Brown Dwarfs : Up Close and Physical

1. Brown Dwarfs : Up Close and Physical In the mass range intermediate between stars and planets are the substellar objects known as brown dwarfs. The first BDs were discovered in 1995. The first confirmations were based in one case on interior properties (the lithium test), and in the other case on external temperature (the methane test). I will concentrate on what we have learned about their physical properties. We are only beginning to directly test masses and evolutionary models, but are learning about temperatures and atmospheric properties. I also touch on rotation, magnetic and accretion activity, in young and old BDs over their entire mass range. I will NOT cover many topics, including search techniques and results, the mass function or space density, binarity, or formation mechanisms. Gl 229

2. Luminosity History of Low Mass Objects Burrows et al Minimum Stellar Temperature You could view brown dwarfs as stars which only have a deuterium main sequence (which is short). Regular stars also have hydrogen and helium main sequences, and massive stars have additional burning phases for heavier elements.

3. History of Substellar Sizes Burrows et al.

4. Core Temperature depends on Age and Mass Lithium & Hydrogen burning limit Deuterium burning limit

5. The Lithium Test Basri 1997

6. The New Cool Spectral Types : L Dwarfs “L” and “T” have been added to cover the changes in spectra at very cool temperatures. The L dwarfs are marked by a change from domination by oxide to hydride molecular species. Refractory metals condense out. This has big ramifications in the optical. 2200K – 1400K Kirkpatrick et al. 1999

7. Spectra of L Dwarfs Geballe et al. 2002

8. The New Cool Spectral Types : T Dwarfs The infrared spectrum shows methane in preference to carbon monoxide; the optical spectrum is dominated by resonance line wings of alkali metals. 1300K – 700K

9. Alkalai Resonance Lines in the Optical In very cool objects, the lines of sodium and potassium dominate the optical opacity. This yields a “magenta” color for brown dwarfs. The first measurement on an extrasolar planet shows sodium Burrows

10. Spectra of T Dwarfs Geballe et al. 2002

11. Spectral Typing by IR Molecular Indices Burgasser et al. 2002

12. Dust formation Allard 1999 The opacities and atmospheric chemistry in brown dwarfs becomes increasingly tied to the physics of condensates. Tsuji 2002 Basri 1997

13. Atmospheric Structure Changes Tsuji 1300K condensation No condensation Marley et al. 2002

14. Cloud Formation in Brown Dwarfs The formation of clouds is poorly understood (not that great even here on Earth). Particle size distributions, saturation regimes, horizontal inhomogeneities, global and turbulent currents are all crucial to how optically thick the clouds will be, what their height of formation, thickness, and covering fraction is, and knowing when precipitation will occur. Observed spectra may reflect a blend of different heights and compositions.

15. Photometric Evidence of Rotation and Weather The vsini of BDs implies that the rotation periods should be hours. Direct evidence for this has been found. Some vary on longer timescales; this could be due to condensation features (clouds). 4.5 hr Several days – not periodic: weather (dust clouds)? Gelino et al. 2002 7.5 hr

16. The Weather Report for Brown Dwarfs Burgasser et al. 2002 Dusty Clear Partly Cloudy Cloudy

17. Effect of Clouds on Spectra Models for T=500K, 1000K, 1500K. Flat spectra result if dust clouds are optically thick; spectral features are for clear atmospheres. Marley et al. 2002 There is evidence for cloud formation and then clearing in the behavior of FeH near the L/T transition. The FeH should disappear as liquid iron droplets form, but it reappears even as the temperature cools further, likely due to breaks in the clouds that expose hotter interior regions. Burgasser et al. 2002

18. Atmospheric Changes with Spectral Type L M L/T T Y?

19. “Fine Analysis” vs Structural Effective Temperatures “Structural” temperatures are defined by measuring the luminosity (from photometry adjusted with a bolometric correction), combined with the parallax, then using theory to define a relation between bolometric luminosity and radius. High resolution spectroscopy yields results that don’t quite seem to agree with theoretical models (problems may be bolometric corrections, atmospheric models). The cooler objects are inferred to be smaller by spectroscopy than in the models. Smith et al. 2003 ApJ

20. Rotation and Magnetic Activity on Brown Dwarfs Solar-type stars form with a variety of rotations, perhaps due to disk-locking. They initially show signs of accretion and outflow. They are active in their youth because of relatively rapid spin. The fields carry off angular momentum, and the stars spin down and become less active. Does this story apply all the way down the main sequence into brown dwarfs? Does this story even extend below the brown dwarf mass limit?

