Download Today Only, A New Exhibit: M,L, and T Dwarfs!

Today Only, A New
Exhibit:
M,L, and T Dwarfs!
Ted Maxwell
Indiana University Astronomy
A540: Stellar Atmospheres
April 1, 2005
Outline
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History
Characteristics
Distributions
Current Understanding
Future questions/goals
History
 Harvard  Morgan and Keenan (MK)
 Only class M2 and earlier
 Before 1940s, observations mostly in Blue-Green 
Spectral Feature saturate quickly with decrease in
Temp
 Yerkes (Morgan and Kuiper) vs Mount Wilson
(Joy) system
 TiO band strength in green vs. blue regime
 Diverge for later-type M dwarfs
 Wolf 359 (Gl 406) was M8 vs. dM6e
History
 Keenan and MacNeil  add wavelength
coverage (1976)
 Boeshaar  6800 A
 KIRKPATRICK et al. (KHM system) (1991)
 6300-9000 A features + Overall slope
 Finally a standard set for late M Dwarfs
 Bessell has competing system, close to KHM,
but less widely used
 Tinney and Reid (1999) expand atlas for later
spectral types
History
 In 1988 – GD 165B discovered  Clearly different
 2MASS + Keck follow up spectroscopy = GD165B no
longer so odd!
 1999 Kirkpatrick et al.  New spectral type is codified,
called L and defined to L8, later expanded to L9.5
 T is suggested for Gl 229B-like methane dwarfs.
 Burgasser et al. and Geballe et al. Provided first
schemes for T dwarfs (2002) in near-IR, Burgasser et al.
2003 in the visible.
 Currently ~400 L dwarfs, ~50 T dwarfs are known
 http://spider.ipac.caltech.edu/staff/davy/ARCHIVE/
 http://research.amnh.org/~adam/tdwarf/
M Dwarfs
 TiO bands to ~ M6, VO dominate after that
 Become fully convective at 0.3 M‫סּ‬, Mbol 8.5,
~M4
 Chromospheric activity remains constant down
to ultra cool M Dwarfs where Hα emission
drops rapidly.
 Magnetic fields from turbulent dynamo?
 Flares still common, but not coupled to
rotation!
L Dwarfs
 TiO and VO bands
disappear
 Hydride bands become
apparent
 H- opacity drops  red
spectrum clears up!
 Na I, K I, Rb I, and Cs I
doublets appear more
strongly
 K I resonance doublet
VERY broad (100s of Å)
 Lots of dust grains, even
clouds
L Dwarfs
T Dwarfs
 CH4 and H20
 Surface temp as low as
700 K
 Color? Magenta due to
huge Na absorption.
 Li lines get very small as
Li  molecules
 Clouds dissipate
 Radii are the same as
for L dwarfs.
 These guys are brown
dwarfs!
Compare sizes/ colors
Sun, M dwarf, hot L dwarf, T Dwarf, visible
http://spider.ipac.caltech.edu/staff/davy/2mass/science/comparison.html
Evolution of Atmosphere
Black spots are M dwarfs
Red are L dwarfs
Blue are T dwarfs.
Purple line is normal, mixed
stellar atmosphere
Right hand black line is
cloudy atmosphere
Left hand black line is cleared
up atomophere
Dark line traces possible
evolution path.
Burgasser, A.J., et al. 2002, ApJ 571, L151
What is a Brown Dwarf?
 Central temp < 3 Million K
 Mass <~ 75 Jupiter Masses which is 8 percent
of our Sun's mass
 Burns Deuterium at >~ 13 Jupiter Masses for ~
10’s of Myrs  heating from contraction
 Highly convective
 Hot Corona may disappear around 2770 K i.e.
LP 944-20
Photometry
 Need a good
distinguisher of
temperature from
colors
 Special narrow band
filters can be used to
measure band
strength and tus give
spectral types
 I-J is a good bet!
Answered Questions
 Are they the missing mass?
 Nope, There may be one for each other star,
but they just don’t have enough mass
 Are their field brown dwarves?
 Yep, plenty!
 Do they have strong magnetic fields?
 Actually yes, but that leads us to..
Unanswered Questions
 Where do the magnetic fields and activity
come from in late dwarfs?
 What exactly is the difference between
how a planet vs. a brown dwarf forms….
 Can we get an even more complete
survey of the local population?
 Can we get better models of these
objects?
Odd assortment of
References
 http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/
03/text/
 http://www.solstation.com/stars/pc10bd.htm
 http://www.solstation.com/stars/lp944-20.htm
 http://www.ipac.caltech.edu/2mass/overview/dwarfs.html
 http://spider.ipac.caltech.edu/staff/davy/PAPERS/kirkpatrick_iaupa
p.ps
 http://www.ipac.caltech.edu/2mass/publications/proceedings.html
 http://www.mpiahd.mpg.de/homes/calj/tdwarfs_parallax_venus2004.pdf
 http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0207/0207672.pdf
 Reid, I. N. et al., 1999, ApJ, 527, 105
 Kirkpatrick, J. D. et al., 1999, ApJ, 519, 802
 Reid and Hawley, New Light on Dark Stars, Springer, 2000