Gravitoturbulence in magnetized protostellar discs

Gravitational instability (GI) features in several aspects of protostellar disc evolution, most notably in angular momentum transport, fragmentation, and the outbursts exemplified by FU Ori and EX Lupi systems. The outer regions of protostellar discs may also be coupled to magnetic fields, which could then modify the development of GI. To understand the basic elements of their interaction, we perform local 2D ideal and resistive magnetohydrodynamics simulations with an imposed toroidal field. In the regime of moderate plasma beta, we find that the system supports a hot gravitoturbulent state, characterized by considerable magnetic energy and stress and a surprisingly large Toomre parameter Q ≳ 10. This result has potential implications for disc structure, vertical thickness, ionization, etc. Our simulations also reveal the existence of long-lived and dense ‘magnetic islands’ or plasmoids. Lastly, we find that the presence of a magnetic field has little impact on the fragmentation criterion of the disc. Though our focus is on protostellar discs, some of our results may be relevant for the outer radii of AGN.Science and Technology Facilities Council (ST/L000636/1)This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw111

Master of Fine Arts

thesisThe following stories were written from 1991 through 1996. My primary objective is to captivate each reader, to gain a complete trust, for the walk across a canyon on a fundamental, yet carefully constructed bridge. The limitation of using the short form requires the most precise selection of details that creates the story where something memorable takes shape, materializes. One advantage of the short story is a limited relief from the responsibility to relate every thing, each detail. This proportionally increases my responsibility as writer, to be honest with each reader. Every detail of scene, character, setting and plot must be interwoven with each segment that both precedes and follows. Every word is required to be essential. Revision is something that does not end. Each time I read one of these pieces, I can see a clearer path over the trail. I know that I err in requiring my reader to assume the same pace as myself. When I revise, I find more color, a clearer view of the landscape or what year a certain pickup truck was made. I know I am successful when my work is read by a friend and I am told, I remember when that happened, as if it were part of their memory. Yet, they have questions, they observe and think. This collection was written as nonfiction, pushed over the edge. Each detail must be considered as a truth in itself and together with every other story that is each of us. Whether as the writer or the reader, there is a complete journey when each is allowed to seek their own way. It is not being led by the hand to the end of the trail

The stress-pressure lag in MRI turbulence and its implications for thermal instability in accretion discs

The classical alpha-disc model assumes that the turbulent stress scales linearly with -- and responds instantaneously to -- the pressure. It is likely, however, that the stress possesses a non-negligible relaxation time and will lag behind the pressure on some timescale. To measure the size of this lag we carry out unstratified 3D magnetohydrodynamic shearing box simulations with zero-net-magnetic-flux using the finite-volume code PLUTO. We impose thermal oscillations of varying periods via a cooling term, which in turn drives oscillations in the turbulent stress. Our simulations reveal that the stress oscillations lag behind the pressure by $\sim 5$ orbits in cases where the oscillation period is several tens of orbits or more. We discuss the implication of our results for thermal and viscous overstability in discs around compact objects

Magnetohydrodynamic convection in accretion discs

Convection has been discussed in the field of accretion discs for several decades, both as a means of angular momentum transport and also because of its role in controlling discs' vertical structure via heat transport. If the gas is sufficiently ionized and threaded by a weak magnetic field, convection might interact in non-trivial ways with the magnetorotational instability (MRI). Recently, vertically stratified local simulations of the MRI have reported considerable variation in the angular momentum transport, as measured by the stress to thermal pressure ratio $\alpha$, when convection is thought to be present. Although MRI turbulence can act as a heat source for convection, it is not clear how the instabilities will interact dynamically. Here we aim to investigate the interplay between the two instabilities in controlled numerical experiments, and thus isolate the generic features of their interaction. We perform vertically stratified, 3D MHD shearing box simulations with a perfect gas equation of state with the conservative, finite-volume code PLUTO. We find two characteristic outcomes of the interaction between the two instabilities: straight MRI and MRI/convective cycles, with the latter exhibiting alternating phases of convection-dominated flow (during which the turbulent transport is weak) and MRI-dominated flow. During the latter phase we find that $\alpha$ is enhanced by nearly an order of magnitude, reaching peak values of $\sim 0.08$. In addition, we find that convection in the non-linear phase takes the form of large-scale and oscillatory convective cells. Convection can also help the MRI persist to lower Rm than it would otherwise do. Finally we discuss how our results help interpret simulations of Dwarf Novae

