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Cox, D. P.

Publications and source records attributed to Cox, D. P..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

The time development of a blast wave with shock-heated electrons

Accurate approximations are presented for the time development of both edge conditions and internal structures of a blast wave with shock heated electrons, and equal ion and electron temperatures at the shock. The cases considered evolve in cavities with power law ambient densities (including the uniform ambient density case) and have negligible external pressure. Account is taken of possible saturation of the thermal conduction flux. The structures evolve smoothly to the adiabatic structures.

Edgar, R. J.↗
NASA NTRS1983

Self-regulated star formation in the galaxy

Assuming that star formation regions are supported against gravity by winds from low mass young objects, the stellar birthrate obtained for winds interacting in the momentum conservation stage is correlated with the molecular gas density of the parent fragment as n to the 13/8 power or n to the 5/8 power, respectively, for rates/unit volume or rates/unit mass. Birthrates derived from protostellar rotationally driven winds are in good agreement with the observed star production in the cloud B18. With the aid of observed Taurus-Auriga complex properties, the present model is extrapolated to the Galaxy as a whole, yielding a Milky Way predicted average rate that is in good agreement with standard estimates based on observations of the solar neighborhood.

Franco, J.↗
NASA NTRS1983

Self-regulating star formation - The rate limit set by ionizing photons

In the present investigation the possibility is explored that during galaxy formation the gas has the capacity to generate stars so rapidly that the back reaction of the young stars on the gas sets the limiting rate. Sources which could supply significant negative feedback include stellar winds, ionizing UV, and supernova explosions. However, only the simplest of these, the heating by ionizing UV, is currently considered. The UV production associated with a given amount of star formation is calculated, and the critical UV production rate above which the UV heating quenches star formation is estimated. It is argued that in the absence of more powerful limiting mechanisms the star formation will stabilize to provide this critical UV emissivity. It is found that this rate is in fact similar to that which galaxy formation models imply has actually occurred.

Cox, D. P.↗
NASA NTRS1983

The time development of a blast wave with shock heated electrons

Accurate approximations are presented for the time development of both edge conditions and internal structures of a blast wave with shock heated electrons, and equal ion and electron temperatures at the shock. The cases considered evolve in cavities with power law ambient densities (including the uniform ambient density case) and have negligible external pressure. Account is taken of possible saturation of the thermal conduction flux. The structures evolve smoothly to the adiabatic structures.

Edgar, R. J.↗
NASA NTRS1983

Self-regulating galaxy formation. Part 1: HII disk and Lyman alpha pressure

Assuming a simple but physically based prototype for behavior of interstellar material during formation of a disk galaxy, coupled with the lowest order description of infall, a scenario is developed for self-regulated disk galaxy formation. Radiation pressure, particularly that of Lyman depha (from fluorescence conversion Lyman continuum), is an essential component, maintaining an inflated disk and stopping infall when only a small fraction of the overall perturbation has joined the disk. The resulting galaxies consist of a two dimensional family whose typical scales and surface density are expressable in terms of fundamental constants. The model leads naturally to galaxies with a rich circumgalactic environment and flat rotation curves (but is weak in its analysis of the subsequent evolution of halo material).

Cox, D. P.↗
NASA NTRS1983

Heating of the interstellar medium by supernova remnants

Models for the mechanisms active during supernova (SN) heating of surrounding regions are examined. SNs heat both the matrix material and material in the nearby interstellar medium, and also accelerate cosmic rays with shock waves. Fountain and wind models vary according to the SN power and mass acquisition rate. Additionally, a warm intercloud medium may be fed by returning fountain material and stellar mass loss. The thermal wind temperature varies with the number of solar massess heated, although remnants have been observed to vanish in the LMC when more than 100 solar masses were heated. The possibility that the solar system is inside a remnant that produces the observed soft X-ray background is considered.

Cox, D. P.↗
NASA NTRS1982

Extended adiabatic blast waves and a model of the soft X-ray background

The suggestion has been made that much of the soft X-ray background observed in X-ray astronomy might arise from being inside a very large supernova blast wave propagating in the hot, low-density component of the interstellar (ISM) medium. An investigation is conducted to study this possibility. An analytic approximation is presented for the nonsimilar time evolution of the dynamic structure of an adiabatic blast wave generated by a point explosion in a homogeneous ambient medium. A scheme is provided for evaluating the electron-temperature distribution for the evolving structure, and a procedure is presented for following the state of a given fluid element through the evolving dynamical and thermal structures. The results of the investigation show that, if the solar system were located within a blast wave, the Wisconsin soft X-ray rocket payload would measure the B and C band count rates that it does measure, provided conditions correspond to the values calculated in the investigation.

Cox, D. P.↗
NASA NTRS1982

Another self-similar blast wave: Early time asymptote with shock heated electrons and high thermal conductivity

Accurate approximations are presented for the self-similar structures of nonradiating blast waves with adiabatic ions, isothermal electrons, and equation ion and electron temperatures at the shock. The cases considered evolve in cavities with power law ambient densities (including the uniform density case) and have negligible external pressure. The results provide the early time asymptote for systems with shock heating of electrons and strong thermal conduction. In addition, they provide analytical results against which two fluid numerical hydrodynamic codes can be checked.

Cox, D. P.↗
NASA NTRS1981

Some properties of adiabatic blast waves in preexisting cavities

Cox and Anderson (1982) have conducted an investigation regarding an adiabatic blast wave in a region of uniform density and finite external pressure. In connection with an application of the results of the investigation to a study of interstellar blast waves in the very hot, low-density matrix, it was found that it would be desirable to examine situations with a positive radial density gradient in the ambient medium. Information concerning such situations is needed to learn about the behavior of blast waves occurring within preexisting, presumably supernova-induced cavities in the interstellar mass distribution. The present investigation is concerned with the first steps of a study conducted to obtain the required information. A review is conducted of Sedov's (1959) similarity solutions for the dynamical structure of any explosion in a medium with negligible pressure and power law density dependence on radius.

