Résultats de recherche pour « Nova$^+$ »

The observed properties of novae before and after eruption are discussed. The distribution of orbital periods of novae shows a concentration near 3.2 h, which resembles that of magnetic cataclysmic variables, and there is some evidence that many of the novae themselves are magnetic near that orbital period. Desynchronisation of polars by nova eruptions can lead to an estimate (~2 x 10^3 y) for the time between eruptions for the strongly magnetic systems; this is much shorter than that found from other methods. The similarity of pre- and post-nova luminosities, at high rates of mass transfer, is ascribed to irradiation of the secondary producing a self-sustained high Mdot state. This slows cooling of the white dwarf after eruption, delays the onset of full scale dwarf nova outbursts in most systems, and delays any descent into a hibernation state of low rate of mass transfer.

This paper is a sequel to an earlier paper devoted to multiple, multicycle nova evolution models (Prialnik & Kovetz, Paper I), which showed that the different characteristics of nova outbursts can be reproduced by varying the values of three basic and independent parameters: the white dwarf mass, M WD , the temperature of its isothermal core, T WD , and the mass transfer rate, . Here we show that the parameter space is constrained by several analytical considerations and find its limiting surfaces. Consequently, we extend the grid of multicycle nova evolution models presented in Paper I to its limits, adding multicycle nova outburst calculations for a considerable number of new parameter combinations. In particular, the extended parameter space that produces nova eruptions includes low mass transfer rates down to 5 × 10 -13 M ☉ yr -1 and more models for low T WD . Resulting characteristics of these runs are added to the former parameter combination results to provide a full grid spanning the entire parameter space for carbon-oxygen white dwarfs. The full grid covers the entire range of observed nova characteristics, even those of peculiar objects, which have not been numerically reproduced until now. Most remarkably, runs for very low lead to very high values for some characteristics, such as outburst amplitude A ≳ 20, high super-Eddington luminosities at maximum, heavy element abundance of the ejecta Z ej ≈ 0.63, and high ejected masses m ej ≈ 7 × 10 -4 M ☉ .

NOvA is a next generation neutrino oscillation experiment designed to search for muon neutrino to electron neutrino oscillations by comparing electron neutrino event rates in a Near Detector at Fermilab with the rates observed in a large Far Detector at Ash River, Minnesota, 810 km from Fermilab. The detectors are totally active, segmented, liquid scintillator detectors and are located 14 mrad off the NuMI beam axis. The Far Detector has begun construction and will begin taking data in early 2013. The experiment aims to measure the neutrino mixing angle theta_13 and will push the search for electron neutrino appearance beyond the current limits by more than an order of magnitude. For non-zero theta_13, it is possible for NOvA to observe CP violation in neutrinos and establish the neutrino mass hierarchy. The NOvA prototype near detector on the surface (NDOS) began running at Fermilab in November and registered its first neutrinos from the NuMI beam in December 2010. An overview and current status of the experiment will be presented.

Aims. Nova Scorpii 2008 was the target of our Director Discretionary Time proposal at VLT+UVES in order to study the evolution, origin and abundances of the heavy-element absorption system recently discovered in 80% of classical novae in outburst.Methods. The early decline of nova Scorpii 2008 was monitored with high resolution echelle spectroscopy at 5 different epochs. The analysis of the absorption and the emission lines show many unusual characteristics.Results. Nova Scorpii 2008 is confirmed to differ from a common classical nova as well as a symbiotic recurrent nova, and it shows characteristics which are common to the so called, yet debated, red-novae. The origin of this new nova remains uncertain.

In this article we review the current status of the stellar population assignment for novae. Observations in the Milky Way and in external galaxies point out the existence of two nova populations: fast and bright novae, mainly originated from massive white dwarfs and associated with the `thin disk/spiral arm' stellar population, and slow and faint novae, originated from lighter white dwarfs and associated with `thick-disk/bulge' population.

