Aims. We investigate the suitability of alpha-viscosity in self-similar models for self-gravitating disks with a focus on active galactic nuclei (AGN) disks. Methods. We use a self-similar approach to simplify the partial di ff erential equations arising from the evolution equation, which are then solved using numerical standard procedures. Results. We find a self-similar solution for the dynamical evolution of self-gravitating alpha-disks and derive the significant quantities. In the Keplerian part of the disk our model is consistent with standard stationary alpha-disk theory, and self-consistent throughout the self-gravitating regime. Positive accretion rates throughout the disk demand a high degree of self-gravitation. Combined with the temporal decline of the accretion rate and its low amount, the model prohibits the growth of large central masses. Conclusions. alpha-viscosity cannot account for the evolution of the whole mass spectrum of super-massive black holes (SMBH) in AGN. However, considering the involved scales it seems suitable for modelling protoplanetary disks.

Rapid rates of coastal wetland loss in Louisiana are widely recognized. One important question of wetland sustainability is how volumetric contributions of roots to wetland soils vary under the influence of different hydrologic regimes. The research presented here specifically investigates the spatial and temporal relationships among the specific gravity of live roots, soil chemistry, and flooding regime for the macrophyte Spartina alterniflora Loisel. in natural, salt marsh, field settings located across southeastern coastal Louisiana. The results of this research propose the existence of a stress-tolerance threshold (beyond which root specific gravity modifications are observed), and highlight the importance of micro-scale factors over macro-scale regional characteristics in determining environmental stresses and the subsequent impact on root specific gravity. A conceptual model is developed linking the interactions of relevant environmental variables, root specific gravity, and the idea of a stress-tolerance threshold.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 106-115). Available also in a digital version from Dissertation Abstracts.

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The focus of this study is the change in accretion disk size in Dwarf Novae (DN), IP Peg. DN systems are a type of cataclysmic variable that experience periodic outbursts. These outbursts are caused by the release of gravitational potential energy from an increased rate of matter flow through the accretion disk. Throughout outburst, the radius of the accretion disk of the DN changes. Recent research done at Ball State University has suggested that the disk radius may not change as the disk instability model predicts. According to the disk instability model, the accretion disk should be at its largest radial size when the DN is at the peak of outburst. IP Peg in September and October of 2006 underwent outburst. It was found that during that particular outburst that the accretion disk was at its largest radial size on the decline from outburst and not peak. Further research into how the accretion disk changes with time is needed. / Department of Physics and Astronomy

