Accretion disc viscosity: how big is alpha?

We consider observational and theoretical estimates of the accretion disc viscosity parameter α. We find that in thin, fully ionized discs, the best observational evidence suggests a typical range α∼ 0.1–0.4, whereas the relevant numerical simulations tend to derive estimates for α which are an order of magnitude smaller. We discuss possible reasons for this apparent discrepancy.     

Color imaging of Mars by the High Resolution Imaging Science Experiment (HiRISE)

HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE.     

Episodic accretion in magnetically layered protoplanetary discs

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 au such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion on to the protostar occurs in ∼10⁴-yr bursts with ̇ ∼10⁻⁵ M_⊙ yr⁻¹, separated by quiescent intervals lasting ∼10⁵ yr where ̇ ≈10⁻⁸ M_⊙ yr⁻¹. Such bursts could lead to repeated episodes of strong mass outflow in young stellar objects. The transition to this episodic mode of accretion occurs at an early epoch ( t ≪1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum-mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at r ∼1 au. The predicted rate of low-mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1–3 au.     

A numerical interpretation of load tests on vibro-piles

The finite element interpretation is discussed of two load tests carried out on instrumented vibro-piles in a granular deposit. A first back analysis, aimed at assessing the improvement of the mechanical characteristics of soil induced by the vibratory construction process, highlights an apparent contradiction between the experimental variation of the axial load along the pile and the numerical results. This suggests introducing as a free variable, in addition to the elastic and shear strength parameters of the granular soil, also the increase of the nominal diameter of pile caused by vibrations. The second back analysis provides some insight into the variation of the diameter with depth and leads to an acceptable interpretation, from the engineering standpoint, of the load tests. On these bases a quantitative comparison is presented between the calculated load–settlement diagram of the vibro-pile and that of a “standard” pile constructed without vibrations in the same granular deposit.     

Eocene simple shear and plio-quaternary pure-shear folding in the Central-Eastern Algerian Atlas

Tertiary folds of the central and eastern part of the Algerian Atlas were studied in order to assess their kinematics and regimes of deformation. Folds developed following two main phases of deformation during the Eocene and the Pliocene. The NE-SW trending Eocene folds show a clockwise rotation of fold axis with depth, noncylindrical geometry and a right-stepping en echelon configuration. Early secondary structures inside each fold are also rotated clockwise with respect to the younger folds. These data are in agreement with progressive ‘simple shear’ deformation. Pliocene folds strike E-W and display a cylindrical geometry. Associated Plio-Quaternary brittle structures show no rotational path. These observations are compatible with ‘pure shear’ style of deformation. These results, in combination with focal mechanism solutions, have important implications for the understanding of the kinematic evolution of the western segment of the African-European plate boundary. For Algeria they imply that transcurrent motion during the Paleogene was followed by coaxial shortening with a NNW direction during Neogene-Quaternary times. Comparison with published data for Morocco and Tunisia suggests that this transcurrent regime developed from west to east continuing from the Middle Jurassic through to the Miocene.     

On the equations of the dynamics of the attracting point masses

On the equations of the dynamics of the attracting point masses     

Can ε Carinae be a triple system?

We examine the possibility that ε Carinae is a triple system. We show that a stable hierarchical system is possible, although perhaps only marginally.
We outline a speculative scenario for the history of the system, in which the giant outburst of the last century is associated with an interchange of components in a dynamically unstable triple system. We point out that the currently observed spectroscopic 'event' could be used to distinguish among the different possible configurations of the system.     

