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At least 523 records · Page 29
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NASA NTRS1982

Line profiles of T Tauri stars - Clues to the nature of the mass flow

A set of high resolution observations of the H-alpha Na D, He I 5876 A, and H-gamma lines in several T Tauri stars is presented. Repeated observations of some of these objects yield some indication of the importance of time-dependent effects. The line profiles should be useful for detailed radiative transfer modeling. A cursory survey of the profiles suggests that 'turbulent' velocities in some cases may not be negligible in comparison with the bulk flow speeds and that many emission lines, such as Na D, may be formed in a narrow layer near the stellar photosphere. It is suggested that the effects of geometrical occulation of narrow line-emitting regions in general outflow can account for some of the peculiarities of the line profiles. However, some aspects of the line profiles, such as the large-scale, highly variable redshifted Na D absorption in RW Aur, clearly require more complicated motions.

Hartmann, L.↗
NASA NTRS1981

Observations and theory of mass loss in late-type stars

The presented review is mainly concerned with the ubiquitous mass loss which occurs during most of a star's existence as a cool giant or supergiant. Observations of mass loss are considered, taking into account wind components and kinematics, and the temperature structure of cool winds. Theories of mass loss are examined, giving attention to radiation pressure on dust, radiation pressure in Lyman alpha, and magnetic wave-driven winds. It is pointed out that the study of mass loss from late-type stars appears to be entering a promising new phase. In this phase, the behavior of cool giants and supergiants is considered from a solar perspective, a perspective which contains important implications concerning the nature of solar activity.

Hartmann, L.↗
NASA NTRS1981

Discovery of multi-ring basins - Gestalt perception in planetary science

Early selenographers resolved individual structural components of multi-ring basin systems but missed the underlying large-scale multi-ring basin patterns. The recognition of multi-ring basins as a general class of planetary features can be divided into five steps. Gilbert (1893) took a first step in recognizing radial 'sculpture' around the Imbrium basin system. Several writers through the 1940's rediscovered the radial sculpture and extended this concept by describing concentric rings around several circular maria. Some reminiscences are given about the fourth step - discovery of the Orientale basin and other basin systems by rectified lunar photography at the University of Arizona in 1961-62. Multi-ring basins remained a lunar phenomenon until the fifth step - discovery of similar systems of features on other planets, such as Mars (1972), Mercury (1974), and possibly Callisto and Ganymede (1979). This sequence is an example of gestalt recognition whose implications for scientific research are discussed.

Hartmann, W. K.↗
NASA NTRS1982

Protostellar mass and angular momentum loss

Recent radio observations have indicated that appreciable, continuing mass loss is occurring from regions of star formation. The conditions under which massive protostellar objects may exhibit strong, rotationally driven winds are investigated. For plausible rotational velocities and magnetic field strengths, ejection of about 10,000 solar masses a year at speeds of 10-100 km/s can be maintained for time scales of about 0.0001 yr; these values are insensitive to the protostellar luminosity. An evolutionary scenario is sketched which suggests that protostellar clouds may be expected to be rapidly rotating and to possess substantial magnetic fields when the clouds have contracted to radii of about 10 to the 14th cm. The centrifugally driven winds proposed present an alternative to fragmentation for providing the angular momentum loss required in the phase of rapid cloud contraction.

Hartmann, L.↗
NASA NTRS1982

Advanced technologies for turbomachinery systems: An overview

Turbomachinery system components and associated technologies are discussed. Specific technologies reviewed include the compressor, turbine, internal flow analysis methods, combustion, fuels, materials, structures, bearings, seals, and lubrication, dynamics and controls, and instrumentation. Analytical procedures as a path to improved performance are discussed. The strong interaction between the various technologies if turbomachinery performance gains are to be realized is reflected.

Hartmann, M. J.↗
NASA NTRS1982

Advanced flight control system study

A fly by wire flight control system architecture designed for high reliability includes spare sensor and computer elements to permit safe dispatch with failed elements, thereby reducing unscheduled maintenance. A methodology capable of demonstrating that the architecture does achieve the predicted performance characteristics consists of a hierarchy of activities ranging from analytical calculations of system reliability and formal methods of software verification to iron bird testing followed by flight evaluation. Interfacing this architecture to the Lockheed S-3A aircraft for flight test is discussed. This testbed vehicle can be expanded to support flight experiments in advanced aerodynamics, electromechanical actuators, secondary power systems, flight management, new displays, and air traffic control concepts.

