BROWN DWARFSThe coldest stars in the

UniverseNgoc Phan-Bao

HCM International University-Vietnam National

University

Image credit: JPL/NASA

Workshop on Astrophysics and CosmologyQuy Nhon, Vietnam August 5-9 2013

Outline of Talk Physical properties of brown dwarfs Most important issues of the brown dwarf

science How do they form? How is their magnetic field morphology? Does the new spectral type “Y” exist? What are the properties of planets forming around

brown dwarfs?

Main sequence stars

O B A F G K M

Sun

1995

L T Y

2011

Rebolo et al.Nakajima et al.

Cushing et al.

T7

Stars, Brown Dwarfs and Planets

Main Sequence:O B A F G K M L

T Y

Sun (G2)

0.075 M(75 MJ)

0.015 M(15 MJ)

Stars PlanetsBrown Dwarfs

hydrogen-burning limit deuteurium-burning limit

(Oh Be A Fine Girl Kiss My Lips Tonight Yahoo!)

Brown Dwarfs

Credit: ESA (Hipparcos)

M dwarf part

Credit: Gemini Observatory/Artwork by Jon Lomberg

Fundamental Parameters(solar metallicity and a few gigayear old)

1) MASS: ● Very low mass stars: below 0.35 M (spectral

type: M3-M4) fully convective stars (Chabrier & Baraffe 1997).

● Brown dwarfs: 13 MJupiter – 75 MJupiter, massive enough for deuterium-burning but below the hydrogen-burning limit.

Chabrier et al. 2000

p + p d + e+ + e Tcrit 3 106 K, Mmin 0.075 M

p + d 3He + Tcrit 5 105 K, Mmin 0.013 M

7Li + p 4He + 4He Tcrit 2.5 x 106 K, Mmin 0.065 M

Brown Dwarf Structure

Burrow et al. 1997

Lithium test

Basri 1998

Pavlenko et al. 1995

Martin et al. 1998

Nguyen Anh Thu’s master thesis

60 MJ, t = 100 Myr

2) TEMPERATURE: ● Very low mass stars: below 3500 K (Chabrier &

Baraffe 1997)● Brown dwarfs: 300 — 2200 K (Burrow et al. 2001)

3) RADIUS: • 0.1-0.3 R for VLM stars• ~0.1 R or 1 RJupiter for all brown dwarfs

Artwork Credit: Dr. Robert Hurt (IPAC/Caltech)

M9, 75 MJupiter

L, 65 MJ T, 35 MJ Jupiter

Where to Find Ultracool Dwarfs?● In the field, freely-floating objects identified by

color-color diagrams, high-proper motion surveys, spectroscopic observations.

1984.915

1996.937

Phan-Bao et al. 2001 (A&A), 2003 (A&A), 2006a (ApJ), 2008 (MNRAS)

An M9 at 8 pc

Vt=4.74 x x d

I = MI + 5 logd - 5

Phan-Bao et al. 2006b

Phan-Bao et al. 2003: Maximum Reduced Proper Motion Method

Color-color Diagram Reduced Proper Motion:

HI = I + 5 log + 5 = MI + 5 log(Vt/4.74)

• In star-forming regions (young substellar objects)

Zapatero Osorio et al. 2000 (Science) Sigma Orionis, 350 pc, 2-4 MyrFreely floating planetary mass objects

● Around nearby starsG2, 18 pc

Potter et al. 2002 Phan-Bao et al. 2006b

Chauvin et al. 2004

Most Important Issues of the Ultracool Dwarf Science

I. How do they form? II. How is their magnetic field

morphology? III. Does the new spectral type “Y”

exist? IV. What are the properties of

planets forming around ultracool dwarfs?

I. How do Ultracool Dwarfs Form? Major issue in making a brown dwarf: Balance

between requirement of very low mass core formation and prevention of subsequent accretion of gas

Two major models: – Star-like models: very low-mass cores formed by

turbulent/gravitational fragmentation (Padoan & Nordlund 2002, Bonnell et al. 2008) are dense enough to collapse

Turbulent fragmentation (Padoan et al.)Collapse & fragmentation (Bonnell et al.)

cartoon from Close et al. 2003

– Ejection model: very low-mass embryos are ejected from multiple systems (Reipurth & Clarke 2001, Bate et al. 2004)

Key Issue: All of these mechanisms might be happening but the key question is “which mechanism dominates in brown dwarf formation?”

Observations: Statistical properties of BDs such as IMF, binarity, velocity dispersion, disks, accretion, jets…show a continuum with those of stars.

A typical picture of star formation

Key to understand early stages of BD formation: Molecular outflows (velocity, size, outflow mass, mass-loss rate) offer a very useful tool to identify and study BD classes 0, I, II.

