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Brown dwarfs: Not the missing mass
Neill Reid, STScI
What is a brown dwarf?
..a failed star
What about `missing mass’
.. actually, it’s missing light....
Originally hypothesised by Zwicky in the 1930s from observations
of the Coma cluster
Missing mass and Coma
Velocities of cluster galaxies
depend on the mass, M
high velocities  high mass
low velocities  low mass
Measuring the brightness gives
the total luminosity, L
(M, L in solar units)
Zwicky computed a mass to light ratio, M/L ~ 500 for Coma
.. Solar Neighbourhood stars give M/L ~ 3
i.e. ~99% of the mass contributes no light  dark matter
Dark matter on other scales
Dark matter is present in galaxy halos:
observations by Rubin & others show
flat rotation curves at large radii
 expect decreasing velocities
Mass of the Milky Way ~ 1012 MSun
~90% dark matter
Local missing mass
Use the motions of stars perpendicular to the Galactic Plane
to derive a dynamical mass estimate
Compare with the local census of stars, gas and dust
The Oort limit
Dynamical mass estimates made by Kapteyn & Jeans in 1920s
First comparison with local census by Oort, 1932
Dynamical mass ~ 0.09 MSun pc-3
Stars
~ 0.04 MSun pc-3
Gas & dust
~ 0.03 MSun pc-3
0.02 MSun pc-3 “missing”
described as ‘dark matter’
distributed in a disk
assumed to be low-mass stars
Oort re-calculated the dynamical mass in 1960 ~ 0.15 MSun pc-3
~ 0.07 MSun pc-3 “missing”
Dark matter on different scales
Three types of missing mass:
1. Galaxy clusters – 99% dark matter, 1014 MSun
distributed throughout the cluster
2. Galaxies
– 90% dark matter, 1012 MSun
distributed in spheroidal halo
3. Local disk
- <50% dark matter, <1010 MSun
distributed in a disk
So what has all this to do with
brown dwarfs?
Solving the missing mass problem requires objects with high
mass-to-light ratios –
Vega – 2.5 solar mass A star:
M/L ~ 0.05
Sun - 1 solar mass G dwarf:
M/L = 1
Proxima – 0.1 solar mass M5 dwarf: M/L ~ 85
Gl 229B – 0.05 solar mass BD:
M/L~ 8000
low mass stars and brown dwarfs have the right M/L
BUT you need lots of them....
Galactic halo dark matter ~ 1012 solar masses
 requires ~ 1014 brown dwarfs
 nearest BD should be within 1 pc. of the Sun
Taking a census
Finding the number of brown dwarfs requires that we determine
the mass function
(M) = No. of stars(BDs) / unit mass / unit volume
= c . M-a
a = 0  NBD/Nstar ~ 0.1, so MBD/Mstar ~ 0.01
a = 1  NBD/Nstar ~ 1,
so MBD/Mstar ~ 0.1
a > 2  NBD/Nstar > 10, so MBD/Mstar > 1
In only the last case are brown dwarfs viable dark matter candidates
How to find low-mass stars/BDs
They’re cool T < 3000 K
 red colours
They’re faint L < 0.001 LSun
 only visible within the immediate vicinity
therefore need to survey lots of sky
Methods
1. Photometric – look for red starlike objects
2. Spectroscopic – look for characteristics absorption bands
3. Motion – look for faint stars which move
4. Companions – look near known nearby stars
Missing mass in the ’60s & ’70s
Oort’s 1960 calculation indicated ~50% of the disk was dark matter
 required 2000 to 5000 undiscovered M dwarfs/brown dwarfs
within ~30 l.y. of the Sun
i.e. 1 to 3 closer than Proxima Cen
Surveys in the 60s were limited to photographic techniques
• Objective prism surveys
• Blue/red comparisons
• Proper motion surveys
Finding low mass stars (1)
Objective prism surveys:
Pesch & Sanduleak
Scan the plates by eye and pick
out and classify cool dwarfs
Finding low mass stars (2)
Wolf 359 .. red
Wolf 359 .. blue
Photometric surveys:
Donna Weistrop
IRIS photometry of
Palomar Schmidt plates
Finding low mass stars (3)
1952
1991
Identify faint stars with large proper motions:
Willem Luyten, using Palomar Schmidt – to ~19th mag.
