FU Ori and Outburst Mechanisms Zhaohuan Zhu Hubble Fellow, Princeton University Collaborators: Lee Hartmann (Umich), Charles Gammie (UIUC), Nuria Calvet (Umich), Jonathan McKinney (UMD), Jaehan Bae (Umich)

Outlines FU Ori observations -High accretion rate inner disks Outburst mechanisms -MRI+GI -Disk fragmentation R*R*

FU Ori FU Orionis objects Light from disk accretion Star Boundary layer Disk λ F Class I/II

FU Ori F,G K,M High mass accretion disk Constant dM/dt inner disk What can we learn from SED? Spectral type: Luminosity : Zhu et al Double peaked absorption lines: Hartmann & Kenyon 1985, 1987 Kenyon, Hartmann & Hewett 1988

FU Ori: Hot inner disk (Zhu et al 2007, 2008) Tools: Disk atmospheric radiative transfer model (Disk structure + Kurucz model)

FU Ori: Hot inner disk (Zhu et al 2007, 2008) Disk atmospheric radiative transfer modeling: Steady accretion model fits SED The hot inner disk extends from 5 R  to AU Decay timescale: t visc ~R 2 /ν  ~ No hot boundary layer emission High mass accretion disk T=6000 K Flared outer disk (silicate emission) AU Log λ (μm) Independent constraints on hot FU Ori disk size (5 R  to AU): MOST satellite suggests short small scale variability ~ days, corresponding to the orbital time at R  (Siwak et al. 2013) Keck Interferometer spatially resolve FU Ori to sub-AU scale, constraining the hot disk size ~0.5 AU (Eisner & Hillenbrand 2011)

(Zhu et al b) 5 µm optical 2 µm Keplerian rotation disk The high Ṁ disk could extend to 0.5 AU FU Ori: Keplerian rotation Central star mass 0.3 M ʘ Produced at ~0.5 AU

1)FU Ori is a high mass accretion rate disk (2x10 -4 M ʘ yr -1 ) from 5 R  to AU around a 0.3 M  star 2) Outbursts last ~100 yrs=>0.02 Msun t visc ~R 2 /ν = decay time α= ) Keplerian rotation disk Observation summary:

Disk accretion mechanisms (MRI & GI): MRIGI (Gammie 1996, Turner et al. 2007, Bai & Goodman 2009) GI can transport angular momentum (Gammie 2001, Durisen et al. 2007) ~1 MRI GI High ionization ratio Toomre Q~1  t cool >Ωt cool <Ω

Log T eff Log Σ S curve can be due to various reasons 1)Hydrogen ionization-``Thermal Instability’’ successful for CV objects, proposed for FU Ori (Bell & Lin 1994) But the outburst radius is ~0.1 AU 2) Different accretion mechanisms at different Σ and T. `S’ curve determines the outburst Heating>cooling Heating<cooling Outburst and ‘S’ curve:

(Zhu et al. 2009a, 2010b) Disk unstable regions: (1)At M>3x10 -5 M ʘ /yr may be subject to thermal instability (2) At M<3x10 -5 M ʘ /yr Non-steady (outbursts) GI pileup->dissipation->MRI (3) At M>10 -6 M ʘ /yr, R>100 AU Gravitationally fragmentation (Rafikov 2007, 2009)...

Outburst mechanisms 1) Accretion of Clumps generated by GI Vorobyov & Basu 2005, 2006, ) Thermal instability Bell & Lin 1994, Lodato & Clarke 2004

GI (Armitage et al. 2001, Zhu, Hartmann, Gammie 2009 a,c) Outburst mechanism: MRI+GI instability 3) MRI+GI (Martin & Lubow 2011)

Axisymmetric viscous fluid where the viscosity parameter (α) of MRI and GI α= α MRI if T>T MRI or  A =100 g/cm 2 α=exp(-Q 2 ) the radiative transfer MRI+GI instability: 2D R-Z simulation Toomre Q

– Maximum mass accretion rate – Outburst duration time – High Ṁ disk size (Zhu et al. 2009c) dM/dt MRI+GI instability: 2D R-Z simulation B 2x10 -4 M ʘ yr -1

MRI+GI instability: 2D R-Z simulation Due to Thermal instability The midplane temperature is 10 5 K Hartmann, Zhu & Calvet 2010

(Bae et al. In prep.) MRI+GI instability: 2D R-Φ simulation

(Bae et al in prep.) MRI+GI instability: 2D R-Φ simulation

Disk Fragmentation:

Disk fragmentation: (Zhu et al. 2012)

Disks fragment under certain conditions:

Clumps could have different fates: Tidal destruction: Gap opening: (Zhu et al. 2012) Boley 2009 Nayakshin 2010

How to test various theories? The synthetic ALMA image for a fragmenting disk. 1 minute integration with Full ALMA with 0.1” resolution Accepted ALMA proposal for Cycle 2 (PI: Lucas Cieza): 3 FU Orionis objects and 5 Exor objects

FU Ori is a high mass accretion rate disk (2x10 -4 M ʘ yr -1 ) from 5 R  to AU around a 0.3 M  star. MRI-GI can explain the outbursts. Disk fragments under certain conditions. Clumps could have different fates. Summary: R*R*