Brown Dwarfs as Chronometers

1 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Brown Dwarfs

Adam J. Burgasser Massachusetts Institute of

Technology

as Galactic Chronometers

© 2009 Adam J. Burgasser

Brown Dwarfs as Chronometers

2 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

~0.075 M

Eff

ectiv

e T

empe

ratu

re (

K)

“Planemos”

Brown Dwarfs

Stars

Time (Gyr)

(Z dependent)

M L

T

Y?

Spectral classes

models from Burrows et al. (2001)

Brown Dwarfs as Chronometers

3 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Magnetic Activity: �A Poor Diagnostic

Fleming et al. (2003); Burgasser & Putman (2006) see also Gizis et al. (2000); West et al. (2004); Osten et al. (2005); Berger (2006); Audard et al. (2007); Schmidt et al. (2007)

M5 L0 L5

X-rays

Hα

radio

log

LR/L

bol log L

Hα /L

bol lo

g L

X/L

bol

(next) talk by A. West

Brown Dwarfs as Chronometers

4 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

No slow rotators for SpT > L2, evidence of mass & temperature dependent spindown timescale (ωcrit → 0).

Do Brown Dwarfs ever spin down? Halo L subdwarf vsini = 65 km/s

Reiners & Basri (2008) see also Mohanty et al. (2002), Mohanty & Basri (2003), Bailer-Jones (2004); Reiners & Basri (2006), Blake et al. (2007)

Brown Dwarfs as Chronometers

5 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Method 1: Companions and Clusters

Gliese 570D (2-5 Gyr) Burgasser et al. (2000)

HD 3651B (0.7-4.7 Gyr) Luhman et al. (2006) Mugrauer et al. (2006)

G 196-3B (30-300 Myr) Rebolo et al. (1998)

Young clusters

MK

J-K

Upp Sco α Per Pleiades Ursa Major & Hyades

Jameson et al. (2008)

Brown Dwarfs as Chronometers

6 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Method 2: �The Li clock

Li is fused to exhaustion in the core for M > 0.065 M, ages > 500 Myr

50-200 Myr “sweet spot”: degree of Li depletion depends on age and mass - the Li depletion boundary.

see Rebolo et al. (1992); Bildsten et al. (1997)

Li I 6708 Å

models from Burrows et al. (2001)

Brown Dwarfs as Chronometers

7 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Pleiades 125 Myr

Luminosity at which Li I line disappears can be matched to a corresponding mass

& age

Stauffer et al. (1998) see also Barrado y Navascués et al. (1999); Stauffer et al. (1999); Oliveira et al. (2003); Manzi et al. (2008)

Brown Dwarfs as Chronometers

8 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Li depletion line

⇒ t > 0.4 Gyr

Time (Gyr)

models from Burrows et al. (2001)

No Li I detected

Li

LiCl

Eff

ecti

ve T

empe

ratu

re (

K)

Brown Dwarfs as Chronometers

9 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Li depletion line

⇒ t > 1 Gyr

Time (Gyr)

Li I detected

models from Burrows et al. (2001)

Li

LiCl

Eff

ecti

ve T

empe

ratu

re (

K)

Brown Dwarfs as Chronometers

10 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Li I detected

⇒ 0.4 < t < 1 Gyr

A Li clock Binary

see Liu & Leggett (2005); Burgasser et al. (2005); Zapatero Osorio et al. (2008)

Eff

ecti

ve T

empe

ratu

re (

K)

Time (Gyr)

No Li I detected

Li depletion line

models from Burrows et al. (2001)

Li

LiCl

Brown Dwarfs as Chronometers

11 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Method 3: Mass Standards

A growing number of brown dwarf binaries have orbital (dynamic) mass measurements.

Binary companions to age-dated stars/in clusters provide tests of evolutionary models.

see also Lane et al. (2001), Bouy et al. (2004); Zapatero Osorio et al. (2004); Stassun et al. (2006); Bouy et al. (2008); Liu et al. (2008)

Talk by T. Dupuy, Poster by M. Liu

Dupuy, Liu & Ireland (2008)

~0.8 Gyr

Brown Dwarfs as Chronometers

12 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

models from Burrows et al. (2001) Effective Temperature (K)

Log

Sur

face

Gra

vity

(cg

s)

Method 4: Surface Gravity

Brown Dwarfs as Chronometers

13 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Teff & gravity → mass & age can be made for individual field objects

Caveat: metallicity

Burgasser, Burrows & Kirkpatrick (2006) see also Knapp et al. (2004); Burgasser (2007); Liebert & Burgasser (2007); Liu et al. (2007); Looper et al. (2007); Warren et al. (2007); Delorme et al. (2008); Leggett et al. (2008); Burningham et al. (2008)

Surface Gravity Diagnostics

Poster by S. Leggett

Brown Dwarfs as Chronometers

14 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Method 5: Kinematics

Are local brown dwarfs young? Zapatero Osorio et al. (2007): 21 L & T dwarfs w/ UVW ⇒ <age> ~ 0.5-4 Gyr

Faherty et al. (2008): >800 M7-T8 dwarfs w/ Vtan ⇒ <age> ~ 3-8 Gyr see also Schmidt et al. (2007); Jameson et al. (2007); Casewell et al. (2008)

Faherty et al. (2008) - 20 pc Zapatero Osorio et al. (2007)

Brown Dwarfs as Chronometers

15 © 2009 Adam J. Burgasser

IAU 258: Ages of Stars

Cluster members & companions - distant and/or rare, can encompass any spectral type, useful benchmarks

Li clock - optimal for 50-200 Myr & resolved binaries (rare); limits for Teff > 1300 K (M and L dwarfs)

Mass standards - very rare, long-term time investment, critical for testing models, challenging benchmarks

Surface gravity - useful in principle any source, ideal for T dwarfs, requires benchmarks

Kinematics - appropriate for large samples, current results controversial (single sources caveat emptor)