The results
Analysis of both objective
prism and imaging surveys
suggested that M dwarfs
were the disk missing mass.
Luyten disagreed ...
“The Messiahs of the Missing Mass”
“The Weistrop Watergate”
“More bedtime stories from Lick Observatory”
The resolution
Both (B-V) and spectral type are poor
luminosity indicators for M dwarfs:
small error in (B-V), large error in MV.
Systematics kill....
Surveys tended to overestimate sp. type
& overestimate redness
underestimate luminosity, distance
overestimate density
By early 80s, M dwarfs were eliminated
as potential dark matter candidates.
Recent analysis indicates there is NO
missing matter in the disk.
Moral: be very careful if you find what you’re looking for.
So what about brown dwarfs?
Some are easier to
find than others...
The HR diagram
Sun
Brown dwarfs are
~15 magnitudes fainter
than the Sun at visual
magnitudes (~106)
Modern method
2MASS
Photographic surveys are
limited to l < 0.8 microns
Flux distribution peaks
at ~ 1 micron
 search at near-IR
wavelengths
SDSS – far-red
DENIS – red/near-IR
2MASS – near-IR
Photo
SDSS
Meanwhile…...
Discovery of
Gl 229B
confirms that
brown dwarfs
exist.
Blue IR colours
due to CH4
 T < 1300K
Field brown dwarfs
New surveys turned up
over 120 ultracool dwarfs.
Some could have been
found photographically.
Two new spectral classes:
OBAFGKM
L 2100  1300K
T < 1300 K
Field T dwarfs
Only ~20 T dwarfs
known;
none visible on
photographic sky
surveys
Cool dwarf spectra
Spectral class L:
decreasing TiO, VO
- dust depletion
increasing FeH, CrH,
water
lower opacities increasingly strong
alkali absorption
Na, K, Cs, Rb, Li
What do brown dwarfs look like?
To scale
The Sun
M8
L5
T4
Jupiter
..and if we had IR-sensitive eyes
A statistical update
Within 8 parsecs of the Sun there are:
Primaries
Companions
• A stars
4
• F stars
1
• G dwarfs
9
• K dwarfs
23
8
• M dwarfs
91
38
• white dwarfs 7
5
• brown dwarfs 1
2 known
A total of 179 stars in 135 systems (including the Sun)
Average distance between systems = 2.5 pc. (~8 l.y.)
How many brown dwarfs might there be?
The stellar mass function
a ~ 1.1 for masses
below 1 MSun
a ~ 3 for higher
masses
The problem
Brown dwarfs fade rapidly
with time;
lower-mass BDs fade faster
than high-mass BDs;
even our most sensitive
current surveys detect a
fraction of the BD population,
preferentially young, high-mass
What lies beneath?
young brown dwarfs –
types M, L + a few Ts
Middle-aged and old
brown dwarfs.....
the majority
A new survey
NStars project with
Kelle Cruz (U.Penn.),
Jim Liebert (U.A),
Davy Kirkpatrick (IPAC)
2MASS 2nd Release includes
~2 x 108 sources over ~47%
of the sky.
Select sources with (J, (J-K))
matching M8 – L8 dwarfs
within 20 parsecs
Preliminary results
2224 sources initially
430 spurious
 1794 viable candidates
cross-reference vs DSS,
IRAS, SIMBAD etc;
KPNO/CTIO spectra
130 M8, M9 dwarfs
 80 L dwarfs, ~30 at d<20 pc
248 targets lack observations
1-3 L dwarfs / 1000 pc3
i.e. 2-6 within 8 pc.
x 10 for T dwarfs
So are BDs dark matter?
No.....
0.5 < a < 1.3 
brown dwarfs may be
twice as common as
H-burning stars
BUT
they only contribute
~10% as much mass
Conclusions
Low-mass stars and brown dwarfs have been postulated as
potential dark matter candidates for over 50 years.
Based on the results from recent, deep, near-infrared surveys,
notably 2MASS and SDSS, both can be ruled out as viable
dark matter candidates.
Brown dwarfs are much more interesting as a link between
star formation and planet formation
The Dutch exclusion principle