Outburst mechanisms of dwarf novae are discussed. There is a rich variety in outburst behaviors of non-magnetic cataclysmic variable stars, starting from non-outbursting nova-like stars to various sub-classes of dwarf novae (i.e., U Gem-type, Z Cam-type, and SU UMa-type). A unification model for dwarf nova outbursts is proposed within the basic framework of the disk instability model in which two different intrinsic instabilities (i.e., the thermal instability and the tidal instability) within accretion disks play an essential role. Non-magnetic cataclysmic variables are classified into four regions in the orbital-period versus mass-transfer-rate diagram, showing different combinations of stability behavior for the two intrinsic instabilities in accretion disks, and their different outburst behaviors are basically understood in this diagram. We discuss several problems concerning the thermal limit-cycle instability model for dwarf novae above the period gap. We then discuss the thermal-tidal instability model for SU UMa stars, dwarf novae below the period gap, in which the coupling of the two intrinsic instabilities in the accretion disk plays a unique role. In particular, a rich variety of outburst behaviors of cataclysmic variables below the period gap (i.e., starting from "permanent superhumpers", to Z Cam-like SU UMa stars, "ER UMa stars", to ordinary SU UMa stars, and finally to WZ Sge stars) is understood by the thermal-tidal instability model.

Despite its fundamental importance, a reliable estimate of the Galactic nova rate has remained elusive. Here, the overall Galactic nova rate is estimated by extrapolating the observed rate for novae reaching $m\leq2$ to include the entire Galaxy using a two component disk plus bulge model for the distribution of stars in the Milky Way. The present analysis improves on previous work by considering important corrections for incompleteness in the observed rate of bright novae and by employing a Monte Carlo analysis to better estimate the uncertainty in the derived nova rates. Several models are considered to account for differences in the assumed properties of bulge and disk nova populations and in the absolute magnitude distribution. The simplest models, which assume uniform properties between bulge and disk novae, predict Galactic nova rates of $\sim$50 to in excess of 100 per year, depending on the assumed incompleteness at bright magnitudes. Models where the disk novae are assumed to be more luminous than bulge novae are explored, and predict nova rates up to 30% lower, in the range of $\sim$35 to $\sim$75 per year. An average of the most plausible models yields a rate of $50_{-23}^{+31}$ yr$^{-1}$, which is arguably the best estimate currently available for the nova rate in the Galaxy. Virtually all models produce rates that represent significant increases over recent estimates, and bring the Galactic nova rate into better agreement with that expected based on comparison with the latest results from extragalactic surveys.

The availability of virtualization features in modern CPUs has reinforced the trend of consolidating multiple guest operating systems on top of a hypervisor in order to improve platform-resource utilization and reduce the total cost of ownership. However, today's virtualization stacks are unduly large and therefore prone to attacks. If an adversary manages to compromise the hypervisor, subverting the security of all hosted operating systems is easy. We show how a thin and simple virtualization layer reduces the attack surface significantly and thereby increases the overall security of the system. We have designed and implemented a virtualization architecture that can host multiple unmodified guest operating systems. Its trusted computing base is at least an order of magnitude smaller than that of existing systems. Furthermore, on recent hardware, our implementation outperforms contemporary full virtualization environments.

Several of the inconsistencies plaguing the field of novae are resolved once we consider novae to be steady state super-Eddington objects. In particular, we show that the super-Eddington shell burning state is a natural consequence of the equations of stellar structure, and that the predicted mass loss in the super-Eddington state agrees with nova observations. We also find that the transition phase of novae can be naturally explained as "stagnating" winds.

We explore the scenario where the core of a neutron star (having experienced a transition to an up and down quark phase) shrinks into the equilibrated quark object after reaching strange quark matter saturation density (where a composition of up, down and strange quarks is the favored state of matter). The overlaying (envelope) material free-falls following the core contraction releasing upto in energy as radiation, partly as a result of the conversion of envelope material to quarks. This phenomena, we named Quark-Nova, leads to a wide variety of ejectae ranging form the Newtonian, “dirty" to the ultra-relativistic fireball. The mass range of the corresponding compact remnant (the quark star) ranges from less than up to a solar mass. We discuss the connection between Quark-Novae and Gamma ray bursts and suggest the recently studied GRB011211 event as a plausible Quark-Nova candidate.