Identifying Deficiencies of Standard Accretion Disk Theory: Lessons from a Mean-Field Approach

Turbulent viscosity is frequently used in accretion disk theory to replace the microphysical viscosity in order to accomodate the observational need for in- stabilities in disks that lead to enhanced transport. However, simply replacing the microphysical transport coefficient by a single turbulent transport coeffi- cient hides the fact that the procedure should formally arise as part of a closure in which the hydrodynamic or magnetohydrodynamic equations are averaged, and correlations of turbulent fluctuations are replaced by transport coefficients. Here we show how a mean field approach leads quite naturally two transport coefficients, not one, that govern mass and angular momentum transport. In particular, we highlight that the conventional approach suffers from a seemingly inconsistent neglect of turbulent diffusion in the surface density equation. We constrain these new transport coefficients for specific cases of inward, outward, and zero net mass transport. In addition, we find that one of the new transport terms can lead to oscillations in the mean surface density which then requires a constant or small inverse Rossby number for disks to maintain a monotonic power-law surface density.Comment: 11 page

Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks

Non-thermal X-ray emission in compact accretion engines can be interpreted to result from magnetic dissipation in an optically thin magnetized corona above an optically thick accretion disk. If coronal magnetic field originates in the disk and the disk is turbulent, then only magnetic structures large enough for their turbulent shredding time to exceed their buoyant rise time survive the journey to the corona. We use this concept and a physical model to constrain the minimum fraction of magnetic energy above the critical scale for buoyancy as a function of the observed coronal to bolometric emission. Our results suggest that a significant fraction of the magnetic energy in accretion disks resides in large scale fields, which in turn provides circumstantial evidence for significant non-local transport phenomena and the need for large scale magnetic field generation. For the example of Seyfert AGN, for which of order 30 per cent of the bolometric flux is in the X-ray band, we find that more than 20 per cent of the magnetic energy must be of large enough scale to rise and dissipate in the corona.Comment: submitted to ApJL, 2 fig

Structure of Protoplanetary Discs with Magnetically-driven Winds

We present a new set of analytical solutions to model the steady state structure of a protoplanetary disc with a magnetically-driven wind. Our model implements a parametrization of the stresses involved and the wind launching mechanism in terms of the plasma parameter at the disc midplane, as suggested by the results of recent, local MHD simulations. When wind mass-loss is accounted for, we find that its rate significantly reduces the disc surface density, particularly in the inner disc region. We also find that models that include wind mass-loss lead to thinner dust layers. As an astrophysical application of our models, we address the case of HL Tau, whose disc exhibits a high accretion rate and efficient dust settling at its midplane. These two observational features are not easy to reconcile with conventional accretion disc theory, where the level of turbulence needed to explain the high accretion rate would prevent a thin dust layer. Our disc model that incorporates both mass-loss and angular momentum removal by a wind is able to account for HL Tau observational constraints concerning its high accretion rate and dust layer thinness.Comment: Accepted for publication in MNRAS, 13 pages, 8 figure

Gravitational microlensing of gamma-ray blazars

We present a detailed study of the effects of gravitational microlensing on compact and distant $\gamma$-ray blazars. These objects have $\gamma$-ray emitting regions which are small enough as to be affected by microlensing effects produced by stars lying in intermediate galaxies. We analyze the temporal evolution of the gamma-ray magnification for sources moving in a caustic pattern field, where the combined effects of thousands of stars are taken into account using a numerical technique. We propose that some of the unidentified $\gamma$-ray sources (particularly some of those lying at high galactic latitude whose gamma-ray statistical properties are very similar to detected $\gamma$-ray blazars) are indeed the result of gravitational lensing magnification of background undetected Active Galactic Nuclei (AGNs).Comment: 30 pages, 27 figures. Four figures are being submitted only as .gif files, and should be printed separately. The abstract below has been shortened from the actual version appearing in the pape

The galactic center black hole as a possible retro-lens for the S2 orbiting star

Holz & Wheeler (\cite{hw}) have recently proposed that a Schwarzschild black hole may act as a retro-lens which, if illuminated by a powerful light source, deflects light ray paths to large bending angles and a series of luminous arcs (or rings in the case of aligned objects) centered on the black hole may form. Obviously, the most convenient geometry to get retro-lensing images would be that of a very bright star close to a massive black hole, say the putative $\sim 4\times 10^6$ M$_{\odot}$ black hole at the galactic center. Recent observations of the galactic center region in the $K$-band have revealed the presence of a very bright main sequence star (labelled S2) with mass $\sim 15$ M$_{\odot}$ orbiting at close distance (130-1900 AU) from Sgr A$^*$. The relatively vicinity of S2 to the central massive black hole may offer a unique laboratory to test the formation of retro-lensing images. The next generation of space-based telescopes in the $K$-band (like NGST) may have high enough limiting magnitude necessary to observe such retro-lensing images.Comment: 4 pages, 2 Postscript figures, accepted for pubblications on Astronomy and Astrophysic

Nonlinear energy transfers in accretion discs MRI turbulence. I-Net vertical field case

