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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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9 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Synthetic Stromgren photometry for F dwarf stars

Recent synthetic spectrum and color calculations for cool dwarf star models are tested by comparison with observation. The accuracy of the computed dependence of the thermal colors B-V and b-y on effective temperature is examined, and H-beta indices are presented and compared with observed values. The accuracy of the predictions of the Stromgren uvby system metal-abundance indicator m1 and luminosity indicator c1 are tested. A new calibration of the c1, b-y diagram in terms of absolute magnitudes is given, making use of recent calculations of stellar isochrones. Observations of very metal-poor subdwarfs are used to study the accuracy of the isochrones. The c1, b-y diagram of the subdwarfs is compared with that of the turnoff-region stars in the very metal-poor globular cluster NGC 6397.

Bell, R. A.↗
NASA NTRS1971

The temperatures, abundances and gravities of F dwarf stars.

Theoretical colors computed from laboratory line data and from model stellar atmospheres have been used to interpret the colors of about 150 F and early G dwarfs. Effective temperatures have been derived from the H-beta index and from R-I, abundances have been obtained from m(sub 1) and from b-y, and gravities have been obtained from c(sub 1) and from b-y. The effective temperatures and gravities are in good agreement with values obtained from spectral scans. Absolute magnitudes have been obtained from the effective temperatures and gravities, the latter being used with assumed stellar masses to yield radii. The present results provide theoretical justification of the empirical formulas given by Crawford and by Stroemgren for the determination of absolute magnitudes and abundances from uvby photometry.

Bell, R. A.↗
NASA NTRS1972

The intrinsic values and color excesses of (B-V) for 115 F-K supergiants

Color excesses in B-V are determined indirectly from a study of Stromgren's b-y color for a sample of F0 - K5 supergiants. The resulting E(B-V)'s are estimated to have an expected precision of + or - 0.05. With the calculated color excesses and the observed values of B-V given in various catalogs, the run of B-V with spectral type is obtained. This B-V/(spectral type) relationship is compared with those found previously by other investigators.

Kelsall, T.↗
NASA NTRS1972

Intrinsic B-V color for galactic cepheids and some comments on the Sandage-Tammann relationship

Transformations are found for converting the b-y color excesses for Cepheids given by Williams (1966) and Kelsall (1971) into B-V excesses. The combination of these results with the E(B-V)'s determined by Sandage and Tammann (1971) gives precise data for eighty-eight galactic Cepheids. The period-color and period-color-(amplitude defect) relationships, that are germane to the LogP intervals 0.4 to 1.4 and 0.4 to 1.3, respectively, are found.

Kelsall, T.↗
NASA NTRS1978

Predicted color excess ratios versus interstellar grain size

Various color excess ratios, the ratio (R) of total to selective extinction, and values of the wavelength of maximum interstellar linear polarization (lambda max) are computed and displayed as functions of mean interstellar grain size and absorption coefficients. E(V-K)/E(B-V)-E(u-b)/E(b-y) is shown to be highly sensitive to changes of mean grain size. The shape of the R versus lambda max curve is shown to depend on the amount of absorption initially present in the grains, and on the absorptivity of the material added to the grains. While no simple model of grain growth has been found to fit all the observations, the two stars with the largest values of lambda max (HD 147889 and W67) support the concept of dielectric mantles growing on dielectric grains. It is further shown that more accurate uby and JKL photometry of heavily reddened stars would permit better discrimination between grain growth theories.

Mcmillan, R. S.↗
NASA NTRS1984

A Statistical Evaluation of the Limitations of Single-channel Intermediate-band Photoelectric Stellar Photometry

Error distribution in differential photometry for a moving solar system object is investigated. The results of differential photometry of several dozen pairs of planetary comparison stars observed since 1972 are reviewed. Each pair of stars was observed along with the corresponding planet or satellite, typically about ten times during the course of a single apparation lasting about four months. The comparison stars are chosen to bracket the opposition position of the planet, the expected opposition magnitude, and solar color. In practice, this means that the range of spectral type was mid-F to early K, the range of B-Y color was about 0.25 to 0.95 mag, the range of differential magnitude was less than 2 mag but most often less than 1.0 mag, and the difference in declination was typically less than 3 deg. The difference in air mass at meridian transit was usually less than 0.03, and rarely exceeded 0.01. Hence, differential extinction effects are negligible, except under extraordinary conditions.

Lockweed, G. W.↗
NASA NTRS1990

Ultraviolet spectra of field horizontal-branch A-type stars. II

The spectra of six additional A-type stars have been obtained at low resolution between 1200 and 1900 A with the IUE. The energy distributions of four of the stars match that of the field horizontal branch (FHB) distribution in Huenemoerder et al. (1984) while those of the other two do not. Three of the FHB stars fall above a line in the C(19 - V)0 vs. (b-y)0 diagram; however, HD 60825 is anomalously blue for its C(19 - V) color.

Philip, A. G. Davis↗
NASA NTRS2004

Cusp and LLBL as Sources of the Isolated Dayside Auroral Feature During Northward IMF

An intense dayside proton aurora was observed by IMAGE FUV for an extensive period of northward interplanetary magnetic field (IMF) on 17 and 18 September, 2000. This aurora partially coincided with the auroral oval and intruded farther poleward into the polar cap, and it showed longitudinal motions in response to IMF $B-y$ variation. Intense magnetosheath-like electron and ion precipitations have been simultaneously detected by DMSP above the poleward portion of the high-latitude dayside aurora. They resemble the typical plasmas observed in the low-altitude cusp. However, less intense electrons and more intense energetic ions were detected over the equatorward part of the aurora. These plasmas are closer to the low-latitude boundary layer (LLBL) plasmas. Under strongly northward IMF, global ionospheric convection derived from SuperDARN radar measurements showed a 4-cell pattern with sunward convection in the middle of the dayside polar cap and the dayside aurora corresponded to two different convection cells. This result further supports two source regions for the aurora. The cusp proton aurora is on open magnetic field lines convecting sunward whereas the LLBL proton aurora is on closed field lines convecting antisunward. These IMAGE, DMSP and SuperDARN observations reveal the structure and dynamics of the aurora and provide strong evidence for magnetic merging occurring at the high-latitude magnetopause poleward from the cusp. This merging process was very likely quasi-stationary.

Chang, S.↗