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

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

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  1. Empirical Constraints on Physical Properties of Young Low-Mass Stars and Brown Dwarfs Keivan Guadalupe Stassun Physics & AstronomyVanderbilt University

  2. Context: Testing and Calibrating PMS Stellar Evolutionary Models Orion Nebula Cluster (Hillenbrand 1997)

  3. 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)

  4. Dynamical Masses of Young Starscirca 2006 N=23 Mathieu et al. (2007)

  5. 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 a=1.0. Hillenbrand & White (2004), updated Mathieu et al. (2007)

  6. 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) 30

  7. Low lithium depletion in V1174 Ori implies low a(inefficient mixing). 1.0 1.5 2.0 Using lithium to probe physics ofstellar interiors V1174 Ori Stassun et al. (2004)

  8. Case Study: 2M0535-05The First Brown-Dwarf Eclipsing Binary Bob Mathieu (Wisconsin) Jeff Valenti (STScI) Yilen Gomez (Vanderbilt) Luiz Paulo Vaz (UFMG, Brazil) Matthew Richardson (Fisk)

  9. 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:

  10. Temperature reversal Oversized radii 2M0535-05: Summary of Results • Non-coeval formation? • Dynamical effects, ejection scenarios • Magnetically suppressed convection? • Decreased surface temperature • Increased radius • Problem with model initial conditions? • Starting gravities usually arbitrary R1 = 0.67 ± 0.03 Rsun R2 = 0.51 ± 0.03 Rsun M1 = 55 ± 5 MJup M2 = 34 ± 3 MJup Stassun et al. (2006, 2007)

  11. Problem with model initial conditions? Baraffe et al. models Mohanty et al. (2004)

  12. 2M0535-05: Summary of Results Temperature reversal Oversized radii • Non-coeval formation? • Dynamical effects, ejection scenarios • Magnetically suppressed convection? • Decreased surface temperature • Increased radius • Problem with model initial conditions? • Starting gravities are arbitrary R1 = 0.67 ± 0.03 Rsun R2 = 0.51 ± 0.03 Rsun M1 = 55 ± 5 MJup M2 = 34 ± 3 MJup Stassun et al. (2006, 2007)

  13. Chandra Orion Ultradeep Project (COUP) Simultaneous optical/X-ray monitoring of 800 TTS Stassun et al. (2006, 2007)

  14. Rotationally modulated X-ray emission: Highly structured, strong surface fields Jardine et al (2006) Flaccomio et al. (2005)

  15. Chromospherically active main-sequence stars: Oversized radii YY Gem Torres et al. (2006) Torres & Ribas (2002) V1016 Cyg

  16. What you should remember…

  17. 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.

  18. 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.

  19. DT  250 K 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 Stassun et al. (in prep.)

  20. Measure: Measure: V, SpT Dynamical mass, Radius Distance B.C. L, Teff SpT-Teff Surface gravities of PMS stars? calibrate Models Mass, age How to Determine Mass and Age of a Young Star

  21. Orion Nebula Cluster (Hillenbrand 1997)

  22. Different Models, Different Answers! Theoretical Masses/Ages for 3800K, 0.5 Lsun young star Including typical observational errors in Teff and L

  23. Techniques for making dynamical mass measurements Single stars • Circumstellar disk “rotation curve” Binary stars • Astrometric • Spectroscopic • Eclipsing

  24. 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)

  25. P = 9.779621 ± 0.000014 days

  26. System Geometry (to scale)

  27. Flare analysis: Solar-type flaring loops Brightest flares require loops ~10 R* in size. Angular momentum losses likely severe. Favata et al. (2005)

  28. Possible importance of rapid stellar rotation? Breakup velocity! Stassun et al. (2003)