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

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

3. Empirical Measurements: Eclipsing Binaries Stassun et al. (2004) V1174 Ori M 1 = 1.01 ± 0.015 M sun M 2 = 0.73 ± 0.008 M sun R 1 = 1.34 ± 0.015 R sun R 2 = 1.07 ± 0.011 R sun

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

5. 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)

6. Tests of Models Limited by Inaccurate Stellar Temperatures

7. Models of Siess et al. (2000) MLT  = 1.9 3 10 30 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

8. Using lithium to probe physics of stellar interiors Stassun et al. (2004) Low lithium depletion in V1174 Ori implies low  (inefficient mixing). 1.0 1.5 2.0 V1174 Ori

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

10. 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:

11. 2M0535-05: Summary of Results Stassun et al. (2006, 2007) M 1 = 55 ± 5 M Jup M 2 = 34 ± 3 M Jup R 1 = 0.67 ± 0.03 R sun R 2 = 0.51 ± 0.03 R sun  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

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

13. 2M0535-05: Summary of Results Stassun et al. (2006, 2007) M 1 = 55 ± 5 M Jup M 2 = 34 ± 3 M Jup R 1 = 0.67 ± 0.03 R sun R 2 = 0.51 ± 0.03 R sun 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

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

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

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

17. What you should remember…

18. 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.

19. 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.

20. Stassun et al. (in prep.) A new low-mass eclipsing binary at ~ 1 Myr: Activity implicated again? M 1 = 0.39 ± 0.03 M sun M 2 = 0.38 ± 0.03 M sun R 1 = 1.21 ± 0.06 R sun R 2 = 1.17 ± 0.06 R sun  T  250 K

22. 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

23. Orion Nebula Cluster (Hillenbrand 1997)

24. Different Models, Different Answers! ModelM (Msun) Age (Myr) D’Antona & Mazzitelli (1998) 0.320.7 Palla & Stahler (1999) 0.622.9 Baraffe et al. (1998) 0.9410.1 Theoretical Masses/Ages for 3800K, 0.5 Lsun young star Including typical observational errors in Teff and L

25. Techniques for making dynamical mass measurements Single stars Circumstellar disk “rotation curve” Binary stars Astrometric Spectroscopic Eclipsing TechniqueMass determined? Mass dependence on distance Luminosity dependence on distance Disk kinematics Mtot DD2D2 Astrometric binary M1 + M2 D3D3 D2D2 Disk kinematics + SB2 M1 M2 DD2D2 Astrometric binary + SB2 M1 M2 D2D2 Eclipsing binary M1 M2

26. 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)

27. P = 9.779621 ± 0.000014 days

28. System Geometry (to scale)

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

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