1. Brown Dwarfs Substellar, low-mass stars
Between least massive star and the most massive planet
Ultra-cool Dwarf
M, L, T, Y Dwqarfs

2. Brown Dwarf Star Joins The Jet-set
Discovery of a Bipolar Outflow from 2MASSW J a 24 MJup Brown Dwarf belongs to the TW Hydrae Association and is therefore about 8 million years old. ", by E.T. Whelan et al

3. Oh Be A Fine Girl Kiss Me Later Tonight Yes
L and T Dwarfs*
Oh Be A Fine Girl Kiss Me Later Tonight Yes
History of discovery
Spectral types/properties
Interiors of low mass stars
Evolution of low mass stars
Photospheres of low mass stars

4. A Little History
Substantial effort in ’80s and early ’90s to find very low mass M dwarfs
Parallax surveys of high proper motion red objects
Companions to M dwarfs, WDs (IR excesses)
Companion to vB8 – NOT
Companion to G29-38 – NOT
Companion to GD165B – YES! the first L dwarf
Spectrum not understood until more found
Gl 229B the first T dwarf
IR Colors surprisingly blue
Note change in slope – H2

5. Brown Dwarfs definition
Stellar
Fusion (at least deutrium) in interior
Never stabilize L or T
Grow fainter and cooler with time
0.072 Mo (75 MJ) with solar composition (90 MJ with zero metallicity) MJ

6. First confirmed BD (1995) Gl 229B : companion of a nearby M star
8 pc Nearby Low-Mass Star Survey :
Nakajima et al (1995),
among 200 targets  only one
CH4, at 2 um (Oppenheimer et al ’95)
 T dwarf  40 MJ (20 – 50), Te = 900

7. Gl229B (HST)and Jupiter

8. First L Dwarf : GD 165B Survey of WD Becklin & Zuckerman (1988)
very red and faint companion
enigmatic spectrum (Kirkpatric et al : probably BD)
photospheric Dust formation
 Hydrides, FeH, CrH, CaH
strong neutral alkalic lines, NaI, KI, CsI

9. M, L, T, Y dwarf Temperature
M dwarf : 3700 – 2100
L dwarf :
L0 – L8 : 2000 – 1400
(Kirkpatric et al ’98)
L0 – L9 : 2200 – 1300
(Martin et al ’99)
T dwarfs :
Y dwarf : ~ 500

10. BDs In M, L, T dwarfs Not all M stars are H-burning stars
Nor all L dwarfs are BDs
All T dwarfs are BDs
BD confirmed by mass and age
Massive BD starts at mid M (> M6-7)
 L Type
 T Type with Time

11. Ultra –cool Dwarfs Old Dwarfs in the Field (> 1 Gyr)  nearby
Their young Progenitors in the nearest star-forming regions and open clusters ( 1 – 100 Myr)
 young
(luminosity class of them is closer to subgiant than dwarf)

12. Searches for Ultra-cool dwarfs
In nearby open clusters and star-forming regions
100 BD candidates

13. IR Color-Sp Types/Color-Color Diagram

14. Flux Densities for a Star with m=0
Band l Fl
J *10-13 Wcm-2mum-2 H
K *10-14
L *10-15

15. Magnitudes of Known BDs
Obj Sp K L’ Ls L L L T T T
L: , L’: , Ls:

16. Photometric Variability at the L/T Boundary
Early L dwarf : atm with thin silicate and
iron cloud
Late L Dwarf : atm with thick cloud
T Dwarf : clean atm
J-Ks  redder for late L type
to blue for T type
Atm change across the L/T types (very small range < 350 도)
 substantial variability
= break of cloud layers

17. Brown Dwarfs Abound! Many L and T dwarfs have now been found
Improved IR detectors
Better spatial resolution (seeing improvements, AO)
IR and multi-color surveys (2MASS, DENIS, and Sloan)
Breakthrough in understanding appearance of spectra
Significant progress in modeling low mass stellar and substellar objects
Understood in the late ’50s (Limber) that
low mass stars must be fully convective
Electron degeneracy must play a role
H2 formation also important (change in slope of main seq. at 0.5 MSun)
Kumar figured out (in the early ’60s) that a minimum mass is needed for H burning
Grossman et al. included deuterium burning (early ’70s)
Recent improvements include better equation of state and grain formation

18. Minimum Mass for H Burning
As protostar collapses, core temperature rises
Low mass stars must collapse to higher densities before temperature high enough for fusion
As density increases, core becomes partially degenerate
An increasing fraction of energy from collapse goes into compressing degenerate gas
Degeneracy stops star from collapsing below 0.1 RSun (and the core temperature can’t get any higher than this)
What happens to the star?
If M>0.09MSun, core fusion is possible and sustainable for many Hubble times
For MSun, degeneracy lowers central temperature, but it’s still hot enough for hydrogen fusion (main sequence)
At Msun, core temperature is initially hot enough, but degeneracy cools the core and fusion stops – “transition object”
For lower masses (M<0.07MSun), the core is never hot enough for fusion, brown dwarf cools to oblivion
Stellar mass limit somewhere between transition object and brown dwarf

19. Brown Dwarfs Stellar evolution Adiabatic contraction (Hayashi tracks)
(1)
(2)
Adiabatic contraction (Hayashi tracks)
Ignition, formation of radiative core, heating – dynamic equilibrium (Henyey tracks)
Settle onto Hydrogen main sequence – radiative equilibrium
(3)
Hayashi (1965)

20. Brown Dwarfs
PPI chain:
p + p → d + e+ + e, Tc = 3  106 K
Below ~0.1 M, e- degeneracy becomes significant in interior (Pcore ~ 105 Mbar, Tcore ~ TFermi) and will inhibit collapse.
Below ~ M, Tcore remains below critical PPI temperature  Cannot sustain core H fusion.
Kumar (1963)

21. Brown Dwarfs 10 20 30 40 50 60 70 75 80 90
With no fusion source, Brown dwarfs rapidly evolve to lower Teff and lower luminosities.
Stars
BDs
“… cool off inexorably like dying embers plucked from a fire.”
A. Burrows

22. Some Brown Dwarf Properties
Interior conditions: ρcore ~ g/cm3, Tcore ~ K, Pcore ~ 105 Mbar, fully convective, largely degenerate (~90% of volume), predominantly metallic H (exotic?).
Atmosphere conditions: Pphot ~ 1-10 bar, Tphot ~ K and lower.
All evolved brown dwarfs have R ~ 1 RJupiter.
Age/Mass degeneracy: old, massive BDs have same Teff, L as young, low-mass BDs.
Below Teff ~ 1800 K, all objects are substellar.
NBD ~ N*, MBD ~ 0.15 M*
Extrapolate Salpeter to 0.001(0.005) Msun => 4(2)x more mass in BDs
Current estimates: NBD ~ N*, MBD ~ 0.15 M*
MJeans ~ 1 Msun
BD/EGP models – Baraffe et al. 2003, Burrows et al. 1997, etc
Degeneracy era – yr, ~1/2 of all objects BDs, ~50 BD collision-made stars in Galaxy at any time

23. Why Brown Dwarfs Matter
Former dark matter candidates - no longer the case.
Important and populous members of the Solar Neighborhood.
End case of star formation, test of formation scenarios at/below MJeans.
Tracers of star formation history and chemical evolution in the Galaxy.
Analogues to Extra-solar Giant Planets (EGPs), more easily studied.
Last source of stars in distant future of non-collapsing Universe - Adams & Laughlin (RvMP, 69, 337, 1997).
Extrapolate Salpeter to 0.001(0.005) Msun => 4(2)x more mass in BDs
Current estimates: NBD ~ N*, MBD ~ 0.15 M*
MJeans ~ 1 Msun
BD/EGP models – Baraffe et al. 2003, Burrows et al. 1997, etc
Degeneracy era – yr, ~1/2 of all objects BDs, ~50 BD collision-made stars in Galaxy at any time

24. M, L, and T dwarfs Three spectral classes encompass Brown Dwarfs:10 20 30 40 50 60 70 75 80 90
Three spectral classes encompass Brown Dwarfs:
M dwarfs ( K): Young BDs and low-mass stars.
L dwarfs ( K): BDs and very low-mass, old stars.
T dwarfs (< 1300 K): All BDs; coolest objects known.

