Program for the ANITA Workshop 2015

نویسندگان

  • Amanda Karakas
  • Christoph Federrath
  • Mark Wardle
چکیده

Book for the ANITA Workshop 2015 Research School of Astronomy and Astrophysics Australian National University ! DAY 1: MONDAY FEBRUARY 9TH! !! SESSION 1:! !! James Gilmore (UNSW)! Title: “Planetary Architecture and Climate”! Abstract: How is solar system architecture connected to climate? Here I examine this question by studying how Jupiter's orbit influences Earth's axial tilt or obliquity. The n-body integrator MERCURY, modified to include post-Newtonian effects, is used to compute the evolution of the Solar System under changes in Jupiter's orbit. I also solve the obliquity equations to form a complete picture of the Milankovitch cycles on Earth. Results indicate that small to moderate changes in Jupiter's orbit can have substantial effects on Earth's obliquity and therefore climate. I will also discuss the implications of this work for exoplanet systems.! !! Mark Hutchison (Swinburne University of Technology)! Title: “SPH and photoevaporation: sweating the small stuff”! Abstract: Smoothed Particle Hydrodynamics (SPH) naturally concentrates particle resolution in high-density regions without wasting computational resources on empty or sparsely populated space. Does this rule out SPH as being a viable tool for simulating phenomena such as photoevaporation—a dispersal mechanism that operates in the tenuous atmospheres of protoplanetary discs? I will discuss my recent attempts at enhancing the SPH resolution for low-density phenomena that are dynamically coupled to objects that are considerably more dense (e.g. photoevaporation).! !! James Wurster (Monash University)! Title: “When Ideal Gas is Not Enough: Collapsing a Cloud to form a Protostar”! Abstract: A simple way to numerically study the formation of a protostar is to model the collapse of a spherical gas cloud. Although much can be learned from performing a parameter study on the basic characteristics (e.g. the magnetic field), additional physics is required in order to obtain more realistic results. I will discuss the effect additional physics (e.g. non-ideal MHD, dust) has on the collapse of the gas cloud. The focus will be on the resulting disc and jet, and how this differs from the results of the simple case of ideal gas. ! !! Daniel Price (Monash University)! Title: “Planet formation it's real and we see it”.! Abstract: In Nov 2014 planet formation became an observational science. I'll discuss our attempts over the summer to reconstruct the ALMA observations of HLTau and what it has taught us about planet formation so far.! !! Page of 5 14 Raquel Salmeron (RSAA, ANU)! Title: “Protostellar accretion disks and their outflows”! Abstract: Magnetocentrifugal jets and magnetically-driven turbulence have been recognized as the leading candidates for transporting the excess angular momentum of protostellar disks, thereby enabling mass accretion onto the central object. It is also clear that magnetic diffusivity plays a central role in the overall disk accretion and outflow processes. However, the impact of magnetic dissipation on the dynamical and thermal structure of these objects, and on their observational signatures, remains poorly understood. In my talk, I will examine the launching of outflows from the surfaces of weakly-ionised protostellar disks, and present models that calculate the structure of the disk and the emerging wind, taking detailed account of magnetic diffusivity effects. These models allow us to study the properties of the disk and wind as a function of disk physics, and compare them with observational data. ! !! SESSION 2:! !! Jan Staff (Macquarie University)! Title: “3D simulations of MHD disk winds: jet rotation at very large scales”! Abstract: I will present the results of three-dimensional magneto-hydrodynamics simulations of disk-winds from young stellar objects. I will discuss magnetic field configurations that are favourable for the formation of two-component rotating jet; a cylindrically shaped outer jet surrounding a much faster, inner jet. For some specific configurations, the underlying Keplerian rotation profile of the disk is preserved to large distances from the source in the inner jet. For other configurations the Keplerian rotation profile is not preserved, and we even find regions in the jet rotating opposite to the disk.! ! Christopher Nolan (RSAA, ANU)! Title: ”Centrifugally driven winds from protostellar accretion disks”! Abstract: One of the outstanding challenges in star formation is the angular momentum problem. For material to be accreted onto a forming protostar, angular momentum must be redistributed to or ejected from the system. One of the ways that this may happen is through winds driven centrifugally from the disk surface, mediated by large scale magnetic fields threading the disk. In this talk I present the first ever 1+1D semi-analytic model of the structure of strongly magnetized wind-driving protostellar disks, including a realistic ionization and diffusion model. Using this model I explore the impact of magnetic field strength, mass accretion rate and surface density profile on the regions of the disk which may launch a wind and the strength of these winds.! !! SESSION 3:! !! Christoph Federrath (RSAA, ANU)! Title: “The role of turbulence and feedback for star formation”! Abstract: I will present hydrodynamical simulations of star cluster formation, including turbulence, magnetic fields and outflow/jet feedback. Comparisons with a theoretical model for the star formation rate and comparisons with observations show encouraging agreement.! Page of 6 14 Andrew Lehmann (Macquarie University)! Title: “Dissipation of Molecular Cloud Turbulence by Magnetohydrodynamic Shockwaves”. ! Abstract: The origin and character of star formation remains a key open problem in astrophysics. This process is intimately related to the supersonic magnetohydrodynamic (MHD) turbulent dynamics of the giant molecular clouds in which stars form. Simulations have shown that shock waves are responsible for a significant amount of turbulent dissipation. As these shocks dissipate by molecular line cooling of the compressed gas, the nature of the turbulence may be probed by studying the radiative signatures of shocks. Fast, intermediate and slow MHD shocks differ in how they compress and heat