Thomas Y. Steiman-Cameron

The quasi-periodic oscillations and very low frequency noise of Scorpius X-1 as transient chaos - A dripping handrail?

We present evidence that the quasi-periodic oscillations (QPO) and low frequency noise (LFN) characteristic of many accretion sources are different aspects of the same physical process. We analyzed a long, high time resolution EXOSAT observation of Sco X-1. The x-ray luminosity varies stochastically on time scales from milliseconds to hours. The nature of this variability -- as quantified with both power spectrum analysis and new wavelet technique, the {it scalegram} - agrees well with the {it dripping handrail} accretion model, a simple dynamical system which exhibits {it transient chaos}. In this model both the QPO and LFN are produced by radiation from blobs with a wide size distribution, resulting from accretion and subsequent diffusion of hot gas, the density of which is limited by an unspecified instability to lie below a threshold.

On the evolution of gravitationally unstable protostellar disks

The formation of single stars from rotating interstellar clouds generally requires that the angular momentum of the cloud be redistributed in such a way that a major fraction of the mass (eventually residing in the star) retains only a minor fraction of the angular momentum, most of the latter being deposited in a circumstellar disk. Gravitational instabilities in a growing disk would promote such a redistribution by the action of self-excited density waves. We have attempted to quantify this process by conducting two-dimensional, N-body simulations of the nonlinear development of long-wavelength instabilities predicted to occur early in the formation stage of the disk, under conditions of controlled energy loss

The remarkable twisted disk of NGC 4753 and the shapes of galactic halos

The complex dust lanes in the S0 galaxy NGC 4753 are shown to be consistent with a disk that is strongly twisted by differential precession. Yet another peculiar S0 can therefore be explained as the result of an accretion event. An evolving disk model is fitted to the observed distribution. This disk is inclined by 15 deg relative to the galaxys equatorial plane and twisted such that its line of nodes changes smoothly by 3.8 pi over a factor of seven in radius. The model indicates that most of the galaxys mass is unseen, is nearly spherically distributed, and has a nearly scale-free spatial distribution. The ellipticity of the total galactic mass distribution must be constant to within 20 percent over the radial extent of the twisted disk - a conclusion which may significantly constrain galaxy formation mechanisms.

FURTHER STUDIES OF GRAVITATIONALLY UNSTABLE PROTOSTELLAR DISKS

Models of the solar nebula reveal that it might have been gravitationally unstable, both early and later in its evolution. Such instabilities produce density waves and associated gravitational torques, which are potent agents of angular momentum transport. In previous work, we conducted a series of numerical simulations designed to quantify the effects of gravitational instabilities in a generalizable way (Tomley, Cassen, & Steiman-Cameron 1991). Here we present a second series of simulations in which we examine disks of greater size, increased star/disk mass ratio, and flatter surface density distribution than those in our initial study. The purpose is to represent disks at a later stage of evolution than those already studied, to test the quantitative relations derived in our earlier work and to explore the effects of mass ratio on the results. The new results indicate that the tendencies for unstable, uncooled disks to heat to stability and for dynamical evolution rates to be proportional to cooling rates are general characteristics of the behavior of gravitationally unstable disks. Nevertheless, there are quantitative, and (for strong cooling) even qualitative differences that are revealed in the new simulations, particularly with regard to the cooling rates at which clumping tends to occur.

Physical Conditions in the Photodissociation Region of NGC 2023

Observations of the far-infrared fine-structure emission lines of [O I] (63, 146 μm), [C II] (158 μm), and [Si II] (35 μm) and the adjacent continua were obtained for the reflection nebula NGC 2023 with the Cryogenic Grating Spectrometer aboard the Kuiper Airborne Observatory. These fine-structure lines originate from a warm, atomic photodissociation region (PDR), where gas has been heated by the illuminating star HD 37903. The measured line and continuum intensities were combined with [C II] and CO observations from the literature to estimate the density and temperature of the PDR. Simple analysis indicates densities of ~2 × 104 cm-3 and temperatures around 400 K for the emitting gas. Detailed models of the PDR in NGC 2023 have also been constructed. Comparisons between observed and predicted line strengths demonstrate that considerable density enhancements (clumps) exist throughout the PDR. A two-component model of dense (total hydrogen density = 105 cm-3) warm (≈ 750 K) clumps with a beam area filling factor of 0.11 embedded in a diffuse (750 cm-3) cooler (≈ 250 K) halo is in good agreement with the observations.

