Docstoc

Brown Dwarfs.ppt

Document Sample
Brown Dwarfs.ppt Powered By Docstoc
					    Brown Dwarfs
            Daniel W. Kittell
Stellar Astrophysics II: Stellar Interiors
          September 9, 2005
                    Summary
   Basic Properties
   Very Low Mass Stars (VLMs)
       Spectral Features of M,L,T Dwarfs
       Convection

       H2 Dissociation

   Recognizing Brown Dwarfs
       Degeneracy & the Minimum Mass for H-
        Burning
       Lithium Depletion




9 Sept 2005         Stellar Astro II : Brown Dwarfs.ppt   2
                   Basic Properties
   Spectral Classification System:
               One                            Old *
               Boy                            Boring *
               And                            Astronomers *
               Five                           Feel *
               Giant                          Greatly *
               Killer                         Knowledgeable *
               Monkeys                        Making *
               Left                           Ludicrous *
               Toledo                         Tests *


        * The Statements in this presentation do not necessarily reflect
        those of the presenter. Any resemblance of, or likeness to, any
        person, real or imagined, is purely coincidental.
9 Sept 2005                  Stellar Astro II : Brown Dwarfs.ppt           3
                        Basic Properties
   Brown Dwarf (BD) Characteristics:
         Not defined by spectral type, but includes late M,L,and T spectral
          types.
         No central, stable H fusion.
         Convection is the dominant form of energy transport.
            (M < 0.3M)
         Defined by mass (but hard to directly determine):
                 15MJup < MBD < 0.08 M
                 No central stable H fusion: not a star
                 Burns deuterium: not a planet
         Teff < 2800 K (M6 spectral type), the presence of lithium proves
          that they are substellar.
         Short luminous lifetimes.
         Candidate for baryonic DM.
         LBD < 10-4 L .
         RBD ~ RJup (Degeneracy).

9 Sept 2005                        Stellar Astro II : Brown Dwarfs.ppt       4
              Basic Properties




9 Sept 2005      Stellar Astro II : Brown Dwarfs.ppt   5
                Basic Properties
   Substellar-mass objects were first theorized by Kumar,S.S. 1963,
    ApJ, 137, 1121.
   Objects renamed BDs by Tarter J.C., 1974, PhD Thesis, Cal-
    Berkeley.
   Very few potential BDs few observed prior to 2MASS, SDSS, &
    other surveys in the mid 1990’s. Wien’s Displacement Law:

                max(m) = 2898 / TBB(K)




9 Sept 2005               Stellar Astro II : Brown Dwarfs.ppt          6
              Very Low Mass Stars
                                                         L: Weakening bands of
                                                         metallic oxides - TiO &
                                                         VO (these are dominant
                                                         in M dwarfs)

                                                         L: Strengthening bands of
                                                         metallic hydrides-CrH &
                                                         FeH; and alkali metals-Na
                                                         I&KI

                                                         T: Exhibit H20 & CH4
                                                         absorption bands




9 Sept 2005        Stellar Astro II : Brown Dwarfs.ppt                             7
              Very Low Mass Stars




9 Sept 2005        Stellar Astro II : Brown Dwarfs.ppt   8
                  Very Low Mass Stars
                                                                           T Dwarfs:
                                                                               Teff < 1200 K
                                                                               Methane
                                                                                absorption similar
                     QuickTime™ and a
                                                                                to Jupiter: causes
                 TIFF (LZW) decompressor
              are neede d to see this picture.
                                                                                a bluer color- see
                                                                                next slide…




9 Sept 2005                                      Stellar Astro II : Brown Dwarfs.ppt                 9
              Very Low Mass Stars
                                      Expect dwarfs to get redder
                                       for late spectral types.
                                      This trend is seen, except for
                                       the T class.
                                      Methane absorption bands
                                       lower observed flux in the Ks
                                       NIR band.




9 Sept 2005        Stellar Astro II : Brown Dwarfs.ppt            10
                       Very Low Mass Stars
        In very low mass (VLM) objects (stars & BDs), convection, not radiation, is
         the dominant form of energy transport.

                   dT(r)    3                                  dT(r)      1 T dP
                                 F(r)                                  (1 )
                    dr     4acT 3
                                                                  dr         P dr
              For VLMs below 0.3 M, the objects are fully convective.
              Convective stability occurs when the adiabatic temp. gradient is less than that for
               radiation:
                                           
                                       radiation > adiabatic

                  adiabatic = ( - 1)/                          = 1 + Rg/Cv
              Temperatures are low enough such that H2 can form. A larger amount of energy
               is needed to raise the temperature due to subsequent H2 dissociation: The
               specific heat rises significantly.
              A small temp gradient allows the pressure to increase. In the case of convection,
               this allows for more efficient energy transport to the envelope thus increasing
               luminosity and Teff. Change in slope of HR?


