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VLBA Orbits of Young Binary Stars

Rosa M. Torres – CRyA, UNAM

Laurent Loinard – CRyA, UNAM

Amy Mioduszewski – DSOC, NRAO

Luis F. Rodríguez – CRyA, UNAM

Andy Boden – Caltech

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Introduction

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

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d = 160 pc

d = 140 pc

d = 130 pc

30 pc

29 pc

36 pcRosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Taurus

Torres et al. 2009

L.Loinard talk last Tuesday

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B~ 1 G

E ~ 1 MeVwww.nrao.edu

• Phase referencing• Continuum mode at 8.42 GHz• Observations every 2 or 3 months• More frequently for orbits• PMS stars (not masers) in Taurus & Ophiuchus

– Low mass and magnetic– Compact and bright sources

• We detect non-thermal emission coming from stellar magnetospheres:– Gyrosyncrotron emission

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Parallax with VLBA

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T Tau

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

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TTau system with NACO in the Ks photometric band on 2006 Oct. 11 (Köhler et al. 2008)

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

T Tau system

12 observations

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• Initial position at some epoch (usually 2000)• Angular velocity the proper motions• Parallax with the projection of the equation

of the ellipse over the axis of coordinates• Angular acceleration (uniform)

(t) = J2000.0 + t + ½ at2 + × f(,,t)

(t) = J2000.0 + t + ½ at2 + × f(,,t)

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Astrometry

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d = 145 ± 2 pc1.4% error

d = 146 ± 0.6 pc0.4% error

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Proper motion

Loinard et al. 2007

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Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Proper motion

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• Near IR

• VLBA

Fit by Dûchene et al. (2006)P = 21.66 ± 0.93 yr

Fit by Köhler et al. (2008)P = 93+115

-55 yr

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Previous orbit

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P = 25.83 ± 7.9 yr

T Tau Sa T Tau Sb

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Our orbit

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V773 Tau

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

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• Brightest radio source in Taurus• The first multiple system to display such

a variety of evolutionary states

V773 Tau BVisual companionClassical T Tauri

at 0.1 arcsec

V773 Tau C“IR companion”

at 0.2 arcsec

V773 Tau ASpectroscopic binaryT Tauri Weak-LinePeriod of 52 days

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

V773 Tau system

19 observations

A. Boden talk last Tuesday

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Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

First six observations

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Boden et al. 2007

RV

RV

RV

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Orbit

VLBI (Phillips, 1996, Boden et al. 2007)

VLBA

Keck Interferometer (Boden et al. 2007)

In the reference frame of V773 Tau Aa

A. Boden talk last Tuesday

Torres et al. 2009

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• Keck + VLBA dynamical masses and distance– MAa = 1.48 ± 0.12 Msun

– MAb = 1.28 ± 0.07 Msun

– d =134.6 ± 3.6 pc• VLBA distance

– d =134.8 ± 3.8 pc• Photometry IR magnitude

• Spectral temperature

HR diagram ages

luminosity

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Physical parameters

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Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Comparison with models A. Boden talk last Tuesday

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13 observations over 1orbital period (~ 52 days)

6 observationsover 1 year

The dotted linesshow the periastronpassages assumingas a initial epoch t0 = 2449330.94 JD

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Flux evolution 19 epochs

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from epoch #7 to epoch #19

(each panel = 1 epoch)

AverageEach hour

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Total intensity

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• V773 Tau AB suggest that the emitted flux is a function of the separation between the two stars

• Accurate orbit determination for PMS binaries– Dynamical masses– Constrain PMS evolutionary models

• Observations of the structure of the stars at higher resolution reveal variation in the structure of the active magnetosphere

Rosa M. Torres New Science Enabled by Microarcsecond Astrometry Socorro, NM – July 23, 2009

Conclusions