Brown Dwarf Formation and Jet Propagation in Core Collapse Simulations

Transcription

1 Brown Dwarf Formation and Jet Propagation in Core Collapse Simulations Knot Knot Cavity (proto-b.d) Observation Riaz et al. (2017) Masahiro N Machida (Kyushu Univ.) Clumps B.D mass object Simulation Collaborators: Basmah Riaz (Max-Planck-Institut), Shingo Hirano (Kyushu Univ.)

2 Formation Scenario of Brown Dwarfs There are some (or many) formation scenarios of Brown Dwarfs (e.g. reviewed by Whitworth et al. 2007) Turbulent fragmentation Collapse and fragmentation of massive prestellar cores to form tiny clumps Disk fragmentation Ejection of protostellar embryos from their natal core etc.. Star and B.D. formation in turbulent environment (Bate 2012) B.D. (or Planet?) formation in circumstellar disk (Stamatellos et al. 2009) Formation of low-mass objects in turbulent disk. Turbulence does not promote disc fragmentation(?, Rice & Nayakshin 2018)

3 Our Study and This Talk Our study or strategy is very (or extremely) simple! B.D. formation is a scale-down version of low-mass star formation We only assume a tiny prestellar core (or clump) with a mass of < 1 solar We calculate the gravitational collapse of such core (without sink cells) Thus, we only set a tiny core as the initial state and wait until the simulation ends without any artificial settings The purpose of this kind of study is to test the hypothesis that BDs can be formed according to the core collapse scenario, as seen in low-mass star formation Contents of this Talk 1. Show some observational evidences 2. Explain the formation of low-mass stars in cloud collapse scenario 3. Discuss the difference between low-mass star and brown-dwarf formations

4 Low-mass Star Formation Process Whole Picture of low-mass star formation : Different spatial scales and timescales Our research area 1. Gravitational collapse of a tiny prestellar core begins 2. Protostar or proto-b.d. appears in the gravitationally collapsing core, and Outflow (low- and high-velocity) flows appear 3. A rotationally supported disk forms and evolves around a protostar or proto-b.d. From text book: An introduction to Star Formation (Ward-Thompson & Whitworth)

5 Properties of Jets and Outflows Outflows and jets are a strong evidence of mass accretion and the existence of circumstellar disk Molecule Outflows Low-velocity (~< km/s) Wide opening angle Steady (or non-steady?) flow Massive (Optical) Jets High velocity (~>10-100km/s) Very good collimation Time variability, episodically driven Not massive Schematic View HST High-velocity component is enclosed by low-velocity component Pudritz & Norman (1986)

6 Observations

7 Observations 1: Episodic Jet and Time variability Non-steady, time-variable accretion? Binary or multiple system? Non-uniform ISM? Lefloch et al. (2015) Plunkett et al. (2015), Nature Yale University, ALMA HH111 7

8 Observations 2: Circumstellar disk, Magnetic Field & Rotation Velusamy &Langer (1998, Nature) Disk Observations indicate Relation between outflow and circumstellar disk Magnetic field related to the outflow driving Rotation? NGC 1333 IRAS 4A Magnetic Field Rotation Bacciotti et al. (2002) Zapata et al. (2016) Tao-Chung Ching et al. (2016) helical structure toroidal field of outflow? Launhardt et al. (2008)

9 Observations 3: Recent ALMA, Proof of Disk Wind Bjerkeli et al. (2016) Outflow rotation Outflow rotation, but high-mass star case Hirota et al Magnetic field structure on the disk (?) Outflow driven by the disk outer edge Alves et al. (2017) Alves et al. (2018)

10 CMF and IMF Andre et al. (2008) The core mass function is similar to the initial mass function Some clumps have sub-stellar masses Our very simple assumption: Low-mass stars form in low-mass clumps Brown dwarfs form in lower-mass clumps Star and B.D. formation in an isolation cloud (I do not discuss the validity of the isolation, Andre et al. (2008) which is out of the scope of our study) dust continuu m IMF CMF Orion A/B cloud complex Ohishi et al. (2002) Host cloud for B.D.? CMF: Core Mass Function IMF: Initial Mass Function

