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At least 145 records · Page 8

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NASA NTRS1985

Periodic orbits of the general three-body problem for the sun-Jupiter-Saturn system

Two families of symmetric periodic orbits of the planar, general, three-body problem are presented. The masses of the three bodies include ratios equal to the sun-Jupiter-Saturn system and the periods of the orbits of Jupiter and Saturn are in a 2:5 resonance. The (linear) stability of the orbits are studied in relation to eccentricity and mass variations. The generation of the two families of periodic orbits follows a systematic approach and employs (numerical) continuation from periodic orbits of the first and second kind in the circular restricted problem to the elliptic restricted problem and from the circular and elliptic problems to the general problem through bifurcation phenomena relating the three dynamical systems. The approach also provides insight into the evolutionary process of periodic orbits continued from the restricted problems to the general problem.

Kwok, J. H.↗

NASA NTRS1985

The rotation period and pole orientation of asteroid 4 Vesta

In 1971 asteroid Vesta was observed in a region of the sky in which it had never been observed before. Its photometric lightcurve had two distinct maxima. Those observations have been the only strong evidence to support a rotation period of about 10 hr 41 min. Lightcurves made in 1982, when Vesta was at the same aspect as 1971, do not show two different maxima. It is concluded that there was a systematic error in the 1971 observations. At this time a definitive statement cannot be made about the true period of Vesta, although the 5 hr 20 min period does appear more plausible. Radar echoes in 1988 and 1992 should resolve the problem. The shorter rotation period was assumed and the photometric astrometry method applied. The sidereal period is 5 hr 20 min 31.68 sec (0.2225889 + or 0.0000002 days), the rotation is prograde, and the coordinates of the north pole are 103 deg longitude and + 43 deg latitude with an uncertainty of about 6 deg.

Taylor, R. C.↗

NASA NTRS1985

A perspective on the evidence for periodic cometary impacts on earth

Recent suggestions of periodic cometary showers coincident with mass extinctions have prompted a review of the terrestrial cratering record. From the point of view of completeness of search and crater retention, the record used to argue for statistical periodicities is incomplete and may not be representative. Estimates of crater ages are of variable accuracy and precision and, in some cases, different dating methods yield discordant results. It is possible to define through time-series analysis a number of periodicities of different magnitude (about 29, 21, 18.5, and 13.5 m.y.) and phase, depending on which craters are considered the most representative sample of the record. Arguing against periodic cometary showers is the observation that estimates of the average cratering rate in recent time and observations on Apollo bodies are equivalent. In addition, siderophile element data from impact melt rocks suggest a variety of projectile types were responsible for relatively large terrestrial craters. Given these observations, it is concluded that statements regarding periodic cometray showers are not categorical and require evidence beyond arguments based on time-series analyses of an incomplete cratering record.

Grieve, R. A. F.↗

NASA NTRS1986

Periodicity of the earth's magnetic reversals

Results are presented from an attempt to perform a relatively comprehensive analysis of the evidence for a periodicity, with harmonics, of the observed regular reversals of the earth's magnetic field. The database considered covers 296 reversals over the past 165 Myr. Histograms with bins 1 Myr apart reveal only 30 Myr reversal patterns. The reversal dates are fitted to a linear periodic function and a spectrum is computed for the residuals at the adopted dates. The possible presence of multiple periodicities is evaluated and over various time intervals. The analysis shows that a recently observed 15 Myr periodicity is probably a harmonic of the 29.5-30.5 Myr period. The calculations do not confirm an inherent magnetic reversal property of the earth. The reversals may arise from tectonic events or from impacts from extraterrestrial objects.

Stothers, R. B.↗

NASA NTRS1987

On the distribution of sunspot cycle periods

A comparison is made between the observed distribution of sunspot cycle periods and distributions based on uniform, normal, and bimodal distributions. The bimodal distribution, composed of short-period and long-period cycles, is found to best describe the observed distribution. Compared to the normal distribution for the most reliably determined cycles (cycles 8-20), the bimodal distribution has a residual (sum of squares of differences) that is about 86 percent smaller. Means for short-period and long-period cycles are estimated to be 122 + or - 4 months and 140 + or - 5 months, respectively.

