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Ayres, Thomas R.

Publications and source records attributed to Ayres, Thomas R..

35 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Visualization techniques to aid in the analysis of multi-spectral astrophysical data sets

The goal of this project was to support the scientific analysis of multi-spectral astrophysical data by means of scientific visualization. Scientific visualization offers its greatest value if it is not used as a method separate or alternative to other data analysis methods but rather in addition to these methods. Together with quantitative analysis of data, such as offered by statistical analysis, image or signal processing, visualization attempts to explore all information inherent in astrophysical data in the most effective way. Data visualization is one aspect of data analysis. Our taxonomy as developed in Section 2 includes identification and access to existing information, preprocessing and quantitative analysis of data, visual representation and the user interface as major components to the software environment of astrophysical data analysis. In pursuing our goal to provide methods and tools for scientific visualization of multi-spectral astrophysical data, we therefore looked at scientific data analysis as one whole process, adding visualization tools to an already existing environment and integrating the various components that define a scientific data analysis environment. As long as the software development process of each component is separate from all other components, users of data analysis software are constantly interrupted in their scientific work in order to convert from one data format to another, or to move from one storage medium to another, or to switch from one user interface to another. We also took an in-depth look at scientific visualization and its underlying concepts, current visualization systems, their contributions, and their shortcomings. The role of data visualization is to stimulate mental processes different from quantitative data analysis, such as the perception of spatial relationships or the discovery of patterns or anomalies while browsing through large data sets. Visualization often leads to an intuitive understanding of the meaning of data values and their relationships by sacrificing accuracy in interpreting the data values. In order to be accurate in the interpretation, data values need to be measured, computed on, and compared to theoretical or empirical models (quantitative analysis). If visualization software hampers quantitative analysis (which happens with some commercial visualization products), its use is greatly diminished for astrophysical data analysis. The software system STAR (Scientific Toolkit for Astrophysical Research) was developed as a prototype during the course of the project to better understand the pragmatic concerns raised in the project. STAR led to a better understanding on the importance of collaboration between astrophysicists and computer scientists.

Brugel, Edward W.↗
NASA NTRS1993

Signal-to-noise ratios in IUE low-dispersion spectra. II - Photometrically corrected images

Photometrically corrected images from the IUE's two intensified vidicon cameras are used to explore the character of detector noise, and a protocol for the derivation of realistic noise models is proposed on the basis of available collections of UV-Flood calibration images. The incomplete removal of the pixel-to-pixel sensitivity pattern can lead to a factor-of-2 enhancement in apparent noise; even with good suppression of pixel granularity, the remaining random noise can exhibit saturation behavior which causes S/N to cease improving with increasing exposure. When all relevant effects are considered, underlying, 'pristine' noise models show virtually no dependence on spatial position.

Ayres, Thomas R.↗
NASA NTRS1993

Goddard high-resolution spectrograph observations of the local interstellar medium and the deuterium/hydrogen ratio along the line of sight toward Capella

HST Goddard High-Resolution Spectrograph observations of the 1216, 2600, and 2800 A spectral regions are analyzed for the spectroscopic binary system Capella, obtained at orbital phase 0.26 with 3.27-3.57 km/s resolution and high SNR. The column densities of H I, D I, Mg II, and Fe II for the local interstellar medium along this 12.5 pc line of sight, together with estimates of the temperature and turbulent velocity are inferred. It is inferred that the atomic deuterium/hydrogen ratio by number is 1.65(+0.07, -0.18) x 10 exp -5 for this line of sight. Galactic evolution calculations indicate that the primordial D/H ratio probably lies in the range of (1.5-3) x (D/H)LISM. If H0 = 80 km/s Mpc, as recent evidence suggests, then the baryonic density in units of the Einstein-de Sitter closure density is 0.023-0.031. Thus the universe is argued to expand forever, unless nonbaryonic matter greatly exceeds the amount of baryonic matter.

Linsky, Jeffrey L.↗
NASA NTRS1993

The hydrogen Lyman-alpha emission of Capella

We describe the hydrogen Ly-alpha emission of the spectroscopic binary Capella (G8 III + GO III) recorded at 0.1 A resolution by the International Ultraviolet Explorer. The overt changes in the composite line shape with orbital phase are controlled by the active GO III star and permit a dissection of the stellar components despite the obliteration of the central portion of the profile by atomic hydrogen and deuterium absorption along the 12.5 pc sightline. The Ly-alpha line shape of the active GO III star is surprisingly asymmetric and possibly is variable. Both characteristics suggest a stellar wind of moderate excitation (20,000-100,000 K), a key component of the coronal evolution scenario of Simon and Drake (1989) for the Hertzsprung-gap giants.