The magnetorotational instability (MRI) is believed to be responsible for most of the angular momentum transport in accretion discs. However, molecular dissipation processes may drastically change the efficiency of MRI turbulence in realistic astrophysical situations. The physical origin of this dependency is still poorly understood as linear and quasi linear theories fail to explain it. In this paper, we look for the link between molecular dissipation processes and MRI transport of angular momentum in non stratified shearing box simulations including a mean vertical field. We show that magnetic helicity is unimportant in the model we consider. We perform a spectral analysis on the simulations tracking energy exchanges in spectral space when turbulence is fully developed. We find that the energy exchanges are essentially direct (from large to small scale) whereas some non linear interactions appear to be non local in spectral space. We speculate that these non local interactions are responsible for the correlation between turbulent transport and molecular dissipation. We argue that this correlation should then disappear when a significant scale separation is achieved and we discuss several methods by which one can test this hypothesis.Comment: 10 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

The Skeletal L-type Ca2+ Current Is a Major Contributor to Excitation-coupled Ca2+ entry

The term excitation-coupled Ca2+ entry (ECCE) designates the entry of extracellular Ca2+ into skeletal muscle cells, which occurs in response to prolonged depolarization or pulse trains and depends on the presence of both the 1,4-dihydropyridine receptor (DHPR) in the plasma membrane and the type 1 ryanodine receptor in the sarcoplasmic reticulum (SR) membrane. The ECCE pathway is blocked by pharmacological agents that also block store-operated Ca2+ entry, is inhibited by dantrolene, is relatively insensitive to the DHP antagonist nifedipine (1 μM), and is permeable to Mn2+. Here, we have examined the effects of these agents on the L-type Ca2+ current conducted via the DHPR. We found that the nonspecific cation channel antagonists (2-APB, SKF 96356, La3+, and Gd3+) and dantrolene all inhibited the L-type Ca2+ current. In addition, complete (>97%) block of the L-type current required concentrations of nifedipine >10 μM. Like ECCE, the L-type Ca2+ channel displays permeability to Mn2+ in the absence of external Ca2+ and produces a Ca2+ current that persists during prolonged (∼10-second) depolarization. This current appears to contribute to the Ca2+ transient observed during prolonged KCl depolarization of intact myotubes because (1) the transients in normal myotubes decayed more rapidly in the absence of external Ca2+; (2) the transients in dysgenic myotubes expressing SkEIIIK (a DHPR α1S pore mutant thought to conduct only monovalent cations) had a time course like that of normal myotubes in Ca2+-free solution and were unaffected by Ca2+ removal; and (3) after block of SR Ca2+ release by 200 μM ryanodine, normal myotubes still displayed a large Ca2+ transient, whereas no transient was detectable in SkEIIIK-expressing dysgenic myotubes. Collectively, these results indicate that the skeletal muscle L-type channel is a major contributor to the Ca2+ entry attributed to ECCE

Uncoupling of ATP-Mediated Calcium Signaling and Dysregulated Interleukin-6 Secretion in Dendritic Cells by Nanomolar Thimerosal

Dendritic cells (DCs), a rare cell type widely distributed in the soma, are potent antigen-presenting cells that initiate primary immune responses. DCs rely on intracellular redox state and calcium (Ca(2+)) signals for proper development and function, but the relationship between these two signaling systems is unclear. Thimerosal (THI) is a mercurial used to preserve vaccines and consumer products, and is used experimentally to induce Ca(2+) release from microsomal stores. We tested adenosine triphosphate (ATP)-mediated Ca(2+) responses of DCs transiently exposed to nanomolar THI. Transcriptional and immunocytochemical analyses show that murine myeloid immature DCs (IDCs) and mature DCs (MDCs) express inositol 1,4,5-trisphosphate receptor (IP(3)R) and ryanodine receptor (RyR) Ca(2+) channels, known targets of THI. IDCs express the RyR1 isoform in a punctate distribution that is densest near plasma membranes and within dendritic processes, whereas IP(3)Rs are more generally distributed. RyR1 positively and negatively regulates purinergic signaling because ryanodine (Ry) blockade a) recruited 80% more ATP responders, b) shortened ATP-mediated Ca(2+) transients > 2-fold, and c) produced a delayed and persistent rise (≥ 2-fold) in baseline Ca(2+). THI (100 nM, 5 min) recruited more ATP responders, shortened the ATP-mediated Ca(2+) transient (≥ 1.4-fold), and produced a delayed rise (≥ 3-fold) in the Ca(2+) baseline, mimicking Ry. THI and Ry, in combination, produced additive effects leading to uncoupling of IP(3)R and RyR1 signals. THI altered ATP-mediated interleukin-6 secretion, initially enhancing the rate of cytokine secretion but suppressing cytokine secretion overall in DCs. DCs are exquisitely sensitive to THI, with one mechanism involving the uncoupling of positive and negative regulation of Ca(2+) signals contributed by RyR1

Symmetries, scaling laws and convergence in shearing-box simulations of MRI driven turbulence

We consider the problem of convergence in homogeneous shearing box simula- tions of magneto-rotationally driven turbulence. When there is no mean magnetic flux, if the equations are non dimensionalized with respect to the diffusive scale, the only free parameter in the problem is the size of the computational domain. The problem of convergence then relates to the asymptotic form of the solutions as the computational box size becomes large. By using a numerical code with a high order of accuracy we show that the solutions become asymptotically inde- pendent of domain size. We also show that cases with weak magnetic flux join smoothly to the zero flux cases as the flux vanishes. These results are consistent with the operation of a subcritical small-scale dynamo driving the turbulence. We conclude that for this type of turbulence the angular momentum transport is a proportional to the diffusive flux and therefore has limited relevance in as- trophysical situations.Comment: 28 pages, 7 figures, submitted to Ap