Wilson, Robert M.↗

NASA NTRS1988

Testing for periodicity of extinction

The statistical techniques used by Raup and Sepkoski (1984 and 1986) to identify a 26-Myr periodicity in the biological extinction record for the past 250 Myr are reexamined, responding in detail to the criticisms of Stigler and Wagner (1987). It is argued that evaluation of a much larger set of extinction data using a time scale with 51 sampling intervals supports the finding of periodicity. In a reply by Sigler and Wagner, the preference for a 26-Myr period is attributed to a numerical quirk in the Harland et al. (1982) time scale, in which the subinterval boundaries are not linear interpolations between the stage boundaries but have 25-Myr periodicity. It is stressed that the results of the stringent statistical tests imposed do not disprove periodicity but rather indicate that the evidence and analyses presented so far are inadequate.

Raup, David M.↗

NASA NTRS1989

Structure and dating errors in the geologic time scale and periodicity in mass extinctions

Structure in the geologic time scale reflects a partly paleontological origin. As a result, ages of Cenozoic and Mesozoic stage boundaries exhibit a weak 28-Myr periodicity that is similar to the strong 26-Myr periodicity detected in mass extinctions of marine life by Raup and Sepkoski. Radiometric dating errors in the geologic time scale, to which the mass extinctions are stratigraphically tied, do not necessarily lessen the likelihood of a significant periodicity in mass extinctions, but do spread the acceptable values of the period over the range 25-27 Myr for the Harland et al. time scale or 25-30 Myr for the DNAG time scale. If the Odin time scale is adopted, acceptable periods fall between 24 and 33 Myr, but are not robust against dating errors. Some indirect evidence from independently-dated flood-basalt volcanic horizons tends to favor the Odin time scale.

Stothers, Richard B.↗

NASA NTRS1990

Orbital periods of recurrent novae

The class of recurrent novae (RN) with thermonuclear runaways contains only three systems (T Pyx, U Sco, and V394 CrA), for which no orbital periods are known. This paper presents a series of photometric observations where the orbital periods for all three systems are discovered. T Pyx is found to have sinusoidal modulation with an amplitude of 0.08 mag and a period of 2.3783 h (with a possible alias of 2.6403 h). U Sco is found to be an eclipsing system with an eclipse amplitude of roughly 1.5 mag and an orbital period of 1.2344 days. V394 CrA is found to have sinusoidal modulation with an amplitude of 0.5 mag and a period of 0.7577 days. Thus two out of three RN with thermonuclear runaways (or five out of six for all RN) have evolved companions.

Schaefer, Bradley E.↗

NASA NTRS1990

Rationalization of Comet Halley's periods

The sense of long axis orientation of Comet Halley during the Vega 1 encounter must be reversed from that deduced by Sagdeev et al. (1986) in order to harmonize the comet nucleus' Vega/Giotto-observed orientations with periodicities extracted from time-series brightness data. It is also demonstrated that Vega/Giotto observations can be satisfied by either a 2.2- or 3.7-day long-axis free precession period. A novel Fourier algorithm is used to reanalyze five independent data sets; strong evidence is adduced for periods harmonically related to a 7.4-day period. The preferred candidate models for Halley's nuclear rotation are characterized by a long-axis precession period of 3.7 days.

Belton, Michael J. S.↗

NASA NTRS1990

Sensitivity of the Oceanic Turbulent Boundary Layer to Cyclic Insolation Change with Response Periods of 23 to 2.5 Ky: an Equatorial Atlantic Record for the Last 200 Ka

Time series of sea-surface temperature in cores sited beneath the region of maximum divergence centered on 10 degrees W are characterized by two sets of periodic signals. The dominant signal is centered on a period of 23 Ky and is coherent with and lags, approx. 2.5 Ky, the precessional component of orbitally controlled insolation. The subdominant periods occur between 4.0 and 2.5 Ky. Both sets of signals record variation in the seasonal intensity of oceanic divergence modulated by variation in tropical easterly intensity. The longer periods are a response to precessional forcing. The forcing responsible for the shorter periods is unknown.

