Magnetic fields and accretion flows on the classical T Tauri ...
T Tauri Stars
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Transcript of T Tauri Stars
T Tauri Stars
Kate BarnesA540
T Tauri Stars: Background Very young, solar-type stars
~107 yrs Low mass 0.5 M☉< M < 2 M☉
Name: T Tauri, found in Taurus-Auriga dark cloud Discovered in the 1940s
Found near molecular clouds Optically visible
Connection between IR sources and MS stars
What makes a T Tauri Optically visible, but pre-Main Sequence Youth inferred from:
Proximity to MCs High Lithium abundances Excess emission – above that of a MS star
Other common features: P Cygni profiles (mass inflow and outflow) Circumstellar Disks Variability
Note: LOTS of variability amongst these characteristics
Basic Model Old model (1980s)
that illustrates a typical T Tauri star
Young, convective star with accretion disk and strong stellar winds and mass loss
NOT ALWAYS TRUE!!! Lots of variation of
features amongst TTs
Observations: Optical Spectra Optical Spectra reveal a
range of features Variation between
emission and absorption features
Continuum “veiling” Emission features:
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 T Tauri stars are grouped into one of two types:
Classical T Tauri Stars (CTTSs) Weak-lined T Tauri Stars (WLTTs)
Grouped by the Wλ of Hα CTTSs have Wλ (Hα) > 10 Å
WLTTs have Wλ (Hα) < 10 Å
Probably similar objects All found near MCs Similar locations on HR diagram
Observations: SEDs & IR Excess
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 All TTs emit in X-Ray
Steady flux Flaring
No correlation between Lx and continuum excess (circumstellar matter) Source must be photospheric
Coronal? Tx too low to be coronal Steady-state flux from unresolved flaring
Observed Features 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
What are the physical mechanisms behind these features?
Line Emission & Stellar Winds
~1/4 of CTTSs show broad Hα profiles
Populated n=3 state but unionized H: 5,000 K < T < 10,000K
Width-> v~200 km/s for thermal broadening T~106 K - would ionize H Bulk motion
~3/4 of CTTSs show blueshifted absorption dip 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
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 Conclusion: Winds are caused by circumstellar disks?
Not necessarily true! Lots of possibilities
Mass Inflow: YY Orionis Stars
Subclass of TTs ~1/2 of CTTSs
Show mass infall!!
Redshifted H absorption at 250km/s
Increasingly deep in Balmer series Einstein A increases w/
Balmer series & traces optical depth
Mass Inflow (cont’d) Absorption increases w/ decreasing optical depth
Infall occurs close to star One idea: Mass falling in on magnetic loops
To measure redshifted absorption must start with broad Hα Limited to CTTSs
Such profiles are highly variable Mass infall fluctuates
Circumstellar Disks 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 Seen in CTTSs
mm emission from disk within 100 AU – low density gas component Seen in both CTTSs and
WLTTs
Circumstellar Disks (cont’d) Disk modelling is v. complicated (ask Dick!!) Important to understand disk dynamics to better
understand TTs Disk contribution to luminosity – Active vs. Passive disks Accretion and winds Magnetic fields
Pose an interesting problem CTTSs and WLTTs are of similar age, but show v.
different disk distribution What is causing this?
Variability Known for decades that
TTs are highly variable – often erratic periods
WLTTs have fairly regular, small amplitude periods on order of days or weeks
Variability due to cool spots 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 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 Stars that show sharp
outbursts of energy with ∆mB=4-6
Fast increase and gradual falloff
V1057 Cyg has TT-like spectrum and exhibits FU Orionis behavior
What causes these? 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
Everything you could ask for!
Outstanding Problems Hard to disentangle effects of the many
components of TTs 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: 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.