How the stars as the Sun formed are? How the planetary systems born are? To answer these questions, astrophysicists must discover the processes which transform vast gas clouds into accretion discs, and then into stars and planets. It would seem that, among the major ingredients, the magnetic field plays a major role. An international team of astrophysicists, led by JF Donati of CNRS (Midday-Pyrenees Observatory in Toulouse - KNOWLEDGE), has just succeeded in charting the arches and the tubes which the magnetic field weaves between young-stars and their accretion discs. These observations, carried out with the ESPaDOnS spectropolarimeter installed in the Canada-France-Hawaii telescope, should make it possible to specify how the stars interact with their accretion disc to form their own planetary system. These results are published in Monthly Notices of the Royal Astronomical Society.
Title: Gap Formation in the Dust Layer of 3D Protoplanetary Disks Authors: S. T. Maddison (1), L. Fouchet (2 and 3), J.-F. Gonzalez (2) ((1) Swinburne, Australia, (2) CRAL, Lyon, France, (3) ETH, Zurich, Switzerland)
We numerically model the evolution of dust in a protoplanetary disk using a two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is non-self-gravitating and locally isothermal. The code follows the three dimensional distribution of dust in a protoplanetary disk as it interacts with the gas via aerodynamic drag. In this work, we present the evolution of a disk comprising 1% dust by mass in the presence of an embedded planet for two different disk configurations: a small, minimum mass solar nebular (MMSN) disk and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the grain size and planetary mass to see how they effect the resulting disk structure. We find that gap formation is much more rapid and striking in the dust layer than in the gaseous disk and that a system with a given stellar, disk and planetary mass will have a different appearance depending on the grain size and that such differences will be detectable in the millimetre domain with ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk while not in the gas disk. We also note that dust accumulates at the external edge of the planetary gap and speculate that the presence of a planet in the disk may facilitate the growth of planetesimals in this high density region.
Title: Magnetic Fields in Protoplanetary Disks Authors: Mark Wardle (Macquarie University) (Version v2)
Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines.
Title: Terrestrial Zone Debris Disk Candidates in h & chi Persei Authors: Thayne Currie (1), Scott J. Kenyon (1), George Rieke (2), Zoltan Balog (2), Benjamin C. Bromley (3) ((1) Harvard-Smithsonian Center for Astrophysics, (2) Steward Observatory/University of Arizona, (3) Dept. of Physics, University of Utah)
We analyse 8 sources with strong mid-infrared excesses in the 13 Myr-old double cluster h & chi Persei. New optical spectra and broadband SEDs (0.36-8 mu_m) are consistent with cluster membership. We show that material with T ~ 300-400 K and Ld/Lstar ~ 10^-4-10^-3 produces the excesses in these sources. Optically-thick blackbody disk models - including those with large inner holes - do not match the observed SEDs. The SEDs of optically-thin debris disks produced from terrestrial planet formation calculations match the observations well. Thus, some h and chi Persei stars may have debris from terrestrial zone planet formation.
Title: Imaging the Ionised Disk of the High-Mass Protostar Orion-I Authors: M. J. Reid, K. M. Menten, L. J. Greenhill, C. J. Chandler
We have imaged the enigmatic radio source-I (Orion-I) in the Orion-KL nebula with the VLA at 43 GHz with 34 mas angular resolution. The continuum emission is highly elongated and is consistent with that expected from a nearly edge-on disk. The high brightness and lack of strong molecular lines from Orion-I can be used to argue against emission from dust. Collisional ionisation and H-minus free-free opacity, as in Mira variables, require a central star with >10^5 Lsun, which is greater than infrared observations allow. However, if significant local heating associated with accretion occurs, lower total luminosities are possible. Alternatively, photo-ionisation from an early B-type star and p+/e- bremsstrahlung can explain our observations, and Orion-I may be an example of ionised accretion disk surrounding a forming massive star. Such accretion disks may not be able to form planets efficiently.
Title: Magnetic Fields in Protoplanetary Disks Authors: Mark Wardle (Macquarie University)
Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines.