Hartmann, G. L.↗
NASA NTRS1982

Remote comets and related bodies - VJHK colorimetry and surface materials

VJHK colors for a number of asteroids and eight comets at various solar distances and levels of activity were obtained, and the observations are interpreted in terms of a two-component mixing model in which outer solar system interplanetary bodies are viewed as mixtures of ice and dark carbonaceous-type (RD and C) dirt. It is inferred that the observed comets have comae, and perhaps surfaces, of dirty ice or ice dirt grains colored by an RD-dirt component. This inference is supported by systematics of an 'alpha index' based on VJHK colors and empirically correlated with albedo and ice/dirt ratio. Among comets the alpha index correlates with solar distance in a way that suggests comets emit dirty ice grains which are stable at large solar distance but from which the ice component sublimes and leaves dirt grains at small solar distance.

Hartmann, W. K.↗
NASA NTRS1983

Constraints on the inclination and masses of the HDE 226868/Cygnus X-1 system from the observations

Analyses of two high-dispersions IUE spectra of HDE 226868 (the optical counterpart of Cygnus X-1) are combined with studies of low-dispersion IUE spectra to provide a more accurate determination of the variation of C IV stellar wind absorption as a function of orbital phase. By incorporating these observational results into an analysis of the structure of the X-ray ionization cavity in the stellar wind, it is found that the orbital inclination must lie between 36 and 67 deg, leading to a mass for the compact object between 5.7 and 11.2 solar masses.

Davis, R.↗
NASA NTRS1983

Chromospheric and coronal emissions from the giants in the Hyades

The visible Ca II K and International Ultraviolet Explorer spectra from the four K0 III stars (theta-1, gamma, delta, and epsilon Tau) in the Hyades, along with a field giant of similar spectral type, beta Gem, have been measured. Among the Hyades giants, the range of the high-temperature emissions can be a factor of 6 or more for C IV and an order of magnitude in the X-ray luminosity measured by Stern et al. (1981). For these presumably coeval cluster giants with similar macroscopic parameters, such as age, mass, effective temperature, gravity, and projected rotational velocity, the observed range in chromospheric and coronal emissions is not easily explained by the dominant factors thought to control these emissions in cool stars. It is possible that the emissions are time variable, on a time scale longer than six months. This would be the first evidence that giants undergo magnetic activity cycles similar to those of dwarf stars.

Baliunas, S. L.↗
NASA NTRS1984

Aeronautical propulsion: Present status and future directions

The advancement of aeropropulsion systems continues to provide technology to various portions of the gas turbine field. It is recognized that this area is undergoing considerale change, which will result in substantially improved gas turbine components and systems. These changes are occurring in a number of technical areas including advanced analytical and physical measurement methods, the application of large scientific computers, the dynamic modeling of components and systems, the application of integrated control systems that optimize and improve performance and system condition monitoring, and the development of new and unique materials and structures. As these areas evolve, the ways in which technology will advance, and factors affecting the design and development of new systems, will probably be considerably different than those of today. It is also anticipated that the necessary skilled work force will be different. Certainly there will be changes, but the nature, extent, and rate of those changes can only be surmised at this time.

Hartmann, M. J.↗
NASA NTRS1983

Combined ultraviolet studies of astronomical sources

Ultraviolet spectra of non-radiataive shock waves grain destruction and elemental abundances in interstellar shocks, carbon abundance in M33 and M31 from supernova remnants, determination of the mass function in the large Magellanic cloud, UV spectra of white dwarf pulsars, stellar flares, availability of the double quasar Q 0957 + 56' AB, spectra of late-F dwarfs and their relation to rotation, dynamics of hot gas surrounding hybrid stars, high resolution study of epsilon coronae Austriual, active regions on solar-type dwarfs as a function of rotation rate and age, coordinated chromospheric synoptic observations of selected late-type stars, the two-component atmosphere of Lambda Andromedae, and activity in Hyades giants are discussed.

