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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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At least 181 records · Page 10
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NASA NTRS1988

The age of the galactic disk

The galactic disk is a dissipative structure and must, therefore be younger than the halo if galaxy formation generally proceeds by collapse. Just how much younger the oldest stars in the galactic disk are than the oldest halo stars remains an open question. A fast collapse (on a time scale no longer than the rotation period of the extended protogalaxy) permits an age gap of the order of approximately 10 to the 9th power years. A slow collapse, governed by the cooling rate of the partially pressure supported falling gas that formed into what is now the thick stellar disk, permits a longer age gap, claimed by some to be as long as 6 Gyr. Early methods of age dating the oldest components of the disk contain implicit assumptions concerning the details of the age-metallicity relation for stars in the solar neighborhood. The discovery that this relation for open clusters outside the solar circle is different that in the solar neighborhood (Geisler 1987), complicates the earlier arguments. The oldest stars in the galactic disk are at least as old as NGC 188. The new data by Janes on NGC 6791, shown first at this conference, suggest a disk age of at least 12.5 Gyr, as do data near the main sequence termination point of metal rich, high proper motion stars of low orbital eccentricity. Hence, a case can still be made that the oldest part of the galactic thick disk is similar in age to the halo globular clusters, if their ages are the same as 47 Tuc.

Sandage, Allan↗
NASA NTRS1989

Optical interferometers for tests of relativistic gravity in space

A space-based astrometric interferometer with a large optical bandwidth is considered. POINTS (Precision Optical INTerferometry in Space) would measure the angular separation of two stars separated by about 90 deg on the sky with a nominal measurement error of 5 microarcseconds (mas). For a pair of mag 10 stars, the observation would require about 10 minutes. It is estimated that the instrument would measure daily the separation of two stars for each of about 60 pairs of stars; a random sequence of such measurements, if suitably redundant, contains the closure information necessary to detect and correct time dependent measurement biases to well below the nominal measurement accuracy. The 90 deg target separation permits absolute parallax measurements in all directions. A redundant observing schedule for 300 stars and 5 quasars would provide extra redundancy to compensate for the quasars' higher magnitude. If a nominal 30-day observation sequence were repeated 4 times per year for 10 years, stellar parameter uncertainties would be obtained of: 0.6 mas, position; 0.4 mas/y, proper motion; and 0.4 mas, parallax. This set of well-observed stars and quasars would form a rigid frame and the stars would serve as reference objects for measurements of all additional targets, as well as being targets of direct scientific interest. The instrument global data analysis since objectives are considered including a relativity test and technology.

Reasenberg, R. D.↗
NASA NTRS1989

Discovery of low mass objects in Taurus

In infrared (2.2 micron, K-band) search of small regions (25 in square) near 26 members of the Taurus star-forming association has revealed 20 dim (K = 13-16 mag) stellar objects near 13 of them. Of these 20 objects, 9 are exceptionally red. It is argued that these 9 are probably also Taurus members. From the luminosities (0.4 to 4 times 10 the -3 power luminosity) and ages (estimated at 10(exp 6) years), masses can be determined by reference to theoretical low-mass cooling curves. The masses are in the range 0.005 to 0.015 solar mass, i.e., low-mass brown dwarfs. Proper motion studies of 7 of the objects visible on the POSS plates conducted by Burton Jones establish that 4 are highly probable Taurus members while 1 is a possible member.

Forrest, W. J.↗
NASA NTRS1989

Pulsar reenergization of old supernova remnant shells

The morphology of several unusual composite remnants are suggested to be affected by previously unrecognized interactions between high-velocity pulsars and old SNR shells, and the case of CTB 80 is pointed out as a likely example of such interactions. The interactions generate a new class of 'composite remnants' and furnish a novel method for the derivation of kinematic distances and SNR ages; this technique is noted to be especially useful when the pulsar has a measured spindown age or proper motion. It is predicted that a number of pulsars may interact with 80-100 pc radius 'superbubbles' produced by the combined action of winds and supernovae in OB associations.

Shull, J. Michael↗
NASA NTRS1990

Cometary compact H II regions are stellar-wind bow shocks

Comet-shaped H II regions, like G34.3 + 0.2, are easily explained as bow shocks created by wind-blowing massive stars moving supersonically through molecular clouds. The required velocities of the stars through dense clumps are less than about 10 km/s, comparable to the velocity dispersion of stars in OB associations. An analytic model of bow shocks matches the gross characteristics seen in the radio continuum and the velocity structure inferred from hydrogen recombination and molecular line observations. The champagne flow model cannot account for these structures. VLBI observations of masers associated with the shells of cometary compact H II regions should reveal tailward proper motions predominantly parallel to the shell, rather than perpendicular. It is predicted that over a decade baseline, high signal-to-noise VLA observations of this class of objects will show headward pattern motion in the direction of the symmetry axis, but not expansion. Finally, shock-generated and coronal infrared lines are also predicted.