Evolution of inclined galactic gas disks. I - A cloud-fluid approach

In this paper, numerical models are developed to follow the time-dependent behavior of inclined gas disks in nonspherical galaxies. Differential equations are developed which describe the time rate of change of the orbits of mass elements composing a thin, smoothly warping and twisting gas disk. These equations include multiple moments due to the nonsphericity of the galactic mass distribution, Coriolis effects arising from determination of orbits in a reference frame which tumbles with the surface figure of the galaxy, and viscous forces arising from cloud-cloud collisions. A cloud-fluid approach is used in determining the viscous forces. Physical arguments are used to determine the maximum permissible local value of the coefficient of kinematic viscosity. For small initial inclinations, analytic expressions are obtained for the inclination as a function of time and for the settling time, and the features of the solution are discussed. 55 references.

Coordinated Optical and X-Ray Observations of the AM Herculis Objects VV Puppis, V834 Centauri, and EF Eridani

We obtained coordinated broadband optical photometry and hard X-ray data on the AM Her objects VV Pup, V834 Cen, and EF Eri. The X-ray data were obtained using the Proportional Counter Array detectors of the Rossi X-Ray Timing Explorer, and the optical data were obtained using the 1 m telescope of the Cerro Tololo Inter-American Observatory. During our observations, VV Pup and V834 Cen were in their high optical luminosity states with average mV ~ 17 and 15.1, respectively. EF Eri was in a very low state at the time of our observation: mV > 17.7. VV Pup and V834 Cen had average 2-10 keV X-ray fluxes of 1.7 × 10-12 and 2.13 × 10-11 ergs cm–2 s–1, respectively. EF Eri was not detected in X-rays. The X-ray spectrum of V834 Cen was adequately modeled using an absorbed bremsstrahlung continuum with a Gaussian emission line. The best-fit model had continuum temperature kTX = 11.8 ± 1.0 keV and absorption column nH = (1.03 ± 0.39) × 1022 cm–2. The Gaussian emission line had line center energy E = 6.7 ± 0.1 keV and equivalent width EW = 0.64 keV. VV Pup was too faint to produce reliable spectral fits. The 2-10 keV flux for VV Pup quoted above was based on the best-fit spectral model. Holding kTX fixed in the fitting process yielded roughly the same flux for kTX = 0.5-30 keV.We detected ~1 Hz optical quasi-periodic oscillations (QPOs) with amplitude ~2.2% from VV Pup and ~1% from V834 Cen. We did not detect optical QPOs over the range 0.2-1 Hz from EF Eri at a 90% confidence upper limit of 4.5%. The QPO properties are consistent with those found in the literature. None of the targets showed X-ray QPO activity. However, only V834 Cen yielded a meaningful upper limit: ~14% at the 90% confidence level over 0.2-1.2 Hz.

