During the Astro-2 mission in March of this year, the Ultraviolet Imaging Telescope obtained large-scale (40 arcmin diameter) images of many regions in the Large and Small Magellanic Clouds. We present far-UV (1300-1800 A) UIT images of the observed fields, which include the regions 30 Doradus, Shapley Constellation III, N 4, N 11, N 70, N 79, NGC 330, and NGC 346 as well as regions containing field populations in the SMC and LMC bar. We present preliminary photometric results for some OB associations, and discuss how the UIT observations will increase our knowledge of the populations of massive stars in the Magellanic Clouds. The OB associations in these regions are of particular interest considering the advantages of studying massive stars in the far-UV. These hot stars emit most of their flux in the UV (because of their high temperatures and also because of decreased line-blanketing due to the relatively low metallicity of the Clouds), so massive stars are strongly selected in the far-UV and the cooler, low-mass stars are suppressed. The proximity of the Clouds allowed us to achieve photometric completeness in the 30 Doradus region to far-UV magnitude of approximately 14.0 mag (or a mass of about 14 solar masses) for a 473.0 sec exposure during Astro-1; during Astro-2 we obtained exposure times in the Clouds up to three times longer. UV photometry of hundreds of stars allows us to study the unusual extinction curves and far-UV rise in the Clouds.
In July of 1995, under the twin banners of the IHW and the Planetary Data System (PDS), we released to the IHW and astronomical communities the last two discs of a 26-volume set containing ground, airborne, earth-orbital, and in situ data of Halley's Comet and comet P/Giacobini-Zinner. All 26 discs were pre-mastered at the NASA/GSFC National Space Science Data Center (NSSDC), and it was at NASA/GSFC and U. Md. where much of the data/metadata preparation and on-disc document-writing also took place. Volumes 25 and 26 contain exclusively in situ data obtained by the Halley Armada (Giotto, Vega-1 & -2, Suisei, and Sakigake) in March of 1986, and by the International Cometary Explorer (ICE) in September of 1985 (during the encounter with P/G-Z). It is our particular hope that completion of the spacecraft data discs will allow, in addition to stand-alone studies, a fuller understanding of remote data obtained concurrently with the space encounters. Volumes 1-24, which contained over 60,000 remote observations obtained all over the world during 1981-1989 using the complete range of cometary observational techniques and disciplines, were released by the IHW in 1992 (Niedner et al. 1992; Grayzeck et al. 1995). The IHW archiving activity is now completed, and the future of cometary data archiving resides in large part with the Small Bodies Node of the PDS (M. A'Hearn, Node Chief). It is a pleasure, once again, to recognize the outstanding efforts of over 1500 observers worldwide who obtained important Halley and P/G-Z data and submitted them to the IHW for permanent archiving.
References:
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Grayzeck, E. J., Jr., Klinglesmith, D. A. III, Niedner, M. B., Jr., and
A'Hearn, M. F. 1995, "Generation of the IHW Comet Halley CD-ROM Archive,"
Planetary Space Science, 43, 225-238.
Niedner, M. B., Jr., Grayzeck, E. J., Jr., Klinglesmith, D. A. III, Warnock,
A. III, and Aronsson, M. 1992, "Comet Halley Archive: User's Guide to the
Remote Data CD-ROMs, Volumes 1-24"
Characteristics of the DIRBE and FIRAS instruments on NASA's Cosmic Background Explorer (COBE) are summarized. Data products likely to be useful in studies of the interstellar medium are described. The DIRBE sky survey is compared with the IRAS survey, which has been used in many such studies over the past decade.
At a distance of 0.84 Mpc, Messier 33 is the closest galaxy harboring giant HII regions of both low and high metallicity. As such, M33 provides the optimal setting for investigating the effects of metal abundance on star formation at the high-mass end. Moreover, by comparing the resolved and composite properties of the giant HII regions in M33, one can establish an empirical basis for interpreting the composite spectrophotometric properties of more distant starbursting systems. In the past year, we have obtained HST/WFPC2 images of 6 giant HII regions in M33. These multi-band images clearly resolve the underlying stellar populations. Comparison of the resulting photometry with composite measurements at optical and UV wavelengths will be discussed.
