3,610 research outputs found
Periodicity makes galactic shocks unstable - I. Linear analysis
No full textWe study the dynamical stability of stationary galactic spiral shocks. The steady-state equilibrium flow contains a shock of the type derived by Roberts in the tightly wound approximation. We find that boundary conditions are critical in determining whether the solutions are stable or not. Shocks are unstable if periodic boundary conditions are imposed. For intermediate strengths of the spiral potential, the instability disappears if boundary conditions are imposed such that the upstream flow is left unperturbed as in the classic analysis of D'yakov and Kontorovich. This reconciles apparently contradictory findings of previous authors regarding the stability of spiral shocks. This also shows that the instability is distinct from the Kelvin-Helmholtz instability, confirming the findings of Kim et al. We suggest that instability is a general characteristics of periodic shocks, regardless of the presence of shear, and provide a physical picture as to why this is the case. For strong spiral potentials, high post-shock shear makes the system unstable also to parasitic Kelvin-Helmholtz instability regardless of the boundary conditions. Our analysis is performed in the context of a simplified problem that, while preserving all the important characteristics of the original problem, strips it from unnecessary complications, and assumes that the gas is isothermal, non-self-gravitating, non-magnetized
3D simulations of RS Ophiuchi: from accretion to nova blast
No full textContext. The binary star system RS Ophiuchi is a recurrent nova,
with outbursts occurring about every 22 years. It consists of a red
giant star (RG) and a wind accreting white dwarf close to the
Chandrasekhar limit. This system is considered a prime candidate for
evolving into an SNIa. For its most recent outbursts in 1985 and
2006, exquisite multiwavelength observational data are available.
Aims. Deeper physical insight is needed regarding the inter-outburst
accretion phase and the dynamical effects of the subsequent nova
explosion in order to improve the interpretation of the observed
data and to shed light on whether the system is an SNIa progenitor.
Methods. We present a 3D hydrodynamic simulation of the quiescent accretion
with the subsequent explosive phase.
Results. The computed circumstellar mass distribution in the quiescent phase
is highly structured with a mass enhancement in the orbital plane of
about a factor of 2 as compared to the poleward directions. The
simulated nova remnant evolves aspherically, propagating faster
toward the poles. The shock velocities derived from the simulations
agree with those derived from observations. For km s-1 and for nearly isothermal flows, we find that 10% of the
mass lost by the RG is transfered to the WD. For an RG mass loss of
yr-1, the orbit of the system decays
by 3% per million years. With the derived mass transfer rate,
multi-cycle nova models provide a qualitatively correct recurrence
time, amplitude, and fastness of the nova.
Conclusions. Our 3D hydrodynamic simulations provide, along with the observations
and nova models, the third ingredient for a deeper understanding of
the recurrent novae of the RS Oph type. In combination with recent
multi-cycle nova models, our results suggest that the WD in RS Oph
will increase in mass. Several speculative outcomes then seem
plausible. The WD may reach the Chandrasekhar limit and explode as
an SN Ia. Alternatively, the mass loss of the RG could result in a
smaller Roch volume, a common envelope phase, and a narrow WD + WD system. Angular momentum loss due to gravitational wave emission could trigger the merger of the two WDs and – perhaps – an SN Ia via the double degenerate scenario
New observations of local seismicity by the SN-1 seafloor observatory in the Ionian Sea, off-shore Eastern Sicily (Italy)
No full textSeismicity in Eastern Sicily as recorded by the Submarine Network-1 seafloor observatory (SN-
1) in the period from 2002 October to 2003 May is examined with the aim of identifying the as
yet poorly known seismogenic zones placed in the Ionian basin, where some of the strongest
earthquakes have occurred. A comparison between the seismic recordings of land networks
and the seafloor station has allowed us to focus on low-magnitude seismicity only recorded by
SN-1.We have analysed 239 high-quality events from among a total of 485 seismic signals not
included in the land-based network bulletins. The waveform features and the possible source
zones for those events are investigated by means of polarization and particle motion techniques.
