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A historical perspective on theoretical studies of brown dwarfs
Peter H. Hauschildt
Department of Physics & Astronomy
Center for Simulational Physics
The University of Georgia
Athens, GA 30602
- The Past
- The Path
- input physics
- enabling technologies
This talk will feature glaring omissions, possible
misinterpretations and my subjective view on the topic.
All errors and blunders are solely mine.
- Publication rate on ``brown dwarfs'' follows the general
increase in the astronomical literature ... almost.
- I will concentrate on theoretical studies of the interior/evolution
and atmospheres of brown dwarfs.
Progress comes from two sides:
- the ``planetary community'' extending their models up in mass.
- the ``stellar community'' extending their models down in mass.
Interior & evolutionary modeling of brown dwarfs:
- Kumar, 1963: electron degeneracy stops collapse of low mass objects
before H-burning temperatures are reached ().
- Peebles, 1964; Hubbard 1968: thermal cooling models of giant planets.
- Grossman, Graboske et al, 1970-1974: evolution of low mass stars (mass limit
set to ).
- Tarter, 1975: cooling of brown dwarfs based on extrapolated sequences
- Alexander, 1975: low temperature Rosseland opacities
- Stevenson, 1975-1978: thermodynamics and evolution of brown dwarfs.
- D'Antona & Mazzitelli, 1982-1986: evolution of low mass stars and brown dwarfs.
- Hubbard & McFarlane, 1983-1986: equation of state for giant planets.
- Nelson, Rappaport & Joss, 1982-86: evolution of brown dwarfs from
early stages to ages of Gyr.
- Alexander et al, 1983, 1994: updated (molecules and grains) low temperature opacities.
- Lunine, Hubbard & Marley, 1986: evolutionary sequence of BDs with grey atmospheres
used as boundary conditions.
- Burrows, Hubbard, Lunine, Saumon, Marley 1989-: continuously
improved evolutionary models.
- Chabrier, Saumon, van Horn: 1990-1995: non-ideal EOS for conditions in low mass
stars, BDs and giant planets.
- Saumon et al, 1994: evolution/interior coupled to zero-metallicity
- Baraffe, Chabrier, 1995: evolutionary sequences coupled to finite
metallicity, non-grey AH95 model atmospheres.
- CNRS/WSU/UGA collaboration, 1996; Tucson group, 1996: Gl229B models.
Model atmospheres for brown dwarfs
- Mould, 1975/1976: convective model atmospheres for based on ATLAS.
- Lunine et al, 1986: grey atmosphere models for giant planets.
- Allard, 1990; Kui 1991: model atmospheres for based on Wehrse's
low white dwarf code.
- Brett & Plez, 1993: low models based on Gustafsson's MARCS code.
- Saumon, 1994: zero-metallicity models for brown dwarfs and giant planets.
- Allard & Hauschildt, 1995: model atmospheres for low mass stars based on the
general PHOENIX code.
- Allard et al, Marley et al, Tsuji et al, 1996: model atmospheres for
substellar objects (Gl229B).
- Allard, Hauschildt & Baron, 1998: unified (NLTE, wind,
...) model atmospheres from to .
The Path: Input Physics (interiors)
- stars with are convective from the center to the
upper atmosphere, simplifying the interior models but complicating the
- structure of the object depends critically on the atmosphere
and its thin radiative layer.
- nuclear process well understood (deuterium burning, Li burning).
- but screening factors are nasty ...
- Equation of State for low temperature, dense material.
- electron degenerate
- pressure ionization
- early work: Salpeter (1961), Hubbard (1973-), Stevenson (1975-).
- State-of-the-art: Saumon, Chabrier & van Horn, 1995: consistent
pressure ionization of H and He, tested with experimental results (Livermore).
The Path: Input Physics (atmospheres)
- Standard classical ``stellar atmosphere'' problem (at least,
that is the usual assumption):
- plane parallel
- radiative plus convective energy equilibrium
- The Problems:
- convection into optically thin layers (no real theory).
- complex equation of state (molecules, dust).
- highly non-grey opacities.
- poorly known opacities (molecules, dust).
- 100's of millions of lines need to be handled (TiO, water vapor).
- strongly depth dependent line broadening (van der Waals) but
mostly unknown interaction constants.
- non-ideal EOS effects occur near the bottom of BD atmospheres.
- radiative transfer.
- State-of-the-art: unified model atmospheres employing direct opacity
sampling with large EOS, dust formation and opacities, and, for hotter
BDs, NLTE effects for various elements (Allard, Hauschildt & Baron, 1998).
- mainframe era: serial scalar processors with virtual memory, 1965-1980's
- allowed more detailed calculations than were previously possible.
- numerical algorithms developed for simulations.
- line opacity included as ODF's and similar techniques (table interpolation).
- allowed direct processing of a few 10k lines with detailed profiles.
- NLTE possible for atomic levels.
- CPU performance and virtual memory limitations severely restricted
what could be done.
- vector computer era: serial vector processors, mid 1980's-early 1990's
- vast increase in raw CPU performance.
- required moderate algorithmic changes.
- allowed direct processing of a few 1M lines with detailed profiles.
- NLTE possible for atomic levels with early ALI RT methods.
- memory (no VM on Crays!) and IO restrictions limit calculations.
- parallel computer era: parallel vector or superscalar processors, 1990's-
- vast increase in number of available CPUs per machine (>64),
architectural design not yet fully evolved.
- requires significant algorithmic changes.
- allowed direct processing of a few 100M lines with detailed profiles.
- NLTE possible for at least 10,000 atomic levels with latest ALI algorithms.
- allow more physically accurate treatment within the simulations.
- memory and IO restrictions significantly reduced, this makes much more
detailed and accurate calculations possible.
- Breakthroughs are typically made when the physics that
is included in the models was improved, either explicitly or
- explicit: EOS, better line lists for molecular lines,
dust condensation and opacities.
- implicit: progress in ``enabling technologies'' (computer and algorithms)
allows a progressively more accurate treatment of the input physics,
leading to better results.
- synergy effects help driving the theory towards better
- In the future:
- better treatment of dust.
- more complete opacities (which means more lines in many cases).
- true 3D models (fun but hard).
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Peter H. Hauschildt