Implications of protostellar disk fragmentation

Transcription

1 Implications of protostellar disk fragmentation Eduard Vorobyov: The Institute of Astronomy, The University of Vienna, Vienna, Austria Collaborators: Shantanu Basu, The University of Western Ontario, Canada Isabelle Baraffe, The University of Exeter, UK Gilles Chabrier, University of Leon, France Mike Dunham, Yale University, USA

2 Gravitational fragmentation of protostellar disks Stamatellos & Whitworth (2009 MNRAS) Various numerical and theoretical studies 1 of protostellar disks have shown that under favorable initial configurations, disk fragmentation is a robust phenomenon. Prerequisites for disk fragmentation in local models relatively massive disks (> 30-50% that of the star) sufficiently large size (> 50 AU) sufficiently fast disk cooling (Ω * t cool < 3-5) no external heating source (massive stars) Prerequisites for disk fragmentation in global models that self-consistently follow core disk transition Initial core mass > Msun initial ratio of rotational to gravitational energy > 0.5% 1 References : Stamatellos, Whitworth, Kroupa, Inutsuka, Gammie, Bate, Boss, Machida, Zhu, Durisen, Nayakshin, Mayer, Wadsley, Kratter, Krumholz, Klein, Hayfield, Lodato, Clarke, Goodwin, Rafikov, Thies, Vorobyov, Basu and many others )

3 In situ formation of massive giant planets and brown dwarfs via disk fragmentation M ~ 8 M J ; R ~330 AU M ~ 3 M J ; R ~ 115 AU M ~ 17 M J ; R ~ 440 AU 1RXS J (Lafrenire et al 2010) (Schmidt et al 2008) Formation of giant planets and brown dwarfs on wide orbits (>50 AU) can most naturally be explained by disk gravitational fragmentation (e.g. Stamatellos et al 2007, Vorobyov & Basu 2010, Thies et al., 2010, Boss 2011). Formation of BDs seem to be favored (e.g. Stamatellos & Whitworth 2009, Kratter et al. 2009) (Vorobyov 2012, in prep) M ~ 25 M J, R ~ 400 AU

4 There is a lot more to disk fragmentation than just planet / brown dwarf formation The Burst Mode of Accretion and FU Orionis eruptions (Vorobyov & Basu 2005, 2006, 2010)

5 Schematic representation of an accreting embedded protostellar object Magnetic fields Jets (~10% of accreted mass) Central star Inner inflow boundary (sink cell ~ 5 AU) Physical processes taken into account in the code: stellar evolution code of Chabrier & Baraffe blackbody cooling from the disk surface stellar and background irradiation of the disk surface viscous and shock heating of the disk interior disk self-gravity frozen-in magnetic fields local vertical hydrostatic equilibrium no sink particles!

6 Disk fragmentation, fragment migration and mass accretion bursts Face-on view of the disk Black regions infalling envelope (off scale) Mass accretion rate onto the star 10-5 М / year Vorobyov & Basu (2006, 2010)

7 FU Orionis and EX Lupi luminosity outbursts Time (Myr) 1000 yr Total luminosity (L 8 ) Time (Myr) EX Lupi FU ori FU-Ori-like V1057 Cyg Julian days

8 Migrating fragments in other numerical studies Full 3D numerical hydrodynamics simulations starting from pre-stellar cores but limited in time scope ( 10 5 yr) Machida, Inutsuka, Matsumoto 2011, ApJ, 729, 42 See also Cha & Nayakshin (2011, MNRAS, 415, 3319) and Zhu et al. (2011,ArXiv: )

9 Accretion Variability and the Luminosity Problem

10 Resolving the luminosity problem 3 & c s 6 M yr -1 for T = K (Shu 1977) M (1 4) 10 G 1 GMM & L = 15 60, for M = 1.0 M and R = 3 R * L ac " * sun * sun 2 R* Dunham & Vorobyov (2011) Key features: 1) Wide spread in luminosities (~ 4 orders of magnitude) 2) Luminosities peak at 3-5 L " Embedded objects in Perseus, Serpens, and Ophiuchus By allowing for a wider range of accretion rates, as predicted by fragmenting disk models, one may expect to obtain a better agreement with observations.

