Post on 27-Jan-2016
description
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%