Van Buren, Dave↗
NASA NTRS1991

Aiming Schedule For Orbiting Astrometric Telescope

Report discusses schedule of observation times and aiming directions of proposed astrometric telescope facility mounted on Space Station in orbit around Earth. Primarily makes repeated observations of apparent positions of each of 127 selected stars with respect to distant reference stars in same field of view known to have unperturbed proper motions. Purpose of observations to detect motions of selected stars about centers of mass and to analyze harmonic content of motions to infer existence, number, masses, and orbital radii of planets.

Mascy, Alfred C.↗
NASA NTRS1990

First images of water vapor masers in the galaxy M33

The first successful VLBI observations of 10 extragalactic H2O masers in the M33 galaxy are reported. A spectral-line VLBI synthesis map is constructed, the first of any extragalactic H2O maser sources. The map has the lowest noise of any K-band spectral line VLBI map yet produced. The maser emission extends over about 300 milliarcseconds and is divided into 14 distinct spatial components, the strongest of which has a correlated flux density of about 0.7 Jy. The relative positions of some of these components are determined accurately enough to provide first-epoch measurements for proper motion studies. The characteristics of the maser are similar to those of the most powerful maser in the Galaxy, W49N. A compact H II region is found close to the maser which is 1 pc in diameter and whose emission measure is about 6 x 10 to the 7th pc/cm exp 6. This region is the compact component of a more extended H II complex that extends over about 100 pc.

Greenhill, L. J.↗
NASA NTRS1990

POINTS - A global reference frame opportunity

POINTS is a space-based optical astrometric interferometer capable of measuring the angular separation of two stars about 90 degrees apart with 5-microarcsec nominal accuracy . During the intended ten-year mission, a repeated survey of a few hundred targets over the whole sky, including a few bright quasars, establish a 'rigid' reference grid with 0.5 microarcsec position uncertainties. At that level, the grid is free of regional biases and tied to the extra-Galactic frame that is the present best candidate for an inertial frame. POINTS will also determine parallaxes and annual proper motions at about the same level. Further, the planetary ephemeris frame is tied through stellar aberration to the grid at about 300 microarcsec. Additional targets of interest, to a limiting magnitude of greater than 20, are observed relative to the grid, yielding determinations with uncertainties depending on the observing schedule. Measurement at the microarcsec/year level of the apparent relative velocities of quasars that are widely separated on the sky severely test the assumption of cosmological quasar distances and may also constrain models of the early universe.

Chandler, J. F.↗
NASA NTRS1990

Lunar optical interferometer

Various design configurations of a lunar optical interferometer are examined to determine the interaction of science objectives and the architecture of an instrument that will have 10 microarcsec imaging resolution and 0.1 microarcsec astrometric accuracy. The imaging of the central energy source of AGNs, the discovery of earth-like planets around nearby stars, and the measurement of the proper motion of extragalactic objects to determine the isotropy of the Hubble expansion are some of the observations possible with a lunar interferometer. This interferometer with kilometric baselines and large collecting area will be able to make astronomical observations with angular resolution and sensitivity many orders of magnitude beyond orbital based or earth based instruments.

Shao, M.↗
NASA NTRS1991

High-precision timing of millisecond pulsars. II - Astrometry, orbital evolution, and eclipses of PSR 1957 + 20

Pulse time-of-arrival measurements have been acquired for PSR 1957 + 20 on 61 days over a 2.7 yr interval, including the times of 44 eclipse disappearance or reappearance events. The timing data provide a reliable measurement of the pulsar's proper motion, 29 +/- 3 mas/yr, at position angle 216 +/- 6 deg; this direction is in excellent agreement with that suggested by the cometlike shape of a surrounding optical emission nebula. The rate of change of the orbital period has been measured: a surprisingly large (-3.9 +/- 0.9) x 10 exp -11, implying that significant orbital evolution must take place on a time scale of 30 Myr. Finally, the present observations extend the eclipse measurements to higher frequencies and provide better statistics for these highly variable phenomena. The eclipse data are used to compute a column-density profile of free electrons in the outer parts of the eclipsing region, and thereby provide an important glimpse into the material being ablated from the companion star.

Ryba, M. F.↗
NASA NTRS1992

Gravitational lensing by a massive black hole at the Galactic center

The manifestations of gravitational lensing by a massive black hole at the Galactic center, with particular attention given to lensing of stars in the stellar cluster that lie behind Sgr A*, and of Sgr A east, a nonthermal extended radio source which is known with certainty to lie behind the Galactic center. Lensing of the stellar cluster produces a deficit of stellar images within 10 mas of the center, and a surplus between 30 and 300 mas. The results suggest that the proper motion of the stars will produce brightness variations of stellar images on a time scale of a few years or less. Both images of such a source should be visible, and will rise and fall in luminosity together.

