Brown dwarfs Not the missing mass Neill Reid

Brown dwarfs: Not the missing mass Neill Reid, STSc. I

What is a brown dwarf? . . a failed star

What about `missing mass’ . . actually, it’s missing light. . Originally hypothesised by Zwicky in the 1930 s 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 1920 s 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 M 5 dwarf: M/L ~ 85 Gl 229 B – 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 Y(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 ’ 60 s & ’ 70 s 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 60 s 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 ~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 MV. Systematics kill. . Surveys tended to overestimate sp. type & overestimate redness underestimate luminosity, distance overestimate density By early 80 s, 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 2 MASS 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 2 MASS – near-IR Photo SDSS

Meanwhile…. . . Discovery of Gl 229 B confirms that brown dwarfs exist. Blue IR colours due to CH 4 T < 1300 K

Field brown dwarfs New surveys turned up over 120 ultracool dwarfs. Some could have been found photographically. Two new spectral classes: OBAFGKM L 2100 1300 K T < 1300 K

Field T dwarfs Only ~20 T dwarfs known; none visible on photographic sky surveys

Cool dwarf spectra Spectral class L: decreasing Ti. O, VO - dust depletion increasing Fe. H, Cr. H, water lower opacities increasingly strong alkali absorption Na, K, Cs, Rb, Li

What do brown dwarfs look like? To scale The Sun M 8 L 5 T 4 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) 2 MASS 2 nd Release includes ~2 x 108 sources over ~47% of the sky. Select sources with (J, (J-K)) matching M 8 – L 8 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 M 8, M 9 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. . . 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 2 MASS 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