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Worden, S. P.

Publications and source records attributed to Worden, S. P..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

The Near-Earth Space Surveillance (NESS) Mission: Discovery, Tracking, and Characterization of Asteroids, Comets, and Artificial Satellites with a Microsatellite

The Near-Earth Space Surveillance (NESS) Mission, a microsatellite dedicated to observing near-Earth (NEO) and interior-to-the-Earth (IEO)asteroids and comets plus artificial satellites, is currently being studied under contract to the Canadian Space Agency. Additional information is contained in the original extended abstract.

Hildebrand, A. R.↗
NASA NTRS1983

The photometric variability of solar-type stars. III - Results from 1981-82, including parallel observations of thirty-six Hyades stars

A photometric survey of main-sequence stars in the Pleiades, Hyades, and Malmquist Field, to investigate variability among solar-type stars, had been continued at Cloudcroft Observatory for a second season. Possibly six of 40 Pleiades stars, two (possibly six) of 49 Hyades stars and possibly four of 42 stars in the Malmquist Field are found to be variable. Parallel observations of Hyades stars were obtained also at Lowell Observatory. These measurements, which achieved better precision, reveal significiant variations in eleven (possibly 16) of 36 Hyades stars, all of spectral type F7 V-K2 V. Agreement between the two surveys is acceptable. The two seasons of the Cloudcroft survey show that changes in the mean annual brightness of the Hyades stars accompany changes in the amplitude of their short-term variability, in the sense that a star tends to become fainter as its variability increases and vice versa. Color effects are absent.

Radick, R. R.↗
NASA NTRS1983

The magnetic field on the RS Canum Venaticorum star Lambda Andromedae

A program to detect and measure magnetic flux on the surfaces of late-type stars is discussed. A technique is adopted to deconvolve magnetically insensitive lines from similar, magnetically sensitive lines to infer the degree of Zeeman splitting in the latter lines. These measurements yield values for the magnetic field strength and filling factor (flux). To illustrate the approach, observations of the RS CVn star Lambda And are presented. At the epoch of observation, 1981 April 26, a field strength of 1290 + or - 320 gauss covering 48 + or - 7 percent of this star's surface is found. This measurement compares with an estimate of coronal magnetic flux in the cooler component of the stellar corona of 1110 gauss with a coronal volume filling factor of 75 percent, based on X-ray data for Lambda And.

Golub, L.↗
NASA NTRS1982

Outer atmospheres of cool stars. XII - A survey of IUE ultraviolet emission line spectra of cool dwarf stars

Quantitative information is obtained on the chromospheres and transition regions of M dwarf stars, in order to determine how the outer atmospheres of dMe stars differ from dM stars and how they compare with the outer atmospheres of quiet and active G and K type dwarfs. IUE spectra of six dMe and four dM stars, together with ground-based photometry and spectroscopy of the Balmer and Ca II H and K lines, show no evidence of flares. It is concluded, regarding the quiescent behavior of these stars, that emission-line spectra resemble that of the sun and contain emission lines formed in regions with 4000-20,000 K temperatures that are presumably analogous to the solar chromosphere, as well as regions with temperatures of 20,000-200,000 K that are presumably analogous to the solar transition region. Emission-line surface fluxes are proportional to the emission measure over the range of temperatures at which the lines are formed.

Linsky, J. L.↗
NASA NTRS1982

Stellar model chromospheres. XIII - M dwarf stars

Single-component, homogeneous model chromospheres that are consistent with high-resolution profiles of the Ca II K line calibrated in surface flux units for three dMe and 2 dM stars observed at quiescent times are constructed. The models reveal several systematic trends. Large values of the ratio of T(min) to T(eff) are derived, indicating a large amount of nonradiative heating present in the upper photospheres of M dwarf stars. It is also found that the lower chromospheric temperature gradient is similar for all the M dwarf stars. Since for the models here the chromospheric K line emission strength is most sensitive to the total amount of chromospheric material present within the approximate temperature range T(min)-6000 K, increasing the emission strength is not simply due to increasing chromospheric temperature gradients. It is also found that both the electron density and electron temperature at one thermalization length in the K line below the top of the chromospheres are greater in the dMe stars than in the dM stars. The M dwarf models here have microturbulent velocities between 1 and 2 km/sec, which are much smaller than for solar chromosphere models.

