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.