Showing papers by "Michael E. Brown published in 1998"
TL;DR: The water ice bands are weaker than those detected on the surface of any other solar system body; the spectrum is well fit with a model surface consisting predominantly of a neutral dark absorbing substance with only ~3% areal coverage of water ice as discussed by the authors.
Abstract: We report the detection of the 1.5 and 2.0 μm absorption bands due to water ice in the near-infrared reflection spectrum of the Centaur 1997 CU_(26), which is currently located just outside the heliocentric distance of Saturn. The water ice bands are weaker than those detected on the surface of any other solar system body; the spectrum is well fit with a model surface consisting predominantly of a neutral dark absorbing substance with only ~3% areal coverage of water ice. The spectrum thus appears very different from that of the Centaur 5140 Pholus, although both objects are of similar brightness and are at similar heliocentric distances.
56 citations
TL;DR: In this article, the authors used the HST Goddard High Resolution Spectrograph (GHRS) to measure the line profile of hydrogen Lyα (H Lyα) at six locations around the coma of the comet, ranging from the nucleus to a displacement of 100,000 km and covering different directions compared with the comet-sun line.
Abstract: Comet Hyakutake (C/1996 B2) provided a target of opportunity for performing a systematic study of water photodissociation products in which we obtained data from three instruments on the Hubble Space Telescope (HST). The HST Goddard High Resolution Spectrograph (GHRS) was used to measure the line profile of hydrogen Lyα (H Lyα) at six locations around the coma of the comet, ranging from the nucleus to a displacement of 100,000 km, and covering different directions compared with the comet-sun line. GHRS yielded line profiles with a spectral resolution (FWHM ~4 km s^(-1)) that was a factor of 2-3 better than any previous H Lyα or Hα ground-based measurements. The Wide Field Planetary Camera 2 (WFPC2) and the Woods filter were used to obtain H Lyα images of the inner coma. The faint object spectrograph (FOS) was used to determine the OH production rate and monitor its variation throughout the HST observing sequence. The GHRS H Lyα line profiles show the behavior of a line profile that is optically thick in the core for positions near the nucleus (<5000 km) and gradually becoming more optically thin at larger displacements and lower column abundances. A composite H Lyα image constructed from four separate WFPC2 exposures is consistent with the relative fluxes seen in GHRS observations and clearly shows the dayside enhancement of a solar illuminated optically thick coma. These data were analyzed self-consistently to test our understanding of the detailed physics and chemistry of the expanding coma and our ability to obtain accurate water production rates from remote observations of gaseous hydrogen (H) and hydroxyl (OH), the major water dissociation products. Our hybrid kinetic/hydrodynamic model of the coma combined with a spherical radiative transfer calculation is able to account for (1) the velocity distribution of H atoms, (2) the spatial distribution of the H Lyα emission in the inner coma, and (3) the absolute intensities of H and OH emissions, giving a water production rate of (2.6 ± 0.4) × 10^(29) s^(-1) on 1996 April 4.
37 citations
TL;DR: In this paper, the motion of sound ray trajectories in deep ocean environments, including internal wave induced scattering, is considered using the empirical Garrett-Munk internal wave spectrum and results from the study of stochastic differential equations.
Abstract: The motion of sound ray trajectories in deep ocean environments, including internal wave induced scattering, is considered Using the empirical Garrett–Munk internal wave spectrum and results from the study of stochastic differential equations, a framework for studying and modeling stochastic ray motion is developed It is argued that terms in the ray equations involving internal wave induced sound speed perturbations δc can be neglected, but those involving ∂δc/∂z cannot It is then shown in that the (Markov) approximation that spatial variations of ∂δc/∂z are delta correlated is remarkably good These results lead naturally to an extremely simple system of coupled stochastic ray equations (in ray depth z, ray slowness p and travel time T) in which stochasticity enters the system only through the equation for p Solutions to the stochastic ray equations—or the corresponding Fokker–Planck equation—describe approximately the density of acoustic energy in range, depth, angle and time Two dimensionless parameters are introduced: (1) an acoustic Peclet number which is a measure of the ratio of the strength of deterministic ray refraction to that of stochastic scattering induced ray diffusion; and (2) a measure of the ratio of the strength of scattering induced ray diffusion to that of wave diffraction Numerical solutions to the stochastic ray equations are compared to full wave simulations These results show that, even in the weak scattering regime (large acoustic Peclet number), the inclusion of internal wave induced scattering may lead to important qualitative corrections to predictions of distributions of acoustic energy
29 citations
TL;DR: The spectrum and albedo of Nereid appear intermediate between those of the Uranian satellites Umbriel and Oberon, suggesting a surface composed of a combination of water ice frost and a dark and spectrally neutral material.
