Day 536

Friday.  Saturday.

1310.6209
Toward a hybrid dynamo model for the Milky Way
Gressel, Elstner, Ziegler

MW's B-field is now modeled with an unprecedented level of detail and complexity, based on the rapidly increasing all-sky data of Faraday rotation measures and polarized synchrotron radiation.  Complement this heuristic approach with a physically motivated, quantitative Galactic dynamo model: a model that allows for the evolution of the system as a whole, instead of just solving the induction equation for a fixed static disc.  Building on the framework of mean-field magnetohydrodynamics and extending it to the realm of hybrid evolution, perform three-dimensional global simulations of the Galactic disc.  Closure coefficients embodying the mean-field dynamo are calibrated against resolved box simulations of SN-driven interstellar turbulence.  The emerging dynamo solutions comprise a mixture of dominant axisymmetric S0 mode, with even parity and a subdominant A0 mode, with odd parity.  Notably, such a superposition of modes creates a strong localized vertical field on one side of the Galactic disc.  Moreover, find significant radial pitch angles, which decay with radius -- explained by flaring of the disc.  In accordance with previous work, magnetic instabilities appear to be restricted to the less-stirred outer Galactic disc.  Their main effect is to create strong fields at large radii such that the radial scale length of the B-field increases from 4 kpc (for the case of a mean-field dynamo alone) to about 10 kpc in the hybrid models.  There remain aspects (e.g., spiral arms, X-syaped halo fields, fluctuating fields) that are not captured by the current model and that will require further development towards a fully dynamical evolution.  Nevertheless, the work presented demonstrates that a hybrid modeling of the Galactic dynamo is feasible and can serve as a foundation for future efforts.

1310.6248
The planetary system to KIC11442793: a compact analogue to the solar system
Cabrera, et al

Announce the discovery of 7 transiting planets around a Kepler target, a current record for transiting systems.  Planets b, c, e and f are new; d, g, h previously reported, revise their orbital parameters and confirm their planetary nature.  Planets h and g are gas giants and show strong dynamical interactions.  Orbit of planet g is perturbed in such a way that its orbital period changes by 25.7h between two consecutive transits during the length of the observations (largest perturbation found so far).  The rest of the planets also show mutual interactions: planets d, e and f are super-Earths close to a mean motion resonance chain (2:3:4), and planets b and c, with sizes below 2 Earth radii, are within 0.5% of the 4:5 mean motion resonance.  This complex system presents some similarities to our Solar System, with small planets in inner orbits and gas giants in outer orbits.  It is, however, more compact.  The outer planet has an orbital distance around 1 AU, and the relative position of the gas giants is opposite to that of Jupiter and Saturn, which is closer to the expected result of planet formation theories.  The dynamical interactions between planets are also much richer.

1310.6261
The link between magnetic fields and filamentary clouds: bimodal cloud orientations in the Gould belt
Li, Fang, Henning, Kainulainen

* Gould belt: a partial ring of stars in the MW, about 3k lyr across, tilted toward the galactic plane by 16-20 degrees.  Conatins many O- and B-type stars, and may represent the local spiral to which the Sun belongs---currently the Sun is about 325 lyrs from the arm's center.  The belt is thought to be from 30 to 50 Myrs old, and of unknown origin.  Belt contains the constellations  Cephus, Lactera, Perseus, Orion, Canis Major, Puppis, Vela, Carina, Crux, Centaurus, Lupus, and Scorpius (MW also passes through most of these constellations).  A recent theory is that the Gould Belt formed about 30 Myrs ago when a blob of DM collided with a molecular cloud in our region.  These is also evidence for similar Gould belts in other galaxies.

The orientations of filamentary molecular clouds in the Gould belt and their local ICM (inter-cloud media) B-fields are studied using NIR dust extinction maps and optical stellar polarimetry data.  These filamentary clouds are few-to-ten parsecs in length, and find that their orientations tend to be either parallel or perpendicular to the mean field directions of the local ICM.  This bimodal distribution is not found in cloud simulations with super-Alfvenic turbulence, in which the cloud orientations should be random.  ICM B-felds that are dynamically important compared to inertial-range turbulence and self-gravity can readily explain both field-filament configurations.  Previous studies commonly recognize that strong B-fields can guide gravitational contraction and result in filaments perpendicular to them, but few discuss the fact that B-fields can also channel sub-Alfvenic turbulence to form filaments aligned with them.  This strong-field scenario of cloud formation is also consistent with the constant field strength observed from ICM to clouds (Crutcher+ 2010) and is possible to explain the "hub-filament" cloud structure (Myers 2009) and the density threshold of cloud gravitational contraction (Kainulainen+ 2009).

1310.6267
The high-ion content and kinematics of low-redshift Lyman limit systems
Fox et al

Study the high-ionization phase and kinematics of the circumgalactic medium around low-z galaxies using a sample of 23 Lyman limit systems (LLSs) at 0.08<z<0.93 observed with the cosmic origins spectrograph onboard HST.  Lehner+ 2013 showed that low-z LLSs have a bimodal metallicity distribution.  Here, extend that analysis to search for differences between the high-ion and kinematic properties of the metal-poor and metal-rich branches.  Find that metal-rich LLSs tend to show higher O VI columns and broader O VI profiles than metal-poor LLSs.  The total H I line width (dv90 stats) in LLSs is not correlated with metallicity, indicating that the H I kinematics alone cannot be used to distinguish inflow from outflow and gas recycling.  Among the 17 LLSs with O VI detections, all but two show evidence of kinematic sub-structure, in the form of O VI-H I centroid offsets, multiple components, or both.  Using various scenarios for how the metallicity in the high-ion and low-ion phases of each LLS compare, constrain the ionized hydrogen column in the O VI phase to lie in the range log N(H II)~17.6-20.  The O VI phase of LLSs is a substantial baryon reservoir, with M(high-ion)~1e8.5-10.9 (R/150 kpc)^2 solar masses, similar to the mass in the low-ion phase.  Accounting for the O VI phase approximately doubles the contribution of low-z LLSs to the cosmic baryon budget.