Ayres, Thomas R.↗
NASA NTRS1992

Extreme ultraviolet spectroscopy of Capella

The X-ray active binary system Capella was observed with a moderate-resolution extreme ultraviolet spectrograph from 200 to 330 A. Two low-level emission features were detected. One most likely is geocoronal He II 304 A emission, while the other probably originates from the corona of Capella. The weak stellar emission at 304 A is in direct conflict with predictions of the intrinsic stellar He II flux based on standard scaling arguments but is consistent with the only previous observation of Capella in the EUV. The most plausible explanation for the lack of stellar 304 A emission is a warm wind from the active G0 III star.

Green, James C.↗
NASA NTRS1991

Digging in the coronal graveyard - A Rosat observation of the red giant Arcturus

A deep exposure of the bright star Arcturus (Alpha Bootis: K1 III) with the Roentgensatellit (Rosat) failed to detect soft X-ray emission from the archetype 'noncoronal' red giant. The 3-sigma upper limit in the energy band 0.1-2.4 keV corresponds to an X-ray luminosity of less than 3 x 10 to the 25th erg/s, equivalent to a coronal surface flux density of less than 0.0001 solar. The nondetection safely eliminates coronal irradiation as a possible mechanism to produce the highly variable He I 10830 feature and emphasizes the sharp decline in solarlike coronal activity that accompanies the evolution of low-mass single stars away from the main sequence. While the most conspicuous object in the Rosat field of view was not visible in X-rays, at least one fainter star is among the about 60 sources recorded: the Sigma Sct variable CN Boo, an A8 giant in the UMa Stream.

Ayres, Thomas R.↗
NASA NTRS1991

The many faces of F stars - A rotational modulation study of Capella, Procyon, and Caph with the International Ultraviolet Explorer

Far-UV spectroscopy of the F stars Capella, Procyon, and Caph is reported. Although all three stars are regard as chromospherically 'active', none exhibited any sensible rotational modulations, indicating a smooth surface distribution of chromospheric emission. Balancing this nonsolar behavior are a variety of puzzling differences among the three stars. On the one hand, the C IV features of Procyon are narrow, while that of the Capella secondary and Caph are broad; differences in v sin i alone cannot explain the dichotomy. On the other hand, the high-excitation emissions of the Capella secondary and Procyon, but not Caph, exhibit statistically significant redshifts. The significance of these results is discussed in the light of the speculation by Simon and Drake (1989) that the X-ray deficient coronae of the early F dwarfs are produced by classical acoustic heating.

Ayres, Thomas R.↗
NASA NTRS1991

CO fundamental bands in late-type stars. II - Spectrum simulations for F-K stars

The procedure of Ayres and Wiedemann (1989) was applied to a range of stellar-atmosphere models to study the CO Delta-v = 1 spectrum and to establish its use as a remote sensor of thermal conditions in late-type stars. Spectra were computed to examine the sensitivity of the CO Delta-v = 1 to fundamental stellar parameters and to assess the errors introduced into the spectrum intepretation by uncertain input parameters and non-LTE effects. Results of the sensitivity study demonstrate that CO fundamental spectra are useful probes for the temperature structure of the outer layers of cool stellar atmospheres, but that their value is limited by the uncertainties introduced by non-LTE effects. However, in stars with surface gravities of log g of about 1.5 and greater, the values of these uncertainties are reasonably small.

Wiedemann, Guenter↗
NASA NTRS1990

Signal-to-noise ratios in IUE SWP-LO spectra of chromospheric emission-line sources

The short-wavelength-prime (SWP) detector of the International Ultraviolet Explorer should operate near the photon-counting limit, but the noise levels in flat-field images are several times higher. The exaggerated noise can be traced to the incomplete removal of the pixel-to-pixel granularity of the television frames by the prevailing spectral image processing system. An empirical noise model for the current-epoch photometric linearization strategy and one for a hypothetical processing system that achieves complete flat fielding of the raw images are derived. A formula is then proposed to predict the signal-to-noise ratio in the measured flux of an emission line (possibly superimposed on a smooth continuum) in an IUE low-dispersion (5 A resolution) far-ultraviolet (1150 A-1950 A) spectrum as recorded with the SWP camera. For illustration, the formula is specialized to the important C IV 1549 A feature of F-K stars. The S/N relation permits one to determine sensitivity limits, upper limits in faint exposures, and optimum exposure times.