11 Observations 4: Jet and Outflow from proto-b.d. High-velocity Jet driven by proto-b.d. (Whelan et al. 2005, 2012,2018, Monin et al. 2012, Morata et al. 2015, Riaz et al. 2017) Low-velocity outflow driven by-proto B.D. (Phan-Bao et al. 2008, Joergens et al. 2012) Disk structure around B.D. (Luhman et al. 2008, 2012 Joergens et al. 2012, Riaz et al. 2016) These indicate that B.D. forms in the same manner as in the low-mass star formation process But, in some observations, outflows were not observed around proto-b.d (Phan-Bao et al. 2014), and no disk was also observed around B.D. (Luhman et al. 2010) Is there any difference in B.D. formation? Riaz et al. (2017) Proto-B.D. Whelan (2014) Proto-B.D. Proto-B.D. Phan-Bao et al. (2008) Molecular outflow from B.D.

12 Formation of Low-mass Star and Brown Dwarf Jets and outflows are significant clues to understand low-mass and brown dwarf formation Jets and outflows are driven by the circumstellar or circum-b.d. disk Mass ejection (rate) is closely related to Mass accretion (rete) Matsushita et al. (2017,2018): Mass ejection rate is proportional to Mass outflow rate About a half of infalling mass is ejected by the protostellar outflow: (dm/dt) out 0.5 (dm/dt) infall M out (M star + M disk ) Thus, we can estimate the mass accretion rate, and protostellar and disk mass from observed outflows. Ratio of the mass outflow rate to the mass infall rate Ratio of M out /M infall does not depend on the mass accretion rate Thus, it can be applicable from very low-mass to high mas stars Mass accretion rates in different models, which are in the range of 10-5 M sun dm infall /dt 10-2 M sun Mass accretion rate onto the protostar Matsushita et al. (2017, 2018)

13 Simulations

14 Settings for Core Collapse Simulations Basic equations: Non-ideal MHD eqs. Protostellar model: resolves the protostar itself with a sufficient spatial resolution but, does not calculate the internal structure of the protostar (, T) P P( ), (Barotropic eos + Protostellar Model) Machida & Nakamura (2015) Protostar Numerical Method: 3D Nested Grid resolves both molecular cloud core (>10 4 AU) and protostar (<0.01 AU), in which the density is varied from ~10 3 cm -3 to ~10 21 cm -3 Molecular Cloud core

15 Magnetic Field Initial Condition A spherical core is set to the computational box A uniform magnetic field B 0 and a rigid rotation Ω 0 are imposed, in which typical observational values of B 0 and Ω 0 are adopted Without turbulence (Unknown turbulence is all-around player, and can explain all the results, thus, we did not include it) Three types of Simulations 1. Initial magnetic field lines are parallel to the initial rotation axis, resolving the protostar 2. Initial magnetic field lines are not parallel to the initial rotation axis, resolving the protostar 3. Molecular Initial magnetic Cloud field core lines are parallel to the initial rotation axis, not resolving the protostar using Sink method to investigate a long-term evolution Initial Condition Ω Sink cell 3D MHD Nested Grid Rotation Axis Bonnor-Ebert Sphere ~10 4 AU Similar simulations, see Tomisaka (2002), Banerjee & Pudritz (2006), Tomida et al. (2013), Bate (2014), Tsukamoto et al. (2015), Wruster et al. (2018), Lewis & Bate (2017,2018), Kuffmeier et al (2017,2018) Ω Calculation time (not CPU time) a huge computational time is necessary (~10 months to 3 years (wall clock time!, not CPU time)

16 Cloud evolution Before protostar formation Isothermal contraction Mag. dissipation 0.01AU Protostar

17 Simulations 1: Parallel case resolving protostar (without sink cells)

18 Cloud evolution after protostar formation in a small scale Low-vel. Outflow driven by the disk High-vel. jets driven near the protostar Machida 14 Low-vel component Hihg-vel component protostar ~500 AU ~5 AU Color: density, Arrows: velocity Contours: velocity white: v r = 0.2 km/s, red: 10 km/s, orange: 50 km/s Contours: velocity red: v r = 10 km/s, orange: 30 km/s, white: 50 km/s Two distinct flows (high- and low-velocity flows) appear High-velocity jets show a high time variability