Mcintyre, Andrew↗

NASA NTRS1990

Changes in the orbital periods of close binary stars

A number of close binary stars show erratic changes in their orbital periods on time scales of order 5-10 yr. Recently it has been proposed that the period changes are the result of changes in the quadrupole moment of one star, caused in turn by an alteration of the internal structure of that star. Magnetic pressure, which either distorts the shape of the star or changes its tidally induced quadrupole moment, is suggested as the driving force behind the alteration. Here, the amount of energy required to distort one component of a binary and match the observed period changes is estimated. The rate at which energy is produced or lost is governed by the thermal time scale of the star, and the estimates indicate that the observed period changes would take at least 1000 yr for the tidal quadrupole mechanism, and of order 60 yr to match a period change in V471 Tau which took only 4 yr.

Marsh, T. R.↗

NASA NTRS1991

A connection between long-term luminosity variations and orbital period changes in chromospherically active binaries

The eclipsing binary CG Cyg provides observational confirmation of three predictions made by Applegate's (1991) improvement on the theory that magnetic cycles cause the quasi-periodic orbital period changes in binaries containing a convective star. The mean brightness outside eclipse and the period vary with the same cycle length of about 50 yr. The light curve and O - C curve are in phase, with maximum light and period increase occurring in early 1980. The chromospherically active star becomes bluer in phase with the brightening. Because a period increase occurs at maximum brightness, the sense of the star's differential rotation is specified: outside rotating faster.

Hall, Douglas S.↗

NASA NTRS1992

The photometric period of the recurrent Nova T Pyxidis

The paper presents 1713 photometric measurements of T Pyx from 1966 to 1990. The light curve of T Pyx shows roughly sinusoidal variations with a typical amplitude of 0.09 mag and a time scale of 2 hr. Discrete Fourier transforms of data from individual nights and runs reveal a highly significant periodic modulation. This modulation is proven to be coherent only on time scales shorter than from 6 d to under 1 d. The present period for the modulation is 0.07616 +/- 0.00017 d. Since other stars have variable photometric periods (possibly related to the superhump phenomenon) which are slightly different from the orbital period, an orbital period of about 0.073 d for T Pyx is suggested.

Schaefer, Bradley E.↗

NASA NTRS1993

Long-period oxygen-rich optical Miras in the solar neighborhood

The spatial distribution of the oxygen-rich Miras with periods longer than 400 days in the neighborhood of the sun were determined using available survey and the K-band period luminosity relationship. It is found that the exponential scale height of these stars is near 240 pc. There is a marked contrast between the Mira population at about 1 kpc from the Galactic center where there are nearly as many long-period oxygen-rich Miras as intermediate-period oxygen-rich Miras. It is hypothesized that, at about 1 kpc from the Galactic center, the main sequence stars with masses larger than 1 solar mass have higher metallicities than main-sequence stars with the same masses in the solar neighborhood. In the solar neighborhood such main sequence stars become carbon-rich on the AGB and in the region near the Galactic center they become long-period oxygen-rich Miras.

Jura, M.↗

NASA NTRS1994

Wavelets, period-doubling, and time-frequency localization with application to organization of convection over the tropical western Pacific

In this paper, preliminary results in using orthogonal and continuous wavelet transform (WT) to identify period doubling and time-frequency localization in both synthetic and real data are presented. First, the Haar WT is applied to synthetic time series derived from a simple nonlinear dynamical system- a first-order quadratic difference equation. Second, the complex Morlet WT is used to study the time-frequency localization of tropical convection based on a high-resolution Japanese Geostationary Meteorological Satellite infrared (IR) radiance dataset. The Haar WT of the synthetic time series indicates the presence and distinct separation of multiple frequencies in a period-doubling sequence. The period-doubling process generates a multiplicity of intermediate frequencies, which are manifested in the nonuniformity in time with respect to the phase of oscillations in the lower frequencies. Wavelet transform also enables the detection of extremely weak signals in high-order subharmonics resulting from the period-doubling bifurcations. These signals are either undetected or considered statistically insignificant by traditional Fourier analysis. The Morlet WT of the IR radiance dataset indicates the presence of multiple timescales, which are localized in both frequency and time. There are two regimes in the variation of IR radiance, corresponding to the wet and dry periods. Multiple timescales, ranging from semidiurnal, diurnal, synoptic, to intraseasonal with embedding structures, are active in the wet regime. In particular, synoptic variability is more prominent during the wet phase of an intensive intraseasonal cycle. These are not only consistent with, but also show more details than, previous findings by using other techniques. The phase-locking relationships among the oscillations with different time-scales suggest that both synoptic and intraseasonal variations may be mixed oscillations due to the interaction of self-excited oscillations in the tropical atmosphere and external forcings such as annual and diurnal solar radiation variations. Both examples show that WT is a powerful tool for analysis of phenomena involving multiscale interactions that exhibit localization in both frequency and time. A discussion on the caveats in the use of WT in geophysical data analysis is also presented.

