The formation of Brown dwarf :

Transcription

1 The formation of Brown dwarf : What do we (not) know/understand? Gilles Chabrier CRAL, ENS-Lyon/U. Exeter

2 Which dominant mechanism(s) determine(s) the IMF? 1) what do OBSERVATIONS say? II) which dominant mechanism/processus/scenario? accretion/ejection? (gravo)turbulence? disk fragmentation?

3 The Brown Dwarf IMF

4 Brown dwarf mass function T Y Reid et al Cruz et al. 07 Burgasser 04 n(t eff ) L Burningham et al. 10 Gizis et al. 00 L T Reylé et al. 10 CFHBS n(m J ) Allen et al. 05 Reid et al Cruz et al. 07 Burgasser 04 Burningham et al. 10 Chabrier, ASSL 2005

5 Brown dwarf mass function T Y Reid et al Cruz et al. 07 Burgasser 04 n(t eff ) L Burningham et al. 10 Gizis et al. 00 Metchev et al. 08 Reylé et al. 10 L T Reylé et al. 10 CFHBS n(m J ) Allen et al. 05 Reid et al Cruz et al. 07 Burgasser 04 Burningham et al. 10 Chabrier, ASSL 2005

6 Brown dwarf mass function T Y Reid et al Cruz et al. 07 Burgasser 04 n(t eff ) L Burningham et al. 10 Gizis et al. 00 Metchev et al. 08 Reylé et al. 10 Kirkpatrick et al. 12 L T Reylé et al. 10 CFHBS n(m J ) Allen et al. 05 Reid et al Cruz et al. 07 Burgasser 04 Burningham et al. 10 Chabrier, ASSL 2005

7 Brown dwarf mass function T Y Reid et al. 04 n(t eff ) L + Cruz et al. 07 <10MJup Burgasser 04 Burningham et al. 10 Gizis et al. 00 Metchev et al. 08 Reylé et al. 10 Kirkpatrick et al. 12 L T Reylé et al. 10 CFHBS n(m J ) Allen et al. 05 Reid et al Cruz et al. 07 Burgasser 04 Burningham et al. 10 Chabrier, ASSL 2005

8 Brown dwarf mass function T Y Reid et al Cruz et al. 07 <10MJup Burgasser 04 n(t eff ) L α=0 Burningham et al. 10 Gizis et al. 00 Metchev et al. 08 Reylé et al. 10 Kirkpatrick et al. 12 L T Reylé et al. 10 CFHBS n(m J ) Allen et al. 05 Reid et al Cruz et al. 07 Burgasser 04 Burningham et al. 10 Chabrier, ASSL 2005

9 Young clusters and SFR: system IMF across the HB limit Stars Brown dwarfs field system IMF

10 Young clusters and SFR: system IMF across the HB limit field system IMF 0.01 M sol 5 M sol 0.01 M sol 5 M sol Jeffries, EAS 2012

11 Young clusters and SFR: system IMF across the HB limit field system IMF 0.01 M sol 5 M sol 0.01 M sol 5 M sol Jeffries, EAS many «free floating» BDs discovered down to ~a few M Jup (< 10 M Jup )

12 NBD/N* = 1/4-1/3 WISE (now!)~1/5 Chabrier 05

13 Andersen et al Combined analysis of 7 SFR s star/bd ratio in the 7 clusters consistent w/ the same underlying IMF

14 Bayo et al. 11: radial dist n of stars and BDs in Lambda-Ori * stars BD s Members confirmed spectroscopically! stars BD s No difference in the distributions of stellar and substellar members

15 Marsh et al. 10: radial dist n of stars and BDs in rho-oph

16 Lodieu et al. 11 : Upper Sco : excess of BD s wrt field? 11

17 Lodieu et al. 11 : Upper Sco : excess of BD s wrt field?? 11

18 Luminosity func<ons are dominated by background stars at fainter levels, making the luminosity func<ons of brown dwarfs uncertain cluster off-field cluster off field Muench et al. 2003

19 Luminosity func<ons are dominated by background stars at fainter levels, making the luminosity func<ons of brown dwarfs uncertain Other issues: -reddening, exctinction,. -underlying models: dn/dm ~ (dn/dmag)x(dmag/dm) -strong bias due to bining! -.. cluster off-field cluster off field Muench et al. 2003

