T Tauri Stars

1. T Tauri Stars Kate Barnes A540

2. 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

3. 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

4. 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

5. 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?

6. 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

7. 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!

8. 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

9. 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?

10. 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

11. 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

12. One Idea of the presence of winds… • Hαand 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

13. 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

14. 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

15. 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

16. 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?

17. 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

18. 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

19. 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 • ???

20. 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!

21. 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?

22. References • Bertout, C. 1989. ARAA, 27, 351 • Stahler, S.W. & Palla, F. The Formation of Stars. 2004: Wiley-VCH.