Accretion of brown dwarfs Clues from spectroscopic variability

Accretion of brown dwarfs: Clues from spectroscopic variability Alexander Scholz (University of Toronto) Ray Jayawardhana (Uo. T) Jochen Eislöffel (Tautenburg) Alexis Brandeker (Uo. T)

Brown dwarfs in T Tauri phase Accretion flow variability

T Tauri stars are variable „T Tauri has a magnitude range of from 9. 4 to 13. 5 or 14, but no regular period has as yet been detected. “ (Knott, 1891)

Example: RW Aur 1945 -2005 Alencar et al. 2005 Joy 1945

Monitoring brown dwarfs Long-term project: five years 1999 -2004 of deep photometric monitoring in young open clusters with ages from 3 to 700 Myr Using data from ESO, TLS, Calar Alto Main goal: evolution of rotation and activity for M<0. 3 Ms

T Tauri lightcurves 11 objects with large amplitudes, partly irregular variability typical `T Tauri lightcurves` Scholz & Eislöffel, A&A, 2005

Brown dwarfs have accretion disks Muzerolle et al. 2003 Natta et al. 2002 Scholz & Eislöffel 2005 Jayawardhana et al. 2003

Origin of variability shockfront (hot gas) + rotation + instabilities = strong, irregular variability (in photometry and emission lines)

Spectroscopic monitoring three observing runs with MIKE/Magellan, Jan-March 2005 six targets: accreting brown dwarfs in star forming regions Hα line: σ(EW) = 22 -90% σ(10%width) = 4 -30%

2 M 1207 Brown dwarf at 8 Myr with wide, planetary-mass companion No NIR (but MIR) colour excess, clear signature of accretion and wind Final stage of accretion?

Profile Variability 4 hours broad emission plus redshifted absorption feature absorption disappears and re-appears on timescales of ~1 day Scholz, Jayawardhana, Brandeker, Ap. JL, 2005

Interpretation: Accretion flow cool, infalling, co-rotating material: accretion column close to edge-on geometry asymmetric flow geometry Scholz, Jayawardhana, Brandeker, Ap. JL, 2005

ISO 217 Scholz & Jayawardhana, Ap. J, 2006 profile asymmetry AND profile variability nonspherical accretion indirect evidence for magnetically funneled flow

Linewidth variations for 2 M 1207 variations in the linewidth by ~30% on a timescale of 6 weeks Scholz, Jayawardhana, Brandeker, Ap. JL, 2005

2 M 1101 -7718 8 hours 10% width: 122 EW: 12 other lines: 24 hours 232 92 + He. I, Ca. II, Hβ 194 km/s 126 Å +He. I, Ca. II, Hβ, Hγ strong variations of the accretion-related emission lines Scholz & Jayawardhana, Ap. J, 2006

Accretion rate variations Natta et al. (2004) Accretion rate changes by 0. 5 -1 order of magnitude in 2 M 1207 and 2 M 1101 clumpy, unsteady accretion flow

Accretion rate vs. mass Mohanty et al. (2005) Natta et al. (2004) accretion rate vs. object mass: ~M 2 large scatter: influenced by variability? variability studies essential to study accretion fundamentals

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

. . . because you never know

Conclusions 1. Brown dwarfs show T Tauri like variability 2. monitoring allows close-up view on accretion behaviour 3. strong accretion rate variations up to one order of magnitudes on timescales of days to weeks 4. profile asymmetry and variability: evidence for asymmetric flow (large-scale magnetic fields? )

Brown dwarfs in T Tauri phase Accretion flow 1 st part: Variability Dusty disk 2 nd part: mm/MIR SEDs

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

Dusty disks Mohanty et al. (2004) Pascucci et al. (2003) constraints from MIR-SEDs: flared disk, flat disks, grain growth but only 1 object with SED from NIR to mm

Connection to disk accretion near-infrared colour excess, emission line spectrum variability related to accretion from circumsubstellar disk Scholz & Eislöffel, A&A, 2004