20 Sumi et al Excluded by new re-analysis Mróz et al (6x larger sample, higher sensitivity at ~1-2days)

21 5 objects! Sumi et al Excluded by new re-analysis Mróz et al (6x larger sample, higher sensitivity at ~1-2days)

22 Multiplicity frequency in the stellar and substellar regime

23 Multiplicity frequency in the stellar and substellar regime

24 Thies & Kroupa 2008 : discontinuity between the stellar and the BD IMF

25 Thies & Kroupa 2008 : discontinuity between the stellar and the BD IMF Salpeter Kroupa 01 Chabrier 03 Chabrier 05 dn dm / m =0.3 1

26 Young BDs vs young star properties same radial velocity dispersion (Luhman et al. 2007; Luhman ARA&A 2012) same spatial distribution in young clusters (see e.g. Bayo et al. 11) consistent with the same IMF wide binary BDs form in low-density environments (e.g. Taurus) accretion + disk signature (large blue/uv excess, large asymetric emission lines, Hα => natural extension of CTTs disk fraction around BDs ~40-60% similar to stars timescales for accretion around BDs ~similar to stars ~1-10 Myrs presence of outflows Observations of isolated proto-bds and pre-bd

27 Young BDs vs young star properties same radial velocity dispersion same spatial distribution in young clusters (see e.g. Bayo et al. 11) consistent with the same IMF wide binary BDs form in low-density environments (e.g. Taurus) accretion + disk signature (large blue/uv excess, large asymetric emission lines, Hα => natural extension of CTTs disk fraction around BDs ~40-60% similar to stars timescales for accretion around BDs ~similar to stars ~1-10 Myrs presence of outflows (Luhman et al. 2007; Luhman ARA&A 2012) Observations of isolated proto-bds and pre-bd BD and star formation : common dominant mechanism

28 Conclusion I IMF similar between the field and young clusters/sfr s (within some scatter <3σ ) consistent w/ the same underlying IMF No obvious discontinuity near the HB limit No (or at least very weak) dependence on the environment : NBD/N* ~ 1/3-1/4 WISE : minimum mass for BDs > 0.01 Msol ruled out!!

29 Conclusion I IMF similar between the field and young clusters/sfr s (within some scatter <3σ ) consistent w/ the same underlying IMF No obvious discontinuity near the HB limit No (or at least very weak) dependence on the environment : NBD/N* ~ 1/3-1/4 WISE : minimum mass for BDs > 0.01 Msol ruled out!! BD and STAR formation: common dominant mechanism

30 Ejection, competitive accretion Reipurth & Clarke; Bate, Bonnell and coll. - Collapse of a cloud -> starts forming small N-body clusters of small (~10-3 Msol) gravitationnally bound entities - then accrete mass, (Bondi-Hoyle) or (tides) Ṁ / M 2 Ṁ / M dynamical interactions responsible for merging (-> high-mass stars) or ejection (LMS/BD) dynamical interactions are essential to build the IMF all stars form in clusters the IMF is determined by the gas-to-star conversion (no correspondence with the CMF). No CMF!!

31 Bate 2012: fragmentation of a cloud w/ radiative feedback 500 Msol; L=0.4 pc ; n ~ pc -3 ; Mach# =14 ~ >5x denser and 5x more turbulent than (standard) Larson s relations! => overfavor dynamical int ns!!

32 Bate 2012: fragmentation of a cloud w/ radiative feedback 500 Msol; L=0.4 pc ; n ~ pc -3 ; Mach# =14 ~ >5x denser and 5x more turbulent than (standard) Larson s relations! => overfavor dynamical int ns!! same cond ns (Mc,Lc) in NGC1333: N +BD (simus)=191 M N +BD (observed)~50 M (Scholz et al. 12)

33 Bate 2012: fragmentation of a cloud w/ radiative feedback 500 Msol; L=0.4 pc ; n ~ pc -3 ; Mach# =14 ~ >5x denser and 5x more turbulent than (standard) Larson s relations! => overfavor dynamical int ns!! same cond ns (Mc,Lc) in NGC1333: N +BD (simus)=191 M N +BD (observed)~50 M (Scholz et al. 12) <σ > ~ 0.4 km/s collapse collisions for nearly isothermal filaments (γ 1) : τff (cores) << τcollapse (filaments) André et al. 09, Walsh et al. 07, Muench et al. 07, Gutermuth et al. 09, Bressert et al. 10 Kawachi & Hanawa 98