11 Combined hydrodynamic and radiative transfer Monte-Carlo simulations (RADMC, Dullemond & Domenik 2004) Cloud masses and angular momenta M cl = M ; β = 0.3% - 2% Hydro code Accretion rates Accretion and photospheric luminosities Disk masses and sizes Stellar radius and mass RADMC code Spectral energy distribution at various viewing angles for given disk and core geometry Variable accretion that both declines with time and features episodic bursts in some models Bolometric luminosities and bolometric temperatures in the Class 0 and I phase

12 Bolometric luminosity bolometric temperature diagram K-S = 0.85 Filled circles embedded (Class 0 and I) sources in nearby star-forming regions Plus signs Class II / III objects Dashed line ZAMS Shaded area results of numerical simulations using many model realizations (Dunham & Vorobyov 2012)

13 Ejection of fragments from protostellar disks (Basu & Vorobyov 2012)

14 Disk fragmentation and ejection of (sub-)stellar-mass fragments Myr 0.27 Myr 0.28 Myr Radial distance (AU) Formation scenarios of isolated brown dwarfs: 1) Scaled-down version of star formation - collapse of very low mass pre-stellar cores of high density (Whitworth & Zinnecker 2004, Padoan & Nordlung 2004) 2) Ejection of finished brown dwarfs from protostellar disks (Kroupa & Bouvier 2003, Bate 2009, Stamatellos 0.29 Myr & Whitworth 2009, 0.3 Thies Myr et al. 2010) Myr 5000 A hybrid scenario for brown dwarf/very-low-mass star formation: ejection of fragments 0 from protostellar disks followed by cooling and contraction to stellar densities Predicts the existence of freely floating proto-bd cores (Luhman et al. 2007). No high ejection velocity tail, in contrast to what is often observed in ejections of finished BDs Number of ejections is roughly Total ejected mass of individual fragments Radial 1 for every (including distance 10 stars circumfragment (AU) (for Kroupa IMF). Consistent with disks) = M. estimated ratio of stars to BDs, 5-10 (Luhman et al. 2007). The final mass of BD/VLMS may be a factor 2-3 smaller. The ejected fragment is surrounded by some material in the form of a minidisk. There Velocities Decreased of ejected efficiency fragments of ejection 0.8 ± for 0.35 low-mass km s are also low-density plumes connecting -1. In fragments agreement with due velocity to tidal dispersion dispersal. of May stars the disk and the ejected fragment. explain clusters. the IMF turnover at BD masses

15 Origin of crystalline silicates in protostellar disks (Vorobyov 2011, ApJL)

16 Origin of crystalline silicates in comets Stardust mission to the comet Wild 2 Deep impact mission to comet Tempel 1 Recent space missions to comets Wild 2 and Tempel 1 have brought interesting results: a notable fraction of the silicate composition of these comets is in the crystalline form (Brownlee et al. 2006). Remote sensing of other comets also suggests the presence of crystalline silicates (Crovisier et al. 2000; Sitko et al. 2004; Wooden et al. 2004). Most silicates in the interstellar medium are in the amorphous form, whereas on Earth they are usually found in the crystalline form. Below are two examples of the crystalline silicates Forsterite (Mg 2 SiO 4 ) Fayalite (Fe 2 SiO 4 )

17 The fact that comets appear to contain crystalline silicates is unexpected. Silicates are amorphous if they are formed at T < 800 K and crystalline at T> 800 K when there is enough mobility in the silicate to form the energetically most favorable crystalline lattice structure. Amorphous lattice structure The problem with comets is that they are believed to originate on the outskirts of the Solar system where temperatures do not exceed a few tens of Kelvin. Crystalline lattice structure

18 The temperature in the depths of the fragment can exceed 800 K, thus initiating the annealing of amorphous silicate grains into the crystalline form Ra dial distance (AU ) massive and hot fragment ρ ρcs.. + ρ as.. cs log gas density log crystalline silicate fraction If fragments are dispersed by tidal torques, they release crystalline silicates at distances from a few AU to hundreds AU, thus providing an explanation for the high crystalline abundance in comets (Vorobyov, 2011, ApJL). See also Boley at al. (2010) and Nayakshin et al. (2011)

19 Key results for the migrating fragments model INWARD MIGRATION. Fragments that are gravitationally torqued onto the star trigger luminosity outbursts similar in magnitude to the FU Ori and EX Lupi outbursts. Variable accretion with episodic bursts, caused by disk fragmentation and migration, can explain a wide luminosity spread in young star forming regions. See also Boley et al (2010), Nayakshin (2010), Cha & Nayakshin (2011) for grain sedimentation and solid core formation inside migrating fragments. EJECTION. Fragments can be ejected to form freely floating brown dwarfs and very low-mass stars. SURVIVAL. Fragments can stabilize on wide orbits and form systems with wide separation giant planets or brown dwarfs as in Fomalhout, HR 8799, CT Cha, and others DESTRUCTION. Fragments, when tidally destroyed, release crystalline silicates at distances from a few AU to hundreds AU, thus explaining the chemical composition of comets in the Solar System. Numerical simulations have been performed on the SHARCNET and ACEnet clusters