Ayres, Thomas R.↗
NASA NTRS1990

CO fundamental lines - Indicators for inhomogeneous atmospheres in cool stars

Carbon monoxide fundamental lines near 4.7 microns are employed to probe the thermal structure of the atmospheres of cool stars. A new non-LTE radiation transfer code is used to analyze high-resolution infrared CO line spectra and derive observation-based stellar atmosphere models. The main results are: (1) the CO-based models developed here deviate strongly from previously published models based on UV/visible observations; (2) varying degrees of agreement between the CO empirical models and predictions based on theoretical radiative-equilibrium atmosphere models are found; and (3) the parameter used to quantify this agreement is anticorrelated with the magnitude of chromospheric activity in the observed stars. These results suggest thermally bifurcated upper atmospheres as the standard case for cool stars.

Wiedemann, Guenter↗
NASA NTRS1989

Non-LTE CO, revisited

A more extensive and detailed non-LTE simulation of the Delta v = 1 bands of CO than attempted previously is reported. The equations of statistical equilibrium are formulated for a model molecule containing 10 bound vibrational levels, each split into 121 rotational substates and connected by more than 1000 radiative transitions. Solutions are obtained for self-consistent populations and radiation fields by iterative application of the 'Lambda-operator' to an initial LTE distribution. The formalism is used to illustrate models of the sun and Arcturus. For the sun, negligible departures from LTE are found in either a theoretical radiative-equilibrium photosphere with outwardly falling temperatures in its highest layers or in a semiempirical hot chromosphere that reproduces the spatially averaged emission cores of Ca II H and K. The simulations demonstrate that the puzzling 'cool cores' of the CO Delta V = 1 bands observed in limb spectra of the sun and in flux spectra of Arcturus cannot be explained simply by non-LTE scattering effects.

Ayres, Thomas R.↗
NASA NTRS1989

How deep can one see into the sun?

Conventional wisdom dictates that the 1.642-micron H(-) opacity minimum is the best window to the depths of the solar photosphere. However, the violet continuum near 0.4 micron exhibits a larger intensity response to small thermal perturbations at depth, and thus might offer an even better view of the subsurface roots of granulation cells and magnetic flux tubes.

Ayres, Thomas R.↗
NASA NTRS1988

Empirical continuum modeling for IUE low-dispersion spectra

A numerical procedure for empirically determining a 'continuum' level in low-dispersion spectra from the IUE is described. The technique was developed for the automated fitting of SWP-LO (1150 A - 2000 A) spectra of stars of late spectral type, but it is more widely applicable.

Bennett, Jeffrey O.↗
NASA NTRS1988

A spectral dissection of the ultraviolet emissions of Capella

Phase-resolved observations of Capella were conducted with the IUE to separate the spectrum of the core-helium-burning primary from that of the fast-rotating, chromospherically active secondary. Characteristics of the primary and secondary spectra are presented. The UV emissions of the primary are compared to those of the four K0 giants of the Hyades, and the field K giants Beta Cet and Beta Gem. The Capella primary is found to be similar to the two active Hyads, Gamma Tauri and Theta Tauri, suggesting that if Capella is coeval with the Hyades nucleus, the age of the cluster might be less than half that commonly assumed.

Ayres, Thomas R.↗
NASA NTRS1988

Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars. II - New, precise measurements of dwarfs and giants

Results are presented from an IUE SWP camera investigation of the occurrence of gasdynamic flows, analogous to the downdrafts of 10 to the 5th K material observed over magnetic active regions of the sun, among stars of late spectral type. The SWP calibration spectra study conducted documents the existence of local, small, persistent distortions of the echelle wavelength scales that are of unknown origin. Attention is given to the enormous widths of the stellar high-excitation emission lines in both the dwarfs and the giants, with respect to the comparatively small, subsonic Doppler shifts; the widths are typically an order of magnitude greater than the redshifts.

Ayres, Thomas R.↗
NASA NTRS1987

Far-ultraviolet observations of the Delta-Scuti variable Beta Cassiopeiae

FUV (115-200-nm) observations of Beta Cas, obtained at resolutions 5 and 0.15 A by the IUE during September 1984, are reported and analyzed. The data are presented graphically and compared with results for Procyon, a sharp-lined F star similar in temperature to Beta Cas. A sharp factor-of-two increase in H I Ly-alpha flux on a time scale comparable to its Delta Sct pulsation period was observed, but the data were insufficient to determine whether the increase was flarelike or pulsation-related. The outer edges of the Beta Cas emission are shown to be significantly wider than those of Procyon.

Ayres, Thomas R.↗