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Van Buren, D.

Publications and source records attributed to Van Buren, D..

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

The spectrum of TMR-1C is consistent with a background star

...This paper presents near-infrared grism spectroscopy to constrain the efective temperature of TMR-1C, obtained with the Near-Infrared Camera (NIRC) at Keck Observatory. To interpret the data, we construct a grid of extincted M dwarf spectra to compare with the low-resolution (RD120) NIRC spectrum. The assumed extinction corresponds to standard interstellar dust. With the additional assumption that no near-infrared dust excess contributes to the spectrum, then M4.5 is the latest spectral type TMR-1C can have within the uncertainties. ...

Padgett, D. L.↗
NASA NTRS1998

A Candidate Protoplanet in the Taurus Star Forming Region

A long narrow filament seems to connect the protobinary to the faint companion TMR-1C, suggesting a physical association. If the sources are physically related then we hypothesize that TMR-1C has been ejected by the protobinary.

HST/NICMOS faint companion protostar filament prot↗
NASA NTRS1997

Astrometric Observation of MACHO Gravitational Microlensing

This paper discusses the prospects for astrometric observation of MACHO gravitational microlensing events. We derive the expected astrometric observables for a simple microlensing event assuming a dark MACHO, and demonstrate that accurate astrometry can determine the lens mass, distance, and proper motion in a very general fashion.

Astrometry MACHO Microlensing↗
NASA NTRS1995

The temporal evolution of the 1-5 micron spectrum of V1974 CYGNI (Nova Cygni 1992)

We present 1-5 micrometer moderate- and high-resolution infrared spectra of the ONeMg nova V1974 (Nova Cygni 1992) obtained at multiple epochs during an approximately 500 day period after outburst. During the first 80 days, the spectra exhibited continuum emission from thermal bremsstrahlung (free-free radiation) with prominent recombination line emission from hydrogen, helium, nitrogen, and oxygen. The measurement Full Width at Half Maximum (FWHM) of these recombination lines suggests that the initial velocity of the ejecta was approximately equals 2400-3400 km/s. We estimate from the hydrogen recombination line ratios that the density of the ionized shell at this epoch was approximately equals 10(exp 9), with an effective electron temperature of approximately equals 5 x 10(exp 3) K. As the temporal evolution of the ejecta progressed, the hydrogen and helium lines diminished in intensity, and coronal lines of aluminum, calcium, magnesium, neon, and sulfur appeared by approximately day 80. The coronal line emission phase persisted for over 400 days. During this epoch, no significant dust formation occurred. We find that the ejecta of V1974 Cyg were overabundant in aluminum with respect to silicon by a factor of approximately equals 5 and in magnesium with respect to silicon by a factor of greater than or equal to 3 relative to the solar photosphere. Comparison of our observed overabundances with recent model predictions of elemental synthesis in ONeMg outbursts suggests that the accreted envelope on V1974 Cyg was close to solar composition and that the precursor ONeMg white dwarf had a mass of approximately equals 1 solar mass.

Woodward, Charles E.↗
NASA NTRS1994

Shocked molecular gas around the extremely young source IRAS 03282+3035

We present observations of the shock tracers H2 and SiO around the young stellar object IRAS 03282+3035. This unusual low-luminosity (L approximately = 2 solar luminosity) source drives a strong highly collimated CO outflow, and it is one of the youngest stellar objects known so far. The near-infrared H2 emission, tracing 2000 K gas, comes from extremely high velocity CO bullets along the axis of the blueshifted lobe of the outflow. The millimeter SiO emission, tracing roughly 100 K gas, arises from lower velocity material at the end of the outflow lobe. The lack of high-temperature and high-velocity gas at the end of the outflow lobe indicates there is no not bow shock at the outflow termination. In the context of current jet models this appears to rule out a bow shock driven by a steady state jet. Possible explanations for the structure include a time-dependent jet or a jet dominated by turbulent entrainment.

Bachiller, R.↗
NASA NTRS1994

Palomar Prime-Focus Infrared Camera (PFIRCAM)

In a joint effort between engineers and scientists at the Jet Propulsion Laboratory and the California Institute of Technology, a near-infrared (0.8 - 2.6 micrometers) direct imaging system has been developed and integrated into the Caltech Palomar Observatory detector series. The camera system has been tested and operated in a science mode at the prime-focus (f/3.3) of the Hale 5-m Telescope. This paper outlines the system components and performance, including discussion of the detector linearity.

Jarrett, T. H.↗
NASA NTRS1992

Development and test of an IR camera for astronomical observation at the Palomar 200-in. observatory using the NICMOS II array

A NICMOS II array, a NIR (0.80 - 2.6 micron) imaging system, was integrated into the California Institute of Technology Palomar Observatory camera. The Palomar camera system which consists of six discrete subsystems including a focal plane, an electrical interface, data acquisition electronics, a dewar, dewar optics, and the software is described. The performance and calibration data for each subsystem are presented. Emphasis is placed on test data on the focal plane performance.

