Limiting eccentricity of sub-parsec massive black hole binaries surrounded by self-gravitating gas discs

We study the dynamics of supermassive black hole binaries embedded in circumbinary gaseous discs, with the SPH code Gadget-2. The sub-parsec binary (of total mass M and mass ratio q=1/3) has excavated a gap and transfers its angular momentum to the self--gravitating disc (M_disc=0.2 M). We explore the changes of the binary eccentricity e, by simulating a sequence of binary models that differ in the initial eccentricity e_0, only. In initially low-eccentric binaries, the eccentricity increases with time, while in high-eccentric binaries e declines, indicating the existence of a limiting eccentricity e_crit that is found to fall in the interval [0.6,0.8]. We also present an analytical interpretation for this saturation limit. An important consequence of the existence of e_crit is the detectability of a significant residual eccentricity e_LISA} by the proposed gravitational wave detector LISA. It is found that at the moment of entering the LISA frequency domain e_LISA ~ 10^{-3}-10^{-2}; a signature of its earlier coupling with the massive circumbinary disc. We also observe large periodic inflows across the gap, occurring on the binary and disc dynamical time scales rather than on the viscous time. These periodic changes in the accretion rate (with amplitudes up to ~100%, depending on the binary eccentricity) can be considered a fingerprint of eccentric sub-parsec binaries migrating inside a circumbinary disc.Comment: 10 pages, 7 figures, accepted for publication in MNRA

Optical and Infrared Photometry of the Unusual Type Ia Supernova 2000cx

We present optical and infrared photometry of the unusual Type Ia supernova 2000cx. With the data of Li et al. (2001) and Jha (2002), this comprises the largest dataset ever assembled for a Type Ia SN, more than 600 points in UBVRIJHK. We confirm the finding of Li et al. regarding the unusually blue B-V colors as SN 2000cx entered the nebular phase. Its I-band secondary hump was extremely weak given its B-band decline rate. The V minus near infrared colors likewise do not match loci based on other slowly declining Type Ia SNe, though V-K is the least ``abnormal''. In several ways SN 2000cx resembles other slow decliners, given its B-band decline rate (Delta m_15(B) = 0.93), the appearance of Fe III lines and weakness of Si II in its pre-maximum spectrum, the V-K colors and post-maximum V-H colors. If the distance modulus derived from Surface Brightness Fluctuations of the host galaxy is correct, we find that the rate of light increase prior to maximum, the characteristics of the bolometric light curve, and the implied absolute magnitude at maximum are all consistent with a sub-luminous object with Delta m_15(B) ~ 1.6-1.7 having a higher than normal kinetic energy.Comment: 46 pages, 17 figures, to be published in Publications of the Astronomical Society of the Pacifi

The Milky Way's Fermi Bubbles: Echoes of the Last Quasar Outburst?

{\it Fermi}-LAT has recently detected two gamma ray bubbles disposed symmetrically with respect to the Galactic plane. The bubbles have been suggested to be in a quasi-steady state, inflated by ongoing star formation over the age of the Galaxy. Here we propose an alternative picture where the bubbles are the remnants of a large-scale wide-angle outflow from \sgra, the SMBH of our Galaxy. Such an outflow would be a natural consequence of a short but bright accretion event on to \sgra\ if it happened concurrently with the well known star formation event in the inner 0.5 pc of the Milky Way $\sim 6$ Myr ago. We find that the hypothesised near-spherical outflow is focussed into a pair of symmetrical lobes by the greater gas pressure along the Galactic plane. The outflow shocks against the interstellar gas in the Galaxy bulge. Gamma--ray emission could be powered by cosmic rays created by either \sgra\ directly or accelerated in the shocks with the external medium. The Galaxy disc remains unaffected, agreeing with recent observational evidence that supermassive black holes do not correlate with galaxy disc properties. We estimate that an accreted mass \sim 2 \times 10^3\msun is needed for the accretion event to power the observed {\it Fermi}--LAT lobes. Within a factor of a few this agrees with the mass of the young stars born during the star formation event. This estimate suggests that roughly 50% of the gas was turned into stars, while the rest accreted onto \sgra. One interpretation of this is a reduced star formation efficiency inside the \sgra\ accretion disc due to stellar feedback, and the other a peculiar mass deposition geometry that resulted in a significant amount of gas falling directly inside the inner $\sim 0.03$ pc of the Galaxy.Comment: 6 pages, 0 figures; accepted for publication in MNRA

Comportamiento de estudiantes de maestro al medir el volumen

Las magnitudes y su medida, por su uso cotidiano, son un contenido de las matemáticas escolares que los estudiantes para maestro de primaria deben dominar. Estos estudiantes presentan lagunas cuando se les proponen tareas para el desarrollo de la competencia de comparación y medida de la capacidad y del volumen. En el presente trabajo se aportan tareas de este tipo y se identifican las estrategias, los errores y dificultades que cometen los estudiantes, al resolverlas. Además, se describe una actuación realizada en el aula para que los estudiantes detecten y superen las estrategias erróneas

Is there really a debris disc around ζ2 Reticuli\zeta^2\,\mathrm{Reticuli} ?

