Accretion of brown dwarfsAccretion of brown dwarfs

Alexander Scholz (University of Toronto)

Ray Jayawardhana, Alexis Brandeker, Ray Jayawardhana, Alexis Brandeker, Jaime Coffey, Marten van Kerkwijk Jaime Coffey, Marten van Kerkwijk

(University of Toronto)(University of Toronto)

Clues from spectroscopic variabilityClues from spectroscopic variability

OutlineOutline

1. Variability as a tool: Rotation, spots, activity1. Variability as a tool: Rotation, spots, activity

2. Accretion: Clues from emission line variations2. Accretion: Clues from emission line variations

Case studies: 2M1207, 2M1101, TWA5ACase studies: 2M1207, 2M1101, TWA5A

3. General implications:3. General implications:

Accretion from solar-mass stars to brown dwarfsAccretion from solar-mass stars to brown dwarfs

Photometric monitoring Photometric monitoring

Conclusions about rotation, spots, magnetic activityConclusions about rotation, spots, magnetic activity

Photometric rotation periodsPhotometric rotation periods

solar-mass stars: ~2000 very low mass objects: ~500 solar-mass stars: ~2000 very low mass objects: ~500

Period vs. MassPeriod vs. Mass

ONC: Herbst ONC: Herbst et al. (2002)et al. (2002)

VLM objects rotate faster than solar-mass starsVLM objects rotate faster than solar-mass starsaverage period correlated with massaverage period correlated with mass

Scholz & EislScholz & Eislöffel, öffel, A&A, 2004, 2005A&A, 2004, 2005

VLM rotation periods VLM rotation periods

Scholz & EislScholz & Eislöffel: öffel: A&A, 2004, 419, 249A&A, 2004, 419, 249A&A, 2004, 421, 259A&A, 2004, 421, 259A&A, 2005, 429, 1007A&A, 2005, 429, 1007

PhD thesis A. ScholzPhD thesis A. Scholz

2003: 6 periods (squares) 2004: 80 periods (large dots)2003: 6 periods (squares) 2004: 80 periods (large dots)

Amplitudes vs. massAmplitudes vs. mass

VLM objects: low amplitudes, low rate of active objectsVLM objects: low amplitudes, low rate of active objects change in spot propertieschange in spot properties

Amplitudes in Amplitudes in young open young open clustersclusters

Spot propertiesSpot properties

cool spots, either symmetric distribution or low spot coveragecool spots, either symmetric distribution or low spot coverage indication for a change in the magnetic field generationindication for a change in the magnetic field generation

Scholz, EislScholz, Eislöffel & öffel & Froebrich, 2005, A&A, Froebrich, 2005, A&A, 438, 675438, 675

Scholz & EislScholz & Eislöffel, A&A, 2005öffel, A&A, 2005

High-amplitude variabilityHigh-amplitude variability

11 objects with large amplitudes, partly irregular variability11 objects with large amplitudes, partly irregular variability `T Tauri lightcurves` - produced by accretion in hot spots`T Tauri lightcurves` - produced by accretion in hot spots

Accretion diskAccretion disk

Spectroscopic monitoringSpectroscopic monitoring

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

Case study: 2M1207Case study: 2M1207Brown dwarf at 8 Myr with Brown dwarf at 8 Myr with wide, planetary-mass wide, planetary-mass companioncompanion

No NIR colour excess, but No NIR colour excess, but clear signature of accretion clear signature of accretion and windand wind

Final stage of accretion?Final stage of accretion?

Profile VariabilityProfile Variability

broad emission plus redshifted absorption feature cool, infalling material, co-rotating accretion columnclose to edge-on geometry, asymmetric flow geometry

4 hours4 hours 4 hours4 hours

Scholz, Jayawardhana, Brandeker, ApJL, 2005Scholz, Jayawardhana, Brandeker, ApJL, 2005

Linewidth variationsLinewidth variations

variations in the linewidth by ~30% on a timescale of 6 weeks

Scholz, Jayawardhana, Brandeker, ApJL, 2005Scholz, Jayawardhana, Brandeker, ApJL, 2005

Accretion rate variationsAccretion rate variationsAccretion rate changes by ~one order of magnitude in 2M1207 and 2M1101

Natta et al. (2004)Natta et al. (2004)

Case study: 2M1101-7718Case study: 2M1101-7718

strong variations in the accretion rate, evidence for clumpy flow

10% width: 122 232 194 km/s10% width: 122 232 194 km/s EW: 12 92 126 EW: 12 92 126 ÅÅ other lines: +HeI,CaII,Hβ +HeI,CaII,Hβ,Hγ other lines: +HeI,CaII,Hβ +HeI,CaII,Hβ,Hγ

