAS G Garay et al. (2007) M flow ~ 110 M Guzman et al. (2010) Bronfman et al. (2008)

28 Systematic surveys have shown that: Bipolar flow is a common phenomenon toward high-mass protostellar objects Characteristics of molecular flows associated with high-mass YSOs Velocity : km s -1 Mass : M Size : pc Mass outflow rate : M yr -1 Momentum rate : M km s -1 yr -1 Shepherd & Churchwell 1996 Zhang et al Beuther et al High-mass outflows are times more massive and energetic than low-mass outflows. Ionized jets are, however, rare!

29 3.3 Why are ionized jets rare? Possible explanations: Different formation mechanism? Obscured by bright Hyper Compact HII region? Short timescale for jet phase? Bipolar outflows in high mass protostar have dynamic ages of 10 5 yrs > longer than the K-H time of the jet/disk stage of 10 4 yrs. jet may turn off and the large scale outflow will still persist as a fossil for a relatively long time. To answer this question Guzman (2011) undertook an unbiased search for jets towards HMYSO s.

30 ATCA survey of jets toward luminous massive proto-stellar objects candidates of harboring jets. Selection criteria: Luminous HMYSO s (L > L ) Positive radio continuum spectral indices Underluminous in radio Results: 38% Collimated ionized winds 38% Hyper compact HII regions 24% Ultracompact HII regions From the rate of occurence of jets in the sample: Jet phase in high-mass protostars last for ~210 4 yrs.

31 Momentum rate 3.4 Relationship between ionized jets and bipolar molecular outflows. There is a strong correlation between jet radio luminosity and momentum rate of molecular bipolar outflow: High-mass jets Low-mass jets Jet luminosity Hint for a common origin of jets in YSO s of all luminosities.

32 3.5 The large scale environments of HMYSO s with jets. Dust continuum observations show that HMYSO s with jets are found within structures with distinctive physical parameters. IRAS mm IRAS mm 1 pc 1 pc 0.87 mm 0.87 mm R= 0.23 pc; M d =1.3x10 3 M R= 0.16 pc; M d =9.1x10 2 M

33 HMYSO s with jets are associated with massive and dense cores. Physical parameters. R ~ 0.25 pc M d ~ 2x10 3 M n(h 2 ) ~ 2x10 5 cm -3 N(H 2 ) ~ 5x10 23 cm -2 T d ~ 35 K Density structure. Density depends with radius as n r p, with <p>=1.7 Mueller et al Hatchell & van der Tak 2003 Williams et al MDC s with jets are highly centrally condensed

34 Dynamical state. IRAS Optically thick lines Optically thin lines large scale infalling motions MDC s with jets are undergoing large scale inflow motions with intense mass infall rate V inf ~1 km s -1 M inf ~ 1x10-3 M yr -1

35 Summary of our current knowledge about ionized jets associated with HMYSO s Jets are found associated with high-mass YSOs (up to luminosities of 2x10 5 L ). They are 10 3 times more energetic and luminous than low-mass jets. They have lifetimes of 3x10 4 yr (this makes them rare). They are associated with massive and energetic bipolar molecular outflows. They are located in the central region of massive and dense cores exhibiting infalling motions with high-mass infall rates.

36 Still far from understanding the details of the jet phenomena. Many open questions The momentum rate of the jet is typically only 10% of the momentum rate of the molecular outflow. Guzman et al. (2012) Which is ionization fraction of the jets? How are the jets launched and collimated? Disk wind? X-wind? Hoop stress? Jet launching zone : 10 AU (10 mas at 1 kpc) Jet acceleration/collimation zones: AU ( mas at 1 kpc)

37 How is the angular momentum transferred from the accretion disk to the jet? Do jets rotate? What determines the opening angle in high-mass outflows? Investigate the morphology & kinematics of the outflowing molecular gas at scales of 10 AU. Which is the strength and geometry of magnetic fields?

38 To answer these questions we need to probe jets with: High spatial resolution (< 10 AU) High sensitivity High velocity resolution (v ~ 0.1 km s -1 ) Of course, the instrument of choice is ALMA! Sub-arcsec angular resolution High-fidelity High spectral resolution

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42 Which is the driving mechanism of massive outflows? Investigate the morphology and kinematics of the molecular gas at scales of 1000 AU. Entrainment by turbulent Jet? Momentum driven by highly collimated jets? Masson & Chernin (1993) Magnetically diverted flows? Fiege & Henriksen (1996) Arce & Goodman (2002)

43 Observational consideration How many massive protostars we expect to see in our Galaxy? Massive stars spend short time in the pre-main sequence: K H M M SUN 3 yr Kelvin-Helmholtz time Rate of massive star formation in the Galaxy: N( M ) M M SUN 3 N PMS ( M ) K H N( M ) M M SUN 6 Massive protostars are very rare

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