The origin of the observed light element abundance anomalies in Population II globular cluster stars remains the subject of much research. Stellar evolution models for the red giant branch (RGB) must invoke some noncanonical mixing mechanism in order to explain the observed anticorrelations in CN and CH and the decrease in both the value of 12C/13C and the overall carbon abundance with increasing luminosity. Such a mixing mechanism is required in order to transport CN processed (and possibly ON processed) material from the hydrogen burning shell outward through the surrounding radiative zone to the convective envelope where it can be rapidly mixed to the surface. It is unclear, however, whether deep mixing can explain the variations observed in other elements. For example, Al and Na have been reported to correlate with CN while Na is anticorrelated with O. These abundance anomalies could be the result of primordial "contamination" by more massive ( 5 Msun) stars that expelled their processed envelopes into the globular cluster environment while the cluster was forming low mass stars. Conversely, as some authors have claimed, deep mixing during the RGB phase could explain these abundance anomalies without replying on primordial effects. We have investigated the latter possibility by using an extensive nuclear reaction network to determine the distribution of the various elemental abundances around the hydrogen shell in a number of stellar evolution models for the RGB phase. In this paper we report the first results of this research. Our models vary in mass from 0.795 Msun to 1.5 Msun as well as in metallicity from Z = 0.0001 to 0.01865. Early indications seem to show that deep mixing can bring certain elements (e.g. Na23) to the surface in low metallicity stars but not in high metallicity stars.
The microwave sky is dominated by relic emission from the cosmic microwave background and local emission from within our Galaxy. The spatial distribution and frequency spectrum of these components probe physical conditions ranging from the early universe to the local interstellar medium. The Diffuse Microwave Emission Survey (DIMES) will measure the frequency spectrum of the diffuse emission at centimeter wavelengths to 0.01% precision, and will map the angular distribution to precision DT/T = 10^{-5} per 6 degree field of view. DIMES will provide quantitative information on processes such as the transition from an ionized universe to a neutral state and the subsequent heating and reionization by the first generation of collapsed objects, the abundance, lifetimes, and decay modes of hypothesized non-baryonic dark matter particles, the heating mechanism and energy balance of the high-latitude interstellar medium, and the acceleration mechanism of cosmic-ray electrons. DIMES has been selected for study under the New Mission Concepts in Astrophysics program; a prototype payload is scheduled to fly from a balloon platform in 1996.
The angular distribution of the cosmic microwave background (CMB) reflects the distribution of matter and energy in the early universe and measures the "initial conditions" of currently-observed large-scale structure in the universe. Topological defect models provide a viable alternative to the standard inflationary cosmology. In these models, the primordial density perturbations result from large-scale ordering effects following a phase transition with broken symmetry. Observational tests of these models are hindered by their nonlinear nature, which precludes exact analytic predictions for the CMB anisotropy. Tests to date have either used time-consuming numerical simulations or approximate analytic models. We use numerical simulations of texture cosmologies to investigate the relationship between ordering dynamics and energy density, and test the validity of assumptions commonly used in approximate analytic methods. We find that the energy density is dominated not by isolated, fully-wound textures, but by the much more numerous partially wound events. We discuss the implications for improved approximate methods.
