Accretion of brown dwarfs

1. Accretion of brown dwarfs Clues from spectroscopic variability Alexander Scholz (University of Toronto) Ray Jayawardhana, Alexis Brandeker, Jaime Coffey, Marten van Kerkwijk (University of Toronto)

2. Outline 1. Variability as a tool: Rotation, spots, activity 2. Accretion: Clues from emission line variations Case studies: 2M1207, 2M1101, TWA5A 3. General implications: Accretion from solar-mass stars to brown dwarfs

3. Photometric monitoring Conclusions about rotation, spots, magnetic activity

4. Photometric rotation periods solar-mass stars: ~2000 very low mass objects: ~500

5. Period vs. Mass ONC: Herbst et al. (2002) Scholz & Eislöffel, A&A, 2004, 2005 VLM objects rotate faster than solar-mass stars average period correlated with mass

6. VLM rotation periods Scholz & Eislöffel: A&A, 2004, 419, 249 A&A, 2004, 421, 259 A&A, 2005, 429, 1007 PhD thesis A. Scholz 2003: 6 periods (squares) 2004: 80 periods (large dots)

7. Amplitudes vs. mass Amplitudes in young open clusters VLM objects: low amplitudes, low rate of active objects  change in spot properties

8. Spot properties Scholz, Eislöffel & Froebrich, 2005, A&A, 438, 675 cool spots, either symmetric distribution or low spot coverage  indication for a change in the magnetic field generation

9. High-amplitude variability 11 objects with large amplitudes, partly irregular variability  `T Tauri lightcurves` - produced by accretion in hot spots Scholz & Eislöffel, A&A, 2005

10. Accretion disk

11. Spectroscopic monitoring How to get from flux(,t) to flux(x,y,z)? degenerated problem: necessarily of speculative nature

12. Case study: 2M1207 Brown dwarf at 8 Myr with wide, planetary-mass companion No NIR colour excess, but clear signature of accretion and wind Final stage of accretion?

13. Profile Variability 4 hours 4 hours broad emission plus redshifted absorption feature cool, infalling material, co-rotating  accretion column close to edge-on geometry, asymmetric flow geometry Scholz, Jayawardhana, Brandeker, ApJL, 2005

14. Linewidth variations variations in the linewidth by ~30% on a timescale of 6 weeks Scholz, Jayawardhana, Brandeker, ApJL, 2005

15. Accretion rate variations Accretion rate changes by ~one order of magnitude in 2M1207 and 2M1101 Natta et al. (2004)

16. Case study: 2M1101-7718 8 hours 24 hours 10% width: 122 232 194 km/s EW: 12 92 126 Å other lines: +HeI,CaII,Hβ +HeI,CaII,Hβ,Hγ strong variations in the accretion rate, evidence for clumpy flow Scholz & Jayawardhana, ApJ, 2006

17. Case study: TWA5A Brandeker et al. 2003 close binary, at least one of the components is accreting Aa + Ab (+ Ac?) = one solar mass

18. Hα variability of TWA5A dashed: broad dotted: narrow profile decomposition: broad and narrow component both components contribute to „flare“ event - delay of broad component? Jayawardhana, et al., ApJL, in prep.

19. Velocity variations broad: P = 19.6 h, FAP = 0.004% narrow: P = 19.2 h, FAP = 0.8% comparable periods in both components either rotation period of Aa or Ab  hot and cool spots or orbital period of a third body Ac Jayawardhana, et al., ApJ, in prep.

20. Accretion flow geometry Scholz & Jayawardhana, ApJ, 2006 profile asymmetry AND profile variability  nonspherical accretion  indirect evidence for magnetically funneled flow

21. Young stars and variability H linewidths for stars in young associations (age 6-30 Myr) `errorbars` show scatter over multi-epoch observations  variability common phenomenon in young stars Jayawardhana et al., ApJ, in prep.