Cox, D. P.↗
NASA NTRS1981

Extended adiabatic blast waves and a model of the soft X-ray background

An analytical approximation is generated which follows the development of an adiabatic spherical blast wave in a homogeneous ambient medium of finite pressure. An analytical approximation is also presented for the electron temperature distribution resulting from coulomb collisional heating. The dynamical, thermal, ionization, and spectral structures are calculated for blast waves of energy E sub 0 = 5 x 10 to the 50th power ergs in a hot low-density interstellar environment. A formula is presented for estimating the luminosity evolution of such explosions. The B and C bands of the soft X-ray background, it is shown, are reproduced by such a model explosion if the ambient density is about .000004 cm, the blast radius is roughly 100 pc, and the solar system is located inside the shocked region. Evolution in a pre-existing cavity with a strong density gradient may, it is suggested, remove both the M band and OVI discrepancies.

Cox, D. P.↗
NASA NTRS1979

Mechanical heating of the interstellar medium. I - The source and rate

The paper presents a model for the evolution of a supernova disturbance in the very low density, high temperature, interstellar matrix in order to explore consequences of such disturbances on the interstellar clouds. It is assumed that higher density material is sufficiently common to impede the velocity field, and that thermal conduction is magnetically quenched between the matrix and H I regions. An estimation is made of the work done by blast waves in compressing interstellar clouds, and it is shown that an individual disturbance is found to lose at least a modest fraction of its energy in this manner.

Cox, D. P.↗
NASA NTRS1978

The structure and spectrum of a colliding-cloud system and its possible relationship to QSOs

A collision between two gas clouds with initial densities of approximately 10 million per cu cm, velocities of about 1000 km/s, and radii of approximately 1 pc is investigated quantitatively by coupling a calculation of the radiation spectrum resulting from the anticipated shock fronts with a computation for the conversion of this high-energy radiation into optical emission in adjacent photoionized regions. The detailed structure of the colliding clouds is discussed, and the effects of an ambient magnetic field are considered. The combined emission-line spectrum is presented along with continuum emission estimates for thermal, synchrotron, and very-high-energy bremsstrahlung mechanisms. It is shown that significant continua can be produced over the range from 300 microns to 3 keV, including a blackbody contribution from a high-density neutral region between the shock fronts, free-free and free-bound radiation from the cooling zones directly behind the shocks, and free-free, free-bound, and two-photon radiation from the photoionized regions immediately ahead of and behind the cooling zones. The theoretical spectrum of the structure resulting from the collision is found to be similar in general and in some details to those observed for typical quasars.

Daltabuit, E.↗
NASA NTRS1977

Galactic infall and cosmic ray acceleration

An estimate is made of the extent to which cosmic rays would be accelerated as a result of their scattering off material entering into the galaxy. A model of the flow is presented, and the assumptions on which the applicability of the model depends are examined. As examples, the maximum acceleration efficiencies are calculated for two ratios of disk gas to total pressure. With these results, the upper limit for power supplied by this process is found to be essentially identical to the cosmic ray power requirement. This circumstance suggests the possibility that the infall might contribute appreciably to the cosmic ray acceleration.

Hedrick, D.↗
NASA NTRS1976

Radiative cooling of a low-density plasma

The paper extends and improves the radiative cooling coefficient calculations of Cox and Tucker (1969) and Cox and Daltabuit (1971) for a low-density optically thin plasma with no molecules or dust, of cosmic abundances in the range from 10,000 to 100,000,000 K. Earlier rates applied to a plasma containing H, He, C, N, O, Ne, Mg, Si, and S; the present study added Ca, Fe, and Ni. For the elements included, many individual lines are calculated which previously has been averaged together. The cooling processes considered are permitted, forbidden, and semiforbidden line transitions, including contributions from dielectronic recombination and bremsstrahlung, radiative recombination, and two-photon continua. The ionization balance is calculated in collisional equilibrium using an approximate auto-ionization following inner-shell excitation and the low-density limit for the dielectronic recombination rate. Cooling in various observational bands in the soft X-ray region is determined.

Raymond, J. C.↗
NASA NTRS1976

Accretion by the Galaxy - Effects of radiative cooling on the flow structure and infall rate

The extreme effects of radiative cooling on the structure of an inflow of material into the Galaxy are explored. Significantly, it is found that cooling cannot prevent the formation of a very hot (about 2 million K) corona in the flow close to the Galaxy. When it is important, however, cooling in the flow considerably increases the infall rate over the adiabatic maximum value for the same set of boundary conditions. Limits set on the flow rate and coronal gas parameters by observations of the Galaxy are discussed.

Cox, D. P.↗
NASA NTRS1974

Large-scale effects of supernova remnants on the Galaxy - Generation and maintenance of a hot network of tunnels

It is found that a supernova rate on the order of 1 per 50 years in the gaseous disk of our Galaxy is sufficient to generate and maintain throughout the interstellar medium a mesh of interconnected tunnels containing very low-density gas. This tunnel system would have a density of approximately 0.01 per cu cm, a temperature of about 1,000,000 K, very low magnetic field strength, tunnel radii of about 10 pc, and would occupy roughly half the interstellar volume. Such a tunnel network may already have been observed in soft X-ray emission, in ultraviolet absorption of O VI against background stars, in the seemingly chaotic distribution of local H I, and in the stringy appearance of velocity-correlated large-scale H I features.

Cox, D. P.↗