Bruce, Carl↗
NASA NTRS1986

Kinetic efficiencies of stellar wind bubbles

The theoretical fraction of a stellar wind's energy converted into the kinetic energy of an expanding swept-up shell is 20 percent in the classical theory of stellar wind bubbles. Observational estimates of this conversion factor based on the amount of ionized material in wind-swept shells about Wolf-Rayet stars generally yield results of 1 percent. If there is a substantial amount of neutral material in the shell, it will not be counted and the kinetic efficiency will be underestimated. Presented here is a dynamical estimate which accounts for this neutral material in deducing the kinetic efficiencies of stellar wind bubbles. Bubbles classified as wind-blown shells have kinetic efficiencies in line with theoretical expectations for energy-conserving evolution in a homogeneous medium. Ringlike nebulae have significantly lower efficiencies, probably because they have been 'poisoned' by the photoevaporation of clouds engulfed during evolution into a cloudy substrate.

Van Buren, D.↗
NASA NTRS1986

Equivalence of active and passive gravitational mass using the moon

Based on an asymmetry in the composition of the moon, a limit is established for the violation of the equity of passive and active gravitational mass. It is suggested that the 2-km offset between the moon's center of figure and center of mass imply an asymmetry in the distribution of Fe and Al, and that the Fe on one side and the Al on the other must attract another with equal force in order for the moon to follow the orbit predicted by classical mechanics. Based on laser ranging data and a model for the moon's interior, the ratios of active to passive mass for Fe and Al are found to be equal to a precision of 4 x 10 to the -12th.

Bartlett, D. F.↗
NASA NTRS1986

Comment on the three-body theory for period changes in RS CVn systems

In the three-body theory for period variations in RS CVn systems, the timing residuals are interpreted as light-travel time differences as the eclipsing system moves about the barycenter of the triple. These residuals can require a larger orbit than Kepler's law allows, given the time scale of the period variations. For only two of eight systems investigated, SV Cam and V471 Tau, is the theory plausible in that the inferred barycentric motion of the binary is smaller than the orbit of the third body, and the inferred properties of the third body are both reasonable and consistent with its remaining hidden. The theory is thus not a general theory for period changes. Observational testing of the theory is straightforward and may lead to the detection of 'brown dwarfs' associated with eclipsing systems through their kinematic effects.

Van Buren, D.↗
NASA NTRS1985

Activity-driven structure variations as a cause of period changes in RS Canis Venaticorum-like systems

Magnetic activity cycles of a convective star in a close binary system may lead to orbit period changes through tidal spin-orbit coupling. An increase in the mean magnetic field throughout the convection zone provides an additional pressure support and increases the star's moment of inertia. In order for the system to conserve angular momentum instantaneously, the convective star must reduce its rotation rate. When this happens, the system is no longer synchronous; in particular, the equilibrium tide now suffers a phase lag. A tidal torque must act, transferring angular momentum from the star's rotation to the orbit, lengthening the period. When the field decays, these events reverse and the period shortens. Observed period changes in RS CVn-like systems are of the order delta p/p = 10 to the -6th, implying variations in the mean field strength at the base of the convection zone of approximately 1000 G over time scales of 10-30 yr. It is predicted that the strength of magnetic activity indicators will track orbital period changes in a given system: the period will be largest during the active portion of the magnetic cycle and smallest during the quiescent part.

Van Buren, D.↗
NASA NTRS1985

The initial mass function and global rates of mass, momentum, and energy input to the interstellar medium via stellar winds

Published observational data are compiled and analyzed, using theoretical stellar-evolution models to determine the global rates of mass, momentum, and energy injected into the interstellar medium (ISM) by stellar winds. Expressions derived include psi = 0.00054 x (M to the -1.03) stars formed/sq kpc yr log M (where M is the initial mass function in solar mass units) and mass-loss = (2 x 10 to the -13th) x (L to the 1.25) solar mass/yr (with L in solar luminosity units). It is found that the wind/supernova injection of energy into the ISM and the mass loss from stars of 5 solar mass or more are approximately balanced by the dissipation of energy by cloud-cloud collisions and the formation of stars, respectively.

Van Buren, D.↗
NASA NTRS1983

Detection of flare like events and their relationship to presumed spot regions on V471 Tau - A solar-stellar connection

Since its discovery as an eclipsing binary with a white dwarf companion by Nelson and Young (1970), V471 Tau (BD + 16 deg 516) has been an object of considerable importance. The K dwarf companion has been the subject of intensive studies because it demonstrates many of the properties normally attributed to classical RS CVn stars. In the present investigation, it is demonstrated that the K dwarf companion exhibits also flaring properties which characterize the BY Dra stars. In a sense, V471 Tau may be the critical link by providing information that the distinctions between RS CVn phenomena and BY Dra phenomena are artificial and misleading, arising entirely from historical processes. Observations in the optical and the X-ray wavelength regions are reported. A study of the occurrence times of flare events in terms of the ephemeris of the photometric wave which is known to migrate through the light curve of V471 Tau reveals a strong correlation, suggesting that the flares occur near regions of the K dwarf where spot groups are inferred.

Klimke, A.↗