2013/01/26 ALMA 北海道大学

Disk stability in low-metallicity star formation ~ 低金属量星形成における降着円盤の安定性 ~ K.Tanaka, K.Omukai (Kyoto-U) Primordial stars massive ~ 100Msun zero-metal. Present-day stars low mass ~1Msun dusty (~1wt%) ？？？？？？

Low metallicity stars ？ Primordial stars massive high-Mdot zero-metal. Present-day stars low mass low-Mdot solar-metal. Low-metal. stars have crucial roles for the evolution of universe!! But we do not understand how they formed… What makes difference? → metal/dust cooling must be important.

Importance of dust cooling Omukai+05 etc. Solar-metal. Zero-metal. low↑ Metallicity high↓ metal/dust cooling Dust cooling makes Jeans-mass smaller! × Free-Fall (compression heating)

Accretion through disk Most material accretes onto stars through disks. Dose the disks are gravitaitonally stable?

Main accretion phase in low metallicity star formation We study the disk structure and its stability in low metallicity star formation. Since most material accretes onto a star through a disk, the disk property is crucial for newborn stellar mass.

Star formation model Step.1. We construct Infalling envelope models at various metallicities Step.2. From the envelope models, we evaluate disk structures and its stability. We apply simple analytic model.

Infalling envelope structure Acc. rate: Radius: Accretion rate and disk size depend on infalling envelope R&M of envelope, from ρ&T in one-zone model. (Hosokawa&Omukai09b) ・ One-zone by Omukai05

Pre-stellar core metal. Core radius Acc. rate Enclosed mass T decreases with metal. ・ Mdot (~T 3/2 ) decreases ・ Rj increases From core model, we estimate disk size & M. ・ We give Ω by hand Ω=0.5xΩ Kep Abel+02 Hosokawa+11

Disk structure Local equilibrium : (Heating) = (Cooling) Heating ： Viscous heating Cooling ： H 2 line, Gas continuum, Dust collision (Dust emission) Parameters Metallicity ： Z=0-Zsun Angular velocity ： Ω=0.5xΩ Kep Viscous parameter ： α=1 Standard disk model

Disk structure zero-metal. Disk temp. is relatively high. Toomre’s Q value Kratter+10 Unstable for Q<1

Disk structure Metallicity affects disk structure zero-metal. low-metal Zsun solar-metal. T[K] Q R[AU] Msun Msun Msun Msun 0.1

Disk stability Low-metal. disk is very unstable!!

infalla accretion Why low-metal. disk is unstable? Disk would be unstable when infall rate exceeds its capacity. Dust cooling efficient at disk rather than envelope. Q-value also tells that disk is unstable when T disk <T core.

If disk is very unstable… “Fragmentation-induced starvation” Infalling material divided into multiple stars → Low-metal. stars form as compact clusters (?) Present-day massive star formation (Peters+10)

Conclusion SF in various metal. via disk acc. In solar-metal. Low-Mdot & Low-T disk → ~stable Single (or binary) low mass star ~1Msun mass is limited by pre-stellar core mass In zero-metal. High-Mdot & High-T disk → ~stable Single (or binary) massive star ~ 100Msun mass is limited by photoionization feedback (Hosokawa-san’s talk) ？？？？？？

Conclusion SF in various metal. via disk acc. In solar-metal. Low-Mdot & Low-T disk → ~stable Single (or binary) low mass star ~1Msun mass is limited by pre-stellar core mass In zero-metal. High-Mdot & High-T disk → ~stable Single (or binary) massive star ~ 100Msun mass is limited by photoionization feedback (Hosokawa-san’s talk) In low-metal ~10 -2 Zsun High-Mdot & Low-T disk → ~unstable Low-mass stellar cluster by disk fragmentation

Future work Dynamical evolutions!! Fragmentation, ejection, episodic accretion with our dust/metal cooling (Omukai+05) Radiative hydro. simulation by Dr.Vorobyov (Vienan-U) Vorobyov&Basu10 etc.