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DOE OSTI2024

From Feast to Famine: A Systematic Study of Accretion onto Oblique Pulsars with 3D GRMHD Simulations

Disk-fed accretion onto neutron stars can power a wide range of astrophysical sources ranging from X-ray binaries, to accretion-powered millisecond pulsars, ultraluminous X-ray sources, and gamma-ray bursts. A crucial parameter controlling the gas–magnetosphere interaction is the strength of the stellar dipole. In addition, coherent X-ray pulsations in many neutron star systems indicate that the star's dipole moment is oblique relative to its rotation axis. Therefore, it is critical to systematically explore the 2D parameter space of the star's magnetic field strength and obliquity, which is what this work does, for the first time, in the framework of 3D general-relativistic magnetohydrodynamics. If the accretion disk carries its own vertical magnetic field, this introduces an additional factor: the relative polarity of the disk and stellar magnetic fields. We find that depending on the strength of the stellar dipole and the star–disk relative polarity, the neutron star's jet power can either increase or decrease with increasing obliquity. For weak dipole strength (equivalently, high accretion rate), the parallel polarity results in a positive correlation between jet power and obliquity, whereas the antiparallel orientation displays the opposite trend. For stronger dipoles, the relative-polarity effect disappears, and jet power always decreases with increasing obliquity. The influence of the relative polarity gradually disappears as obliquity increases. Highly oblique pulsars tend to have an increased magnetospheric radius, a lower mass accretion rate, and enter the propeller regime at lower magnetic moments than aligned stars.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1974

Planetary accretions in jet streams

The problem of planetary accretion in a jet stream is studied using the model developed by Alfven and Arrhenius. We find that there are basically three types of planetary accretion, corresponding to cases where the characteristic time of the occurrence of catastrophic accretion is less than, equal to, or greater than the time-scale of mass injection to the planetary system (300 m.y.). These different time scales of accretion are found to be closely related to the primordial thermal profiles and equatorial inclinations of the planets. Finally, Venus' retrograde rotational spin is shown to be a possible result of accretion process in a jet stream.

Ip, W.-H.↗

NASA NTRS1976

Accretion of the terrestrial planets. II

The theory of gravitational accretion of the terrestrial planets is examined. The concept of a 'closed feeding zone' is somewhat unrealistic, but provides a lower bound on the accretion time. A velocity relation for planetesimals which includes an initial velocity component is suggested. The orbital parameters of the planetesimals and the dimensions of the feeding zone are related to their relative velocities. The assumption of an initial velocity does not seriously change the accretion time. Mercury, Venus, and the earth have accretion times on the order of 100 million years. Mars requires well over one billion years to accrete by the same assumptions. The lunar cratering history makes a late formation of Mars unlikely. If Mars is as old as the earth, nongravitational forces or a violation of the feeding zone concept is required. One such possibility is the removal of matter from the zone of Mars by Jupiter's influence. The final sweeping up by Mars would result in the scattering of a considerable mass among the other terrestrial planets. The late postaccretional bombardments inferred for the moon and Mercury may have had this source.

Weidenschilling, S. J.↗

NASA NTRS1977

Accretion of rotating fluids by barytropes - Numerical results for white-dwarf models

Numerical sequences of rotating axisymmetric nonmagnetic equilibrium models are constructed which represent the evolution of a barytropic star as it accretes material from a rotating medium. Two accretion geometries are considered - one approximating accretion from a rotating cloud and the other, accretion from a Keplerian disk. It is assumed that some process, such as Ekman spin-up or nonequilibrium oscillations, maintains nearly constant angular velocity along cylinders about the rotation axis. Transport of angular momentum in the cylindrically radial direction by viscosity is included. Fluid instabilities and other physical processes leading to enhancement of this transport are discussed. Particular application is made to zero-temperature white-dwarf models, using the degenerate electron equation of state. An initially nonrotating 0.566-solar-mass white dwarf is followed during the accretion of more than one solar mass of material. Applications to degenerate stellar cores, to mass-transfer binary systems containing white dwarfs, such as novae and dwarf novae, to Type I supernovae, and to galactic X-ray sources are considered.

Durisen, R. H.↗

NASA NTRS1977

Numerical calculations of the cross-section for the accretional capture of the moon by the earth

If a primary body is increasing in mass the hyperbolic orbit of a secondary body can become an elliptic orbit. In order to determine the cross-section for the accretional capture of the moon by the earth, a series of numerical calculations has been carried out. Calculations have been carried out for various orbital ellipticities and separations and for various accretion rates. Accretional capture is favored if significant accretion occurs in a period of less than 100 yr. A window for accretional capture occurs if the minimum initial separation of the earth and moon (in astronomical units) is nearly equal to the initial heliocentric lunar eccentricity.

