T Tauri Stars Kate Barnes A540

T Tauri Stars: Background Very young, solar-type stars ~10 7 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: W λ of H α T Tauri stars are grouped into one of two types: Classical T Tauri Stars (CTTSs) Weak-lined T Tauri Stars (WLTTs) Grouped by the W λ of H α CTTSs have W λ (H α ) > 10 Å WLTTs have W λ (H α ) < 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 L x and continuum excess (circumstellar matter) Source must be photospheric Coronal? T x too low to be coronal Steady-state flux from unresolved flaring

Observed Features WLTTs do not emit in H α 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 H α profiles Populated n=3 state but unionized H: 5,000 K < T < 10,000K Width-> v~200 km/s for thermal broadening T~10 6 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 M yr -1

One Idea of the presence of winds… H α 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 (contd) 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 H α 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 (contd) 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 ~10 3 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 m B =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: Whats 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 ARAA, 27, 351 Stahler, S.W. & Palla, F. The Formation of Stars. 2004: Wiley-VCH.