Weng, Hengyi↗

NASA NTRS1994

OH/IR stars near the Galactic Center: Pulsation periods, luminosities, and polarimetry

17 stars in the direction of the Galactic Center, 15 of which are OH/IR stars, have been monitored at infrared wavelengths over a period of nearly eight years. Pulsation periods, bolometric luminosities, and light curves for 14 OH/IR stars are presented. The Galactic Center OH/IR stars range in luminosity between M(sub Bol) = -4.5 to M(sub Bol) = -6, implying main sequence progenitors with masses less than 3 solar mass. When compared to optically visible long period variables in the Large Magellanic Cloud (LMC) with similar bolometric luminosities, the Galactic Center OH/IR stars have pulsation periods on average 30% longer. This shift to longer periods is consistent with the current picture of late asymptotic giant branch evolution, placing the OH/IR stars in a phase immediately following the optically visible Mira variable phase during which the star dramatically increases its mass loss rate, becoming invisible at optical wavelength. Infrared polarimetry of 11 of the stars is also presented. The polarization for all of the stars is consistent with purely interstellar polarization, with little evidence for a significant intrinsic component. When compared to OH/IR stars in the galactic plane, the Galactic Center OH/IR stars appear similar in photometric characteristics, except none of the Galactic Center OH/IR stars shows the extremely thick dust shells or very high intrinsic polarization found in the more extreme galactic plane OH/IR stars.

Jones, Terry Jay↗

NASA NTRS1994

Dripping handrails and the quasi-periodic oscillations of the AM Herculis objects

AM Her objects exhibit periodic, quasi-periodic, and aperiodic variability on timescales ranging from seconds to years. Here, we investigate a process for the production of aperiodic and quasi-periodic accretion rate fluctuations. We consider the nonlinear dynamical model known as the dripping handrail (DHR). The DHR, basically a model for certain types of spatially extended systems and loosely based on water condensing on and dripping off a handrail, has recently been used as a model for the quasi-periodic oscillations (QPO) and very low frequency noise of the low-mass X-ray binary Sco X-1. Here, we show that (1) the DHR is a robust QPO generation process in that it leads to QPO production under a wide range of conditions and assumptions; (2) the phenomenology of the DHR is consistent with the observed aperiodic and quasi-periodic varibility of the AM Her QPO source VV Pup over timescales ranging from 16 ms to 20 s; and (3) a single DHR model can produce both broadband QPOs and features with quality Q greater than 20 as observed in several AM Her QPO sources.

Steiman-Cameron, Thomas Y.↗

NASA NTRS1994

Recurrence times and periodicities in 4U 1608-52 as observed by Vela 5B

We report on the Vela 5B 10 year history of the soft X-ray transient 4U 1608-52, and on the characteristics of its soft X-ray outbursts. The Vela 5B satellite observed the four known outbursts in 1975, 1977, and 1979, and four new outbursts in 1970 and 1971, altering the recurrence pattern of outbursts from this source. One of the 1970 outbursts is symmetric in its intensity profile, while the two outbursts in 1971 have short exponential profiles separated by 80 days. Despite suggestive recurrence periods of approximately 85 and approximately 150 days evident in the time intervals between the outbursts, there is no single statistically significant recurrence time on which the outbursts recur consistently. In the 1970 symmetric event, there is evidence for a period of either 4.10 or 5.19 days. Drawing upon the analogy with SU Ursa Majoris dwarf novae, we suggest that the short period is orbital and any longer period would be associated with a precession period of the accretion disk.

Lochner, James C.↗