A 1. 3 mm survey in Taurus IRAM 30 m telescope with MAMBOII, Pico Veleta (Spain) 20 sources with Sp. T>M 6, noise level <1 m. Jy for all objects

Fluxes and disk masses 20 sources, 6 detections, flux levels: <0. 7. . . 7 m. Jy transformation to disk masses: <0. 4. . . 2. 4 Jupiter masses Scholz, Jayawardhana, Wood, Ap. J, 2006

Disk mass vs. object mass Relative disk masses comparable from 0. 02 to 3 Ms No trend to lower disk masses in the brown dwarf regime

Enter Spitzer IRAC photometry: 3 -8 μm IRS spectroscopy: 8 -13 μm MIPS photometry: 24 μm IRAC+MIPS available for all Taurus sources: NIR (2 MASS) + MIR (Spitzer) +mm (IRAM)

SED modeling Minimum outer disk radius for objects with mm detection: 10 AU >25% of the disks have radii >10 AU NO evidence for truncated disks Ejection excluded as dominant formation mechanism Scholz, Jayawardhana, Wood 2006

Evolution of brown dwarf disks Spitzer GO program to study 35 brown dwarf disks in Up Sco IRS spectroscopy + MIPS photometry Inner disks and chemistry after 5 Myr All observations finished

Young and old Taurus Up. Sco 2 Myr - strongly flared disk more to come! 5 Myr - dust settling

Origins of brown dwarfs ‘in situ’ formation - ultra-low-mass stars - ejection as embryos - failed stars - signature of formation: binarity, kinematics, accretion disks

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. , Ap. J, in prep.

Case study: TWA 5 A Brandeker et al. 2003 close binary, at least one of the components is accreting Aa + Ab = one solar mass

Hα variability of TWA 5 A dashed: broad dotted: narrow profile decomposition: broad and narrow component both components contribute to „flare“ event - delay of broad component? Jayawardhana, et al. , Ap. JL, in prep.

Velocity variations broad: P = 19. 6 h, FAP = 0. 004% narrow: P = 19. 2 h, FAP = 0. 8% comparable periods in both components possible scenario: rotation period of Aa or Ab two co-rotating spots from accretion and activity Jayawardhana, et al. , Ap. J, in prep.

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

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

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

Period vs. Mass I Pleiades (+ literature) IC 4665 (+ literature) VLM objects rotate faster than solar-mass stars

Period vs. Mass II Pleiades (+ Terndrup et al. ) IC 4665 VLM regime: period decreases with mass

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

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 MS only deuterium burning 0. 013 MS no deuterium burning

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

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

Stellar winds SOHO TRACE

The clusters σOri, εOri 3 -10 Myr Pleiades 125 Myr Scholz & Eislöffel, A&A, 2004, 419, 249 Scholz & Eislöffel, A&A, 2005, 429, 1007 Scholz & Eislöffel, A&A, 2004, 421, 259 Praesepe 700 Myr IC 4665 36 Myr Eislöffel & Scholz 2002, ESO-Conf. Time series imaging with TLS Schmidt, ESO/MPG WFI, Calar Alto 1 Myr 100 Myr 1 Gyr

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

Period vs. Mass II Pleiades (+ Terndrup et al. ) IC 4665 VLM regime: period decreases with mass

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 B) α(t) = (t / ti ) ½ C) α(t) = exp((t – ti) / ) only contraction Skumanich law (d. L/dt ~ω3) exponential braking (d. L/dt ~ ω)

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

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)

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

Accretion vs. rotation Basri, Mohanty & Jayawardhana, in prep. Scholz & Eislö Eisl ffel 2004

Breakup period models not adequate for fastest rotators

Rotational evolution

Only contraction angular momentum loss necessary to explain slow rotators

Contraction + Skumanich braking is too strong

Contraction + exponential braking best agreement of model and observations

Multi-filter monitoring Calar Alto Observatory, 1. 2 m and 2. 2 m telescope simultaneous monitoring with two telescopes in I, J, H

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