19 Cloud evolution After protostar formation in different scales and cutting planes edge-on view (~3000 AU) edge-on view (~100 AU) Strong mass ejection Creation of Cavity Growth of rotationally supported disk Opening angle of jets widens with time consistent with Observations (?) Protostellar mass reaches ~0.04 Msun at the end of the simulation (within the brown dwarf mass range) edge-on view (~10 AU) face-on view

20 Episodic mass accretion and ejection: Gravitational instability & Magnetic effects color:plasma b, contour: density (left), b (right) b P th /P mag =8πP/B 2 Plasma b b>10 3 (very weak B) Jet b<10 (Strong B) high b region Keplerian Disk Equatorial plane Non-axisymetric strcuture due to G.I. y=0 plane low b region Protostar Magnetic field dissipates in a high density gas region, which has a extremely low ionization degree Angular momentum transfer mechanisms: >~3 AU (magnetic coupling region): Magnetic braking, Low-velocity Outflow ~0.5 < r < 3 AU (magnetic decoupling region): Non-axisymmetric structure by G.I <~0.5 AU (magnetic coupling region): (tower) Jet, Magnetic braking, MRI (disk surface)

21 Gravitational torque/magnetic torque Mechanism of Angular Momentum Transfer: Gravitational Torque vs. Magnetic Effects G.I. t grav : gravitational torque t mag : magnetic torque t grav << t mag t grav >> t mag t grav << t mag The periodicity of jet (or jet timescale) is determined by Keplerian timescale of inner and outer edge of grav. unstable region Magnetic dissipation disk radius Early accretion phase high-surface density protostellar mass is very low Q~1 (Inutsuka et al. 2010) Q c s G ~M * 1/2

22 Time variability of Outflow and Jet Episodic Accretion causes Episodic Jets (a) Outflow mass, (b) Momentum, (c) Kinetic energy (d) Outflow and inflow rates Low-velocity component (< 3km/s) High-velocity component (>100km/s) Ejected Mass Green: mass accretion rate Red: mass ejection rate

23 Schematic View Schematic view High-velocity jet is driven near the protostar Low-velocity outflow is driven by the outer disk region High-velocity jet is enclosed by the low-velocity outflow Relatively massive rotationally supported disk Large scale Simulation Small scale Tsukamoto et a. 14 Tomida et al. 14

24 Simulations 2: Non-alighment case resolving protostar (without sink Ω cells) Bonnor-Ebert Sphere

25 Cloud evolution After protostar formation in different scales and different cutting planes Outflow and Jet driving mechanisms are the same as in the parallel case Shell-like structures are created by episodic jets Jet direction does not align with the initial direction of B-field X=0 cutting plane (~50 AU) y=0 cutting plane (~50 AU) Initial B 0 Initial W 0 Protostellar mass is ~0.04 Msun at the end of the simulation y=0 cutting plane (~600 AU) z=0 cutting plane (~2 AU)

26 Precession and warped disk Bow shock Some observations show the precession of jet and time variability of the jet direction, which were reproduced in simulations Ejected clump Cavity (alignment case) The jet direction varies with time, which is cause by warped disk time variability of disk normal different radii of the jet driving Warped disk

27 Time variation of Jet direction Large scale Small scale Blue: low-velocity component Red, green: high-velocity component

28 Structures of Magnetic Fields Alves et al. (2018) Galametz et al. (2018) Girart et al. (2006) Disk scale Middle scale Cloud (large) scale B-field We only changed the viewing angle and scale in a snapshot Various structures can be easily reproduced only by one snapshot adjusting viewing angle

