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
Stassun et al. (2004)
V1174 Ori
M1 = 1.01 ± 0.015 Msun
M2 = 0.73 ± 0.008 Msun
R1 = 1.34 ± 0.015 Rsun
R2 = 1.07 ± 0.011 Rsun
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Dynamical Masses of Young Starscirca 2006
N=23
Mathieu et al. (2007)
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Comparison of Dynamical Masses to Theoretical Models
Above 1 Msun:
•Good agreement: Mean difference 10% (1.6
Below 1 Msun:
•Poorer agreement: Mean difference as large as 40% (2.5)
•Tendency to underestimate masses
Best overall agreement is with Baraffe et al:
•Overall consistency to 1.4, though with large scatter, for MLT =1.0.
Hillenbrand & White (2004), updated Mathieu et al. (2007)
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Models of Siess et al. (2000)
MLT = 1.93
1030
1.0
0.7
Stassun et al. (2004)
1.0
0.7
3
10
30
Models of Baraffe et al.
(1998)
MLT = 1.0
1 Myr
1
V1174 Ori
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Using lithium to probe physics ofstellar interiors
Stassun et al. (2004)
Low lithium depletion in V1174 Ori implies
low (inefficient mixing).
1.0
1.5
2.0
V1174 Ori
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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
Stassun et al. (2006, 2007)
M1 = 55 ± 5 MJup
M2 = 34 ± 3 MJup
R1 = 0.67 ± 0.03 Rsun
R2 = 0.51 ± 0.03 Rsun
Non-coeval formation? Dynamical effects, ejection scenarios
Magnetically suppressed convection? Decreased surface temperature Increased radius
Problem with model initial conditions? Starting gravities usually arbitrary
Temperature reversal
Oversized radii
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Mohanty et al. (2004)
Problem with model initial conditions?
Baraffe et al. models
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2M0535-05: Summary of Results
Stassun et al. (2006, 2007)
M1 = 55 ± 5 MJup
M2 = 34 ± 3 MJup
R1 = 0.67 ± 0.03 Rsun
R2 = 0.51 ± 0.03 Rsun
Temperature reversal
Non-coeval formation? Dynamical effects, ejection scenarios
Magnetically suppressed convection? Decreased surface temperature Increased radius
Problem with model initial conditions? Starting gravities are arbitrary
Oversized radii
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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
Flaccomio et al. (2005)
Jardine et al (2006)
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Torres & Ribas (2002)
Chromospherically active main-sequence stars: Oversized radii
Torres et al. (2006)
YY Gem
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 ).
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 models.
2M0535-05: Temperature reversal and oversized radii suggest suppressed convection.
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Stassun et al. (in
prep.)
A new low-mass eclipsing binary at ~ 1 Myr:Activity 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
T 250 K
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How to Determine Mass and Age of a Young Star
Dynamical mass, Radius
Measure:
B.C.
SpT-Teff
Surface gravities of PMS stars?
Distance
Measure:
Mass, age
L, Teff
Models
V, SpT
calibrate
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Orion Nebula Cluster
(Hillenbrand 1997)
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Different Models, Different Answers!
Model M(Msun)
Age (Myr)
D’Antona & Mazzitelli (1998)
0.32 0.7
Palla & Stahler (1999)
0.62 2.9
Baraffe et al. (1998)
0.94 10.1
Theoretical Masses/Ages for 3800K, 0.5 Lsun young star
Including typical observational errors
in Teff and L
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Techniques for making dynamical mass measurements
Single stars Circumstellar disk
“rotation curve”
Binary stars Astrometric Spectroscopic Eclipsing
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
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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?
Stassun et al.
(2003)
Breakup velocity!