Dupree, A. K.↗
NASA NTRS1984

Low frequency variability and tropical-extratropical interactions

In this investigation of relationships between mid-latitude 500 mb anomalies and tropical heating anomalies on the time scales of weeks to seasons are studied using outgoing IR to characterize the tropical convection heating. The data are first used to form 5-day means and the mean seasonal cycle and the interannual variability of seasonal means are then removed. The time scales considered are then between about 10 days and 90 days. By correlating the mid-latitude height anomalies with the tropical precipitation anomalies it can be shown that, on the time scales considered here, the predominant correlation patterns are those associated with Rossby waves propagating from mid-latitudes into the tropics and influencing the tropical convection. The mid-latitude Rossby waves can significantly influence convection to within 5 degrees of the equator.

Hartmann, D. L.↗
NASA NTRS1984

Does saturation cratering exist in the solar system?

A number of researchers have concluded that saturation equilibrium cratering exists nowhere in the solar system, and therefore that diameter distributions in even the most heavily cratered provinces reveal initial production functions related to impacting bodies. Based on this premise, different populations of impactors are identified in different epochs and regions of the solar system. These hypotheses are clearly crucial to interpreting planetary history and need further independent examination. The production function in the outer solar system may differ from that in the inner solar system, but it is also possible that viscous relaxation of ice craters or immediate flooding of craters that penetrate through an ice lithosphere into watery substrate may explain the greater deficiency of large craters on icy moons. This problem is controversial and needs more study.

Hartmann, W. K.↗
NASA NTRS1984

Study of the system of middle atmosphere-ionosphere using remote-sensing data

In the present investigation, the methods of statistical spectral analysis are employed for a study of the quasi-periodic changes of state parameters of the middle atmosphere and the ionosphere, taking into account oscillations with periods of several days. The considered oscillations are typical for planetary waves. The theory and empirical findings regarding transient planetary waves are utilized as a basis for the concepts employed in the data analysis and for the interpretation of the results. The results of the investigation show a presence of coherent variations in the time series of radiation density measurements and other state parameters for the middle atmosphere and the ionosphere. The existence of presumably dynamic coupling processes can be recognized in oscillations with periods of about 16 or 5 days.

Hass, H.↗
NASA NTRS1984

Ultraviolet and optical observations of metal deficient red giants and chromospheric models

Three metal deficient field stars were observed in the ultraviolet and optical spectral regions: HD 165195, HD 110281, and HD 232078. High dispersion spectra near H alpha, and low dispersion, long wavelength IUE spectra were obtained. The H alpha profiles have strong asymmetric emission with absorption cores that are frequently asymmetric. The surface flux of Mg II lines is similar to that of luminous Pop I stars in spite of the lower metal abundance. Semi-empirical atmospheric models suggest that the characteristic emission in the wings of the H alpha line can arise within static chromospheres. Radial expansion gives an asymmetric, blue-shifted H alpha core accompanied by greater emission in the red line wing than the blue wing. Wind models with extended atmospheres suggest mass loss rates - 2 billion M/yr. Thus H alpha provides no evidence that steady mass loss is substantial enough to significantly affect the evolution of stars on the red giant branch of globular clusters.

Duprele, A. K.↗
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

Star formation in blue galaxies. I - Ultraviolet, optical, and infrared observations of NGC 4214 and NGC 4670

NGC 4214 and NGC 4760 have been observed between 1150 A and 1950 A with the IUE satellite, and new infrared and optical broad-band photometry have been obtained. The spectra of both galaxies are dominated by O stars, and, in the spectrum of NGC 4214, the C IV 1550 A line exhibits a P Cygni profile indicative of hot stars with large mass-loss rates. Although the optical spectra of both galaxies are dominated by strong narrow emission lines, no strong emission lines are seen in the UV (the observed Ly-alpha flux is dominated by geocoronal emission). Both galaxies contain giant H II region complexes in which the energy (in particular, the H-beta luminosity and the ultraviolet continuum) is provided by several hundred O stars. The comparison of the observed UVOIR energy distributions with evolutionary population models indicates that the regions observed consist of young 'bursts' of star formation superposed on an underlying old population. The initial mass function for this epoch of star formation is constrained to be similar in slope to or flatter than the Salpeter function.

Geller, M. J.↗