Magnetorotational instability and dynamo action in gravito-turbulent astrophysical discs

Though usually treated in isolation, the magnetorotational and gravitational instabilities (MRI and GI) may coincide at certain radii and evolutionary stages of protoplanetary discs and active galactic nuclei. Their mutual interactions could profoundly influence several important processes, such as accretion variability and outbursts, fragmentation and disc truncation, or large-scale magnetic field production. Direct numerical simulations of both instabilities are computationally challenging and remain relatively unexplored. In this paper, we aim to redress this neglect via a set of 3D vertically stratified shearing-box simulations, combining self-gravity and magnetic fields. We show that gravito-turbulence greatly weakens the zero-net-flux MRI. In the limit of efficient cooling (and thus enhanced GI), the MRI is completely suppressed, and yet strong magnetic fields are sustained by the gravitoturbulence. This turbulent `spiral wave' dynamo may have widespread application, especially in galactic discs. Finally, we present preliminary work showing that a strong net-vertical-flux revives the MRI and supports a magnetically dominated state, in which the GI is secondary.Comment: 23 pages, 16 figures, accepted in MNRA

Spiral structures in gravito-turbulent gaseous disks

CONTEXT. Gravitational instabilities can drive small-scale turbulence and large-scale spiral arms in massive gaseous disks under conditions of slow radiative cooling. These motions affect the observed disk morphology, its mass accretion rate and variability, and could control the process of planet formation via dust grain concentration, processing, and collisional fragmentation. AIMS. We study gravito-turbulence and its associated spiral structure in thin gaseous disks subject to a prescribed cooling law. We characterize the morphology, coherence, and propagation of the spirals and examine when the flow deviates from viscous disk models. METHODS. We used the finite-volume code PLUTO to integrate the equations of self-gravitating hydrodynamics in three-dimensional spherical geometry. The gas was cooled over longer-than-orbital timescales to trigger the gravitational instability and sustain turbulence. We ran models for various disk masses and cooling rates. RESULTS. In all cases considered, the turbulent gravitational stress transports angular momentum outward at a rate compatible with viscous disk theory. The dissipation of orbital energy happens via shocks in spiral density wakes, heating the disk back to a marginally stable thermal equilibrium. These wakes drive vertical motions and contribute to mix material from the disk with its corona. They are formed and destroyed intermittently, and they nearly corotate with the gas at every radius. As a consequence, large-scale spiral arms exhibit no long-term global coherence, and energy thermalization is an essentially local process. CONCLUSIONS. In the absence of radial substructures or tidal forcing, and provided a local cooling law, gravito-turbulence reduces to a local phenomenon in thin gaseous disks

Mass return to the interstellar medium from highly-evolved carbon stars

Data produced by the Infrared Astronomy Satellite (IRAS) was surveyed at the mid- and far-infrared wavelengths. Visually-identified carbon stars in the 12/25/60 micron color-color diagram were plotted, along with the location of a number of mass-losing stars that lie near the location of the carbon stars, but are not carbon rich. The final sample consisted of 619 objects, which were estimated to be contaminated by 7 % noncarbon-rich objects. The mass return rate was estimated for all evolved circumstellar envelopes. The IRAS Point Source Catalog (PSC) was also searched for the entire class of stars with excess emission. Mass-loss rates, lifetimes, and birthrates for evolved stars were also estimated

Detection of OH+ and H_2O+ towards Orion KL

We report observations of the reactive molecular ions OH+, H_(2)O+, and H_(3)O+ towards Orion KL with Herschel/HIFI. All three N = 1-0 fine-structure transitions of OH+ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho-H_(2)O+ 1_(11)–0_(00) transition at 1115 and 1139 GHz were detected; an upper limit was obtained for H_(3)O+. OH+ and H_(2)O+ are observed purely in absorption, showing a narrow component at the source velocity of 9 km s^(-1), and a broad blueshifted absorption similar to that reported recently for HF and para-H_(2)^(18)O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH+ and H_(2)O+ for the 9 km s^(-1) component of 9 ± 3 × 10^(12) cm^(-2) and 7 ± 2 × 10^(12) cm^(-2), and those in the outflow of 1.9 ± 0.7 × 10^(13) cm^(-2) and 1.0 ± 0.3 × 10^(13) cm^(-2). Upper limits of 2.4 × 10^(12) cm^(-2) and 8.7 × 10^(12) cm^(-2) were derived for the column densities of ortho and para-H_(3)O+ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate