1. 갈색왜성
L and T dwarfs

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 Ye!!
L , T and Y Dwarfs*
Oh Be A Fine Girl Kiss Me Later Tonight Ye!!
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 Substellar, low-mass stars
Between least massive star and the most massive planet
Ultra-cool Dwarf

6. 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 M_J) with solar composition (90 M_J with zero metallicity) M_J

7. 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 M_J (20 – 50), Te = 900

8. Gl229B (HST)and Jupiter

9. 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

10. M, L, T, and 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 dwarfs < 500, ultra-cool BD

11. Y dwarfs
As of early 2009, the coolest known brown dwarfs have estimated effective temperatures between 500 and 600 K, and have been assigned the spectral class T9.
CFBDS J ,
ULAS J , and
ULAS J −
The spectra of these objects display absorption around 1.55 micrometers. Delorme et al. have suggested that this feature is due to absorption from ammonia and that this should be taken as indicating the T-Y transition, making these objects of type Y0. However, the feature is difficult to distinguish from absorption by water and methane, and other authors have stated that the assignment of class Y0 is premature.
In April 2010, two newly discovered ultracool brown subdwarfs (UGPS and SDWFS [10]) were proposed as prototypes for spectral class Y0.

12. BDs In M, L, T dwarfs Not all M stars are H-burning stars
Not all L dwarfs are BDs
All T dwarfs are BDs
All Y dwarfs are BDs
BD confirmed by mass and age
Massive BD starts at mid M (> M6-7)
 L Type
 T Type with Time

13. BD : M, L, T Dwarfs(visible)

14. BD : M, L, T Dwarfs(IR)

18. 8 pc Solar-Neighborhood
4 A stars, 1 F stars, 5 G stars,
22 K stars,
87 M stars
9 WD
Few M, L, and
Lots T BDs

19. Currently Known L, T Dwarfs(2002. 7)
Survey L Dwarf T Dwarf
2MASS
SDSS
DENIS
Others
~50

20. 2MASS, DENIS, SDSS 2MASS ; 1.3 m NICMOS 256*256 2”/pixel 8.5’*8.5’
J=16.3, H=15.3, K=14.9
DENIS : 1 m
I(0.82)=18.5, J=16.5 Ks=14.0
SDSS : 2.5m, u,g,r,I,z
r=22.7, I=21.8, z=20.3

21. 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)

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

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

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

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

26. 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

27. 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

28. 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

29. Mass vs Radius
The mass range, from about 0.1 M to 0.001M , has an essentially constant radius, because at the high-mass end the degeneracy pressure leads to the slow function R ~ M-1/3, whereas at
the low-mass end the Coulomb pressure, which is characterized by constant density ( r ~ M/R3 , which implies R ~ M+1/3 ), begins to dominate over degeneracy; the net result is that approximately R ~M0 (Burrows and Liebert 1993).

30. 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

31. 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

33. 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!)

34. 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

35. 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

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

37. Lithium Test Very Low Mass Star : fully convective
little over 108 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

38. 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

39. T Dwarf Spectrum (NIR)

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

41. Mid IR Spectrum (T, L Dwarfs)

42. 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

43. 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

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

47. 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)

48. 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

49. 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

50. 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

51. 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

53. 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

54. 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.
See Burrows et al. 1997, Ap.J. 491, 85

55. 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

56. 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

57. 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

58. 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.

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

60. 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.

61. summary

62. Li