Empirical Constraints on Physical Properties of Young Low-Mass Stars and Brown Dwarfs

Keivan Guadalupe Stassun

Physics & AstronomyVanderbilt University

Context: Testing and Calibrating PMS Stellar Evolutionary Models

Orion Nebula Cluster

(Hillenbrand 1997)

Empirical Measurements: Eclipsing Binaries

Stassun et al. (2004)

V1174 Ori

M1 = 1.01 ± 0.015 Msun

M2 = 0.73 ± 0.008 Msun

R1 = 1.34 ± 0.015 Rsun

R2 = 1.07 ± 0.011 Rsun

Dynamical Masses of Young Starscirca 2006

N=23

Mathieu et al. (2007)

Comparison of Dynamical Masses to Theoretical Models

Above 1 Msun:

•Good agreement: Mean difference 10% (1.6

Below 1 Msun:

•Poorer agreement: Mean difference as large as 40% (2.5)

•Tendency to underestimate masses

Best overall agreement is with Baraffe et al:

•Overall consistency to 1.4, though with large scatter, for MLT =1.0.

Hillenbrand & White (2004), updated Mathieu et al. (2007)

Models of Siess et al. (2000)

MLT = 1.93

1030

1.0

0.7

Stassun et al. (2004)

1.0

0.7

3

10

30

Models of Baraffe et al.

(1998)

MLT = 1.0

1 Myr

1

V1174 Ori

Using lithium to probe physics ofstellar interiors

Stassun et al. (2004)

Low lithium depletion in V1174 Ori implies

low (inefficient mixing).

1.0

1.5

2.0

V1174 Ori

Case Study: 2M0535-05The First Brown-Dwarf Eclipsing Binary

Bob Mathieu (Wisconsin)

Jeff Valenti (STScI)

Yilen Gomez (Vanderbilt)

Matthew Richardson (Fisk)

Luiz Paulo Vaz (UFMG, Brazil)

Prior to 2M0535-05

Dynamical mass measurements of brown dwarfs: GJ 1245 c: 0.074 ± 0.013 Msun 2M0746 b: 0.066 ± 0.006 Msun GJ 802 b: 0.058 ± 0.021 Msun GJ 569 c: 0.052 ± 0.018 Msun

Direct radius measurements of brown dwarfs:

2M0535-05: Summary of Results

Stassun et al. (2006, 2007)

M1 = 55 ± 5 MJup

M2 = 34 ± 3 MJup

R1 = 0.67 ± 0.03 Rsun

R2 = 0.51 ± 0.03 Rsun

Non-coeval formation? Dynamical effects, ejection scenarios

Magnetically suppressed convection? Decreased surface temperature Increased radius

Problem with model initial conditions? Starting gravities usually arbitrary

Temperature reversal

Oversized radii

Mohanty et al. (2004)

Problem with model initial conditions?

Baraffe et al. models

2M0535-05: Summary of Results

Stassun et al. (2006, 2007)

M1 = 55 ± 5 MJup

M2 = 34 ± 3 MJup

R1 = 0.67 ± 0.03 Rsun

R2 = 0.51 ± 0.03 Rsun

Temperature reversal

Non-coeval formation? Dynamical effects, ejection scenarios

Magnetically suppressed convection? Decreased surface temperature Increased radius

Problem with model initial conditions? Starting gravities are arbitrary

Oversized radii

Chandra Orion Ultradeep Project (COUP)

Simultaneous optical/X-ray monitoring of 800 TTS Stassun et al. (2006, 2007)

Rotationally modulated X-ray emission: Highly structured, strong surface fields

Flaccomio et al. (2005)

Jardine et al (2006)

Torres & Ribas (2002)

Chromospherically active main-sequence stars: Oversized radii

Torres et al. (2006)

YY Gem

V1016 Cyg

What you should remember…

Take-Away Message #1

Empirical constraints on the fundamental physical properties of young, low-mass stars and brown dwarfs are improving.

Masses and radii accurate to ~ 1% (eclipsing binaries), including first masses and radii for young brown dwarfs.

Take-Away Message #2

Evidence for magnetically suppressed convection in young, low-mass stars and brown dwarfs:

Empirical mass determinations: Best matched by theoretical models with inefficient convection (i.e. low ).

Lithium: Low levels of depletion imply inefficient mixing.

X-rays from PMS stars: Most consistent with highly structured, strong surface fields.

Magnetically active main-sequence binaries: Show oversized radii, most consistent with low models.

2M0535-05: Temperature reversal and oversized radii suggest suppressed convection.

Stassun et al. (in

prep.)

A new low-mass eclipsing binary at ~ 1 Myr:Activity implicated again?

M1 = 0.39 ± 0.03 Msun

M2 = 0.38 ± 0.03 Msun

R1 = 1.21 ± 0.06 Rsun

R2 = 1.17 ± 0.06 Rsun

T 250 K

How to Determine Mass and Age of a Young Star

Dynamical mass, Radius

Measure:

B.C.

SpT-Teff

Surface gravities of PMS stars?

Distance

Measure:

Mass, age

L, Teff

Models

V, SpT

calibrate

Orion Nebula Cluster

(Hillenbrand 1997)

Different Models, Different Answers!

Model M(Msun)

Age (Myr)

D’Antona & Mazzitelli (1998)

0.32 0.7

Palla & Stahler (1999)

0.62 2.9

Baraffe et al. (1998)

0.94 10.1

Theoretical Masses/Ages for 3800K, 0.5 Lsun young star

Including typical observational errors

in Teff and L

Techniques for making dynamical mass measurements

Single stars Circumstellar disk

“rotation curve”

Binary stars Astrometric Spectroscopic Eclipsing

Technique Mass determined?

Mass dependence on

distance

Luminosity dependence on

distance

Disk kinematics

Mtot D D2

Astrometric binary

M1 + M2 D3 D2

Disk kinematics +

SB2

M1M2

D D2

Astrometric binary + SB2

M1M2

D2

Eclipsing binary

M1M2

Measuring Accurate Stellar Temperatures: A Pressing Issue

Need to securely anchor stars in the HR diagram

Current SpTy errors ± 1 spectral subtype = 150 K

SpTy-Temp scale at least doubles this uncertainty

Detailed spectral synthesis and modeling: ~ 50 K

Detailed study underway (Stassun & Doppmann in prep.)

Doppmann et al. (2005)

P = 9.779621 ± 0.000014 days

System Geometry (to scale)

Flare analysis: Solar-type flaring loops

Brightest flares require loops ~10 R*

in size. Angular momentum losses likely severe.

Favata et al. (2005)

Possible importance of rapid stellar rotation?

Stassun et al.

(2003)

Breakup velocity!