Most of the 239 events (213) are characterized by high values of rectilinearity typical of P- and
S-arrival particle motions, while the remaining 26 events have different polarization features,
with an emergent first phase and prevalently planar polarization.We have interpreted the latter
signals as being associated to submarine landslides. From particle motion analysis, we have
determined the azimuthal distribution of the events and the incidence angles of P waves in
respect to the Observatory with the aim of determining their distribution in relation to the active
but scarcely known structural setting of the off-shore area. Moreover, the integrated locations
of some earthquakes occurring in the study area and recorded by SN-1 and land stations was
performed to determine the apparent P-wave velocity necessary to calculate source-station
distances. As an additional result of the integration, we have obtained more accurate locations
of earthquakes occurring in the coastal and off-shore areas of Eastern Sicily, associated with
reduced horizontal and vertical errors and significantly lower values of azimuthal gaps. Lastly,
a location distribution of the 213 analysed events was obtained by setting two conditions:
(1) a maximum epicentral distance to a fixed depth coinciding with the depth of the seafloor
station and (2) a minimum epicentral distance associated to the maximum depth of events.
Accordingly, two patterns of seismicity were determined for the maximum and the minimum
expected spread of the hypocentres. The main features of both patterns are a diffuse seismicity
in the Western Ionian basin with a major epicentre density SE of SN-1 and a depth of most
of the events within 60 km. Local magnitude determination was also performed, taking into
account an attenuation law proposed for Southeastern Sicily. Despite the uncertainties in the
location distribution using single-station recordings, the results show diffuse seismicity all
around SN-1 and, in particular, in the off-shore area.Published490-5011.4. TTC - Sorveglianza sismologica delle aree vulcaniche attiveJCR Journalreserve
Ordering of organic molecules on templated surfaces
No full textThis thesis describes the controlled growth of molecular nanostructures using modified metallic and semiconductor surfaces. The Ag/Si(lll)-(root3 x root3),the Sn/Cu(100) surface alloy system and the Bi/Si(100) nanolines and (2xn) surfaces were all investigated as suitable substrates for the controlled growth of pentacene, (C22H14) or trimesic acid, (C6H3(COOH)3) organic molecules. The following techniques were used in this study; Scanning Tunnelling Microscopy (STM), Low Energy Electron Diffraction (LEED), Normal Incident X-Ray Standing Waves (NIXSW) and Temperature Programmed Desorption (TPD).
The room temperature growth and ordering of trimesic acid on the AgfSi(ll1)-(root3 x root3) surface was investigated. An oblique unit cell was determined and a model proposed for the highly ordered close-packed domains.
The discovery of a new submonolayer phase on Sn/Cu(100) and the re-examined known phase are discussed. New models for these reconstructions are proposed. Adsorption of trimesic acid at room temperature on the clean substrate the lowest Sn coverage phase were studied. Two new Sn coverage dependent structures were discovered and bonding schemes in upright and flat orientations are discussed.
BifSi(100)-(2xn) surface was exploited as a template for the ordered growth of pentacene, which exhibited orientation specific adsorption. The Bi/Si(100)-(2xn) single domain surface created on vicinal silicon was used to test the suitable of Daresbury 4.2 beamline for NIXSW Imaging experiments and the quality of the results are discussed
A trio of gamma-ray burst supernovae : GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu
No full textWe present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu. For GRB 130215A/SN 2013ez, we also present optical spectroscopy at t − t0 = 16.1 d, which covers rest-frame 3000–6250 Å. Based on Fe ii λ5169 and Si ii λ6355, our spectrum indicates an unusually low expansion velocity of ~4000–6350 km s-1, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A/SN 2013fu, we used our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we took advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Mészáros (2001, ApJ, 552, L35), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P = 12 ms and a magnetic field of B = 1.1 × 1015 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs
Silica supported Sn catalysts with tetrahedral Sn sites for selective isomerization of glucose to fructose