Nordmann, J. C.↗

NASA NTRS1978

Conclusions derived from the evidence on accretion in meteorites

A study of the evidence on accretion in meteorites shows that gross chemical features of chondritic meteorite type were established by accretion of materials with characteristic chemical and isotopic compositions. Some meteorites have been subjected to intensive mechanical and/or thermal processes. Mixing has occurred among meteorite types, presumably late in the accretion sequence. From the materials which appear to have escaped substantial modification it can be concluded that the relative velocities between accreting grains varied but in some cases were very low. Accretion took place after most, if not all, of the gaseous components had been separated from the condensed portion of solar matter.

Herndon, J. M.↗

NASA NTRS1983

The Eddington limit and supercritical accretion. II - Time-dependent calculations

Spherically symmetric, time-dependent accretion of an ionized hydrogen plasma onto a neutron star is calculated for accretion rates in excess of the Eddington limit. The coupled hydrodynamic and frequency integrated radiative transfer equations are solved on an Eulerian grid for these supercritical accretion flows. Our results indicate that steady state flows are limited to rates at or below the critical rate, with emergent luminosities equal to or less than the Eddington luminosity. Initially supercritical accretion rates generate a large pulse of radiation which reduces the accretion rate to the critical value and produces an extended quasi-static envelope.

Burger, H. L.↗

NASA NTRS1985

An impact-induced terrestrial atmosphere and iron-water reactions during accretion of the Earth

Shock wave data and theoretical calculations were used to derive models of an impact-generated terrestrial atmosphere during accretion of the Earth. The models showed that impacts of infalling planetesimals not only provided the entire budget of terrestrial water but also led to a continuous depletion of near-surface layers of water-bearing minerals of their structural water. This resulted in a final atmospheric water reservoir comparable to the present day total water budget of the Earth. The interaction of metallic iron with free water at the surface of the accreting Earth is considered. We carried out model calcualtions simulating these processes during accretion. It is assumed that these processes are the prime source of the terrestrial FeO component of silicates and oxides. It is demonstrated that the iron-water reaction would result in the absence of atmospheric/hydrospheric water, if homogeneous accretion is assumed. In order to obtain the necessary amount of terrestrial water, slightly heterogeneous accretion with initially 36 wt% iron planetesimals, as compared with a homogeneous value of 34 wt% is required.

Lange, M. A.↗

NASA NTRS1986

In-flight photogrammetric measurement of wing ice accretions

A photographic instrumentation system was developed for the Lewis icing research aircraft to measure wind ice accretions during flight. The system generates stereo photographs of the accretions which are then photogrammetrically measured by the Air Force Arnold Engineering and Development Center. The measurements yield a survey of spatial coordinates of an accretion's surface to an accuracy of at least + or - 0.08 cm. The accretions can then be matched to corresponding icing cloud and aerodynamic measurements. The system is being used to measure rime, mixed, and clear natural ice accretions.

Mcknight, R. C.↗

NASA NTRS1986

In-flight photogrammetric measurement of wing ice accretions

Mcknight, R. C.↗

NASA NTRS1987

Energetic particle acceleration in spherically symmetric accretion flows and shocks

Steady state, spherically symmetric solutions of the cosmic-ray transport equation describing the acceleration of energetic particles in galactic accretion flows onto neutron stars, black holes, white dwarfs, and protostars are studied. The results indicate that astrophysical accretion flows can be partitioned into distinct classes depending upon whether the accretion rate lies above or below a critical value of a few times 10 to the -7th stellar masses/yr. When the particle transport is convection-dominated, both classes of accretion flows exhibit a spectral index appropriate for first-order Fermi acceleration at a plane shock in the absence of losses. As the particle transport becomes diffusion-dominated, both classes show a break and precipitous falloff in the particle spectrum due to the escape of these particles from the accretion flow. The precise nature of the spectrum depends on the relationship between the particle momentum and the spatial diffusion coefficient.

Webb, G. M.↗

NASA NTRS1988

Investigation of surface water behavior during glaze ice accretion

Microvideo observations of glaze ice accretions on 1-in-diameter cylinders in a closed-loop refrigerated wind tunnel were obtained to study factors controlling the behavior of unfrozen surface water during glaze ice accretion. Three zones of surface water behavior were noted, each with a characteristic roughness. The effect of substrate thermal and roughness properties on ice accretions was also studied. The contact angle and hysteresis were found to increase sharply at temperatures just below 0 C, explaining the high resistance to motion of water beads observed on accreting glaze ice surfaces. Based on the results, a simple multizone modification to the current glaze ice accretion model is proposed.

Hansman, R. John, Jr.↗

NASA NTRS1988

Accretion flows onto supermassive black holes

The radiative and hydrodynamic properties of an angular momentum-dominated accretion flow onto a supermassive black hole depend largely on the ratio of the accretion rate to the Eddington accretion rate. High values of this ratio favor optically thick flows which produce largely thermal radiation, while optically thin 'two-temperature' flows may be present in systems with small values of this ratio. Observations of some AGN suggest that thermal and nonthermal sources of radiation may be of comparable importance in the 'central engine'. Consideration is given to the possibilities for coexistence of different modes of accretion in a single flow. One intriguing possibility is that runaway pair production may cause an optically thick 'accretion annulus' to form at the center of a two-temperature inflow.

