Brown dwarfs: Not the missing mass

Neill Reid, STScI

..a failed star

What is a brown dwarf?

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 galaxiesdepend on the mass, Mhigh velocities high masslow velocities low mass

Measuring the brightness givesthe 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 ~ 3i.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 Planeto derive a dynamical mass estimateCompare with the local census of stars, gas and dust

The Oort limit

Dynamical mass estimates made by Kapteyn & Jeans in 1920sFirst 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 cluster2. Galaxies – 90% dark matter, 1012 MSun

distributed in spheroidal halo3. 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 highmass-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~ 8000low mass stars and brown dwarfs have the right M/LBUT 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 determinethe mass function(M) = No. of stars(BDs) / unit mass / unit volume = c . M

BD/Nstar ~ 0.1, so BD/Mstar ~ 0.01 = 1 BD/Nstar ~ 1, so BD/Mstar ~ 0.1 > 2 BD/Nstar > 10, so BD/Mstar > 1

In only the last case are brown dwarfs viable dark matter candidates

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 skyMethods1. Photometric – look for red starlike objects2. Spectroscopic – look for characteristics absorption bands3. Motion – look for faint stars which move4. Companions – look near known nearby stars

How to find low-mass stars/BDs

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

Missing mass in the ’60s & ’70s

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)

Photometric surveys: Donna Weistrop IRIS photometry of Palomar Schmidt plates

Wolf 359 .. red

Wolf 359 .. blue

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 rednessunderestimate luminosity, distanceoverestimate 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 tofind than others...

The HR diagram

Brown dwarfs are ~15 magnitudes fainterthan the Sun at visualmagnitudes (~106)

Sun

Modern method

Photographic surveys are limited to < 0.8 micronsFlux distribution peaks at ~ 1 micron search at near-IR wavelengths SDSS – far-red DENIS – red/near-IR 2MASS – near-IR

2MASS

SDSS

Photo

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?

The Sun M8 L5 T4 Jupiter

To scale

..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

~ 1.1 for massesbelow 1 MSun

~ 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 oldbrown 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 spectra130 M8, M9 dwarfs 80 L dwarfs, ~30 at d<20 pc 248 targets lack observations1-3 L dwarfs / 1000 pc3

i.e. 2-6 within 8 pc. x 10 for T dwarfs

So are BDs dark matter?

No..... 0.5 << 1.3 brown dwarfs may be twice as common as H-burning starsBUT 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