Brown dwarfs: Not the missing mass
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Transcript of Brown dwarfs: Not the missing mass
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Brown dwarfs: Not the missing mass
Neill Reid, STScI
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..a failed star
What is a brown dwarf?
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What about `missing mass’
.. actually, it’s missing light....Originally hypothesised by Zwicky in the 1930s from observations of the Coma cluster
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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
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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
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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
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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”
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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
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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
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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
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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
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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
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Finding low mass stars (1)
Objective prism surveys: Pesch & Sanduleak
Scan the plates by eye and pick out and classify cool dwarfs
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Finding low mass stars (2)
Photometric surveys: Donna Weistrop IRIS photometry of Palomar Schmidt plates
Wolf 359 .. red
Wolf 359 .. blue
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Finding low mass stars (3)
1952 1991
Identify faint stars with large proper motions: Willem Luyten, using Palomar Schmidt – to ~19th mag.
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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”
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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.
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So what about brown dwarfs?
Some are easier tofind than others...
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The HR diagram
Brown dwarfs are ~15 magnitudes fainterthan the Sun at visualmagnitudes (~106)
Sun
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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
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Meanwhile…...
Discovery of Gl 229B confirms that brown dwarfs exist. Blue IR colours due to CH4
T < 1300K
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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
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Field T dwarfs
Only ~20 T dwarfs known; none visible on photographic sky surveys
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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
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What do brown dwarfs look like?
The Sun M8 L5 T4 Jupiter
To scale
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..and if we had IR-sensitive eyes
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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?
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The stellar mass function
~ 1.1 for massesbelow 1 MSun
~ 3 for higher masses
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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
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What lies beneath?
young brown dwarfs –
types M, L + a few Ts
Middle-aged and oldbrown dwarfs..... the majority
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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
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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
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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
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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
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The Dutch exclusion principle