Title: Spitzer Limits On Dust Emission and Optical Gas Absorption Variability Around Nearby Stars with Edge-On Circumstellar Disk Signatures Authors: Seth Redfield, Jacqueline E. Kessler-Silacci, Lucas A. Cieza
We present Spitzer observations and McDonald Observatory Smith Telescope and Anglo-Australian Telescope high spectral resolution optical observations of 4 nearby stars with variable or anomalous optical absorption, likely caused by circumstellar material. The optical observations of CaII and NaI cover a 2.8 year baseline, and extend the long term monitoring of these systems by previous researchers. In addition, mini-surveys of the local interstellar medium (LISM) around our primary targets provide a reconstruction of the intervening LISM along the line of sight. We confirm that the anomalous absorption detected toward alpha Oph is not due to circumstellar material, but to a small filamentary cloud <14.3 pc from the Sun. The three other primary targets, beta Car, HD85905, and HR10 show both short and long term variability, and little of the observed absorption can be attributed to the LISM along the line of sight. The Spitzer observations did not detect infrared excesses. We are able to place upper limits on any possible fractional infrared luminosity, which range from L_IR/L_star < 2-5 10^-6, for our three disk stars. No stable gas absorption component centred at the radial velocity of the star is detected for any of our targets. Based on simple assumptions of the variable gas absorption component, we estimate limits on the circumstellar gas mass causing the variable absorption, which range from 0.4-20 10^-8 M_Earth. These multiwavelength observations place strong limits on any possible circumstellar dust, while confirming variable circumstellar gas absorption, and therefore are interesting targets to explore the origins and evolution of variable circumstellar gas.
Title: New Low-Mass Stars and Brown Dwarfs with Disks in Lupus Authors: P. R. Allen, K. L. Luhman, P. C. Myers, S. T. Megeath, L. E. Allen, L. Hartmann, G. G. Fazio
Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the Spitzer Space Telescope, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 \micron. We present photometry in these bands for the 41 previously known members that are within our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 \micron colours that are indicative of circumstellar disks. We have performed optical spectroscopy on 6 of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 M_\odot for ages of ~1 Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf (M~0.03 M_\odot) with a spectral type of M8. In contrast to their results, we measure an earlier spectral type of M5.75±0.25 for this object, indicating that it is probably a low-mass star (M~0.1 M_\odot). In fact, according to its gravity-sensitive absorption lines and its luminosity, 2M 1541-3345 is older than members of the Lupus clouds (\tau~1 Myr) and instead is probably a more evolved pre-main-sequence star that is not directly related to the current generation of star formation in Lupus.
Title: Disks around Brown Dwarfs and Cool Stars Authors: Daniel Apai, Kevin Luhman, Michael Liu
We review the current picture of disks around cool stars and brown dwarfs, including disk fractions, mass estimates, disk structure and dispersal, accretion, dust composition, and the debris disk phase. We discuss these in the framework of recent planet formation models.
Title: New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets Authors: C. Beichman, G. Bryden, K. Stapelfeldt, et al
Using the MIPS instrument on the Spitzer telescope, we have searched for infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence field stars, along with a small number of nearby M stars. These stars were selected for their suitability for future observations by a variety of planet-finding techniques. These observations provide information on the asteroidal and cometary material orbiting these stars - data that can be correlated with any planets that may eventually be found. We have found significant excess 70um emission toward 12 stars. Combined with an earlier study, we find an overall 70um excess detection rate of 13 ± 3% for mature cool stars. Unlike the trend for planets to be found preferentially toward stars with high metallicity, the incidence of debris disks is uncorrelated with metallicity. By newly identifying 4 of these stars as having weak 24um excesses (fluxes ~ 10% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24um excess is associated with a strong 70um excess. Interestingly, we find no evidence for debris disks around 23 stars cooler than K1, a result that is bolstered by a lack of excess around any of the 38 K1-M6 stars in 2 companion surveys. One motivation for this study is the fact that strong zodiacal emission can make it hard or impossible to detect planets directly with future observatories like the Terrestrial Planet Finder (TPF). The observations reported here exclude a few stars with very high levels of emission, > 1,000 times the emission of our zodiacal cloud, from direct planet searches. For the remainder of the sample, we set relatively high limits on dust emission from asteroid belt counterparts.