8 hours8 hours 24 hours24 hours

Scholz & Jayawardhana, ApJ, 2006Scholz & Jayawardhana, ApJ, 2006

Case study: Case study: TWA5ATWA5A

close binary, at least one of the components is accretingclose binary, at least one of the components is accreting

Aa + Ab (+ Ac?) = one solar massAa + Ab (+ Ac?) = one solar mass

Brandeker et al. 2003Brandeker et al. 2003

HHα variability of TWA5Aα variability of TWA5A

both components contribute to „flare“ both components contribute to „flare“ event - delay of broad component?event - delay of broad component?

profile decomposition: profile decomposition: broad and narrow broad and narrow

componentcomponent

dashed: broad dashed: broad dotted: narrowdotted: narrow

Jayawardhana, et al., ApJL, in prep.Jayawardhana, et al., ApJL, in prep.

Velocity variationsVelocity variations

comparable periods in both componentscomparable periods in both componentseither rotation period of Aa or Ab either rotation period of Aa or Ab hot and cool spots hot and cool spots

or orbital period of a third body Acor orbital period of a third body Ac

broad: P = 19.6 h, FAP = 0.004%broad: P = 19.6 h, FAP = 0.004% narrow: P = 19.2 h, FAP = 0.8%narrow: P = 19.2 h, FAP = 0.8%

Jayawardhana, et al., ApJ, in prep.Jayawardhana, et al., ApJ, in prep.

Accretion flow geometryAccretion flow geometry

profile asymmetry AND profile variability nonspherical accretion

indirect evidence for magnetically funneled flow

Scholz & Jayawardhana, Scholz & Jayawardhana, ApJ, 2006ApJ, 2006

Young stars and variabilityYoung stars and variabilityH 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., Jayawardhana et al., ApJ, in prep.ApJ, in prep.

Accretion rate vs. massAccretion rate vs. mass

accretion rate proportional to object masslarge scatter mainly due to variability

Mohanty et al. (2005)Mohanty et al. (2005) Natta et al. (2004)Natta et al. (2004)

Most important Most important conclusion:conclusion:

Keep an eye Keep an eye on them...on them...

... because you ... because you never knownever know

ConclusionsConclusions

1. Photometric variability:1. Photometric variability:

primary tool to study stellar rotation and activityprimary 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:2. Spectroscopic variability:

close-up view on accretion behaviourclose-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: SpitzerOutlook: Spitzer

Spitzer provides means to Spitzer provides means to study the dust in the inner part study the dust in the inner part of the diskof the disk

GO program for 35 brown GO program for 35 brown dwarfs in UpSco:dwarfs in UpSco:- IRS spectra from 8-14 - IRS spectra from 8-14 mm- MIPS photometry at 24 - MIPS photometry at 24 mm

Dusty disks of brown dwarfsDusty disks of brown dwarfs

without disk with diskwithout disk with disk

more to come!more to come!

Period vs. Mass IPeriod vs. Mass I

Pleiades (+ literature) IC4665 (+ literature)Pleiades (+ literature) IC4665 (+ literature)

VLM objects rotate faster than solar-mass starsVLM objects rotate faster than solar-mass stars

Period vs. Mass IIPeriod vs. Mass II

VLM regime: period decreases with massVLM regime: period decreases with mass

Pleiades (+ Terndrup et al.) IC4665Pleiades (+ Terndrup et al.) IC4665

Period vs. Mass IIIPeriod vs. Mass III

Median period decreases with mass, even at very young agesMedian period decreases with mass, even at very young ages

σOri + Herbst et al. (2001) Ori + Herbst et al. (2001) εOri + Herbst et al. (2001)Ori + Herbst et al. (2001)

The physics of VLM objectsThe physics of VLM objects

0.35 M0.35 MSS objects are fully convective objects are fully convective

0.15 M0.15 MSS degeneracy pressure dominates degeneracy pressure dominates

(radius independent of mass)(radius independent of mass)

0.075 M0.075 MSS no stable hydrogen burning no stable hydrogen burning

(substellar limit) (substellar limit)

0.060 M0.060 MSS only deuterium burningonly deuterium burning

0.013 M0.013 MSS no deuterium burningno deuterium burning

Interior structureInterior structure

fully convectivefully convective

VLM objectVLM objectsolar-type starsolar-type star

Consequences for magnetic fields, activity, rotationConsequences for magnetic fields, activity, rotation

radiative zoneradiative zone

Rotation and stellar evolutionRotation and stellar evolution

´Disk locking´´Disk locking´

Stellar windsStellar windsBouvier et al. 1997

Stellar windsStellar winds

TRACETRACE

SOHOSOHO

1Myr 10Myr 100Myr1Myr 10Myr 100Myr 1Gyr

σOri, εOri3-10 MyrScholz & Eislöffel, A&A, 2004, 419, 249Scholz & Eislöffel,A&A, 2005, 429, 1007

IC466536 Myr Eislöffel & Scholz 2002, ESO-Conf.