29 Directions of Magnetic field, Disk, low- and high-velocity flows An initial slight difference between B field and rotation axis causes a large difference of the directions in a small scale B-field direction Large (Cloud) scale Hull et al (2013) Small (Disk) scale Difference of B- field in difference scales During a very early stage of S.F. Disk normal direction with different radii differs Large scale: B field determines the disk normal Small scale: Rotation determines the disk normal Precession: Lorentz, centrifugal forces and gravity Jet and outflow directions also change Precession Disk direction Outflow, Jet directions

30 Long term evolution and Large scale structures with Sink Cells: Brown Dwarf Case

31 Disappearance of Outflow As the infalling envelope dissipates, the outflow weakens and finally disappears After the main accretion phase end, the outflow gradually weakens Over half of the cloud mass is ejected by the wide-angle low-velocity outflow The outflow can limit the star formation efficiency to e < Machida, Inutsuka & Matsumoto (2009) B.D formation case: Initial cloud mass is 0.2 M sun

32 Mass accretion rate, and Mass of B.D., Outflow and Disk Mass accretion rate outflow escaped from the host cloud Initial Cloud Mass = 225 M Jup Mass accretion rate dm ~ 10 dt ~ 10 Final mass time after the protostar formation 6 12 M M sun sun yr 1 yr 1 for t 10 for t yr yr Initial core(m ini ) = Protostar (M ps ) + Disk (M disk ) + Outflow (M out ) + Accreting gas (M acc ) M ini = 225 M Jup M ps = 45 M Jup (20%) M disk = 13 M Jup (6%) M out = 128 M Jup (57%) M acc = 3 M Jup (1%) s.f.e. e=0.2

33 Difference between low-mass star and B.D. formation in core collapse simulations There is no qualitative difference between low-mass star and B.D. formation In B.D formation process, Pseudo disk, Circumstellar disk, Outflow, and Jet appear, as seen in the low-mass star formation process Only the difference is the duration of the main accretion phase Low-mass star formation: ~10 5 yr (main accretion phase) B.D. formation: ~10 4 yr (main accretion phase) A high density is required to have a less massive cloud core (<1 M sun ) with a nearly equilibrium state (e.g. Bonnor-Ebert sphere, Jeans mass) a higher density causes a shorter freefall timescale a short duration of main mass accretion phase during which the accretion luminosity is emitted by a central star However, a central star (or proto-b.d) darkens in a short duration In the framework of B.D. formation, Core collapse simulations indicate that the main accretion phase is very short do not support that a low-accretion rate lasts from the beginning by any mechanism do support a sudden drop of the mass accretion rate at ~10 4 yr after the proto.b.d. formation

34 Minimum mass accretion rate Theoretically, the mass accretion rate is represented by dm dt f 3 cs G f=0.975 for Shu (1977), f =46.9 for Hunter (1977) All another effects such as magnetic field, rotation, and turbulence increase the mass accretion rate (Larson 2003) The minimum accretion rate before the dissipation of the infalling envelope dm dt ( Msun / yr) Using T=10K, and Shu s solution Shorter period of main accretion phase for B.D. formation

35 Formation of Brown Dwarfs according to Core Collapse Scenario t ~ yr t ~ 0 t ~ 10 4 yr t ~ 10 5 yr Substellar clump Gravitational collapse Detection Probability Proto-B.D. formation Small-sized outflow and disk Main accretion phase Outflow, Jet and Disk evolves Bright proto-b.d due to a large accretion luminosity Gradual Disappearance of Outflow and Jet Proto-B.D darkens due to the lack of accretion luminosity Low (short timescale) Very Low (very short timescale) Low (short timescale) High (long timescale)

36 Summary Core collapse simulation can reproduce some of observational phenomena such as outflow, jet, cavity, ejected clumps and rotationally supported disk Thus, some brown dwarfs may be able to be explained in the frame work of the low-mass star formation Only the differences are the initial prestellar cloud mass and the duration of the main accretion phase However, it is difficult to explain the formation of all of the brown dwarfs by core collapse scenario We need to further compare observations and simulations to confirm or establish the formation process of brown dwarfs It is important to observe the very early phase of the star and B.D. formation, in order to identify the mass ejection and disk formation mechanisms We need to clarify whether tiny clumps, which are the parent for brown dwarfs, really exist and the environment around such clumps