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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.

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what is a brown dwarf
What is a brown dwarf?

..a failed star

what about missing mass
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
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 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
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
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
Dark matter on different scales
  • Three types of missing mass:
  • Galaxy clusters – 99% dark matter, 1014 MSun
  • distributed throughout the cluster
  • 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
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
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

  • = 0  NBD/Nstar ~ 0.1, so MBD/Mstar ~ 0.01
  • = 1  NBD/Nstar ~ 1, so MBD/Mstar ~ 0.1
  • > 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
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


  • Photometric – look for red starlike objects
  • Spectroscopic – look for characteristics absorption bands
  • Motion – look for faint stars which move
  • Companions – look near known nearby stars
missing mass in the 60s 70s
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
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
Finding low mass stars (2)

Photometric surveys:

Donna Weistrop

IRIS photometry of

Palomar Schmidt plates

Wolf 359 .. red

Wolf 359 .. blue

finding low mass stars 3
Finding low mass stars (3)

1952 1991

Identify faint stars with large proper motions:

Willem Luyten, using Palomar Schmidt – to ~19th mag.

the results
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
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
So what about brown dwarfs?

Some are easier to

find than others...

the hr diagram
The HR diagram

Brown dwarfs are

~15 magnitudes fainter

than the Sun at visual

magnitudes (~106)


modern method
Modern method


Photographic surveys are limited to l < 0.8 microns

Flux distribution peaks

at ~ 1 micron

 search at near-IR


SDSS – far-red

DENIS – red/near-IR

2MASS – near-IR




Discovery of

Gl 229B

confirms that

brown dwarfs


Blue IR colours

due to CH4

 T < 1300K

field brown dwarfs
Field brown dwarfs

New surveys turned up

over 120 ultracool dwarfs.

Some could have been

found photographically.

Two new spectral classes:


L 2100  1300K

T < 1300 K

field t dwarfs
Field T dwarfs

Only ~20 T dwarfs


none visible on

photographic sky


cool dwarf spectra
Cool dwarf spectra

Spectral class L:

decreasing TiO, VO

- dust depletion

increasing FeH, CrH,


lower opacities -

increasingly strong

alkali absorption

Na, K, Cs, Rb, Li

what do brown dwarfs look like
What do brown dwarfs look like?

To scale

The Sun M8 L5 T4 Jupiter

a statistical update
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
The stellar mass function
  • ~ 1.1 for masses
  • below 1 MSun
  • a ~ 3 for higher
  • masses
the problem
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
What lies beneath?

young brown dwarfs –

types M, L + a few Ts

Middle-aged and old

brown dwarfs.....

the majority

a new survey
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
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
So are BDs dark matter?


0.5 < a < 1.3 

brown dwarfs may be

twice as common as

H-burning stars


they only contribute

~10% as much mass


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