Pleiades 125 Myr Scholz & Eislöffel, A&A, 2004, 421, 259

The clustersThe clusters

Praesepe 700 Myr

Time series imaging with TLS Schmidt, ESO/MPG WFI, Calar AltoTime series imaging with TLS Schmidt, ESO/MPG WFI, Calar Alto

LightcurvesLightcurves

90% of all variable objects: regular, periodic variability90% of all variable objects: regular, periodic variability

VLM star in the Pleiades Brown Dwarf in VLM star in the Pleiades Brown Dwarf in εεOriOri

Period vs. Mass IIPeriod vs. Mass II

VLM regime: period decreases with massVLM regime: period decreases with mass

Pleiades (+ Terndrup et al.) IC4665Pleiades (+ Terndrup et al.) IC4665

ModelsModels

P(t) = P(t) = αα(t)(t) (R(t)/R (R(t)/Rii))22 P Pii

A) A) αα(t) (t) = const. = 1 = const. = 1 only contractiononly contraction

B) B) αα(t) (t) = (t / t= (t / ti i ) ) ½ ½ Skumanich law (dL/dt Skumanich law (dL/dt ~~ωω33))

C) C) αα(t) (t) = exp((t – t= exp((t – tii) / ) / )) exponential braking (dL/dt exponential braking (dL/dt ~ ~ ωω))

Period evolution between 3 and 750 Myr determined by… Period evolution between 3 and 750 Myr determined by… - hydrostatic contractionhydrostatic contraction- rotational braking by stellar windsrotational braking by stellar winds- disk-locking (not important)disk-locking (not important)

Surface features: Magnetic spotsSurface features: Magnetic spots

Amplitudes of variability determined by spot propertiesAmplitudes of variability determined by spot properties

Spot configurationSpot configurationHow do the surfaces of VLM objects look like?How do the surfaces of VLM objects look like?

Lamm (2003)Lamm (2003) Barnes & Collier Cameron (2001)Barnes & Collier Cameron (2001)

b) Only polar spotsb) Only polar spots

c) Low spot coveragec) Low spot coverage

d) High symmetryd) High symmetry

e) Low contraste) Low contrast

Disks around VLM objectsDisks around VLM objects

NIR colour excess Strong emission linesNIR colour excess Strong emission lines

but: disk frequency only 5-15% in but: disk frequency only 5-15% in Ori clusterOri cluster

Colour-colour diagram Optical spectroscopyColour-colour diagram Optical spectroscopy

Accretion vs. rotationAccretion vs. rotation

Scholz & EislScholz & Eislöffel 2004ffel 2004

Basri, Mohanty & Basri, Mohanty & Jayawardhana, in prep.Jayawardhana, in prep.

Breakup periodBreakup period

models not adequate for fastest rotatorsmodels not adequate for fastest rotators

Rotational evolutionRotational evolution

Only contractionOnly contraction

angular momentum loss necessary to explain slow rotatorsangular momentum loss necessary to explain slow rotators

Contraction + SkumanichContraction + Skumanich

Skumanich braking is too strong Skumanich braking is too strong

Contraction + exponential brakingContraction + exponential braking

best agreement of model and observationsbest agreement of model and observations

Multi-filter monitoringMulti-filter monitoring

simultaneous monitoring with two telescopes in I, J, Hsimultaneous monitoring with two telescopes in I, J, H

Calar Alto Calar Alto Observatory, Observatory, 1.2m and 2.2m 1.2m and 2.2m telescopetelescope

Magnetic field generationMagnetic field generation

Fully convective objects:Fully convective objects:

no interface layerno interface layer solar-type solar-type ωω--dynamo,dynamo, only small-scale magnetic only small-scale magnetic fields?fields?

inefficient wind brakinginefficient wind braking fast rotationfast rotation

symmetric spot distributionsymmetric spot distribution small amplitudessmall amplitudes

Spectroscopic monitoringSpectroscopic monitoring

accretion = strong emission line variabilityaccretion = strong emission line variability

Hα line: σ(Hα line: σ(EW) = 22-90% EW) = 22-90% σ(σ(10%width) = 4-30%10%width) = 4-30%