갈색왜성 L and T dwarfs

Brown Dwarf Star Joins The Jet-set Discovery of a Bipolar Outflow from 2MASSW J1207334-393254 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. 2007. 05. 25

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

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

Brown Dwarfs Substellar, low-mass stars Between least massive star and the most massive planet Ultra-cool Dwarf

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) --- 13 M_J

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

Gl229B (HST)and Jupiter

First L Dwarf : GD 165B Survey of WD Becklin & Zuckerman (1988) very red and faint companion enigmatic spectrum (Kirkpatric et al. 1999 : probably BD) photospheric Dust formation  Hydrides, FeH, CrH, CaH strong neutral alkalic lines, NaI, KI, CsI

M, L, T, and Y dwarf Temperature M dwarf : 3700 – 2100 L dwarf : 2000 - 1300 L0 – L8 : 2000 – 1400 (Kirkpatric et al ’98) L0 – L9 : 2200 – 1300 (Martin et al ’99) T dwarfs : 1300 - 600 Y dwarfs < 500, ultra-cool BD

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 J005910.90-011401.3, ULAS J133553.45+113005.2, and ULAS J003402.77−005206.7. 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 0722-05 and SDWFS 1433+35[10]) were proposed as prototypes for spectral class Y0.

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

BD : M, L, T Dwarfs(visible)

BD : M, L, T Dwarfs(IR)

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

Currently Known L, T Dwarfs(2002. 7) Survey L Dwarf T Dwarf 2MASS 155 18 SDSS 72 11 DENIS 9 0 Others 9 3 2004.2 ~50

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

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)

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

IR Color-Sp Types/Color-Color Diagram

Flux Densities for a Star with m=0 Band l Fl J 1.22 3.15*10^-13 Wcm-2mum-2 H 1.63 1.14 K 2.19 3.96*10^-14 L 3.45 7.1*10^-15

Magnitudes of Known BDs Obj Sp K L’ Ls 1159384+005727 L0 12.80 11.87 12.60 0756252+124456 L6 14.89 13.33 14.18 1632291+190441 L8 14.01 12.58 13.52 0837172-000018 T1 15.97 14.42 15.71 1021097-030420 T3 15.40 13.53 15.53 1624144+002916 T6 15.66 13.22 15.43 L:2.5-4.1, L’:3.5-4.1, Ls:2.5-3.5

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

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

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 0.08-0.085 MSun, degeneracy lowers central temperature, but it’s still hot enough for hydrogen fusion (main sequence) At 0.075 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

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).

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

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

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!)

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

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

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

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 : 30 - 400 M years

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

T Dwarf Spectrum (NIR)

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

Mid IR Spectrum (T, L Dwarfs)

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

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~1500-1200) CO is depleted by methane formation (CH3) – the transition from L to T dwarfs

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

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)

Dust Dust formation is important in M, L, and T dwarfs Depletes metals, including Ti Dust includes Corundum (Al2O3) Perovskite (CaTiO3), condensing at T < 2300-2000K Iron (Fe) VO, condensing at T < 1900-1700 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

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

And More Dust As temperature falls: CO depleted to form methane at temperatues < 1500-1200 K But Na may condense onto “high albite” (NaAlSi3O8) CrH condenses at T=1400 K Alkali elements expected to form chlorides at T < 1200

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

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

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

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

Confirmed Brown Dwar Name: Confirmation Method: Gliese 229B. : PPl15 (brown dwarf binary). Teide 1. Calar 3. Kelu 1. DENIS-P J1228.2-1547. LP944-20. Methane Lithium Lithium Lithium Lithium Lithium Lithium

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

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.

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

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.

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Li