25. Evolutionary Models Deuterium burning Hydrogen burning
Transition objects may burn for ~10 Gyr
At a given luminosity, it is hard to distinguish between young brown dwarfs and older stars

27. M Dwarf Spectral Types Molecular species switch from MgH to TiO
CaOH appears in later M dwarfs
Prominent Na D lines
Spectral types determined in the blue

29. Later Spectral Classes
TiO disappears to be replaced by water, metal hydrides (FeH, CrH)
Alkali metal lines strengthen (note K I in the L8 dwarf)
Spectral types determined from red, far red spectra (blue too faint!)

30. L-type Spectral Sequence
K I line strength increases with later spectral type
Li I appears in some low mass stars (m < 0.06 solar masses)
Appearance of FeH, CrH
Strength of Cs I
Strength of water
Disappearance of TiO
Absence of FeH, CrH in T dwarf, much increased strength of water

31. M, L, and T Dwarfs in the IR
M dwarfs are dominated by TiO, VO, H2O, CO absorption plus metal/alkali lines.
L dwarfs replace oxides with hydrides (FeH, CrH, MgH, CaH) and alkalis are prominent.
T dwarfs exhibit strong CH4 and H2O and extremely broadened Na I and K I.

32. Li in Brown Dwarfs Li I appears in about a third of L dwarfs
EQW from 1.5 to 15 Angstroms
Li I can be used to distinguish between old, cooled brown dwarfs and younger, lower mass dwarfs

33. Lithium Test for Brown Dwarfs' confirmation
Destruction of lithium nuclei in true stars, but not in brown dwarfs.
The high temperatures in star's core promote high-energy collisions between a lithium-7 nucleus (consisting of three protons and four neutrons) and a proton, producing two helium-4 nuclei. Since even the coolest stars (red dwarfs) attain sufficient temperatures to destroy lithium through the burning of hydrogen, all true stars lack this element. In contrast, since brown dwarfs cannot sustain the burning of hydrogen they do not destroy lithium

34. Evolution of Lithium
At a given Teff,Stars with Li are lower mass than stars with Li depleted.

35. Lithium Test Very Low Mass Star : fully convective
little over 10^8 year Li depleted
BD : Strong Li resonance line 6707 A
T = 3000 – 1500
un-depleted Li  Young BDs
de-saturation starts~ 10 M years
Li boundary star/BD
Useful age dating : M years

36. IR Spectra L dwarf IR spectra are dominated by water and CO
T dwarf IR spectra dominated by water and methane
H2O
H2O
H2O
methane
methane

37. T Dwarf Spectrum (NIR)

38. As a T dwarf becomes cooler (i. e
As a T dwarf becomes cooler (i.e., methane and water absorptions increase) or more distant…
SDSS detects it only at z’ band
2MASS detects it only at J band

39. SIRTF (IRAC, 3.6, 4.5, 5.8 & 8.0 micron, T. L Dwarfs)

40. Mid IR Spectrum (T, L Dwarfs)

41. M Dwarf Spectra Are a Mess
Observed spectrum of M8 V dwarf VB10
Black body and H- continuum spectra shown as dashed lines
Real spectrum doesn’t match either
Spectrum dominated by sources of opacity

42. Opacities Bound-bound opacities – molecules Bound-free opacities
TiO, CaH + other oxides & hydrides in the optical
H2O, CO in the IR
~109 lines!
Bound-bound molecular line opacities dominate the spectrum
Bound-free opacities
Atomic ionization, molecular dissociation
Free-free opacities – Thomson and Rayleigh scattering
In metal-poor low mass stars, pressure induced absorption of H2-H2 is important in the IR (longer than 1 micron)
H2 molecules have allowed transitions only at electric quadrupole and higher order moments, so H2 itself is not significant
Also significant van der Waals collisional (pressure) broadening of atomic and molecular lines, making these lines much stronger than they would otherwise be
At even cooler temperatures (T~ ) CO is depleted by methane formation (CH3) – the transition from L to T dwarfs