the molecular gas. Fast shocks dissipate most of the turbulent energy when the turbulence is driven, whereas slow shocks dissipate most of the energy when the turbulence is decaying. This means that observational differences between fast and slow shocks also distinguish between driven and decaying turbulence. Here we use a two-fluid MHD model to compare the characteristics of one-dimensional fast and slow shocks. Fast shocks are magnetically driven, forcing ion species to stream through the neutral gas ahead of the shock front. This magnetic precursor heats the gas sufficiently to create a large transition zone where all the fluid variables only weakly change in the shock front. In contrast, slow shocks are driven by gas pressure where neutral species collide with ion species in a thin hot slab that closely resembles an ordinary gas dynamic shock. Low-velocity shocks dominate the dissipation of turbulence. For these shocks, we searched for observational diagnostics of the different shock types by following the abundances of a variety of molecules through the shocks and computing the line strengths of carbon monoxide and water. We found that the high-J CO rotational lines differ strongly between fast and slow shocks and therefore are an important probe of turbulence. ! ! Mark Wardle (Macquarie University)! Title: “Alfven waves in partially ionised plasmas”! Abstract: Magnetic flux freezing, an excellent approximation in many settings, breaks down in the partially ionised environment of molecular clouds, protoplanetary disks, and the solar chromosphere. In protoplanetary disks and the solar chromosphere the situation is particularly complicated due to the competing influences of ohmic resistivity, ambipolar diffusion, and Hall drift. Sophisticated numerical simulations are now attacking the important dynamical consequences of decoupling and drift of magnetic flux in these systems, including generation and dissipation of MHD turbulence, angular momentum and magnetic flux transport, and jet launching. Surprisingly, despite the importance of understanding fluid dynamics in these environments, there has not yet been a systematic study of waves in a partially ionised medium. Even for the simplest case, Alfven waves, the existence of an evanescent band of wavelengths is currently under debate in the literature.! ! Here I present an overview of the physics underlying the propagation of Alfven waves in an ion-electron-neutral plasma and present an intuitive description of the results of a detailed analysis. There are three circularly polarised modes, each damped by ion-neutral and electron-neutral collisions. Their character changes several times over different wavelength ranges but there is *no* evanescent region. At short wavelengths, two of the modes correspond to ion and electron cyclotron waves in a fully ionised plasma, but are heavily damped by collisions with neutrals. The remaining neutral-dominated mode has a much lower frequency and is slowly damped. This mode dominates in weakly-ionised environments such as protoplanetary disks, where its destabilisation results in MHD turbulence. I conclude with some remarks on the implications of this analysis for understanding the dynamics of partially ionised plasmas in astrophysical settings.! Page of 7 14 Carolyn Doherty (Monash University)! Title: “The s/r process in super-AGB stars”! Abstract: Super-AGB stars are in the mass range ~ 6.5-10 Msun and are characterised by off-centre carbon ignition prior to a thermally pulsing AGB phase. The final fate of these stars is highly uncertain and depending on a variety of factors they may end life as either a white dwarf or a neutron star after undergoing a supernova explosion. In the most massive super-AGB stars a dredge-out event occurs in which very high neutron densities are achieved, intermediate between those found in the slow “s” and rapid “r” processes. The resultant heavy element distribution from this “s/r” process may provide a unique signature to distinguish the differing evolutionary histories near the end of the carbon burning phase.! !! SESSION 4:! !! Luke Shingles (RSAA, ANU)! Title: "An element in the room: How helium alters stellar evolution and chemical yields”! Abstract: There is now strong evidence that some stars are born with Helium-enhanced initial compositions such as in the globular cluster Omega Centauri. However, little is known about the advanced evolutionary phases, chemical yields, and final fates of these stars. In this talk, I will outline the evidence for He-rich stellar populations and present new theoretical models of He-enhanced stars at [Fe/H] = -1.4 and helium fractions up to Y=0.40. My analysis will include changes to stellar evolution during the thermally pulsing asymptotic giant branch phase and implications for the chemical yields, including heavy elements produced via the slow neutron capture process.! !! Athira Menon (Monash University) ! Title: “The quest for blue supergiants : Possible evolutionary tracks for the progenitor of SN 1987A.”! Abstract: SN 1987A is historically, one of the most remarkable supernova explosions ever to be seen from Earth. Due to the proximity of its location in the LMC, it was closely monitored during the progress of its light curve. It was also particularly fortunate that the progenitor of this supernova, SK69 202, was observed prior to its explosion. ! ! What makes SN 1987A truly standout however, is that it was the first Type II SN to have been observed to have exploded while its progenitor was a blue supergiant, contradictory to prior theoretical and observational of Type II SNe originating from red supergiants. Since then only two more Type II SNe have been observed to have had blue supergiant progenitors, making SN 1987A a rare and unique event. Even today it is not entirely understood what the stellar evolution history might be, that causes a blue supergiant to explode as a Type II SN. The most commonly held idea is that their origin may lie in the merger of massive binary star systems. According to Ivanova & Podsiadlowski (2002), binary systems that undergo a 'moderate merger' over a period of 50-100 years, become blue supergiants with hydrogen and nitrogen rich envelopes, that then explode as Type II SNe. ! ! (Continued on the next page)! !

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تاریخ انتشار 2015