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

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

Photometric monitoring Conclusions about rotation, spots, magnetic activity

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

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

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)

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

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

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

Accretion disk

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

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?

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

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

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

Case study: 2M strong variations in the accretion rate, evidence for clumpy flow 10% width: km/s 10% width: km/s EW: Å EW: Å other lines: +HeI,CaII,Hβ +HeI,CaII,Hβ,Hγ other lines: +HeI,CaII,Hβ +HeI,CaII,Hβ,Hγ 8 hours 24 hours Scholz & Jayawardhana, ApJ, 2006

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

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

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

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

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.

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

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

... because you never know

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

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

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

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

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

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

The physics of VLM objects 0.35 M S objects are fully convective 0.15 M S degeneracy pressure dominates (radius independent of mass) (radius independent of mass) M S no stable hydrogen burning (substellar limit) (substellar limit) M S only deuterium burning M S no deuterium burning

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

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

Stellar winds TRACE SOHO

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

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

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

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

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

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

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

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

Breakup period models not adequate for fastest rotators models not adequate for fastest rotators

Rotational evolution

Only contraction angular momentum loss necessary to explain slow rotators

Contraction + Skumanich Skumanich braking is too strong

Contraction + exponential braking best agreement of model and observations

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

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

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