43. Opacities at 2800K
Solar metallicity
[Fe/H]=-2.5

44. Stellar Models General assumptions include
Plane parallel geometry
Homogeneous layers
LTE
Surface gravities: log g ~ 5.0
Convection using mixing length
Convection is important even at low optical depth (t<0.01)
Strength of water absorption depends on detailed temperature structure and treatment of convection
For Teff < 3000 K, grains become important in atmospheric structure (scattering)

45. Dust Dust formation is important in M, L, and T dwarfs
Depletes metals, including Ti
Dust includes
Corundum (Al2O3)
Perovskite (CaTiO3), condensing at T < K
Iron (Fe)
VO, condensing at T < K
Enstatite (MgSiO3)
Forsterite (Mg2SiO4)
Double-metal absorbers weaken (VO, TiO)
Hydride bands dominate
Dust opacity causes greenhouse heating – outgoing IR radiation trapped by extra dust-grain opacity
Heating dissociates H2O, giving weaker water bands
Dust settles gravitationally, depleting metals and leaving reduced opacities (time scales unclear)
Dusty models fit observed flux distributions better

46. Alkali Lines
Alkali lines very prominent in L dwarf spectra (Li, Na, K, Cs, Rb)
Strong because of very low optical opacities
TiO, VO are gone
Dust formation also removes primary electron donors, so H- and H2- opacities are also reduced
High column density due to low optical opacity leads to very strong lines
K I lines at 7665 and 7699 A have EQWs of several hundred Angstroms
Na D lines also become very strong

47. And More Dust As temperature falls:
CO depleted to form methane at temperatues < K
But Na may condense onto “high albite” (NaAlSi3O8)
CrH condenses at T=1400 K
Alkali elements expected to form chlorides at T < 1200

48. Dust and Clouds in Brown Dwarfs
Cool brown dwarf atmospheres have the right conditions to form condensates or dust.
Observations support the idea that these condensates form cloud structures.
Cloud structures are probably not uniform, likely disrupted by atmospheric turbulence.
Clouds have significant effects on the spectral energy distributions of these objects and analogues (e.g., Extra-solar giant planets).

49. Condensation in BD Atmospheres
At the atmospheric temperatures and pressures of late-M and L dwarfs, many gaseous species are capable of forming condensates.
e.g.:
TiO → TiO2(s), CaTiO3(s)
VO → VO(s)
Fe → Fe(l)
SiO → SiO2(s), MgSiO3(s)
Marley et al. (2002)

50. Evidence for Condensation - Spectroscopy
Relatively weak H2O bands in NIR compared to models require additional smooth opacity source.
The disappearance of TiO and VO from late-M to L can be directly attributed to their accumulation onto condensate species.
Kirkpatrick et al. (1999)

51. Evidence for Condensation - Photometry
The NIR colors of late-type M and L dwarfs are progressively redder – can only be matched by models that allow dust formation in their atmospheres.
However, bluer colors of T dwarfs require a transparent atmosphere – dust must be removed.
Dusty
Gliese 229B
Cond
Chabrier et al. (2000)

52. Evidence for Rainout - AbundancesL
Without the rainout of dust species, Na and K would form Feldspars and atomic species would be depleted in the late L dwarfs.
Burrows et al. (2002)

53. Evidence for Rainout - AbundancesL
With rainout, Na and K persist well into the T dwarf regime.
Burrows et al. (2002)

54. Evidence for Rainout - Abundances
K I (and Na I) absorption is clearly present in the T dwarfs  dust species must be removed from photosphere.
Burgasser et al. (2002)

