BROWN DWARFS
The coldest stars in the Universe
Ngoc Phan-Bao
HCM International University-Vietnam National University
Workshop on Astrophysics and Cosmology
Quy Nhon, Vietnam  August 5-9 2013
Image credit: JPL/NASA
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
Cushing et al.
1995 2011
Rebolo et al.
Nakajima et al.
Sun
O
B
A F G K M L TY
T7
Stars, Brown Dwarfs and Planets
Main Sequence:
O
B
A
(Oh Be A
F
G
K
M
L T Y
Fine Girl Kiss My Lips Tonight Yahoo!)
Sun
(G2)
Stars
0.075 M
(75 MJ)
hydrogen-burning limit
Brown Dwarfs
Planets
0.015 M
(15 MJ)
deuteurium-burning limit
M dwarf part
Brown Dwarfs
Credit: ESA (Hipparcos)
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 hydrogenburning limit.
Chabrier et al. 2000
Brown Dwarf Structure
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
Tcrit  2.5 x 106 K, Mmin  0.065 M
+ p  4He + 4He
Burrow et al. 1997
Lithium test
Pavlenko et al. 1995
Basri 1998
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
M9, 75 MJupiter
L, 65 MJ
T, 35 MJ
Jupiter
Artwork Credit: Dr. Robert Hurt (IPAC/Caltech)
Where to Find Ultracool Dwarfs?
● In the field, freely-floating objects identified by color-color
diagrams, high-proper motion surveys, spectroscopic
observations.
An M9 at 8 pc
1984.915
1996.937
Phan-Bao et al. 2001 (A&A), 2003 (A&A), 2006a (ApJ), 2008 (MNRAS)
Color-color Diagram
Reduced Proper Motion:
Vt=4.74 x  x d
I = MI + 5 logd - 5
HI = I + 5 log + 5 = MI + 5 log(Vt/4.74)
Phan-Bao et al. 2006b
Phan-Bao et al. 2003: Maximum Reduced
Proper Motion Method
• In star-forming regions (young substellar objects)
Zapatero Osorio et al. 2000 (Science)
Sigma Orionis, 350 pc, 2-4 Myr
Freely floating planetary mass objects
● Around nearby stars
Potter et al. 2002
G2, 18 pc
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.
 IV.
Does the new spectral type “Y” exist?
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

Two major models:
requirement of very low mass core formation and prevention
of subsequent accretion of gas
–
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.)
–
Ejection model: very low-mass embryos are ejected from
multiple systems (Reipurth & Clarke 2001, Bate et al. 2004)
cartoon from Close et al. 2003
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; L1014IRS 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
Submilimeter Array and Combined Array for
Research in Millimeter-Wave Astronomy
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)
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
5.2
250
191
1.31
1.3
125
319
0.392
0.39
62
383
0.161
0.16
31
383
0.081
0.081
8
383
0.021
0.021
2
383
0.0052
0.005
ISO-Oph 102, 60 MJ,  Ophiuchi
CO J=21 MAP (230 GHz)
Phan-Bao et al. 2008, ApJL
Lee et al. 2000
MHO 5, 90 MJ, Taurus
CO J=21 MAP (230 GHz)
Phan-Bao et al. 2011, ApJ
Observations of brown dwarf outflows and disks
with SMA, CARMA and ALMA
Target
Array
Mass
Log Ṁacc
(MJ)
(M/yr)
LogMoutflow
(M)
LogṀmass-loss
(M/yr)
-3.8
Reference
papers
ISO-Oph 102
SMA
60
-9.0
-8.9 PB2008, ApJL
2M 0441
CARMA
35
-11.3
-
-
ISO-Oph 32
SMA
40
-10.5
-
-
2M 0439
CARMA
50
-11.3
-
-
MHO 5
SMA
90
-10.8
2M 0414
CARMA
75
-10.0
-
-
PB2013,
2M 0438
CARMA
70
-10.8
-
-
submitted
GM Tau
SMA
73
-8.6
-4.2
-4.9
PB2011, ApJ
-9.1
-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)
-9
-10 M /yr (low-mass stars:
 Mass-loss rate: 10 -10

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 1001000) 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)
presented in Constellation10, Tenerife, 2010
Core
Class 0 (?)
Class I (?)
Class II BD
?
Phan-Bao et al., in prep.
Bourke et al. 2005
Phan-Bao et al. 2008
Now, Hot Planets and Cool Stars, Munich, 2012
Core
Oph-B 11
Class 0
Class I
3 Class II BDs
SMA-PBD1
?
Phan-Bao et al.
Bourke et al. 2005
Andre et al. 2012 Kauffmann et al. 2011
Palau et al. 2012
Phan-Bao et al. 2008
Phan-Bao et al. 2011
Phan-Bao et al. submitted
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
field morphology
•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.
Theory
field morphology
•1997, 1999: Kuker & Rudiger
have developed the 2 dynamo
(Robert & Stix 1972) for T
Tauri stars: non-axisymmetric
fields, no differential rotation.
•2006: (Chabrier & Kuker)
large-scale, non-axisymmetric,
no differential rotation.
•2006: (Dobler et al.) largescale, 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 largescale 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.
Mapping the Magnetic Field of G 164-31, M4
Espadons
CFHT
•Magnetic flux (ZDI): Bf=0.68 kG
•Applying the synthetic spectrum
fitting technique (Johns-Krull &
Valenti 1995): Bf = 3.20.4 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
III. Does the new spectral type “Y” exist?
M
M dwarfs: Strong TiO
& VO in optical
L L dwarfs: TiO & VO
disappear
T T dwarfs: presence
of CH4 in nearinfrared
Kirkpatrick et al. 1999 Cushing et al. 2006
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 dwarfs
Cushing 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: 73 mJy, an excellent target for ALMA
Enstatite (MgSiO3)
• Disk Mass: 8 MJ
• Outer disk radius:
80 AU
Forsterite (Mg2SiO4)
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
Continuum Maps of ISO-Oph 102
0.89 mm
3.2 mm
Image: ESO
Summary
 BDs 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 smallscale 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