The IUE Project has enhanced the processing methods and improved the accuracy of the calibrations for IUE data for the production of the IUE Final Archive. The software used to process the data for the present archive (IUESIPS) has undergone several modifications and enhancements since launch, and the archive reflects these discontinuities. In addition, many new techniques for image processing have been developed in the 19 years since the original software was designed. The primary aim of the IUE Project is to create a uniformly processed and fully calibrated and documented spectral dataset that will enable future investigators to compare data from different epochs of the mission with substantially improved accuracy and to use the IUE archives with minimal need for assistance. Production of the IUE Final Archive is well underway, with most of the SWP and LWP low dispersion images completed. The IUE Final Archive processing system has been demonstrated to provide significant improvements in the signal-to-noise ratio of the extracted spectral data for most images in the IUE archive and allow fundamental new scientific analyses from these data These improvements have been achieved by the development of new image processing techniques, more accurate and complete calibration methods, and a uniform production processing environment. Improvement in signal-to-noise of the extracted spectral data has been shown to range from 10-50% for most images, with factors of 2-4 improvement in some cases. Signal-to-noise improvement for high dispersion data processed for the Final Archive is more dramatic than for low dispersion commonly ranging from 50% to 100%.
It is shown that slopes of p(R)=Omega(R)+/-kappa(R)/m curves at Lindblad resonances determine its widths. The ability of galactic disks to respond on torques exerted at ILRs by perturbers (bar, density wave, galaxy-satellite, etc.) is determined by widths of ILRs. Widths of ILRs vary along the Hubble sequence of normal and barred galaxies. Galaxies having the bulge to disk ratio of masses and radii similar to the Milky Way's could have wide ILRs if they are formed at the region of 2-4 kpc from their centers. A wide range of possible perturbers (4 < omega(p) < 26 km/sec/kpc) could excite an ILR at this region of the Milky Way. Probably, the ILR of the Milky Way's grand design is located at the same region. The hole in the galactic H2 disk is also located at this region. The mechanism responsible for the origin of this hole could be similar to that opening gaps in the planetary rings.
The International Ultraviolet Explorer Regional Data Analysis Facility (RDAF) was established in 1981 to assist users in the analysis and interpretation of IUE data. In October 1992, the Colorado RDAF was closed, and the Goddard RDAF was renamed the International Ultraviolet Explorer Data Analysis Center (IUEDAC). Programs written using the Interactive Data Language (IDL), marketed by Research Systems Incorporated (RSI) allow users to reduce and analyze IUE spectral data processed through IUESIPS and NEWSIPS, display images, perform various database searches and convert IUE data sets into various formats. IUEDAC personnel support US Guest Observers with 19th episode IUE observing programs and are actively involved with LASP scientists working to reformat and distribute data from the Orbiting Astronomical Observatory 3 (Copernicus) mission. The IUEDAC staff also maintains several homepages on the World Wide Web. These pages provide WWW users access to both information and services provided by the IUEDAC. Users may also access the IUEDAC either by visiting the DAC in person or by remote logins via the internet.
A number of Fourier-Transform hard X-ray imagers have been constructed and/or proposed for development by Goddard and other institutions. It is desirable, and usually essential, to verify the end-to-end X-ray imaging performance on the ground before flight. Until now, it has been believed that such a test using hard X-rays would be either prohibitively expensive or impossible. The primary difficulty in developing an appropriate technique has been the apparent requirement that the X-ray source must lie at a great distance from the telescope. It turns out that, for a certain broad class of Fourier-imaging instruments, the source distance need not be large, and laboratory tests are feasible. We shall outline an appropriate technique and describe existing hardware that can perform such tests.
Even at soft X-ray wavelengths, Fourier transform telescopes with angular resolution of a few arcseconds or less are limited by diffraction effects. At optical wavelengths, diffraction is, obviously, even more important. Because the effects of diffraction are calculable, it is, nevertheless, possible to compensate for the effects of diffraction by choosing the parameters: collimator length, grid pitch, and wavelength of the light source to yield an integral multiple of \pi for the argument of the periodic response function (Lindsay 1978). This approach was used to calibrate the High Energy Imaging Device (HEIDI) balloon payload prior to its flight on 1993 June 22 (Gaither et al. 1995). We will describe a facility being developed for calibrating a hard X-ray and gamma-ray telescope of the design being studied for the High Energy Solar Imager (HESI).
