BROWN DWARFS The coldest stars in the UniverseNgoc Phan-BaoHCM International University-Vietnam National University Workshop on Astrophysics and Cosmology QuyNhon, 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 L T Y M B A F G K T7
Stars, Brown Dwarfs and Planets Main Sequence: O B A F G KM L T Y (Oh Be AFine Girl Kiss My Lips Tonight Yahoo!) Sun (G2) Stars Brown Dwarfs Planets 0.075 M (75 MJ) 0.015 M (15 MJ) hydrogen-burning limit deuteurium-burning limit
M dwarf part Brown Dwarfs Credit: ESA (Hipparcos)
Fundamental Parameters(solar metallicity and a few gigayear old) • 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.
Brown Dwarf Structure p + p d + e+ + eTcrit 3 106 K, Mmin 0.075 M p + d 3He + Tcrit 5 105 K, Mmin 0.013 M 7Li + p 4He + 4HeTcrit 2.5 x 106 K, Mmin 0.065 M 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 — 2200K (Burrow et al. 2001)
3) RADIUS: • 0.1-0.3 R for VLM stars • ~0.1 R or 1 RJupiter for all brown dwarfs
L, 65 MJ T, 35 MJ M9, 75 MJupiter 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 1996.937 1984.915 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
G2, 18 pc • Around nearby stars Potter et al. 2002 Phan-Bao et al. 2006b
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.)
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.
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
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
ISO-Oph 102, 60 MJ, Ophiuchi CO J=21 MAP (230 GHz) Phan-Bao et al. 2008, ApJL Lee et al. 2000
MHO 5, 90 MJ, Taurus CO J=21 MAP (230 GHz) Phan-Bao et al. 2011, ApJ
Observations of brown dwarf outflows and disks with SMA, CARMA and ALMA
Summary • BD Outflow Properties: • Compact: 500-1000 AU • Low velocity: 1-2 km/s • Outflow mass: 10-4-10-5M (low-mass stars: 10-1M) • Mass-loss rate: 10-9-10-10M/yr (low-mass stars: 10-7M/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)
presented in Constellation10, Tenerife, 2010 Core Class 0 (?) Class I (?) Class II BD ? Phan-Bao et al., in prep. Phan-Bao et al. 2008 Bourke et al. 2005 Now, Hot Planets and Cool Stars, Munich, 2012 Core Class I Class 0 3 Class II BDs Oph-B 11 ? SMA-PBD1 Phan-Bao et al. Kauffmann et al. 2011 Palau et al. 2012 Bourke et al. 2005 Phan-Bao et al. 2008 Phan-Bao et al. 2011 Phan-Bao et al. submitted Andre et al. 2012
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? Fully convective stars Sun’s structure
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. field morphology • 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 • 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: Browning’s simulations resulted in large-scale fields, weak differential rotation, but toroidal. field morphology • 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.
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 = 3.20.4 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
III. Does the new spectral type “Y” exist? M dwarfs: Strong TiO & VO in optical M L L dwarfs: TiO & VO disappear T T dwarfs: presence of CH4 in near-infrared Kirkpatrick et al. 1999 Cushing et al. 2006
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