Disks around young O-B (proto)stars: Observations and theory

1. Disks around young O-B (proto)stars: Observations and theory R. Cesaroni, D. Galli, G. Lodato, C.M. Walmsley, Q. Zhang • The importanceof disksin massive (proto)stars • The searchfor disks: methods and tracers • The result: “real” disks found in B (proto)stars • The stability of disks accretion rate onto star • The apparentlack of “real” disks in O stars: observational bias, short lifetime, or different star formation scenario?

2. Disks in young (proto)stars Disks seem natural outcome of star formation: collapse+angular momentum conservation  flattening+rotation speed up  disk • Disks detected in low- & intermediate-mass (< 8 MO) pre-main-sequence stars (Simon et al. 2000; Natta et al. 2000) • Disks of a few AU found in young ZAMS B stars (Bik & Thi 2004) • Disks disappear rapidly in intermediate-mass (2-8 MO), pre-main-sequence stars (Fuente et al. 2003)

3. Disks and high-mass star formation Two relevant timescales in star formation: accretion:tacc = Mstar/(dM/dt)acc contraction: tKH = GMstar/RstarLstar M > 8-14 MOtacc > tKH (Palla & Stahler 1993) High-mass stars reach ZAMS still accreting! Spherical symmetry: radiation pressure > ram pressure  stars > 8-14 MOshould not form!??

4. Disk + outflow may be the solution (Yorke & Sonnhalter, Kruhmolz et al.): Outflow  channels stellar photons   lowers radiation pressure Disk  focuses accretion   boosts ram pressure Detection of accretion disks would support O-B star formation by accretion, otherwise other mechanisms are needed

5. The search for disks in massive YSOs Disks are likely associated with outflows: outflow detection rate = 40-90% in massive YSOs (Osterloh et al., Beuther et al., Zhang et al., …) • disks should be widespread! BUT… What to search for…?

6. disk outflow outflow Theorist’s definition: Disk = long-lived, flat, rotating structure in centrifugal equilibrium Observer’s definition: Disk = elongated structure with velocity gradient perpendicular to outflow axis core

7. Which tracer…?

9. Toroids M > 100 MO R ~ 10000 AU L > 105 LO (dM/dt)star > 10-3 MO/yr trot~ 105 yr tacc~ M/(dM/dt)star~ 104 yr tacc << trot non-equilibrium, circum-cluster structures Disks M < 10 MO R ~ 1000 AU L ~ 104 LO (dM/dt)star ~ 10-4 MO/yr trot~ 104 yr tacc~ M/(dM/dt)star~ 105 yr tacc >> trot equilibrium, circumstellar structures Results of disk searchTwo types of objects found:

10. Examples of rotating toroids: • G10.62-0.38 (Keto et al. 1988) • G24.78+0.08 (Beltràn et al. 2004, 2005) • G28.20-0.05 (Sollins et al. 2005) • G29.96-0.02 (Olmi et al. 2003, Gibb et al. 2004) • G31.41+0.31 (Beltràn et al. 2004, 2005) • IRAS 18566+0404 (Zhang et al. 2005) • NGC 7538 (Sandell et al. 2003)

11. Examples of rotating disks:

12. Sako et al. (2005) 13CO(1-0) 0.07 pc 0.01 pc M17 Chini et al. (2004) 2.2 micron disk

13. H2O masers prop. motions IRAS 20126+4104 Cesaroni et al. Hofner et al. Moscadelli et al. M*=7 MO

14. IRAS 20126+4104 Edris et al. (2005) Sridharan et al. (2005) NIR & OH masers disk

15. Ltot Mdisk RdiskMstar(dM/dt)out tout 104LO 1-15MO6-25MO a few103AU 10-4MO/yr 104yr DISKS IN MASSIVE (PROTO)STARS  Disks do exist in B-type (proto)stars

16. Open questions… • Mdisk~ Mstar Are disks stable? • Can disks sustain accretion rate onto star? • Are there disks in O-type stars?

17. Disk stability • Stability: Toomre’s parameter Q > 1 • Q(H/R,Mdisk/Mtot) with H=disk thickness and Mtot= Mdisk+ Mstar Fiducial values (e.g. IRAS 20126+4104): H/R = 0.4 Mdisk/Mtot = 4 MO /11 MO = 0.4  Q=2  the disk is stable to axisymmetric perturbations, but…

19. Disk stability • Stability: Toomre’s parameter Q > 1 • Q(H/R,Mdisk/Mtot) with H=disk thickness and Mtot= Mdisk+ Mstar Fiducial values (e.g. IRAS 20126+4104): H/R = 0.4 Mdisk/Mtot = 4 MO /11 MO = 0.4 Q=2  the disk is stable to axisymmetric perturbations, but…