V2301 Ophiuchi: An X-Ray-bright Eclipsing AM Herculis Object

We present hard X-ray and optical observations of the eclipsing AM Her system V2301 Oph. The X-ray data were obtained using the PCA detector of the Rossi X-Ray Timing Explorer satellite during 1997 May, and the optical data were obtained using the 1 and 1.5 m telescopes of the Cerro Tololo Inter-American Observatory during 1996 May and 1997 June. V2301 Oph was bright in both the optical and hard X-rays during our observations. This, when coupled with its eclipsing nature, makes V2301 Oph an ideal testbed for theories of the large-scale topology of AM Her flows and the radiative shocks in AM Her systems. The X-ray emission from V2301 Oph was modulated strongly on the orbital period. During the bright orbital phases, the X-ray flux was Fx≈3.6 × 10−11 ergs cm-2 s-1 over the energy range E=2-10 keV. The X-ray emission did not go to zero during the faint orbital phases; it was ~10% of the bright phase level. The X-ray spectrum could be fitted by (1) optically thin thermal bremsstrahlung (temperature kTx≈9-19 keV) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV, and (2) power-law continuum (index ≈2) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV. The absorption columns were large for all fits, nH~(3-10) × 1022 cm-2. The nH are model dependent, but their large sizes are secure because they are set by the rollover in the X-ray spectrum at 3-4 keV. The hardness of the X-ray spectrum was roughly constant during the bright orbital phases. During the faint orbital phases, the X-ray spectral properties were not well determined, but it did appear that the spectrum hardened. There were total eclipses in both the X-ray and optical light curves. The X-ray light curves and eclipses were consistent with a dominant hot spot and a secondary hot spot. The dominant hot spot was not a point source; it had to cover about 50° in longitude on the surface of the white dwarf. We argue that the X-ray light curve and eclipse shape also suggest that the accretion occurs in a sheetlike geometry rather than in a columnar geometry. The optical light curves and eclipses were consistent with emission from the white dwarf photosphere, an extended emission region that sat above the surface of the white dwarf, and the X-ray-heated face of the companion star.

Discovery of Quasi-periodic Oscillations in the AM Herculis Object BL Hydri

We obtained high-speed optical photometry of the AM Her object BL Hyi at the Las Campanas Observatory and the Cerro Tololo Inter-American Observatory during 1989-1996. BL Hyi was in its faint-luminosity state in 1989; it subsequently brightened and was in its high-luminosity state for our 1994-1996 observations. We discovered broad, 0.2-0.8 Hz quasi-periodic oscillations (QPOs) and narrower QPOs superposed on the broad QPOs when BL Hyi was in its high-luminosity state. The broad QPOs had widths of Δf/fp ~ 0.5-1 and rms pulsed amplitudes of ~1%-4%, where fp is the frequency of the QPO peak. The narrow QPOs had widths of Δf/fp < 0.1 and rms pulsed amplitudes of less than 1%. BL Hyi showed stronger QPOs and was slightly brighter in 1994 than in 1995-1996. The amplitudes of the broad and narrow QPOs were modulated on the orbital period of the system; they were strongest during the bright orbital phase. BL Hyi is the fifth AM Her system to show the short-period QPOs discovered by Middleditch.

High-State Observations of the Eclipsing AM Herculis Object UZ Fornacis

We present high-speed optical photometric observations of the eclipsing AM Her object UZ For that were obtained during 1995 February and October at the Cerro Tololo Inter-American Observatory. On both occasions, UZ For was in its high-luminosity state. In 1995 February, UZ had peak visual magnitude mV ≈ 15.9 ± 0.1 mag, which is roughly 0.5-1 mag brighter than previous reports of its high-state brightness level. In 1995 October, UZ For had peak magnitude mV ≈ 16.75 ± 0.25 mag, which is more similar to earlier reports of its high-state brightness level. The 1995 February light curves and eclipses are different in appearance from those for the lower luminosity states of UZ For reported in the literature. The eclipses are, however, similar to those of other AM Her systems, e.g., they are similar in shape to the eclipses of RE 2107-05 (HU Aqr) and to the ultraviolet eclipses of UZ For. The significance of the last statement is that in both of these systems, the accretion stream contributes a significant fraction of the overall luminosity of the system. In the low-luminosity state of UZ For, the accretion hot spot and the white dwarf photosphere dominate the optical emission. Here, we conclude that in the high-luminosity state, the accretion stream makes a significant contribution (and sometimes dominates) the optical emission from UZ For.