55. Cloudy Models for BD Atmospheres
Condensate clouds dominate visual appearance and spectrum of every Solar giant planet – likely important for brown dwarfs.
Condensates in planetary atmospheres are generally found in cloud structures.
Requires self-consistent treatment of condensable particle formation, growth, and sedimentation.
Ackerman & Marley (2001); Marley et al. (2002); Tsuji (2002); Cooper et al. (2003); Helling et al. (2001); Woitke & Helling (2003)

56. Evidence for Cloud Disruption - Variability
Many late-type L and T dwarfs are variable, P ~ hours, similar to dust formation rate.
Atmospheres too cold to maintain magnetic spots  clouds likely.
Periods are not generally stable  rapid surface evolution.
Enoch, Brown, & Burgasser (2003)

57. Evidence for Cloud Disruption - Spectroscopy
Strengthening of K I higher-order lines around 1m  reduced opacity at these wavelengths from late L to T.
Burgasser et al. (2002)

58. Evidence for Cloud Disruption - Spectroscopy
Reappearance of condensate species progenitors (e.g., FeH)  detected below cloud deck.
Burgasser et al. (2002)

59. Evidence for Cloud Disruption - Spectroscopy
Presence of CO in Gliese 229B’s atmosphere 16,000x LTE abundance  upwelling convective motion.
Oppenheimer et al. (1998)

60. A Partly Cloudy Model for BD Atmospheres
An exploratory model.
Linear interpolation of fluxes and P/T profiles of cloudy and clear atmospheric models.
New parameter is cloud coverage percentage (0-100%).
Burgasser et al. (2002), ApJ, 571, L151

61. The Transition L → T Dramatic shift in NIR color (ΔJ-K ~ 2).
Dramatic change in spectral morphology.
Loss of condensates from the photosphere.
Objects brighten at 1 mm.
Apparently narrow temperature range: Gl 584C (L8) ~ 1300 K MASS 0559 (T5) ~ 1200 K.

62. Success…? Cloud disruption allows transition to brighter T dwarfs.
Requires very rapid rainout at L/T transition, around 1200 K.
Data fits, model is physically motivated, but is it a unique solution?
Burgasser et al. (2002)

63. Temperature Calibration
Spectral Type
Teff
(K)
Radius
(R/Rsun)
Mass
L/LSun
Log g M0
3800
0.62
0.60
0.072
4.65 M2
3400
0.44
0.023
4.8 M4
3100
0.36
0.20
0.006
4.9 M6
2600
0.15
0.10
0.0009
5.1 M8
2200
0.12
~0.08
0.0003
5.2 L0
2000
~0.1 L2
1900 L4
1750 L6
1600 L8
1400 T
<1200

64. Loooooooooong Term Evolution
After 1400 Gyr, increased He fraction in core causes temperature increase, more complete H burning
Surface temperature increases
After 5740 Gyr, only 16% of H is left, opacity is lower, radiative core develops
H burning shell forms
Teff, L continue to rise until 6000 Gyr
When H depleted, degenerate He star with thin (1% by mass) H envelope finally cools

67. Formation of Brown Dwarfs
Evolution of a brown dwarf begins with its formation in a fragment of a giant molecular cloud with a radius of about 1013 centimeters (or 10-5 light-years). Within the first one million years the cloud fragment condenses into a brown dwarf with an accretion disk (with a radius about 25x109 centimeters) and a peak temperature of about 2,600 degrees Kelvin. In some instances, a planet may form in orbit around the brown dwarf from the material in the accretion disk. After a few million years, the brown dwarf begins a long cooling period as it slowly radiates its heat to space. During the following 10,000 million years the brown dwarf becomes progressively more compact and cooler. Astronomers can get a general estimate of a brown dwarf's age by its temperature and it

68. Structure of Brown Dwarfs
Both red dwarfs and brown dwarfs mix the contents of their cores and their surfaces through convective heating and cooling, but the absence of thermonuclear reactions in the brown dwarf permits the presence of fragile particles such as lithium.
In general, red dwarfs and brown dwarfs are not chemically differentiated throughout their depths. In contrast, because p lanets are formed in the agglomeration of smaller solid bodies they should be chemically differentiated at different depths, including a solid "metallic" core and gaseous upper layers.
The challenge for astronomers is to devise methods to "see" the interiors of these objects. See Burrows et al. 1997, Ap.J. 491, 85

