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 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 ~ M Sun ~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 M Sun pc -3 Stars ~ 0.04 M Sun pc -3 Gas & dust ~ 0.03 M Sun pc -3  0.02 M Sun 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 M Sun pc -3 ~ 0.07 M Sun pc -3 “missing”

Dark matter on different scales Three types of missing mass: 1.Galaxy clusters – 99% dark matter, M Sun distributed throughout the cluster 2.Galaxies – 90% dark matter, M Sun distributed in spheroidal halo 3. Local disk - <50% dark matter, <10 10 M Sun 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 ~ solar masses  requires ~ 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     BD /N star ~ 0.1, so  BD /M star ~ 0.01  = 1   BD /N star ~ 1, so  BD /M star ~ 0.1  > 2   BD /N star > 10, so  BD /M star > 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 < L Sun  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 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 red Wolf blue

Finding low mass stars (3) Identify faint stars with large proper motions: Willem Luyten, using Palomar Schmidt – to ~19 th 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 M V. 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 Brown dwarfs are ~15 magnitudes fainter than the Sun at visual magnitudes (~10 6 ) Sun

Modern method Photographic surveys are limited to < 0.8 microns Flux 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 CH 4  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 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 masses  below 1 M Sun  ~ 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 2 nd Release includes ~2 x 10 8 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 pc 3 i.e. 2-6 within 8 pc. x 10 for T dwarfs

So are BDs dark matter? No <  < 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