34 Bate 2012: fragmentation of a cloud w/ radiative feedback 500 Msol; L=0.4 pc ; n ~ pc -3 ; Mach# =14 ~ >5x denser and 5x more turbulent than (standard) Larson s relations! => overfavor dynamical int ns!! same cond ns (Mc,Lc) in NGC1333: N +BD (simus)=191 M N +BD (observed)~50 M (Scholz et al. 12) <σ > ~ 0.4 km/s collapse collisions for nearly isothermal filaments (γ 1) : τff (cores) << τcollapse (filaments) André et al. 09, Walsh et al. 07, Muench et al. 07, Gutermuth et al. 09, Bressert et al. 10 Kawachi & Hanawa 98 suggest little dynamical evolution during prestellar core formation

35 Bate 2012: fragmentation of a cloud w/ radiative feedback 500 Msol; L=0.4 pc ; n ~ pc -3 ; Mach# =14 ~ >5x denser and 5x more turbulent than (standard) Larson s relations! => overfavor dynamical int ns!! same cond ns (Mc,Lc) in NGC1333: N +BD (simus)=191 M N +BD (observed)~50 M (Scholz et al. 12) <σ > ~ 0.4 km/s collapse collisions for nearly isothermal filaments (γ 1) : τff (cores) << τcollapse (filaments) André et al. 09, Walsh et al. 07, Muench et al. 07, Gutermuth et al. 09, Bressert et al. 10 Kawachi & Hanawa 98 suggest little dynamical evolution during prestellar core formation NO Core

36 Bate 2012: fragmentation of a cloud w/ radiative feedback 500 Msol; L=0.4 pc ; n ~ pc -3 ; Mach# =14 SIMULATIONS ARE A TOOL. ~ >5x denser and 5x more turbulent than (standard) Larson s relations! => overfavor dynamical int ns!! NOT A PROOF! same cond ns (Mc,Lc) in NGC1333: N +BD (simus)=191 M N +BD (observed)~50 M (Scholz et al. 12) <σ > ~ 0.4 km/s collapse collisions for nearly isothermal filaments (γ 1) : τff (cores) << τcollapse (filaments) André et al. 09, Walsh et al. 07, Muench et al. 07, Gutermuth et al. 09, Bressert et al. 10 Kawachi & Hanawa 98 suggest little dynamical evolution during prestellar core formation NO Core

37 Other issues w/ competitive accretion / ejection scenario - how do you preserve the correspondence between CMF and IMF? - how do you form BDs in low-density environments (eg Taurus)? - how do you preserve wide BD binaries? (eg 2MASS J /J AU (Radigan et al. 09) 2MASS AU (Todorov et al. 10) - how do you preserve (large) disks, outflows in ejected embryos (proto-bds)? eg Ricci et al how do you end up having a universal IMF? (Bate 2009 : feedback from LMS leads to a narrow range of Jeans mass in the irradiated gas ) - how do you form BD s in isolation (eg Fu Tau A,B Luhman et al. 09)? - all stars must form in clusters : contradicts obs (Bressert 10, 12) + formation of isolated massive stars (Bontemps et al. 10, Bestenlehner et al. 11, Bressert 12) + formation of isolated proto-bds (Palau et al. 12) and pre-bd cores (André et al. 12)

38 Disk Fragmentation BD s :Vorobyov & Basu; Stamatellos & Whitworth; Bate et al.; GP s : Mayer et al.; Boss - Disk massive and cool enough to become gravitationally unstable, according to the Toomre and Gammie criteria Q = c S apple G < 1 t cool. apple 1 P rot 2

39 Masson et al. A&A 2016 Ideal MHD Ambipolar diff. decreases growth of B ϕ ; induces magnetic reconnection => decreases further magnetic breaking less small-scale org n in J; generates large scale ordered flows : turbulence diffusivity affects the accretion history

40 Masson et al. A&A 2016 Md ~ 0.03 Msol R ~ 50 AU Ideal MHD Ambipolar diff. decreases growth of B ϕ ; induces magnetic reconnection => decreases further magnetic breaking less small-scale org n in J; generates large scale ordered flows : turbulence diffusivity affects the accretion history

41 ANALYTICAL ESTIMATE Hennebelle, Commerçon, Chabrier, Marchand, ApJL, 2016 Far ' B h B z v bk ' 4 hv B z B diff ' 4 h2 c 2 AD rot ' 2 r v