The presence of a debris disc around the Gyr-old solar-type star $\zeta^2\,\mathrm{Reticuli}$ was suggested by the $\mathit{Spitzer}$ infrared excess detection. Follow-up observations with $\mathit{Herschel}$/PACS revealed a double-lobed feature, that displayed asymmetries both in brightness and position. Therefore, the disc was thought to be edge-on and significantly eccentric. Here we present ALMA/ACA observations in Band 6 and 7 which unambiguously reveal that these lobes show no common proper motion with $\zeta^2\,\mathrm{Reticuli}$. In these observations, no flux has been detected around $\zeta^2\,\mathrm{Reticuli}$ that exceeds the $3\sigma$ levels. We conclude that surface brightness upper limits of a debris disc around $\zeta^2\,\mathrm{Reticuli}$ are $5.7\,\mathrm{\mu Jy/arcsec^2}$ at 1.3 mm, and $26\,\mathrm{\mu Jy/arcsec^2}$ at 870 microns. Our results overall demonstrate the capability of the ALMA/ACA to follow-up $\mathit{Herschel}$ observations of debris discs and clarify the effects of background confusion.Comment: 6 pages, 2 figures, 2 table

A Computational Modeling Approach of Fracture-Induced Acoustic Emission

Acoustic Emission (AE) has become a prominent Nondestructive Testing (NDT) technique with capabilities to be used for Structural Health Monitoring (SHM) applications that entail in-service monitoring, detecting damage-prone areas, and establishing damage prognostics of structures. The next generation of acoustics-based techniques for SHM will rely upon the reliable and quantitative characterization of AE signals related to dominant damage mechanisms. In this context, the forward problem of simulating AE activity is addressed herein by proposing advanced finite element models for damage-induced stress wave generation and propagation. Acoustic emission for this purpose is viewed as part of the dynamic process of energy release caused by damage initiation. To form the computational approach, full field experimental information obtained from monitoring the damage initiation process using digital image correlation is used to construct constitutive laws, e.g. traction-separation law, and to define other damage related parameters. Subsequently, 3D FE simulations based on such experimental data are implemented using cohesive zone modeling and extended finite element method to create an initial failure. Numerically simulated AE signals from the dynamic response due to the onset of damage are evaluated in the context of the inverse problem of source identification and localization. The results successfully demonstrate material and geometry effects of the propagating source and describe completely the AE process from crack-induced isolated source to transient and steady-state dynamic response. Furthermore, the computational model is used to provide quantified measures of the energy release associated with crack. In addition, the effect of plasticity on simulated traveling waves ahead of the crack tip was investigated and revealed nonlinear interactions that had been postulated to exist. Ultimately, the forward AE methodology is applied to an aerospace structural component to recreate the debonding process and associated stress release propagation. All damage-induced wave propagation simulations presented in this dissertation create a pathway for the quantitative comparison between experimental and theoretical predictions of AE.Ph.D., Mechanical Engineering and Mechanics -- Drexel University, 201

Validation of a cyclic plasticity computational method using fatigue full-field deformation measurements

The evolution of crack tip displacement and strain fields during uniaxial, room temperature, low-cycle fatigue experiments of Nickel superalloy compact tension specimens was measured by a digital image correlation approach and was further used to validate a cyclic plasticity model and corresponding deformation calculations made by a finite elements methodology. The experimental results provided data trends for the opening displacements and near crack tip strains as function of cycles. A finite element model was developed to capture test conditions for a measured crack size. The model captures crack tip plasticity by using a constitutive model calibrated against stress-strain measurements performed on a round bar. Similar quantities were extracted from the model predictions to compare with the digital image correlation measurements for model validation purposes. This type of direct comparison demonstrated that the computational model was capable to adequately capture the crack opening displacements at various stages of the specimen's fatigue life, providing in this way a tool for quantitative cyclic plasticity model validation. In addition, this integrated experimental-computational approach provides a framework to accelerate our understanding related to interactions of fatigue test data and models, as well as ways to inform one another

Self-gravitating fragmentation of eccentric accretion disks

We consider the effects of eccentricity on the fragmentation of gravitationally unstable accretion disks, using numerical hydrodynamics. We find that eccentricity does not affect the overall stability of the disk against fragmentation, but significantly alters the manner in which such fragments accrete gas. Variable tidal forces around an eccentric orbit slow the accretion process, and suppress the formation of weakly-bound clumps. The "stellar" mass function resulting from the fragmentation of an eccentric disk is found to have a significantly higher characteristic mass than that from a corresponding circular disk. We discuss our results in terms of the disk(s) of massive stars at ~0.1pc from the Galactic Center, and find that the fragmentation of an eccentric accretion disk, due to gravitational instability, is a viable mechanism for the formation of these systems.Comment: 9 pages, 7 figures. Accepted for publication in Ap