69. Cooler Than T Dwarfs…
Proposed spectral class for ultra-cool dwarfs - Y stars
None yet discovered
Cooler than 770K (the coolest subclass of T dwarf)
Not clear (yet) whether the atmospheric chemistry will change enough to warrant a new spectral class
May be discovered with the next generation of deep IR surveys
not detected with DENIS (K<16.5) or 2MASS (K<15.8)
May be detected with UKIRT LAS (J<19.7) and UDS (J<24)
These surveys will also find many more L & T dwarfs

70. Searching for Brown Dwarfs
Three methods :
1. The brown dwarf Gliese 229B, a faint companion orbiting the red dwarf Gliese 229A, was discovered with a new device (the coronograph) that permits astronomers to see dim objects that may be hiding in the bright glare of a nearby star.
2. Teide 1 was discovered with an extremely sensitive charge-coupled device (CCD) in a search through the Pleiades star cluster for progressively fainter components. Teide 1's presence in the Pleiades suggests that it must be a relatively young object, perhaps less than 100 million years old.
3. Kelu-1 was discovered in the course of a wide-field search for white dwarfs .
The three strategies are complementary, each providing information about brown dwarfs that the others cannot

71. Demo Leads to Discovery!
New brown dwarf candidate confirmed spectroscopically with Keck Observatory

72. Brown Dwarf Candidate Search
Scientific Motivation: The search for brown dwarfs has been revolutionized by the latest deep sky surveys. A key attribute to discovering brown dwarfs is the federation of many surveys over different wavelengths. Such matching of catalogs is currently laborious and time consuming. This matching problem is generic to many areas of astrophysics.
Data Resources: Sloan Digital Sky Survey (SDSS) Early Data Release (15 million objects) 2-Micron All Sky Survey (2MASS) 2nd Incremental Point Source Catalog (162 million objects)
What the VO Brings: Today, doing datasets is user-intensive and is replicated by many different users. Also, the correlation of these two datasets can take years of CPU time if not done correctly. The NVO brings two key aspects to this problem. First, it removes the need for the user to download large data to their machine, making direct use of distributed data. Second, the matching algorithm used here is computationally efficient and designed to give answers in minutes rather than hours; results can be returned to the user in real-time.
Sloan z magnitudes vs. 2MASS J magnitudes, with brown dwarf candidates in red.  Data are from the SDSS Early Data Release and 2MASS 2nd Incremental Release.
Future Prospects: Catalog matching of large datasets is a generic problem in astrophysics. Therefore, making the matching facility available to any user for use on any dataset will greatly enhance the productivity of scientists. Standard I/O formats allow developers to create tools to use the matched data and easily integrate with existing visualization and analysis tools (anomaly detector). Bringing these data together on remote machines with enough CPU to perform analysis (Grid technology) will allow cross-comparisons of unprecedented scale.

73. Confirmed Brown Dwar Name: Confirmation Method: Gliese 229B.
PPl15 (brown dwarf binary).
Teide 1.
Calar 3.
Kelu 1.
DENIS-P J LP
Methane Lithium Lithium Lithium Lithium Lithium Lithium

74. Confirmed Brown Dwarf: Teide 1
Teide 1 sits among many faint stars in the Pleiades open cluster. Teide 1 is a relatively young brown dwarf (about 100 million years old) with a mass as great as 55 Jupiters and an atmospheric temperature approaching that of a red dwarf.

75. Brown Dwarf LP 944-20: Chandra Captures Flare From Brown Dwarf

76. LP 944-20 : only 16 ly away, ~ 500 Myears old, a 60Jupiter mass(6 % of solar mass)
This is the first flare at any wavelength detected from a brown dwarf.
The energy emitted in the flare was comparable to a small solar flare, and was a billion times greater than observed X-ray flares from Jupiter.
The flaring energy is thought to be produced by a twisted magnetic field.