Denis F. Cioffi

Dynamics of radiative supernova remnants

A high-resolution numerical simulation is used to study the evolution of a SNR evolving in a homogeneous uniform medium. Emphasis is placed on the transition from the adiabatic stage to the radiative pressure-driven snowplow stage, along with the possible further establishment of a momentum-conserving snowplow state. In most cases the momentum-conserving snowplow is found to be delayed beyond the merger of the remnant with the interstellar medium. 39 references.

Overpressured cocoons in extragalactic radio sources

It is shown that the cocoons of shocked gas which surround powerful double radio sources can have significantly higher pressures than the surrounding intergalactic medium. The pressures can be high enough to confine the jets in these sources, obviating the need for magnetic confinement. The cocoon pressure and the age of a radio source may be estimated from observable quantities, as demonstrated here for the radio galaxy Cygnus A. It is suggested that overpressured cocoons in high-redshift radio galaxies engulf and compress circumgalactic clouds, driving them over the Jeans limit and triggering star formation. It is proposed that this process leads to the observed alignments of optical continuum emission with radio source axes. 28 refs.

A MILLION-SECOND CHANDRA VIEW OF CASSIOPEIA A

We introduce a million second observation of the supernova remnant Cassiopeia A with the Chandra X-Ray Observatory. The bipolar structure of the Si-rich ejecta (northeast jet and southwest counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

The evolution of cocoons surrounding light, extragalactic jets

If the mass density of supersonic, collimated material is less than that of the surrounding medium, a so-called light jet will be enveloped by a cocoon of overpressured shocked gas. Hydrodynamical simulations are used to understand the evolution of the cocoon. The cocoons evolution is also compared to a simple analytic theory. To reconcile the theory with the simulations, the growth of the jet head must be taken into account. The overpressured cocoon stage exists for a relatively short astronomical time, after which only the region of the cocoon near the jet head remains overpressured. The spatial distribution of the optical emission often observed in distant extragalactic jet systems can be explained with this improved understanding of cocoon evolution.

A practical method of determining project risk contingency budgets

Given a collection of accepted risks with corresponding impacts and probabilities over the life of a project (or a relevant portion), a method to estimate the total potential impact at a given certainty is presented. Project sponsors and managers can decide their risk tolerance and set aside corresponding contingency funds. This analytic method uses a binomial distribution with a probability equal to the risks’ average probability. At a desired statistical confidence level, the number of risks to consider is determined, and risks are ranked in descending order of either impact or expected value of impact. The ranked risks’ impacts are then summed over the specified number of risks, leading to an appropriate contingency budget. The budget found with this method compares extremely well with the one ascertained from numerical simulations of the risk occurrences.

Completing projects according to plans: an earned-value improvement index

To finish in accordance with its originally planned duration or cost, a project that is not meeting its schedule or budget requires improved performance in the work that remains. This paper shows how to calculate the new productivity in terms of the average earned-value performance over the first part of a project, and it shows how the new productivity rate depends on the fraction of tasks completed. The completed task fraction beyond which recovery is impossible is calculated, and a point beyond which recovery is highly unlikely is suggested. Using an analytic approximation for an S-curve, these calculations are transformed into a temporal domain.

Program Risk Contingency Budget Planning

A pragmatic procedure is suggested to determine the size of a projectpsilas, a programpsilas, or a systems risk contingency budget at any specified level of certainty, e.g., 99% confidence. The size of the budget is a function of the number of risks expected at the specified confidence level. The number of risks used for developing the contingency budget depends on the total number of risks to be considered and the estimated probabilities of occurrence of the risks. The exact number, derived from the binomial distribution, is given in a table or, when the number of risks to be considered is greater than about 20, by a simple formula. The findings suggest that a specific contingency budget amount can be set aside for the small fraction of risks that typically materialize. The specific numbers would depend on the risk tolerance of the organization.

The growth of density perturbations in radiative shocks

The paper presents a new investigation of the growth of thermal instabilities behind radiative shocks. The analytic and numerical results disagree with McCray, Stein, and Kafatos, who were attempting to explain the filamentary structure commonly seen in supernova remnants, and proposed that the growth of density perturbations would lead to gross condensations in the postshock cooling region. Hydrodynamical simulations are generated which corroborate arguments in favor of no growth in the long-wavelength limit. These simulations also agree with a renewed analytic approach in the short-wavelength limit, and show that the region of rapid growth will remain inconspicuous in the overall density rise toward the back of the shock. It is noted that these calculations are in accord with some recent observations of supernova remnants where the observed filaments do not seem to have been produced by any local thermal instability process. 32 refs.

Supernova Remnants as Probes of the Interstellar Medium

Astronomers can use supernova remnants to learn about the environment in which they exist. Simple thermodynamics is first used to describe the evolution of supernova remnants from early to late times. A dynamical formalism is also presented. I then describe a method for calculating the cooling, and hence the luminosity, of the hot shocked gas. Possible complications to this simple picture are considered. The utilization of these theoretical models employing supernova remnants in nearby galaxies is discussed.

X-ray emission from reverse-shocked ejecta in supernova remnants

A simple physical model of the dynamics of a young supernova remnant is used to derive a straightforward kinematical description of the reverse shock. With suitable approximations, formulae can then be developed to give the X-ray emission of the reverse-shocked ejecta. The results are found to agree favorably with observations of SN1006.