No full textLewis acid catalyzed isomerization of glucose to fructose is an important reaction for production of renewable chemicals. Here, we show the synthesis of an active and selective Lewis acid catalyst for this reaction by controlling Sn dispersion on SBA15. Sn loading of 1 wt. % over SBA15 (Sn/SBA15) maximized the formation of tetrahedral Sn species on the catalyst surface. Increasing the loading or changing support caused formation of SnO2 clusters which reduced fructose selectivity. A mechanism based on condensation of Sn with silanol group of SBA15 is proposed. The catalyst showed high selectivity of 93 % after 2 h with 57 % fructose yield. The Lewis acid catalyzed isomerization of glucose was proven by isotopic tracer study using D-glucose-2-d. The catalyst deactivated in the third cycle owing to byproduct deposition, but the activity was restored by recalcining the catalyst
Sn-Based Electrocatalyst Stability: A Crucial Piece to the Puzzle for the Electrochemical CO<sub>2</sub>Reduction toward Formic Acid
No full textNowadays, Sn-based electrocatalysts for the electrochemical CO2 reduction reaction (eCO2RR) toward formic acid have been reported to reach industrially relevant current densities and Faradaic efficiencies approaching 100%. However, electrocatalyst stability remains inadequate and appears to be a crucial piece to the puzzle, as lifetimes in the range of several thousands of hours should be reached for practical application and economic viability. Here, we provide insights into stability issues related to Sn-based electrocatalysts and electrolyzers for formic acid production. By determining the chemical and physical phenomena that occur during the electrochemical reduction reaction on the surface and bulk of Sn-based catalysts, we intend to elucidate the most common degradation mechanisms that impair long-term electrocatalytic activity of these catalysts. Moreover, highlighting the importance of correctly selected process conditions and an optimized reactor design allows us to unveil all necessary aspects for a stable Sn-based eCO2RR toward formic acid.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Large Scale Energy Storag
Influences of Nanostructures of Sn and Ir for the Oxygen Evolution Reaction in Polymer Electrolyte Membrane Water Electrolysis
No full textThe influence of nanostructures and interaction of Sn and Ir in oxygen evolution catalysts in a polymer electrolyte membrane electrolyzer were investigated. For this aim, two synthesis methods, namely, the one-step solution combustion method and the precipitation-deposition method with sodium borohydride reduction, were evaluated to prepare distinct nanostructures. Sn addition to Ir-based oxygen evolution reaction catalysts has been reported to yield materials with higher activity; however, in our case, this was observed only for Sn/Ir catalysts prepared by the precipitation-deposition method. The nanolayer of Sn/SnO2 deposited over metallic Ir particles was identified to enhance the interfacial contacts, resulting in synergistic interactions. By deconvolution of the polarization curves into constituting contributions, the performance improvement was attributed to the higher exchange current density of the Sn/Ir powder as a consequence of a higher number of surface reaction sites created by the Sn-Ir interactions.ChemE/Catalysis Engineerin
Surfactant-assisted synthesis of Sn nanoparticles via solution plasma technique
No full textWe have adopted a solution plasma synthesis for preparing Sn nanoparticles (Sn-NPs) directly from metallic Sn electrode. The Sn-NPs were synthesized in the presence of the surfactant, cetyltrimethylammonium bromide (CTAB), and the effect of the concentration of CTAB on the Sn-NPs was investigated. Without CTAB addition, SnO plates were precipitated. Sn-NPs with less than 200 nm were synthesized at a high concentration of 200 x 10(-6) g ml(-1) of CTAB. Electrochemical properties of SnO plates and Sn-NPs were analyzed for use as an anode material in Li-ion batteries. A composite of Sn-NPs and graphite enhanced the cyclic stability owing to the buffer space provided by the graphite for volume expansion. In the case of the 30 wt% loaded Sn-NPs, the capacity was measured to be 414 mA h g(-1) after 20 cycles
Investigation on Viscoplastic Properties of Au-Sn Die-attach Solder
No full textReliability is one of the major research topics of modern electronic packaging technology. The characterization of Au-Sn dia-attach solder material properties, including viscoplastic properties is requested for fast and accurate reliability assessment by industry. In this thesis, a detailed experimental based methodology to characterize Au-Sn solder's viscoplastic properties, including raw material introduction, test specimen preparation, shear creep experiments and test data analyze is provided. Furthermore, a study of microstructure effect on Au-Sn solder viscoplastic performance by experiments and following reliability analysis is presented.Electrical Engineerin
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