Empirical Constraints on Physical Properties of Young Low-Mass Stars and Brown Dwarfs

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Empirical Constraints on Physical Properties of
Young Low-Mass Stars and Brown Dwarfs
Keivan Guadalupe Stassun Physics
AstronomyVanderbilt University
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Context Testing and Calibrating PMS Stellar
Evolutionary Models
Orion Nebula Cluster (Hillenbrand 1997)
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Empirical Measurements Eclipsing Binaries
M1 1.01 0.015 Msun M2 0.73 0.008 Msun R1
1.34 0.015 Rsun R2 1.07 0.011 Rsun
V1174 Ori
Stassun et al. (2004)
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Dynamical Masses of Young Starscirca 2006
N23
Mathieu et al. (2007)
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Comparison of Dynamical Masses to Theoretical
Models

• Above 1 Msun
• Good agreement Mean difference 10 (1.6s)
• Below 1 Msun
• Poorer agreement Mean difference as large as 40
  (2.5s)
• Tendency to underestimate masses
• Best overall agreement is with Baraffe et al
• Overall consistency to 1.4s, though with large
  scatter, for MLT a1.0.

Hillenbrand White (2004), updated Mathieu et
al. (2007)
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Tests of Models Limited by Inaccurate Stellar
Temperatures
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1.0
V1174 Ori
0.7
1 Myr
Models of Siess et al. (2000) MLT a 1.9
3
10
30
1.0
Models of Baraffe et al. (1998) MLT a 1.0
0.7
1
3
10
Stassun et al. (2004)
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Using lithium to probe physics ofstellar
interiors
V1174 Ori
Stassun et al. (2004)
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Case Study 2M0535-05The First Brown-Dwarf
Eclipsing Binary
Bob Mathieu (Wisconsin)
Jeff Valenti (STScI)
Yilen Gomez (Vanderbilt)
Matthew Richardson (Fisk)
Luiz Paulo Vaz (UFMG, Brazil)
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Prior to 2M0535-05

• Dynamical mass measurements of brown dwarfs
• GJ 1245 c 0.074 0.013 Msun
• 2M0746 b 0.066 0.006 Msun
• GJ 802 b 0.058 0.021 Msun
• GJ 569 c 0.052 0.018 Msun
• Direct radius measurements of brown dwarfs

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2M0535-05 Summary of Results
R1 0.67 0.03 Rsun R2 0.51 0.03 Rsun
M1 55 5 MJup M2 34 3 MJup

• Non-coeval formation?
• Dynamical effects, ejection scenarios
• Magnetically suppressed convection?
• Decreased surface temperature
• Increased radius
• Problem with model initial conditions?
• Starting gravities usually arbitrary

Stassun et al. (2006, 2007)
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Problem with model initial conditions?
Baraffe et al. models
Mohanty et al. (2004)
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2M0535-05 Summary of Results
Temperature reversal
Oversized radii
R1 0.67 0.03 Rsun R2 0.51 0.03 Rsun
M1 55 5 MJup M2 34 3 MJup

• Non-coeval formation?
• Dynamical effects, ejection scenarios
• Magnetically suppressed convection?
• Decreased surface temperature
• Increased radius
• Problem with model initial conditions?
• Starting gravities are arbitrary

Stassun et al. (2006, 2007)
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Chandra Orion Ultradeep Project (COUP)
Simultaneous optical/X-ray monitoring of 800
TTS Stassun et al. (2006, 2007)
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Rotationally modulated X-ray emission Highly
structured, strong surface fields
Jardine et al (2006)
Flaccomio et al. (2005)
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Chromospherically active main-sequence stars
Oversized radii
YY Gem
Torres et al. (2006)
Torres Ribas (2002)
V1016 Cyg
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What you should remember
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Take-Away Message 1

• Empirical constraints on the fundamental physical
  properties of young, low-mass stars and brown
  dwarfs are improving.
• Masses and radii accurate to 1 (eclipsing
  binaries), including first masses and radii for
  young brown dwarfs.

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Take-Away Message 2

• Evidence for magnetically suppressed convection
  in young, low-mass stars and brown dwarfs
• Empirical mass determinations Best matched by
  theoretical models with inefficient convection
  (i.e. low a).
• Lithium Low levels of depletion imply
  inefficient mixing.
• X-rays from PMS stars Most consistent with
  highly structured, strong surface fields.
• Magnetically active main-sequence binaries Show
  oversized radii, most consistent with low a
  models.
• 2M0535-05 Temperature reversal and oversized
  radii suggest suppressed convection.

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A new low-mass eclipsing binary at 1
MyrActivity implicated again?
M1 0.39 0.03 Msun M2 0.38 0.03 Msun
R1 1.21 0.06 Rsun R2 1.17 0.06 Rsun
Stassun et al. (in prep.)
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How to Determine Mass and Age of a Young Star
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Orion Nebula Cluster (Hillenbrand 1997)
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Different Models, Different Answers!
Theoretical Masses/Ages for 3800K, 0.5 Lsun
young star
Model M(Msun) Age (Myr)
DAntona Mazzitelli (1998) 0.32 0.7
Palla Stahler (1999) 0.62 2.9
Baraffe et al. (1998) 0.94 10.1
Including typical observational errors in Teff
and L
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Techniques for making dynamical mass measurements
Technique Mass determined? Mass dependence on distance Luminosity dependence on distance
Disk kinematics Mtot D D2
Astrometric binary M1 M2 D3 D2
Disk kinematics SB2 M1M2 D D2
Astrometric binary SB2 M1M2 D2
Eclipsing binary M1M2

• Single stars
• Circumstellar disk rotation curve
• Binary stars
• Astrometric
• Spectroscopic
• Eclipsing

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Measuring Accurate Stellar Temperatures A
Pressing Issue

• Need to securely anchor stars in the HR diagram
• Current SpTy errors 1 spectral subtype 150
  K
• SpTy-Temp scale at least doubles this uncertainty
• Detailed spectral synthesis and modeling 50 K
• Detailed study underway (Stassun Doppmann in
  prep.)

Doppmann et al. (2005)
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P 9.779621 0.000014 days
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System Geometry (to scale)
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Flare analysis Solar-type flaring loops
Brightest flares require loops 10 R in size.
Angular momentum losses likely severe.
Favata et al. (2005)
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Possible importance of rapid stellar rotation?
Breakup velocity!
Stassun et al. (2003)