Abstract: We report the detection of the 1.5 and 2.0 μm absorption bands of water ice in the near-infrared reflection spectrum of Neptune's distant irregular satellite Nereid. The spectrum and albedo of Nereid appear intermediate between those of the Uranian satellites Umbriel and Oberon, suggesting a surface composed of a combination of water ice frost and a dark and spectrally neutral material. In contrast, the surface of Nereid appears dissimilar to those of the outer solar system minor planets Chiron, Pholus, and 1997 CU_26. The spectrum thus provides support for the hypothesis that Nereid is a regular satellite formed in a circumplanetary environment rather than a captured object.
27 citations
TL;DR: In this paper, the authors used speckle interferometry at the Keck telescope, obtaining diffraction-limited resolution (55 mas) at the K band, and found that the entire K-band excess is attributable to a single point-like companion.
Abstract: The white dwarf Giclas 29-38 has attracted much attention on account of its large infrared excess and the suggestion that excess might be due to a companion brown dwarf. We observed this object using speckle interferometry at the Keck telescope, obtaining diffraction-limited resolution (55 mas) at the K band, and found it unresolved. Assuming that the entire K-band excess is attributable to a single pointlike companion, we place an upper limit on the binary separation of 30 mas, or 0.42 AU at the star's distance of 14.1 pc. This result, combined with astroseismological data and other images of G29-38, supports the hypothesis that the source of the near-infrared excess is not a cool companion but a dust cloud.
26 citations
TL;DR: In this article, the authors used spatially resolved high-resolution spectra of the 5890 and 5896 A sodium D lines in the nuclear regions of Comet Hale-Bopp to determine the sources of cometary sodium.
15 citations
TL;DR: In this paper, the authors used the Keck telescope to search nearby stars for dust disks that are analogous to the disk of zodiacal dust that fills the interior of our solar system.
Abstract: We have begun an observational program to search nearby stars for dust disks that are analogous to the disk of zodiacal dust that fills the interior of our solar system. We imaged six nearby main‐sequence stars with the Keck telescope at 11.6 μm, correcting for atmosphere‐induced wavefront aberrations and deconvolving the point spread function via classical speckle analysis. We compare our data to a simple model of the solar zodiacal dust based on COBE/DIRBE observations and place upper limits on the concentration of exo‐zodiacal dust around these stars.
11 citations
TL;DR: In this paper, both full wave and ray-based predictions of acoustic wavefields at long range in deep ocean environments are compared to their measured counterparts, including the SLICE89 and AET data sets.
Abstract: Both full wave and ray‐based predictions of acoustic wavefields at long range in deep ocean environments are compared to their measured counterparts, including the SLICE89 and AET data sets. The environment for these experiments was characterized by weak climatological range‐dependent sound‐speed structure, weak mesoscale structure, and moderately strong (typical) internal‐wave‐scale variability. The acoustic measurements are used to provide constraints on the statistics of the internal wave field, using the Garret–Munk spectrum as a starting point. Our attention is focused on the following parameters to which our simulations are sensitive: the strength parameter E, the mode number cutoff jmax, and the horizontal wave‐number cutoff values kmin and kmax. [Work supported by ONR.] a)The ATOC Group: A. B. Baggeroer, T. G. Birdsall, C. Clark, J. A. Colosi, B. D. Cornuelle, D. Costa, B. D. Dushaw, M. Dzieciuch, A. M. G. Forbes, B. M. Howe, D. Menemenlis, J. A. Mercer, K. Metzger, W. H. Munk, R. C. Spindel, P. F...
TL;DR: In this article, the photometric variability of the impact spots of comet Shoemaker-Levy 9 (SL 9) in the Jovian atmosphere was studied and two well-observed and isolated spots, H and Q1, were found to experience a photometric growth that initially approximates a power law in time of index 0.3 and then appears to level off in the case of the H spot.
Abstract: We present our findings on the photometric variability of the impact spots of comet Shoemaker-Levy 9 (SL 9) in the Jovian atmosphere. Extensive imaging and differential photometry of impact spots in the methane band (8950/90 A) have enabled us to conservatively extract their photometric contribution from that of Jupiter. In this methane band, the SL 9 spots appear brighter than the surrounding Jovian surface because they lie above the main concentrations of methane that overlie the cloud tops. Our observations of two well-observed and isolated spots, H and Q1, indicate that spots experience a photometric growth that initially approximates a power law in time of index 0.3 and then appears to level off in the case of the H spot. We consider two explanations for the brightening of spots: (1) the dispersal of high optical depth core regions and (2) the coagulation of dust from the atomized or fragmented cometary ejecta. The former has been rejected because one spot, H, which was disrupted by a cyclonic storm, exhibited no discontinuity in its rate of photometric growth that would correspond to the disruption. We adopt the second as a working hypothesis because the growth of grains is consistent with our observations and has been cited as the cause for the observed changes in the integrated optical depth in the near-IR and the UV. We propose a simple model in which sites for grain nucleation are provided by the chemical interaction of cometary material and the shocked Jovian atmosphere and in which grain growth is restrained by the diminishing availability of raw materials for grain formation on timescales inversely proportional to the original aerosol density. This model, applied to a volumetric power-law distribution of aerosols, can produce an integrated scattering amplitude growth rate closely resembling our observations.