Observations of brown dwarf outflows and disks with SMA, CARMA and ALMA

A. Molecular Outflows Overview:

3 detections of molecular outflow from Very-Low Luminosity Objects (VELLOs): IRAM 04191+1522; L1014-IRS and L1521F-IRS

only one L1014-IRS (class 0/I, Bourke et al. 2005) whose the outflow process is characterized

the sources are embedded in dust and gas, so it is very difficult to determine the mass of the central objects and their final mass

We search for molecular outflows from class II young BDs that are reaching their final mass

Sample: 2 BDs in Ophiuchi and 6 (1 VLM star + 5 BDs) in Taurus (mass range: 35 MJ – 90 MJ). All are in class II.

Observations: 2008-2010 with SMA

and CARMA We search for CO 2-1

(230 GHz, 1.2 mm) SMA: compact,

3.6’’x2.5’’, 0.25 km/s CARMA: D, 2.8’’x2.5 ‘’, 0.18 km/s

SMA (Ho, Morran & Lo 2004) ● A joint project between Academia Sinica IAA and CfA

● Eight 6-m antennas on Mauna Kea

● 4 receiver bands: 230, 345, 400 and 690 GHz● Bandwidth: 2 GHz and 4 GHz ● Configurations: subcompact, compact, extended, very extended● Angular resolutions: 5’’-0.1’’ (at 345 GHz)

Submilimeter Array and Combined Array for Research in Millimeter-Wave

Astronomy

CARMA● A university-based millimeter array at Cedar Flat (US)● six 10.4-meter, nine 6.1-meter, and eight 3.5-meter antennas

● 3 receivers: 27-35 GHz (1 cm), 85-116 GHz (3 mm) and 215-270 GHz (1 mm); 8 bands available.

● Configurations: A, B, C, D, E

● Angular resolutions: 0.3", 0.8", 2", 5", or 10" at 100 GHz

Bandwidth (MHz)

Channels (per sideband)

Chan. width (MHz)

dV [1mm] (km/s)

500 96 5.21 5.2250 191 1.31 1.3125 319 0.392 0.3962 383 0.161 0.1631 383 0.081 0.0818 383 0.021 0.0212 383 0.0052 0.005

CO J=21 MAP (230 GHz)

Phan-Bao et al. 2008, ApJL

ISO-Oph 102, 60 MJ, Ophiuchi

Lee et al. 2000

CO J=21 MAP (230 GHz)

Phan-Bao et al. 2011, ApJ

MHO 5, 90 MJ, Taurus

Observations of brown dwarf outflows and disks with SMA, CARMA and ALMA

Target Array Mass (MJ)

Log Ṁacc

(M/yr)

LogMoutflow

(M)

LogṀmass-

loss (M/yr)Reference

papers

ISO-Oph

102

SMA 60 -9.0 -3.8 -8.9 PB2008,

ApJL

2M 0441 CARM

A

35 -11.3 - - PB2011, ApJ

ISO-Oph 32 SMA 40 -10.5 - -

2M 0439 CARM

A

50 -11.3 - -

MHO 5 SMA 90 -10.8 -4.2 -9.1

2M 0414 CARM

A

75 -10.0 - - PB2013,

submitted

2M 0438 CARM

A

70 -10.8 - -

GM Tau SMA 73 -8.6 -4.9 -10.3

Summary BD Outflow Properties:

Compact: 500-1000 AU Low velocity: 1-2 km/s Outflow mass: 10-4-10-5 M (low-mass stars: 10-1

M) Mass-loss rate: 10-9-10-10 M/yr (low-mass stars:

10-7 M/yr) Episodic: active episodes of t ~ 2000-5000 yr

What we can learn from our observations: BD outflow is a scaled down version (a factor of

100-1000) of the outflow process in stars Supporting the scenario that BDs form like stars BD outflow properties are used to identify/study

BD formation at earlier stages (class 0, I)

Class II BDClass I (?)Class 0 (?)

?

Core

Phan-Bao et al. 2008Bourke et al. 2005Phan-Bao et al., in prep.

Now, Hot Planets and Cool Stars, Munich, 2012

3 Class II BDsClass IClass 0

?

Core

Phan-Bao et al. 2008Phan-Bao et al. 2011Phan-Bao et al. submitted

Bourke et al. 2005Phan-Bao et al.Kauffmann et al. 2011Palau et al. 2012

presented in Constellation10, Tenerife, 2010

Andre et al. 2012

Oph-B 11 SMA-PBD1

ALMA

NRAO/AUT & ESO Whitworth et al. 2007●ALMA is 10-100 times more sensitive and 10-100 times

better angular resolution than the current mm/submm arrays.To achieve the same continuum sensitivity, ALMA only needs 1 sec while SMA needs 8 hours. An excellent instrument to search for proto-brown dwarfs/planetary mass objects class II, I, 0, BD-cores, and the BD disk structure.

II. How is their magnetic field morphology?

Sun’s structure Fully convective stars

The dynamo in the Sun

II. How is their magnetic field morphology?

● Partially convective stars (e.g., Sun): The dynamo results in predominantly magnetic flux

● Fully convective stars (UDs): The lack of radiative core precludes the dynamo Question: What type of dynamo produces strong magnetic activity (X-rays, H, radio) as observed in UDs?

– An 2 dynamo has been proposed by Dobler et al. 2006, Chabrier & Küker 2006 for UDs. In general, all these models agree with observations on large-scale, strong magnetic fields of 1-3 kG.

– They disagree with observations on some properties of the field, e.g., toroidal vs. poloidal, differential vs. no differential rotation.

Theory

• 1993: Durney et al. proposed

turbulent dynamo small-scale fields weakly depending on vsini.

Observation

• 1994: Saar measured magnetic in M3-M4 dwarfs, f ~ 60-90%, B ~ 4 kG.

1995: Johns-Krull & Valenti measured B ~ 4 kG, f ~ 50% in M3-M4 dwarfs.

field morphology

Theory• 1997, 1999: Kuker &

Rudigerhave developed the 2 dynamo (Robert & Stix 1972) for T Tauri stars: non-axisymmetricfields, no differential rotation.

• 2006: (Chabrier & Kuker) large-scale, non-axisymmetric, no differential rotation.

• 2006: (Dobler et al.) large-scale, axisymmetric, differential rotation.

• 2008: Browning’s simulations resulted in large-scale fields, weak differential rotation, but toroidal.

Observation

• 2006: Donati et al.’s observations indicated a axisymmetric large-scale poloidal field, no differential rotation in an M4 dwarf.

• 2006: Phan-Bao et al. independently claimed the first detection of Zeeman signatures, implying a large-scale field in an M3.5 dwarf.

• 2008: Morin and Donati’s observations: mainly toroidal, differential rotation in 6 early-M dwarfs; axisymmetric poloidal in 5 mid-M dwarfs with no differential rotation.

field morphology

Mapping the Magnetic Field of G 164-31, M4

CFHT

Espadons

• Magnetic flux (ZDI): Bf=0.68 kG

• Applying the synthetic spectrum

fitting technique (Johns-Krull & Valenti 1995): Bf = 3.20.4 kG

using both techniques can describe a better image of the field morphology of UDs.

Conclusion: The magnetic field morphology of UDs in reality

might include:● An axisymmetric large-scale poloidal field and ● Small-scale structures containing a significant part

of magnetic energy

M

L

T

Cushing et al. 2006

III. Does the new spectral type “Y” exist?

M dwarfs: Strong TiO & VO in optical

L dwarfs: TiO & VOdisappear

T dwarfs: presence of CH4 in near-infrared

Kirkpatrick et al. 1999

T7

Theory: The “Y” spectral type has been proposed for ultracool

BDs cooler than about 600 K.

Observation: Some ultracool BDs with temperature estimates

about 550-600 K such as CFBDS J0059, ULAS J1335, ULAS

J0034 have been found but they don’t show any new spectral

features in their spectrum (to trigger a new spectral type,

e.g. “Y”).

Leggett et al. 2009

Credits: NASA/JPL

First Y dwarfsCushing et al. 2011, ApJ

Credits: JPL

IV. What properties of planets form around BDs?

Orion HST

i) Detections of grain growth, crystallization, and dust settling occurring in brown dwarf disks have been reported (e.g., Apai et al. 2005, Riaz 2009):

These detections demonstrate that planets can form around BDs

Apai et al. 2005

ii) BD disk properties from SEDs: 20 M6-M9 in Taurus (Scholz et al. 2006):

Disk masses: 0.4 – 1.2 MJ

Disk radii: >20% with >10 AU (10-100 AU) It is unlikely that Jupiter-mass planets are frequent around BDs

iii) So far, no direct imaging of any BD disks has been done: Therefore it is important to map a BD disk to obtain disk parameters, hence to understand how planets form around BDs

ISO-Oph 102: 73 mJy, an excellent target for ALMA

Enstatite (MgSiO3) Forsterite (Mg2SiO4)

• Disk Mass: 8 MJ• Outer disk radius: 80 AU

How does the disk of ISO-Oph 102 look like with ALMA?

Photo: ASIAA

The target’s disk size: 160 AU (~1.3’’), 16 mJy at 345 GHz

Band 7 (345 GHz or 0.8 mm) Configuration C32-6, ~0.2’’ With a total time on source of 5.3 min (sensitivity ~ 0.1

mJy), an expected detection level of 22

Simulation from Wolf & D’Angelo 2005

0.89 mm 3.2 mm

Continuum Maps of ISO-Oph 102

Image: ESO

SummaryBDs can form like low-mass stars in a scaled down

version with a factor of 100-1000. Some giant planets close to the brown dwarf-planet boundary can form like BDs

An axisymmetric large-scale poloidal field and small-scale structures containing a significant part (60%) of magnetic energy

Cooler BDs needed to be detected for new “Y” spectral types

Rocky planets can form around BDs