42 ANALYTICAL ESTIMATE Hennebelle, Commerçon, Chabrier, Marchand, ApJL, 2016 Far ' B h B z v bk ' 4 hv B z B diff ' 4 h2 c 2 AD rot ' 2 r v R AD ' 18 AU ( AD 0.1s )2/9 ( B z 0.1G ) 4/9 ( M d + M? 0.1M )1/3

43 ANALYTICAL ESTIMATE Hennebelle, Commerçon, Chabrier, Marchand, ApJL, 2016 Far ' B h B z v bk ' 4 hv B z B diff ' 4 h2 c 2 AD rot ' 2 r v R AD ' 18 AU ( AD 0.1s )2/9 ( B z 0.1G ) 4/9 ( M d + M? 0.1M )1/3 R hydro ' 106 AU ( 0.02 )( g cm 3 ) 1/3 ( M d + M? 0.1M )1/3 ( = R /3 0 R 3 0 G)

44 A. Maury s in prep. : 16 Class 0 stars (PdBI), < 300 pc, res AU

45 A. Maury s in prep. : 16 Class 0 stars (PdBI), < 300 pc, res AU at most ~25% of Class-0 disks have R 60 AU

46 Discs around Class 0 protostars VANDAM VLA survey 6 Class 0 protostars: all with r disc < AU 8mm Best-fit Modeling Results Source q γ R c (AU) χ 2 reduced SVS13B Per-emb Per-emb Per-emb HH211-mms IC348 MMS Per-emb And several other individual studies find also small upper-limit for disc sizes in the youngest protostars... Segura-Cox+ (2016)

47 Discs around Class 0 protostars VANDAM VLA survey 6 Class 0 protostars: all with r disc < AU all small disks! 8mm Best-fit Modeling Results Source q γ R c (AU) χ 2 reduced SVS13B Per-emb Per-emb Per-emb HH211-mms IC348 MMS Per-emb And several other individual studies find also small upper-limit for disc sizes in the youngest protostars... Segura-Cox+ (2016)

48 Statistical constraints from D.I. apply both to BD s and planets! Lafreniere et al. (2007) Janson et al. (2012) <23% of stars have >2 M J planets at AU <9% of stars have >5 M J planets at AU <10% of stars host ~Jupiter-mass objects formed by disk instability Janson et al. 12, 13 Chabrier, Johansen, Janson & Rafikov, PPVI rev. 2014

49 199 FGK stars, < 100 pc AU, MJup

50 199 FGK stars, < 100 pc AU, MJup + Durkan et al. (2016) w/ Spizter/IRAC : <9% objects 5-13 MJup in AU range

51 Conclusion II Which dominant mechanism(s) determine the Brown Dwarf IMF? Personal Conclusions Accretion-ejection scenario - Raises many issues (see Chabrier 14 PPVI review; Luhman ARA&A 12) - NOT supported by obs ns (CMF? Filaments (Könyves+ 15) t collapse << t collision, ) - present simulations overfavor dynamical interactions " Disk fragmentation - Large, massive Class-0 disks prone to frag n VERY rare. Due to B. - Excluded as a dominant mechanism for star/bd f n (e.g. by Direct Imaging)

52 turbulence sets up the density fluctuations at the very early stages of star formation (see CH A&A 11) the IMF derives from the CMF and is imprinted in the very cloud conditions (<n>, T, Mach) Gravoturbulent fragmentation Padoan & Nordlund; Hennebelle & Chabrier; Hopkins - Large-scale turbulence (density PDF lognormal) sets up a lognormal distribution of overdense regions at all scales in the cloud -> leads to the CO clump dist n - Virial: regions M(R) with Egrav (R) > (Eth+Erms(R)+Emag) collapse (introduces a scale dependence) -> leads to the core mass function (CMF) - magneto-centrifugally outflows expel ~30-50% of the mass -> leads to the star mass function (IMF)

53 No free parameter! turbulence determines the slope (high-mass part) of the IMF, not the shape of the low-mass part!! (see CH A&A 2011) see also Hopkins 12

54 No free parameter! turbulence determines the slope (high-mass part) of the IMF, not the shape of the low-mass part!! (see CH A&A 2011) see also Hopkins 12 Chabrier HC M 7 Mtran n ~ MJ (see HC08) thermal turbulent M 5 HC 09, 13 Include thermodynamics of the gas ( ) + time-dependence ( ff ( ) / 1/ p ) 1 =( ) 1 on the IMF!! ( )

55 Is the gravo-turbulent scenario excluded by simulations? <n> = 300 cm -3, Mc=500 M, Lc=3 pc Bertelli Motta et al. 2016

56 Is the gravo-turbulent scenario excluded by simulations? <n> = 300 cm -3, Mc=500 M, Lc=3 pc λjeans ~ 1pc ~ Lc αvir ~ 2-8 MJeans ~ 4 M Bertelli Motta et al No theory predicts forming stars under such cond ns!

