The cosmic ray streaming instability and the propagation of galactic cosmic rays

Magnetized Accretion and Dead Zones in Protostellar Disks.

Natalia Dzyurkevich (Max Planck Institute für Astronomie, Heidelberg)

L269 (former D18, second floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

The strong impact of magnetic diffusion in the dusty and poorly ionized circumstellar disks leads to the appearance of the so called 'dead zone' in the midplane. Several recent works have proposed the edges of the dead zones in proto-stellar disks as locations for trapping planetesimals and forming planets. Magneto-driven turbulence is expected to excavate gas from active, well-ionized regions and to pile it up in the dead zone.

I present the results of 3D global non-ideal MHD simulations which include inner edge of the dead zone, and explain the appearance of a pressure trap there. The follow-up work investigates the location of the outer edge of dead zone with the help of chemical network. The magnetically-active and 'dead' regions are mapped in a proto-stellar disk around a solar-type star, for various disk temperatures, surface density radial power-law indexes, and dust-to-gas ratios.  We also consider stellar masses between 0.4 and 2 $M_\odot$, with corresponding adjustments in the disk masses and temperatures. The dead zone's size and shape are estimated by computing the full tensor conductivity from the abundances of ions, electrons, and charged fractal dust aggregates.

The results reveal the shape of dead zone which is in most cases defined by the ambipolar diffusion. A variety of shapes of the dead zones appear when viewed in meridional cross-section, including a fish-tail outer edge and islands located on or off the midplane. The corresponding accretion rates vary with radius, indicating locations where the surface density will pile-up over time. We show that the dead zone's outer edge is too broad and diffuse to trap planetesimals in power-law surface density profiles.

Water absorption towards H~II regions in the Milky Way: Herschel/HIFI insights on the history of the gas from the PRISMAS key program

Nicolas Flagey (Jet Propulsion Laboratory, Caltech)

L269 (former D18, second floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

The water molecule plays an essential role in the chemistry and physics of the dense regions of the interstellar medium (ISM). It is an important reservoir of oxygen and cooling agent. It is found in vapor or condensed in iced mantle on grains in the ISM. One specificity of the water molecule is  its asymmetry: a molecule of water, at a given energy level, as defined by its quantum numbers, can either be ortho or para. The ratio between the population of ortho and para water is 3 in the high temperature limit. At lower temperature, the ortho/para ratio can be lower than 2. Collisions and interaction between the molecule and the grain may affect the ortho/para ratio. Consequently, measurements of the ortho/para ratio in water can provide insights on the history of the molecule and its environment. 

The Herschel Guaranteed Time Key Progamme PRISMAS (P.I. Maryvonne Gerin) observed 25 molecules, including nine transitions of water (H2O and H2^18O), towards eight sources with strong dust emission in the Galactic plane with the HIFI instrument. We present the multiple absorption features of water due to the foreground clouds that are known to intersect those lines of sight. We model the observations, taking into account line emission due to the continuum sources. We then infer optical depth along the line of sight and the ratio between ortho and para water column densities. We discuss the results and their implications on our understanding of physical and chemical processes.

Modelling low mass star formation at high redshifts: The role of supernova feedback on gas fragmentation.

by Harpreet Dhanoa (UCL, London)

Room L269 (former D18, second floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

The first stars are thought to be massive, luminous objects that formed around redshift z~20. When these metal-free stars went supernova, heavy metals were introduced to the universe, increasing the cooling efficiency of the ambient gas. This chemical enrichment process led to the formation of lower mass stars and influenced the formation of the first galaxies (z~10). We study the impact of supernova feedback on low mass star formation within one of these early galaxies. If a star of mass ~ 0.8 M_solar could be formed in this low metallicty environment, it would have survived until today and therefore may explain the observations of extremely metal poor stars in our galaxy. Results from our chemico-dynamic model will be presented, showing the importance of molecular and dust cooling on the pressure driven formation of clumps in low metallicty environments.

Depletion in the Gould Belt Clouds

Helen Christie (UCL, London)

Room L269 (former D18, 2nd floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

In dense, cold, star-forming regions molecules in the gas phase freeze-out onto dust grains forming icy mantles on their surfaces, a process we refer to as depletion. The extent of this depletion depends on a complicated chemistry which varies with time and physical environment. The strong dependance of depletion on the past evolution of a region could make it a strong probe of, for example, starless and pre-stellar core history. Depletion is, however, very hard to quantify by way of either observation or theory. We describe a study of CO depletion in five star-forming clouds within the Gould Belt. CO gas phase abundances are compared with dust column densities in order to estimate how much CO is missing from the gas phase. Using data for a large number of starless and pre-stellar cores we attempt a statistical comparison of our five regions.

Convection is a key process in stellar interiors. It is characterized by large Reynolds numbers, implying a highly turbulent regime. I will show how multidimensional hydrodynamical simulations allow us to get a physical insight of this complex phenonemon. The numerical results are obtained with a fully implicit method, and in the talk I will present the general framework and discuss the advantages of implicit methods in tackling stellar hydrodynamical problems.

Star Formation in Galactic Disks: The Efficiency Problem

By Eve Ostriker (University of Maryland)

Room L269 (former D18, 2nd floor) at ENS, 24 rue Lhomond, 11:00 to 12:00

In galactic disk systems, star formation rates (SFRs) are related to the gas content by empirical Kennicutt-Schmidt (KS) laws. Recent surveys have shown that these laws take on different scaling behavior in outer disks, mid disks, and inner disks, and that in some regions there are also strong correlations of SFRs with stellar and dark matter content.  In all regimes, star formation is observationally characterized as highly inefficient, in the sense that gas mass depletion timescales far exceed both global and local dynamical timescales.  I will argue that when viewed from the perspective of energy and momentum production rather than mass consuption, star formation is instead highly efficient, and that observed SFR scalings (and normalization) are a consequence of ISM self-regulation mediated by energy feedback from massive stars.  In models we have recently developed, SFRs evolve to meet the demands of the local galactic environment. In equilibrium, ISM heating balances cooling, total pressure balances gravity, and turbulent driving balances dissipation. These simple models yield remarkably good agreement with observations in all three regimes of star formation, and have been confirmed and calibrated using multiphase numerical hydrodynamic simulations.