Begelman, Mitchell C.↗

NASA NTRS1989

Pre-main-sequence disk accretion in Z Canis Majoris

It is suggested that the pre-main-sequence object Z CMa is a luminous accretion disk, similar in many respects to the FU Orionis variables. Z CMa shows the broad, doubled optical absorption lines expected from a rapidly rotating accretion disk. The first overtone CO absorption detected in Z CMa is blue-shifted, suggesting line formation in a disk wind. Accretion at rates about 0.001 solar mass/yr over 100 yr is required to explain the luminosity of Z CMa. The large amount of material accreted (0.1 solar mass/yr) indicates that Z CMa is in a very early stage of stellar evolution, possibly in an initial phase of massive disk accretion.

Hartmann, L.↗

NASA NTRS1989

Numerical studies of asymmetric adiabatic accretion flow - The effect of velocity gradients

A numerical study of the time variation of the angular momentum and mass capture rates for a central object accreting from a uniform medium with a velocity gradient transverse to the direction of the mean flow is presented, covering a range of velocity asymmetries and Mach numbers in the incident flow. It is found that the mass accretion rate in a given evolutionary sequence varies in an irregular manner, with the matter accreting onto the central object from either a continuously moving accretion wake or from an accretion disk. The implications of the results from the study of short-term fluctuations observed in the pulse period and luminosity of X-ray pulsars are discussed.

Taam, Ronald E.↗

NASA NTRS1990

Investigation of surface water behavior during glaze ice accretion

A series of experimental investigations that focused on isolating the primary factors that control the behavior of unfrozen surface water during glaze ice accretion were conducted. Detailed microvideo observations were made of glaze ice accretions on 2.54 cm diam cylinders in a closed-loop refrigerated wind tunnel. Distinct zones of surface water behavior were observed; a smooth wet zone in the stagnation region with a uniform water film, a rough zone where surface tension effects caused coalescence of surface water into stationary beads, and a zone where surface water ran back as rivulets. The location of the transition from the smooth to the rough zone was found to migrate towards the stagnation point with time. Comparative tests were conducted to study the effect of the substrate thermal and roughness properties on ice accretion. The importance of surface water behavior was evaluated by the addition of a surface tension reducing agent to the icing tunnel water supply, which significantly altered the accreted glaze ice shape. Measurements were made to determine the contact angle behavior of water droplets on ice. A simple multizone modification to current glaze ice accretion models was proposed to include the observed surface roughness behavior.

Hansman, R. John, Jr.↗

NASA NTRS1991

Accretion disk emission from a BL Lacertae object

It is suggested here that the UV and X-ray emission of BL Lac objects may originate in an accretion disk. Using detailed calculations of accretion disk spectra, the best-measured ultraviolet and soft X-ray spectra of the BL Lac object PKS 2155-304 are fitted, and the mass and accretion rate required is determined. The ultraviolet through soft X-ray continuum is well fitted by the spectrum of an accretion disk, but near-Eddington accretion rates are required to produce the soft X-ray excess. A hot disk or corona could Comptonize soft photons from the cool disk and produce the observed power-law spectrum in the 1-10 keV range. The dynamic time scale in the disk regions that contribute most of the observed ultraviolet and soft X-ray photons are consistent with the respective time scales for intensity variations observed in these two wave bands; the mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard X-ray variability.

Wandel, Amri↗

NASA NTRS1992

Prediction of ice accretion on a swept NACA 0012 airfoil and comparisons to flight test results

In the winter of 1989-90, an icing research flight project was conducted to obtain swept wing ice accretion data. Utilizing the NASA Lewis Research Center's DHC-6 DeHavilland Twin Otter aircraft, research flights were made into known icing conditions in Northeastern Ohio. The icing cloud environment and aircraft flight data were measured and recorded by an onboard data acquisition system. Upon entry into the icing environment, a 24 inch span, 15 inch chord NACA 0012 airfoil was extended from the aircraft and set to the desired sweep angle. After the growth of a well defined ice shape, the airfoil was retracted into the aircraft cabin for ice shape documentation. The ice accretions were recorded by ice tracings and photographs. Ice accretions were mostly of the glaze type and exhibited scalloping. The ice was accreted at sweep angles of 0, 30, and 45 degrees. A 3-D ice accretion prediction code was used to predict ice profiles for five selected flight test runs, which include sweep angle of zero, 30, and 45 degrees. The code's roughness input parameter was adjusted for best agreement. A simple procedure was added to the code to account for 3-D ice scalloping effects. The predicted ice profiles are compared to their respective flight test counterparts. This is the first attempt to predict ice profiles on swept wings with significant scalloped ice formations.

Reehorst, Andrew L.↗