The infrared emission from galaxies is an important contributor to the cosmic infrared background. Its intensity distribution is anisotropic and exhibits angular correlations due to the spatial clustering of galaxies. The amplitude, and the evolution with redshift, of these correlations depend upon the evolutionary history of galaxies. Specifically, they depend upon the evolution of the spectrophotometric and clustering properties of galaxies. I discuss the expected correlations in the near IR bands in various evolutionary models, and compare these with the DIRBE estimates.
The Goddard Fabry-Perot Imager (GFPI) is an optical scanning interferometer and CCD imaging system developed at the Goddard Space Flight Center's Laboratory for Astronomy and Solar Physics for use at various astronomical observatories. Besides being transportable, its most notable characteristic is the relatively low spectral resolution (3-30 Angstrom bandpass). The instrument may be used in scanning mode for objects with a large range of velocities but is most often employed to match the observed wavelength of a Doppler shifted emission feature for objects with any velocity.
We report on the detection of a large scale radio structure and plasma flow associated with a bright point flare observed on 1993 July 11. The bright point (BP) flare was simultaneously imaged by the Nobeyama radioheliograph at 17 GHz and the Soft X-ray Telescope on board the Yohkoh mission. The microwave emission consists of a large scale structure and a compact moving source. The large scale component seems to be the radio counterpart of large scale loop structures sometimes observed in association with BP flares in X-rays. The compact source moved from the location of the X-ray BP flare with a speed of about 60 km/s which suggests a plasma flow. Spatial comparison between X-ray and radio data shows that the BP flare had different manifestations in the two wavelength domains. The emission peaks in the two wavelength domains did not coincide, suggesting cool plasma flow along the large scale radio structure. We were able to determine the temperature and emission measure of the BP flare plasma from the X-ray data and hence computed the expected radio flux from the X-ray emitting plasma. We found that the computed radio flux was much smaller than the total observed radio flux.
The goals of the NASA/GSFC Astronomical Test Facility (ATF) are, in order of priority, to: (1) provide a conveniently located 36" telescope for the evaluation of new astronomical instrumentation with space flight or lunar potential by testing them under actual observing conditions without the time and expense of going to remote sites; (2) provide hands-on experience for high school and undergraduate students as observing assistants; (3) obtain correlative data to support spacecraft observations; (4) demonstrate feasibility of research ideas for space projects and larger ground-based telescopes. The telescope is available to Goddard and other scientists (e.g., U Maryland, JHU) for extended periods of time.
A scientifically productive Moon-based observatory can be established in the near term (3-5 years) by robotic spacecraft. Such a project is affordable even taking into account NASA's currently very tight budget. In fact the estimated cost of a lunar telescope is sufficiently low that it can be financed by private industry, foundations, or wealthy individuals. The key factor is imaginative use of new technologies and new materials. Since the Apollo era, many new areas of space technology have been developed in the US by NASA, the military, academic and industry sectors, ESA, Japan, and others. These include ultralite optics, radiation tolerant detectors, precision telescope drives incorporating high temperature superconductors, smart materials, active optics, dust and thermal control structures, subminiature spectrometers, tiny radio transmitters and receivers, small rockets, innovative fuel saving trajectories, and small precision landers. The combination of these elements makes possible a lunar observatory capable of front line astrophysical research in UV-Vis-IR imaging, spectrometry, and optical interferometry, at a per unit cost comparable to that of Small Explorer (SMEX) class missions. We describe work in progress at NASA GSFC and elsewhere, applications to other space projects, and spinoff benefits to ground-based astronomy, industry, and education.
The detection of spatial anisotropy in the Cosmic Microwave Background Radiation (CMBR) by COBE at angular scales of ~7 degrees represents the first evidence that the seeds of structure formation existed in the early universe. At scales of 2 degrees and smaller, gravity may enhance the CMBR spatial anisotropy and observations on these angular scales may provide constraints on both large scale structure formation in the universe and fundamental cosmological parameters. We discuss recent measurements of the Medium-Scale Anistropy Measurement (MSAM), a balloon-borne radiometer, flown from Palestine, TX in 1992, 1994, and 1995 and show that significant temperature differences are observed at angular scales of ~0.5 degrees.