T tauri stars
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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

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T Tauri Stars

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T tauri stars

T Tauri Stars

Kate Barnes

A540


T tauri stars background

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

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

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

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 of h

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

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

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

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

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

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

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


Mass inflow yy orionis stars

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

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

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

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

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

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

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

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

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

References

  • Bertout, C. 1989. ARAA, 27, 351

  • Stahler, S.W. & Palla, F. The Formation of Stars. 2004: Wiley-VCH.


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