On the absence of jet features in the optical spectra of cataclysmic variables

Blue- and redshifted hydrogen and helium satellite recombination lines have recently been discovered in the optical spectra of at least two supersoft X-ray sources (SSSs), RX J0513−069 and RX J0019.8+2156, and, tentatively, also in one short-period cataclysmic variable star (CV), the recurrent nova T Pyx. These features are thought to provide evidence for the presence of highly collimated jets in these systems. No similar spectral signatures have been detected in the spectra of other short-period CVs, despite a wealth of existing optical data on these systems. Here, we ask if this apparent absence of 'jet lines' in the spectra of most CVs already implies the absence of jets of the kind that appear to be present in the SSSs and perhaps T Pyx, or whether the current lack of jet detection in CVs can still be ascribed to observational difficulties.
To answer this question, we derive a simple, approximate scaling relation between the expected equivalent widths (EWs) of the observed jet lines in both types of systems and the accretion rate through the disc, EW(line)∝˙M^4/3_acc. We use this relation to predict the strength of jet lines in the spectra of 'ordinary' CVs, i.e. systems characterized by somewhat lower accretion rates than T Pyx. Making the assumption that the features seen in T Pyx are indeed jet lines, and using this system as a reference point, we find that, if jets are present in many CVs, they may be expected to produce optical satellite recombination lines with EWs of a few hundredths to a few tenths of an angstrom in suitably selected systems. A similar prediction is obtained if the SSS RX J0513−069 is used as a reference point. Such EWs are small enough to account for the non-detection of jet features in CVs to date, but large enough to allow them to be detected in data of sufficiently high quality, if they exist.     

Constraints on mechanisms for the growth of gully alcoves in Gasa crater, Mars, from two-dimensional stability assessments of rock slopes

The value of slope stability analyses for gaining insight into the geologic conditions that would facilitate the growth of gully alcoves on Mars is demonstrated in Gasa crater. Two-dimensional limit equilibrium methods are used in conjunction with high-resolution topography derived from stereo High Resolution Imaging Science Experiment (HiRISE) imagery. These analyses reveal three conditions that may produce observed alcove morphologies through slope failure: (1) a ca. >10 m thick surface layer that is either saturated with H₂O ground ice or contains no groundwater/ice at all, above a zone of melting H₂O ice or groundwater and under dynamic loading (i.e., seismicity), (2) a 1-10 m thick surface layer that is saturated with either melting H₂O ice or groundwater and under dynamic loading, or (3) a >100 m thick surface layer that is saturated with either melting H₂O ice or groundwater and under static loading. This finding of three plausible scenarios for slope failure demonstrates how the triggering mechanisms and characteristics of future alcove growth would be affected by prevailing environmental conditions. HiRISE images also reveal normal faults and other fractures tangential to the crowns of some gully alcoves that are interpreted to be the result of slope instability, which may facilitate future slope movement. Stability analyses show that the most failure-prone slopes in this area are found in alcoves that are adjacent to crown fractures. Accordingly, crown fractures appear to be a useful indicator of those alcoves that should be monitored for future landslide activity.     

Experimental analysis of different software packages for orientation and digital surface modelling from UAV images

The use of UAVs finds application in a variety of fields, among which are the small scale surveys for environmental protection application. In this frame some experimental tests were carried out at Politecnico di Milano to assess metric accuracies of images acquired by UAVs and derived photogrammetric products. A block of 5 strips and 49 photos was taken by fixed wing system SenseFly, carrying a camera Canon Ixus 220HS on a rural area included in an Italian Park. Images are processed through bundle adjustment, automatic DEM extraction and orthoimages production steps with several software packages, with the aim to evaluate their characteristics, capabilities and weaknesses. The software packages tested were Erdas-LPS, EyeDEA (University of Parma), Agisoft Photoscan, Pix4UAV, PhotoModeler Scanner. For the georeferencing of the block 16 pre-signalized ground control points were surveyed in the area through GPS (NRTK survey). Comparison of results is given in terms of differences among orientation parameters and their accuracies. Moreover, comparisons among different digital surface models are evaluated. Furthermore, exterior orientation parameters, image points and ground points coordinates, obtained by the various software packages, were used as initial values in a comparative adjustment made by scientific in-house software. Paper confirms that computer vision software are faster in computation and, even if their main goal is not to pursue high accuracy in points coordinates determination, they seems to produce results comparable to those obtainable with standard photogrammetric approach. Agisoft Photoscan seems in this case to yield the best results in terms of quality of photogrammetric products.