21. Ha falls at the bottom of the Main Sequence There is a dramatic fall-off in activity at the cool end of the M spectral type. Is this due to rotation, or something else? Gizis et al. 2001 Activity in L dwarfs is very minimal; almost none have detected Ha or X-rays. All objects cooler than about L3 are brown dwarfs (and significant fraction above).

22. Rotation in very cool objects Basri Mohanty 2000,2003 Very Low Mass Stars Stellar and Substellar Objects Brown Dwarfs The decrease in activity is clearly NOT due to slow rotation! Rather, the increase in spindown times is due to temperature. The atmospheres are becoming very neutral, and cannot couple to the magnetic field to remove angular momentum.

23. Accretion and Activity in Young BDs:Ha Strength vs Width Going to very early ages, activity is generally stronger (the objects are warmer), and some of them show accretion from disks. The width of Ha can be used as a direct accretion diagnostic in high-resolution spectra. Jayawardhana, Mohanty, Basri 2003 These are late M types (5.5-9.5)

24. Rotation vs Ha strength in Young BDs Evidence for disk-locking? Accretion line

25. Deriving Fundamental Physical Parameters • For objects in a star-forming region, one might hope to get their fundamental stellar parameters (testing the untested evolutionary calculations for low masses and young ages). • The procedure is: • Find an effective temperature from a spectroscopic diagnostic that is largely temperature-dependent • Find a surface gravity from a pressure-sensitive line • Get the radius from the luminosity (which obtains from the observed brightness, coupled with a known distance to the region) and derived temperature • Find the mass from the radius and surface gravity • Assume all objects are coeval and check with isochrones Note: there have been no fundamental mass determinations for visible substellar objects, nor has there been confirmation of the claims that some of these are below the fusion boundary.

26. Sensitivities to Teff and log(g) TiO is sensitive primarily to temperature. NaI is sensitive to both temperature and gravity. Mohanty et al. 2004

27. Breaking the Degeneracy One can get good fits for different combinations of T and g in both TiO and NaI. For NaI an increase of log(g)=0.5 can be offset by an increase of T=200K. There is only one set of parameters that works for both. This is further confirmed by checking the TiO region surrounding NaI.

28. Deriving Fundamental Physical Parameters • For objects in a star-forming region, one might hope to get their fundamental stellar parameters (testing the untested evolutionary calculations for low masses and young ages). • The procedure is: • Find an effective temperature from a spectroscopic diagnostic that is largely temperature-dependent • Find a surface gravity from a pressure-sensitive line • Get the radius from the luminosity (which obtains from the observed brightness, coupled with a known distance to the region) and derived temperature • Find the mass from the radius and surface gravity • Assume all objects are coeval and check with isochrones Note: there have been no fundamental mass determinations for visible substellar objects, nor has there been confirmation of the claims that some of these are below the fusion boundary.

29. Mass-Luminosity Relation GG Tau Ba (!) GG Tau Bb We confirm that the lowest free-floating objects being found may be below the D-burning limit!

30. Radius and Temperature vs Mass GG Ba GG Bb Once again we find a problem between temperatures found by high resolution spectroscopy and models. The slope of the M-T relation is wrong, and the radii of very low-mass objects are too small in the evolutionary models. It is amazing that the model spectra can fit so well in detail if the model atmospheres are wrong (and clouds don’t form). GG Bb

31. Model Gravities and Ages GG Tau Ba GG Tau Bb Mohanty et al. 2004

32. Evolutionary Model Uncertainties A somewhat arbitrary starting point is used (without accounting for accretion effects): >30 jupiter start at D ignition; <30 jupiter start at log(g)=3.5 (higher than what we measure). While D burning is occurring, the collapse of the object is slowed, so this can cause objects to remain at lower gravity and larger radius. These initial conditions will cause very low mass objects to appear too young for the first 1.5 Myr. This problem should be gone, however, by the age of Upper Sco If D burning really starts at lower gravity (3.25) for the lowest mass objects, they take a very long time to complete it (>20 Myr), so they could be hung up in the state we find them (while 30 jupiter objects would be done by 5 Myr). The gravity at which D-burning starts has decreased by 40% in the last 10 years in the D’Antona/Mazzitelli models.

33. Conclusions • We have learned a lot about “substellar” objects in 8 years! • We have seen a large range of masses, temperatures, and ages for substellar objects, down to the substellar mass limit. • Model atmospheres do amazingly well at reproducing spectra, but there is still cause for refinements (especially in the infrared). • Dust and cloud formation, along with precipitation and meteorology, are key to understanding the appearance of some objects, but are very complicated and much work remains in this area. • The magnetic and angular momentum history of these objects is very different from all but the lowest mass stars. • Evolutionary models have many good features, but we cannot consider them well-tested yet.

34. Thank You!