57 Is the gravo-turbulent scenario excluded by simulations? <n> = 300 cm -3, Mc=500 M, Lc=3 pc λjeans ~ 1pc ~ Lc αvir ~ 2-8 MJeans ~ 4 M No theory predicts forming stars under such cond ns! Bertelli Motta et al Does turbulence determine the initial mass function? Liptai et al Missed the point!: neither HC nor Hopkins theory suppose 1-1 mapping between PDF and IMF/CMF turbulence determines the SLOPE of the IMF, i.e. the high-mass (> ~Mjeans) part of the IMF (HC08, CH11) what sets the scale (power-law -> lognormal) is gravity, NOT turbulence PDF only partly affects the low-mass part (eos, gravity, ) decaying turbulence in the simulations => I.C. probably forgotten BUT. HC or similar theories certainly incomplete!!!

58 Chabrier IMF arbitrary norml n conditions ~similar to Bertelli Motta et al. HC IMF exact norml n from <n> HC 09

59 1000 AU 200 AU 50 AU 10 AU 1 AU Vaytet et al. in prep

60 Vaytet+ 2018

61 Observed isolated Pre-Brown dwarf core Oph B-11 M~30 MJup R<460 AU n~ cm -3 Fig. 1. Interferometric 3.2 mm continuum image of the candidate pre-brown dwarf Oph B-11. The angular resolution (or synthesized half-power beamwidth) achieved with the CD array of the IRAM PdBI is 7.8 "2.8 as shown at the upper left. The rms noise is!! 0.05 mjy/beam; the contours are -2! (dotted), 2!, 3!, 4!, 5!, and 6!. The SCUBA position (21) of Oph B-11 is marked by a cross whose size reflects the +-3 pointing uncertainty of JCMT. The PdBI continuum peak is located at RA(J2000) = 16 h 27 m 13 s s, DEC(J2000) = -24 o , or at &% = , &' = with respect to the SCUBA position. It has a peak flux density S 3.2mm, peak! 0.32 mjy/beam (6.2! detection). André et al., 2012, Science 337, 69

62 Conclusion III Which dominant mechanism(s) determine the CMF/IMF? Personal Conclusions Gravo-turbulent star/bd formation depends on 1 single parameter (turbulence P(k)~k n ) + gravity => «universal» process can lead to binary properties consistent w/ observations (Jumper & Fisher 12) emerging observations of fragmentation at the Class-0 phase fragmentation of cores seems to be limited (Bontemps et al. 10; Longmore et al. 11) emerging observations of isolated proto-bd and pre-bd cores : " VeLLO L1148 IRS (Kauffmann et al. 11); IRAS (Wiseman et al.) J J MJup (Palau et al. 12); L328-IRS (C-W Lee et al.) -> observation of pre-bd core: Oph B-11 (André et al. 12) supports the idea that a BD forms like a star

63 Conclusion III Which dominant mechanism(s) determine the CMF/IMF? Personal Conclusions Gravo-turbulent star/bd formation depends on 1 single parameter (turbulence P(k)~k n ) + gravity => «universal» process can lead to binary properties consistent w/ observations (Jumper & Fisher 12) emerging observations of fragmentation at the Class-0 phase fragmentation of cores seems to be limited (Bontemps et al. 10; Longmore et al. 11) emerging observations of isolated proto-bd and pre-bd cores : " VeLLO L1148 IRS (Kauffmann et al. 11); IRAS (Wiseman et al.) J J MJup (Palau et al. 12); L328-IRS (C-W Lee et al.) -> observation of pre-bd core: Oph B-11 (André et al. 12) supports the idea that a BD forms like a star Stellar radiative feedback (Bate 09, Krumholz 11, Guszejnov et al. 16) - Worth exploring! Puzzle: IMF in ONC and Taurus very similar! Stock 8: surrounded by several massive stars: same IMF (Jose et al. 16)!