     9 Sept 2005                         Stellar Astro II : Brown Dwarfs.ppt                    11
              Very Low Mass Stars
   More H2 dissoc. Means larger Teff means
    bluer color?




                                                         Note change
                                                         in slope – H2




9 Sept 2005        Stellar Astro II : Brown Dwarfs.ppt             12
    Recognizing a Brown Dwarf
       As a protostar collapses, core temperature rises.
       Low mass stars must collapse to higher densities: Without fusion to
        support the collapse, the low mass stars attain higher densities.
       As density increases, core becomes partially degenerate: An increasing
        fraction of energy from collapse goes into compressing degenerate gas.
        (Mass X Volume = Constant)
       Degeneracy stops star from collapsing below 0.1 R (and the core
        temperature can’t get any higher than this).


                                        Smaller Mass  Smaller Radius

                                        Smaller Mass  Larger Radius

                                          At 0.1 M, Electron Degeneracy
                                          becomes the dominant source of
                                          pressure support.




9 Sept 2005                  Stellar Astro II : Brown Dwarfs.ppt                 13
    Recognizing a Brown Dwarf
                                                                                Solid: boundaries
                                                                                Dotted: .5 M
                                                                                Dash: .085 M
                                                                                Dot-dash: 0.05 M
                                                                                Degeneracy
                                                                                 becomes
                                                                                 increasingly
                     QuickTime™ and a                                            important for
                 TIFF (LZW) decompresso r
              are neede d to see this picture.                                   decreasing mass!!




9 Sept 2005                            Stellar Astro II : Brown Dwarfs.ppt                           14
    Recognizing a Brown Dwarf
   VLM objects evolve along 4 possible paths: H fuses at 3x106K.
     1. H fusion begins and is sustained: M > 0.09 M. No degeneracy.
           Stable low mass star is produced.
      2.   Degeneracy reduces the temperature, but still sustains fusion: M > 0.08
           M. Stable low mass star is produced.
      3.   Fusion begins, but degeneracy lowers temperature: transition object
           M ~ 0.075 M
      4.   Fusion never becomes a significant source of energy: M < 0.07 M. BD
           is produced.
   Stellar mass limit somewhere between transition object and brown
    dwarf. This is arbitrarily placed at a mass where Lnuc/Ltot never
    exceeds 50%.




9 Sept 2005                     Stellar Astro II : Brown Dwarfs.ppt              15
    Recognizing a Brown Dwarf

                                                    MHBL = 0.073 M




9 Sept 2005   Stellar Astro II : Brown Dwarfs.ppt                     16
    Recognizing a Brown Dwarf
                                                       Deuterium
                                                        burning


                                                       Hydrogen
                                                        burning


                                                       At a given
                                                        luminosity,
                                                        it is hard to
                                                        distinguish
                                                        between
                                                        young
                                                        brown
                                                        dwarfs and
                                                        older stars




9 Sept 2005   Stellar Astro II : Brown Dwarfs.ppt                17
    Recognizing a Brown Dwarf
   Lithium (Li) is a Big Bang nucleosynthetic product, so every VLM
    object contains Li at the beginning of its life.
   As mentioned, objects below ~ 0.3 M are fully convective: all of the
    material is exposed to the hottest temperatures at the core of the
    object.
   The minimum core temperature for Li to burn is Tcrit = 3x106 K,
    corresponding to a minimum mass of 0.06 M. Thus, Li is quickly
    destroyed in dwarfs whose mass exceeds 0.06 M.
   Therefore, VLM objects with detectable Li absorption must NOT be
    undergoing fusion, and are therefore classified as Brown Dwarfs.




9 Sept 2005               Stellar Astro II : Brown Dwarfs.ppt           18
    Recognizing a Brown Dwarf




                        QuickTime™ and a
                    TIFF (LZW) decomp resso r
                 are neede d to see this picture.




9 Sept 2005   Stellar Astro II : Brown Dwarfs.ppt   19
    Recognizing a Brown Dwarf
                                               Presence of Li
                                                absorption (6708Å
                                                lithium doublet) means
                                                the mass is below ~
                                                0.06 M. Thus, no
                                                fusion.
                                               For spectral types later
                                                than M6, this is
                                                conclusive evidence of
                                                a BD.




9 Sept 2005   Stellar Astro II : Brown Dwarfs.ppt                    20
                 References
   New Light of Dark Stars, I. Neill Reid and
    Suzanne L. Hawley, Springer 2000. (and
    references therein)
   Kirkpatrick, Reid, Liebert, ApJ 519, 802, 1999




9 Sept 2005         Stellar Astro II : Brown Dwarfs.ppt   21

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:2
posted:12/21/2012
language:Unknown
pages:21
handongqp handongqp
About