22. Accretion rate vs. mass Mohanty et al. (2005) Natta et al. (2004) accretion rate proportional to object mass large scatter mainly due to variability

23. Most important conclusion: Keep an eye on them...

24. ... because you never know

25. Conclusions 1. Photometric variability: primary tool to study stellar rotation and activity - positive correlation between rotation period and mass - rotational evolution determined by contraction + winds - change of dynamo in very low mass regime 2. Spectroscopic variability: close-up view on accretion behaviour - strong accretion rate variations in stars and brown dwarfs - evidence for asymmetric flow geometry

26. Outlook: Spitzer Spitzer provides means to study the dust in the inner part of the disk GO program for 35 brown dwarfs in UpSco: - IRS spectra from 8-14 m - MIPS photometry at 24 m

27. Dusty disks of brown dwarfs without disk with disk more to come!

28. Period vs. Mass I Pleiades (+ literature) IC4665 (+ literature) VLM objects rotate faster than solar-mass stars

29. Period vs. Mass II Pleiades (+ Terndrup et al.) IC4665 VLM regime: period decreases with mass

30. Period vs. Mass III σOri + Herbst et al. (2001) εOri + Herbst et al. (2001) Median period decreases with mass, even at very young ages

31. The physics of VLM objects 0.35 MS objects are fully convective 0.15 MS degeneracy pressure dominates (radius independent of mass) 0.075 MS no stable hydrogen burning (substellar limit) 0.060 MSonly deuterium burning 0.013 MSno deuterium burning

32. Interior structure solar-type star VLM object radiative zone fully convective Consequences for magnetic fields, activity, rotation

33. Rotation and stellar evolution ´Disk locking´ Bouvier et al.1997 Stellar winds

34. Stellar winds SOHO TRACE

35. The clusters σOri, εOri 3-10 Myr Scholz & Eislöffel, A&A, 2004, 419, 249 Scholz & Eislöffel, A&A, 2005, 429, 1007 Pleiades 125 Myr Scholz & Eislöffel, A&A, 2004, 421, 259 Praesepe 700 Myr IC4665 36 Myr Eislöffel & Scholz 2002, ESO-Conf. Time series imaging with TLS Schmidt, ESO/MPG WFI, Calar Alto 1Myr 10Myr 100Myr1Gyr

36. Lightcurves VLM star in the Pleiades Brown Dwarf in εOri 90% of all variable objects: regular, periodic variability

37. Period vs. Mass II Pleiades (+ Terndrup et al.) IC4665 VLM regime: period decreases with mass

38. Models • Period evolution between 3 and 750 Myr determined by… • hydrostatic contraction • rotational braking by stellar winds • disk-locking (not important) • P(t) = α(t) (R(t)/Ri)2 Pi • A) α(t) = const. = 1 only contraction • B) α(t) = (t / ti ) ½ Skumanich law (dL/dt ~ω3) • C) α(t) = exp((t – ti) / ) exponential braking (dL/dt ~ ω)

39. Surface features: Magnetic spots Amplitudes of variability determined by spot properties

40. Spot configuration How do the surfaces of VLM objects look like? b) Only polar spots c) Low spot coverage d) High symmetry e) Low contrast Lamm (2003) Barnes & Collier Cameron (2001)

41. Disks around VLM objects Colour-colour diagram Optical spectroscopy NIR colour excess Strong emission lines but: disk frequency only 5-15% in Ori cluster

42. Accretion vs. rotation Basri, Mohanty & Jayawardhana, in prep. Scholz & Eislöffel 2004

43. Breakup period models not adequate for fastest rotators

44. Rotational evolution

45. Only contraction angular momentum loss necessary to explain slow rotators

46. Contraction + Skumanich Skumanich braking is too strong

47. Contraction + exponential braking best agreement of model and observations

48. Multi-filter monitoring Calar Alto Observatory, 1.2m and 2.2m telescope simultaneous monitoring with two telescopes in I, J, H

49. Magnetic field generation Fully convective objects: no interface layer  solar-type ω-dynamo,  only small-scale magnetic fields? inefficient wind braking  fast rotation symmetric spot distribution  small amplitudes

50. Spectroscopic monitoring accretion = strong emission line variability Hα line: σ(EW) = 22-90% σ(10%width) = 4-30%