Wardle, Mark↗
NASA NTRS1992

Young globular clusters in the Milky Way Galaxy

Ruprecht 106 and Pal 12 are two known globular clusters in the Milky Way that are unequivocally younger than other clusters of similar metallicity. The Galactic coordinates of Ruprecht 106 place it near to the Magellanic Stream in projection, suggesting a tidal capture from the Magellanic Clouds. It is demonstrated that a family of orbits for both clusters can be constructed that are consistent with this capture hypothesis and that these then lead to a prediction of 3 milliarcseconds per year for the proper motions of both Ruprecht 106 and Pal 12.

Lin, D. N. C.↗
NASA NTRS1991

High-resolution observations of the solar granulation

Digitally-enhanced high-resolution images from space and ground observatories have yielded data on the solar granulation which show it to be a more complex combination of phenomena than previously suspected. These new data have been used to detect large-scale horizontal flows over the solar surface through direct measurements of granule proper motions. The results thus obtained may have important implications for the heating of the solar chromosphere and corona. The observed intensity pattern on the surface of the sun is noted to be due not only to granulation, but the superposition of different types of waves as well. All such phenomena are modified in the presence of magnetic fields, becoming smaller and of reduced contrast.

Topka, K. P.↗
NASA NTRS1992

Water masers in W49N - The youngest stellar jet?

Observations by Gwinn et al. (1992) of the proper motions of water masers in W49N show that they have an elongated distribution expanding from a common center. Features with high space velocity only occur far from the center, while low-velocity features occur at all distances. It is proposed that water masers in star-forming regions occur in expanding shells swept up by high-velocity winds from young, massive stars during the early phases of the expansion. In W49N, confinement of the bubble by a density distribution with an axial cavity can explain both the velocity field and the shape of the maser distribution. A fully dynamical calculation of the expanding bubble is presented which provides a satisfactory fit for the observations and suggests that this system is only about 250 yr old. Thus these observations may show the very first stages of the formation of a jet from a young stellar object.

Mac Low, MORDECAI-M.↗
NASA NTRS1993

Survey incompleteness and the evolution of the QSO luminosity function

We concentrate on a type of QSO survey which depends on selecting QSO candidates based on combinations of colors. Since QSO's have emission lines and power-law continua, they are expected to yield broadband colors unlike those of stellar photospheres. Previously, the fraction of QSO's expected to be hiding (unselected) within the locus of stellar (U-J, J-F) colors was estimated at about 15 percent. We have now verified that the KK88 survey is at least 11 percent incomplete, but have determined that it may be as much as 34 percent incomplete. The 'missing' QSO's are expected to be predominantly at z less than or = 2.2. We have studied the proper motion and variability properties of all stellar objects with J less than or = 22.5 or F less than or = 21.5 in the SA 57 field which has previously been surveyed with a multicolor QSO search by KK88.

Majewski, Steven R.↗
NASA NTRS1992

The synchrotron bays of the Crab Nebula - A magnetic structure associated with a presupernova circumstellar disk

We investigate the nature of two large conical indentations in the Crab Nebula's synchrotron emission, commonly known as the east and west 'bays'. Optical images spanning nearly nine decades (1899-1988) reveal the bays to be long-lasting remnant features. Proper motions of the bays away from the remnant's center of expansion are consistent with their locations near the SN progenitor site in A.D. 1054. Based on their morphologies and polarization properties, the bays appear to be portions of a torus encircling the remnant's center of expansion. The nebular magnetic field wrapped around this torus blocks the pulsar's relativistic particles, resulting in the observed lack of synchrotron emission. We propose that this torus is the result of a presupernova disklike ejection not less than 0.1 solar mass by the Crab progenitor, similar to the ring around SN 1987 A. A 0.5 solar mass disk of He-rich filaments along the equatorial plane is probably also related to presupernova mass loss, which might explain the wide range of abundances seen among filaments.

Fesen, Robert A.↗
NASA NTRS1992

Carbon star luminosity indicators

Results are presented of a proper motion survey of known faint high-latitude C stars using plates spanning a 30-yr baseline that reveals at least one additional new dwarf C star and also confirms the utility of the proposed luminosity indicators. Kinematic simulations suggest that to a limit of V of about 18, at least 10 percent of faint high-latitude C stars are dwarfs. The local space density of dwarf C stars may thus substantially exceed that of luminous C stars.

Green, Paul J.↗
NASA NTRS1992

High-precision VLBI astrometry of the radio-emitting star Sigma CrB - A step in linking the Hipparcos and extragalactic reference frames

VLBI observations of the optically bright radio-emitting star Sigma Coronae Borealis (Sigma CrB) have yielded its position relative to an angularly nearby quasar, its annual proper motion and its trigonometric parallax with formal uncertainties slightly better than 0.2 milliarcsec. This is the first result of the VLBI astrometric program we are conducting on 11 radio stars similar to Sigma CrB. We plan to obtain comparable results for all stars when enough epochs of observations are acquired. These stars will be used to astrometrically link the Hipparcos and VLBI extragalactic reference frames.

Lestrade, Jean-Francois↗