Download T Tauri Stars

T Tauri Stars
Kate Barnes
A540
T Tauri Stars: Background
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Very young, solar-type stars
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~107 yrs
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Low mass 0.5 M☉< M < 2 M☉
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Name: T Tauri, found in Taurus-Auriga dark cloud
Discovered in the 1940s
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Found near molecular clouds
Optically visible
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Connection between IR sources and MS stars
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What makes a T Tauri
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Optically visible, but pre-Main Sequence
Youth inferred from:
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Other common features:
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Proximity to MCs
High Lithium abundances
Excess emission – above that of a MS star
P Cygni profiles (mass inflow and outflow)
Circumstellar Disks
Variability
Note: LOTS of variability amongst these
characteristics
Basic Model
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Old model (1980s)
that illustrates a
typical T Tauri star
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Young, convective star
with accretion disk
and strong stellar
winds and mass loss
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NOT ALWAYS TRUE!!!
Lots of variation of
features amongst TTs
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Observations: Optical Spectra
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Optical Spectra reveal a
range of features
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Emission features:
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Variation between
emission and
absorption features
Continuum “veiling”
Balmer Emission
Neutral & singly ionized
metals (Ca II H & K)
(few) forbidden lines
Where is emission
coming from?
Why so different?
Are these objects really
similar?
Classification Scheme: Wλ of Hα
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T Tauri stars are grouped into one of two types:
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Grouped by the Wλ of Hα
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Classical T Tauri Stars (CTTSs)
Weak-lined T Tauri Stars (WLTTs)
CTTSs have Wλ (Hα) > 10 Å
WLTTs have Wλ (Hα) < 10 Å
Probably similar objects
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All found near MCs
Similar locations on HR diagram
Observations: SEDs & IR Excess
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Energy distributions
show IR (and UV)
excess
CTTSs ~10%
WLTTs – no
Recall: Optically
visible -> not a
spherical distribution
of dust
Must be a disk!
Observations: X-Ray
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All TTs emit in X-Ray
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Steady flux
Flaring
No correlation between Lx and continuum
excess (circumstellar matter)
 Source
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must be photospheric
Coronal?
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Tx too low to be coronal
Steady-state flux from unresolved flaring
Observed Features
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WLTTs do not emit in Hα and must be
detected in X-Rays
Emission lines (or lack of in WLTT)
 IR and UV excess
 X-Ray emission
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What are the physical mechanisms behind
these features?
Line Emission & Stellar Winds
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~1/4 of CTTSs show broad
Hα profiles
Populated n=3 state but
unionized H:
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Width-> v~200 km/s for
thermal broadening
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5,000 K < T < 10,000K
T~106 K - would ionize H
Bulk motion
~3/4 of CTTSs show
blueshifted absorption dip
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Outflowing opaque
material -> represent
stellar winds
~70 km/s
Forbidden Lines
Emission from [O I] 6300 Å shows winds
with similar velocities
 [S II] 6716 & 6731 Å => electron
densities
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Used in conjunction with [O I] luminosity and
crafty physics…
Mass loss from winds of ~ 8 x 10-9 M☉yr-1
One Idea of the presence of winds…
Hα and Forbidden line emission (trace
stellar winds) are only found in CTTSs
 IR Excess (traces circumstellar disks) are
also only found in CTTSs
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 Conclusion:
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Winds are caused by circumstellar disks?
Not necessarily true! Lots of possibilities
Mass Inflow: YY Orionis Stars
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Subclass of TTs
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~1/2 of CTTSs
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Show mass infall!!
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Redshifted H
absorption at 250km/s
Increasingly deep in
Balmer series
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Einstein A increases w/
Balmer series & traces
optical depth
Mass Inflow (cont’d)
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Absorption increases w/ decreasing optical depth
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Infall occurs close to star
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One idea: Mass falling in on magnetic loops
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To measure redshifted absorption must start with
broad Hα
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Limited to CTTSs
Such profiles are highly variable
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Mass infall fluctuates
Circumstellar Disks
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Originally theorized to
explain the IR excess seen
in TT SEDs
Observed in IR and mm
around a number of TTs!!
IR emission from disk
within 10 AU – denser dust
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Seen in CTTSs
mm emission from disk
within 100 AU – low
density gas component
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Seen in both CTTSs and
WLTTs
Circumstellar Disks (cont’d)
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Disk modelling is v. complicated (ask Dick!!)
Important to understand disk dynamics to better
understand TTs
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Disk contribution to luminosity – Active vs. Passive disks
Accretion and winds
Magnetic fields
Pose an interesting problem
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CTTSs and WLTTs are of similar age, but show v.
different disk distribution
What is causing this?
Variability
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Known for decades that
TTs are highly variable –
often erratic periods
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WLTTs have fairly regular,
small amplitude periods on
order of days or weeks
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Variability due to cool
spots
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Signifies presence of
magnetic fields
Other tests show B ~103 G
Variability of CTTSs
Much higher amplitude than WLTT
 Highly erratic
 Astronomers believe these contain hot
spots instead of cool spots
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Occur where infalling matter hits the stellar
surface elevating temps through shock heating
Likely the results of mass moving along
magnetic loops
FU Orionis Stars
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Stars that show sharp
outbursts of energy with
∆mB=4-6
Fast increase and gradual
falloff
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V1057 Cyg has TT-like
spectrum and exhibits FU
Orionis behavior
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What causes these?
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Not IR sources before
brightening
Should be convective and
stably decreasing in
luminosity
???
Summary: What’s going on in a TT star?
Mass accretion (onto star and/or disk)
 Mass loss through stellar winds
 Flaring seen in X-Ray
 Heating from shocks in disk and winds
 Circumstellar Disks (or not)
 Variability from cool spots or hot spots
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Everything you could ask for!
Outstanding Problems
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Hard to disentangle effects of the many
components of TTs
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Are winds originating in disks or is there another
explanation for this correlation
What are the transport mechanisms for mass infall?
Why are CTTSs so aperiodic?
What causes the massive flaring of FU Orionis outbursts?
Little understanding why CTTSs and WLTTs have
such different features and are evolutionarily so
similar
Post T Tauri star problem:
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Few stars found in intermediate stage between TT and
MS
Why is this evolution occuring so quickly?
References
Bertout, C. 1989. ARAA, 27, 351
 Stahler, S.W. & Palla, F. The Formation of
Stars. 2004: Wiley-VCH.
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