In the case of cold accretion disk, coupling between charge neutral gas and magnetic field is too weak such that the magneto-rotational instability will be less effective or even stop working. In such a situation it is of prime interest to investigate the pure hydrodynamic turbulence and transport phenomenon. As the Reynolds number increases, the relative importance of the non-linear term in the hydrodynamic equation increases and in the case of accretion disk where molecular viscosity is too small the Reynolds number is large enough for the non-linear term to bring new effects. We investigate a scenario, the ‘weakly non-linear’ evolution of amplitude of linear mode when the flow is bounded by two parallel walls. The unperturbed flow is similar to plane Couette flow but with Coriolis force included in the hydrodynamic equation. Although there is no exponentially growing eigenmode, due to self-interaction the least stable eigenmode will grow in an intermediate phase. Later on this will lead to higher order non-linearity and plausible turbulence. Although the non-linear term in the hydrodynamic equation is energy conserving, within the weakly non-linear analysis it is possible to define a lower bound of the energy needed for flow to transform to turbulent phase. Such an unstable phase is possible only if the Reynolds number ≥ 103−4. In Chapter-2 we set up equation of amplitude for the hydrodynamic perturbation and study the effect of weak non-linear evolution of linear mode for general angular momentum distribution, where Keplerian disk is obtained as a special case. As we know that to explain observed hard X-rays the choice of Keplerian angular momentum profile is not adequate, we consider the sub-Keplerian regime of the disk. In Chapter-3 we assume that the cooling mechanism is dominated by bremsstrahlung process (without any strict knowledge of the magnetic field structure).We show that in a range of Shakura-Sunyaev viscosity 0.2 ≥ α ≥ 0.0005, flow behavior varies widely, particularly by means of the size of disk, efficiency of cooling and corresponding temperatures of ions and electrons. We also show that the disk around a rotating black hole is hotter compared to that around a Schwarzschild black hole, rendering a larger difference between ion and electron temperatures in the former case. We finally reproduce the observed luminosities(L) of two extreme cases—the under-fed AGNs and quasars and ultra-luminous X-ray sources at different combinations of mass accretion rate, ratio of specific heats, Shakura-Sunyaev viscosity parameter and Kerr parameter. In Chapter-4 we investigate the viscous two temperature accretion disk flows around rotating blackholes. We describe the global solution of accretion flows, unlike that in Chapter-3, with a sub-Keplerian angular momentum profile, by solving the underlying conservation equations including explicit cooling processes self-consistently. Bremsstrahlung, synchrotron and inverse comptonization of soft photons are considered as possible cooling mechanisms. We focus on the set of solutions for sub-Eddington, Eddington and super-Eddington mass accretion rates around Schwarzschild and Kerr black holes with a Kerr parameter 0.998. We analyse various phases of advection–general advective paradigm to radiatively inefficient paradigm. The solution may potentially explain the hard X-rays and γ-rays emitted from AGNs and X-ray binaries. We also compare the solutions for two different regimes of viscosity. We finally reproduce the observed luminosities of the under-fed AGNs and quasars, ultra-luminous X-ray sources at different combinations of input parameters such as mass accretion rate and ratio of specific heats.

The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and secondly, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. Furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. Thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. The next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here.

We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution less than or similar to 0'.18 (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust-trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the (CO)-C-13 and (CO)-O-18 (J = 2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is less than or similar to 0.12 M-circle plus, which gives a disk mass ratio in dust between the two components of M-dust,M-SR 24S/M-dust,M-SR 24N greater than or similar to 840. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to millimeter sizes is inhibited there and that only warm gas, as seen by rovibrational CO emission inside the truncation radii of the binary, is present.

Using Keck/HIRES spectra (Delta v similar to 7 km s(-1)) we analyze forbidden lines of [O I] 6300 angstrom, [O I] 5577 angstrom. and [S II] 6731 angstrom. from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high-velocity component (HVC) associated with microjets, we study the properties of the low-velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (magnetohydrodynamic) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected [O. I] but only in two out of eight with detected [S. II], so our analysis is largely based on the properties of the [O. I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The full width at half maximum of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 au and 0.5 to 5 au, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC, however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.

Variable emission from Sgr A*, the luminous counterpart to the super-massive black hole at the center of our Galaxy, arises from the innermost portions of the accretion flow. Better characterization of the variability is important for constraining models of the low-luminosity accretion mode powering Sgr A*, and could further our ability to use variable emission as a probe of the strong gravitational potential in the vicinity of the 4 x 10(6) M-circle dot black hole. We use the Herschel Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr. A* at wavelengths that are difficult or impossible to observe from the ground. We find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal structure that is highly correlated across these wavelengths. While the variations correspond to < 1% changes in the total intensity in the Herschel beam containing Sgr. A*, comparison to independent, simultaneous observations at 0.85 mm strongly supports the reality of the variations. The lowest point in the light curves, similar to 0.5 Jy below the time-averaged flux density, places a lower bound on the emission of Sgr. A* at 0.25 mm, the first such constraint on the THz portion of the spectral energy distribution. The variability on few hour timescales in the SPIRE light curves is similar to that seen in historical 1.3 mm data, where the longest time series is available, but the distribution of variations in the sub-mm do not show a tail of large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from XMM-Newton shows no significant variation within our observing period, which may explain the lack of very large submillimeter variations in our data if X-ray and submillimeter flares are correlated.