Presentation on theme: "BROWN DWARFS The coldest stars in the Universe Ngoc Phan-Bao HCM International University-Vietnam National University Image credit: JPL/NASA Workshop on."— Presentation transcript:
BROWN DWARFS The coldest stars in the Universe Ngoc Phan-Bao HCM International University-Vietnam National University Image credit: JPL/NASA Workshop on Astrophysics and Cosmology Quy Nhon, Vietnam August
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 BAFGK M Sun 1995 LTY 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) M (75 M J ) M (15 M J ) StarsPlanetsBrown Dwarfs hydrogen-burning limitdeuteurium-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 M Jupiter – 75 M Jupiter, massive enough for deuterium-burning but below the hydrogen- burning limit.
Chabrier et al. 2000
p + p d + e + + e T crit K, M min M p + d 3 He + T crit K, M min M 7 Li + p 4 He + 4 He T crit 2.5 x 10 6 K, M min M Brown Dwarf Structure Burrow et al. 1997
Lithium test Basri 1998 Pavlenko et al Martin et al. 1998
Nguyen Anh Thus master thesis 60 M J, t = 100 Myr
2)TEMPERATURE: Very low mass stars: below 3500 K (Chabrier & Baraffe 1997) Brown dwarfs: K (Burrow et al. 2001)
3)RADIUS: R for VLM stars ~0.1 R or 1 R Jupiter for all brown dwarfs
Artwork Credit: Dr. Robert Hurt (IPAC/Caltech) M9, 75 M Jupiter L, 65 M J T, 35 M J Jupiter
Where to Find Ultracool Dwarfs? In the field, freely-floating objects identified by color-color diagrams, high-proper motion surveys, spectroscopic observations Phan-Bao et al (A&A), 2003 (A&A), 2006a (ApJ), 2008 (MNRAS) An M9 at 8 pc
V t =4.74 x x dI = M I + 5 logd - 5 Phan-Bao et al. 2006b Phan-Bao et al. 2003: Maximum Reduced Proper Motion Method Color-color DiagramReduced Proper Motion: H I = I + 5 log + 5 = M I + 5 log(V t /4.74)
In star-forming regions (young substellar objects) Zapatero Osorio et al (Science) Sigma Orionis, 350 pc, 2-4 Myr Freely floating planetary mass objects
Around nearby stars G2, 18 pc Potter et al. 2002Phan-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 – 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 ; 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 M J – 90 M J ). All are in class II. Observations: with SMA and CARMA We search for CO 2-1 (230 GHz, 1.2 mm) SMA: compact, 3.6x2.5, 0.25 km/s CARMA: D, 2.8x2.5, 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: (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: GHz (1 cm), GHz (3 mm) and 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)
CO J=2 1 MAP (230 GHz) Phan-Bao et al. 2008, ApJL ISO-Oph 102, 60 M J, Ophiuchi Lee et al. 2000
CO J=2 1 MAP (230 GHz) Phan-Bao et al. 2011, ApJ MHO 5, 90 M J, Taurus
Observations of brown dwarf outflows and disks with SMA, CARMA and ALMA TargetArray Mass (M J ) Log acc (M /yr) LogM outflow (M ) Log mass-loss (M /yr) Reference papers ISO-Oph 102SMA PB2008, ApJL 2M 0441CARMA PB2011, ApJ ISO-Oph 32SMA M 0439CARMA MHO 5SMA M 0414CARMA PB2013, submitted 2M 0438CARMA GM TauSMA
Summary BD Outflow Properties: Compact: AU Low velocity: 1-2 km/s Outflow mass: M (low-mass stars: M ) Mass-loss rate: M /yr (low-mass stars: M /yr) Episodic: active episodes of t ~ yr What we can learn from our observations: BD outflow is a scaled down version (a factor of ) 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 BD Class I (?)Class 0 (?) ? Core Phan-Bao et al Bourke et al Phan-Bao et al., in prep. Now, Hot Planets and Cool Stars, Munich, Class II BDs Class I Class 0 ? Core Phan-Bao et al Phan-Bao et al Phan-Bao et al. submitted Bourke et al. 2005Phan-Bao et al. Kauffmann et al Palau et al presented in Constellation10, Tenerife, 2010 Andre et al Oph-B 11 SMA-PBD1
ALMA NRAO/AUT & ESO Whitworth et al ALMA is times more sensitive and 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? Suns 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 & 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.) large- scale, axisymmetric, differential rotation. 2008: Brownings 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 Donatis 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 , M4 CFHT Espadons
Magnetic flux (ZDI): Bf=0.68 kG Applying the synthetic spectrum fitting technique (Johns-Krull & Valenti 1995): Bf = 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 III. Does the new spectral type Y exist? M dwarfs: Strong TiO & VO in optical L dwarfs: TiO & VO disappear T dwarfs: presence of CH 4 in near- infrared Kirkpatrick et al. 1999
Theory: The Y spectral type has been proposed for ultracool BDs cooler than about 600 K. Observation: Some ultracool BDs with temperature estimates about K such as CFBDS J0059, ULAS J1335, ULAS J0034 have been found but they dont show any new spectral features in their spectrum (to trigger a new spectral type, e.g. Y). Leggett et al. 2009
First Y dwarfs Cushing et al. 2011, ApJ
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 M J Disk radii: >20% with >10 AU ( 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 (MgSiO 3 )Forsterite (Mg 2 SiO 4 ) Disk Mass:8 M J Outer disk radius:80 AU
How does the disk of ISO-Oph 102 look like with ALMA? Photo: ASIAA The targets 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 & DAngelo 2005
0.89 mm3.2 mm Continuum Maps of ISO-Oph 102
Summary BDs can form like low-mass stars in a scaled down version with a factor of 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