Giampapa, M. S.↗
NASA NTRS1981

The Mg 2 h and k lines in a sample of dMe and dM stars

Both Mg II h and k line fluxes are presented for a sample of 4 dMe and 3 dM stars obtained with the IUE satellite in the long wavelength, low dispersion mode. The observed fluxes are converted to stellar surface flux units and the importance of chromospheric non radiative heating in this sample of M dwarf stars is intercompared. In addition, the net chromospheric radiative losses due to the Ca II H and K lines in those stars in the sample for which calibrated Ca II H and K line data exist are compared. Active region filling factors which likely give rise to the observed optical and ultraviolet chromospheric emission are estimated. The implications of the results for homogeneous, single component stellar model chromospheres analyses are discussed.

Giampapa, M. S.↗
NASA NTRS1981

UV chromospheric and circumstellar diagnostic features among F supergiant stars

A survey of F supergiant stars to evaluate the extension of chromospheric and circumstellar characteristics commonly observed in the slightly cooler G, K, and M supergiant is discussed. An ultraviolet survey was elected since UV features of Mg II and Fe II might persist in revealing outer atmosphere phenomena even among F supergiants. The encompassed spectral types F0 to G0, and luminosity classes Ib, Ia, and Ia-0. In addition, the usefulness of the emission line width-to-luminosity correlation for the G-M stars in both the Ca II and Mg II lines is examined.

Stencel, R. E.↗
NASA NTRS1981

A heating mechanism for the chromospheres of M dwarf stars

The atmospheric structure of the dwarf M-stars which is especially important to the general field of stellar chromospheres and coronae was investigated. The M-dwarf stars constitute a class of objects for which the discrepancy between the predictions of the acoustic wave chromospheric/coronal heating hypothesis and the observations is most vivid. It is assumed that they represent a class of stars where alternative atmospheric heating mechanisms, presumably magnetically related, are most clearly manifested. Ascertainment of the validity of a hypothesis to account for the origin of the chromospheric and transition region line emission in M-dwarf stars is proposed.

Giampapa, M. S.↗
NASA NTRS1979

Stellar model chromospheres. IX - Chromospheric activity in dwarf stars

High-resolution Ca II K line profiles are used to model the upper photospheres and lower chromospheres of eight main-sequence stars ranging in spectral type from F0 to M0 and exhibiting different degrees of chromospheric activity. The model chromospheres are studied as a function of spectral type and activity for stars of similar spectral type in order to obtain evidence of enhanced nonradiative heating in the upper-photospheric models and in the ratio of minimum temperature at the base of the chromosphere to effective temperature, a correlation between activity and temperature in the lower chromospheres, and a correlation of the width at the base of the K-line emission core and at the K2 features with activity. Chromospheric radiative losses are estimated for the modelled stars and other previously analyzed main-sequence stars. The results obtained strengthen the argument that dMe flare stars exhibit fundamentally solar-type activity but on an increased scale.

Kelch, W. L.↗
NASA NTRS1979

Nucleosynthesis of Li-7 in flares on UV Ceti stars

The possible production of Li-7 by nuclear reactions in UV Ceti flares has been considered. By utilizing solar observations and theory, a relationship is derived between flare energy and production rates for Li-7; approximately 100 erg of total flare energy is found to denote the formation of a Li-7 atom. Based on this value and best estimates of UV Ceti-type flare rates, it is concluded that less than 10% of the Li-7 observed in the intestellar medium may have been produced by this mechanism. Formation of significant amounts of interstellar deuterium by this method is ruled out.

Karpen, J. T.↗