20. Lodato and Rice (2004): • thick (H ~ R/2) and massive (Mdisk~ Mstar/2) disks with Q > 1 develop non-axisymmetric instabilities (over trot~ 104 yr) towards Q ~ 1 marginal stability Lodato, Rice, and Armitage (2005): • disk cooling  development of spiral structure or disk fragmentation • upper limit to the transport of matterthrough the disk by viscosity  maximum accretion rate through the disk onto the star, for disk surface density ~ R-1: (dM/dt)star = 0.38 (H/R)2 Mdisk/trot ~10-5 MO/yr

21. Disk accretion rate Problem: (dM/dt)star=10-5 MO/yr too small • Estimated mass accretion rate on star much smaller than accretion on disk: (dM/dt)disk= Mcloud/tfree-fall = 10-4-10-3 MO/yr • Star formation timescale too long: tstar = Mstar/(dM/dt)star = 106-107 yr >> 105 yr (e.g. Tan & McKee 2004).

22. Possible solutions: • For surface density ~ R-p (dM/dt)star = 0.38/(2-p) (H/R)2 Mdisk/trot may be very large for p ~ 2 • Massive disks (Mdisk~ Mstar)  strong episodes of spiral activity (dM/dt)starenhanced by10 times over trot ~ 104 yr (Lodato & Rice 2005) • Magnetised disks also develop enhancements of accretion rate (Fromang et al. 2004)

23. Are there disks in O stars? • In Lstar~ 104 LO (B stars) true disks found • In Lstar > 105 LO (O stars) no true disk (only toroids) found - but distance is large (few kpc) • Orion I (450 pc) does have disk, but luminosity is unclear (< 105 LO???) • Difficult to detect massive disks in O (proto)stars. Why?

24. Observational bias? • For Mdisk= Mstar/2, a Keplerian disk in a 50 MO star can be detected up to: • continuum sensitivity: d < 1.7 [Mstar(MO)]0.5 ~12 kpc • line sensitivity: d < 6.2 Mstar(MO) sin2i/W2(km/s)~8 kpc • spectral + angular resolution: d < 14 Mstar(MO) sin2i/[D(’’)W2(km/s)]~~19 kpc

25. all disks detectable up to galactic center Caveats!!! One should consider also: • rarity of O stars • confusion with envelope • chemistry • confusion with outflow/infall • non-keplerian rotation • disk flaring • inclination angle • …

26. On the other hand, if O protostars do not have disks, a physical explanation is required: • O-star disks “hidden” inside toroids • O-star disk lifetime too short, i.e. less than rotation period: • photo-evaporation by O star (Hollenbach et al. 1994) • tidal destruction by stellar companions (Hollenbach et al. 2000) In both cases we assume Mdisk=Mstar/2 and disk surface density ~ R-1, i.e. MdiskRdisk:

27. photo-evaporation tidal destruction rotational period

28. Photoionosation: inefficient disk destruction mechanism, for all spectral types (if Mdisk comparable to Mstar) • Tidal interaction with the stellar companions: more effective to destroy outer regions of disks in O stars than in B-stars Disks in O (proto)stars might be shorter lived, and/or more deeply embedded than those detected in B (proto)stars

29. Conclusions • Found about ~10 disks in B (proto)starsstar formation by accretion as in low-mass stars • No disk found yet (only massive, rotating toroids) in O (proto)stars • observational bias (confusion, distance, rarity,…) • disks hidden inside toroids and/or destroyed by tidal interactions with stellar companions • disks do not exist;alternative formation scenarios for O stars needed: coalescence of lower mass stars, competitive accretion (see Bonnell, Bate et al.)

31. G192.16-3.82 Shepherd & Kurtz (1999) 2.6mm cont. disk CO outflow

32. G192.16-3.82 Shepherd & Kurtz (1999) Shepherd et al. (2001) 3.6cm cont. & H2O masers

33. Simon et al. (2000): TTau stars Velocity maps (CO J=21)

34. Fuente et al. (2003): mm continuum in Herbig Ae/Be stars (age ~ 106 yr) Mdisk(B) << Mdisk(A)

35. Bik & Thi (2005): CO first overtone in four B5-O6 stars fitted with Keplerian disk

36. Patel et al. (2005) Cep A HW2 Torrelles et al. (1998) … but see Comito & Schilke for a different interpretation

37. IRAS 18089-1732 Beuther et al. (2004, 2005)

38. Gibb et al. (2002) Olmi et al. (2003) Olmi et al. (1996) Furuya et al. (2002) Beltran et al. (2004)

39. Furuya et al. (2002) Beltran et al. (2004)

40. Furuya et al. (2002) Beltran et al. (2004)

41. Furuya et al. (2002) Beltran et al. (2004)

42. CH3CN(12-11) Gibb et al. (2002) Olmi et al. (2003) Beltran et al. (2005)

43. Olmi et al. (1996) Beltran et al. (2004) 1200 AU

44. Disks &Toroids B stars O stars