Engineering topics

Sonneborn, George

Publications and source records attributed to Sonneborn, George.

At least 73 records · Page 4

NASA NTRS1998

High-Velocity Ly(Alpha) Emission from SMR 1987A

The high-velocity Ly(Alpha) emission from SN 1987A observed with the Space Telescope Imaging Spectrograph (STIS) evidently comes from a reverse shock formed where the outer envelope of SN 1987A strikes ionized gas inside the inner circumstellar ring. The observations can be explained by a simple kinematic model, in which the Ly(Alpha) emission comes from hydrogen atoms with radial velocity approximately 15,000 km s(exp -1) crossing a reverse shock in the shape of a slightly prolate ellipsoid with equatorial radius 4.8 x 10(exp 17) cm or approximately 80% of the distance to the inner surface of the inner ring. N v double Lambda 1239, 1243 emission, if present, has a net luminosity approximately less than 30% times that of the Ly(Alpha) emission. Future STIS observations should enable us to predict the time of impact with the inner ring and to determine unambiguously whether or not N v emission is present. These observations will offer a unique opportunity to probe the structure of SN 1987A's circumstellar environment and the hydrodynamics and kinetics of very fast shocks.

Michael, Eli↗

NASA NTRS1994

A new systems engineering approach to streamlined science and mission operations for the Far Ultraviolet Spectroscopic Explorer (FUSE)

The Mission Operations and Data Systems Directorate (MO&DSD, Code 500), the Space Sciences Directorate (Code 600), and the Flight Projects Directorate (Code 400) have developed a new approach to combine the science and mission operations for the FUSE mission. FUSE, the last of the Delta-class Explorer missions, will obtain high resolution far ultraviolet spectra (910 - 1220 A) of stellar and extragalactic sources to study the evolution of galaxies and conditions in the early universe. FUSE will be launched in 2000 into a 24-hour highly eccentric orbit. Science operations will be conducted in real time for 16-18 hours per day, in a manner similar to the operations performed today for the International Ultraviolet Explorer. In a radical departure from previous missions, the operations concept combines spacecraft and science operations and data processing functions in a single facility to be housed in the Laboratory for Astronomy and Solar Physics (Code 680). A small missions operations team will provide the spacecraft control, telescope operations and data handling functions in a facility designated as the Science and Mission Operations Center (SMOC). This approach will utilize the Transportable Payload Operations Control Center (TPOCC) architecture for both spacecraft and instrument commanding. Other concepts of integrated operations being developed by the Code 500 Renaissance Project will also be employed for the FUSE SMOC. The primary objective of this approach is to reduce development and mission operations costs. The operations concept, integration of mission and science operations, and extensive use of existing hardware and software tools will decrease both development and operations costs extensively. This paper describes the FUSE operations concept, discusses the systems engineering approach used for its development, and the software, hardware and management tools that will make its implementation feasible.

Butler, Madeline J.↗

NASA NTRS1994

The early spectral evolution of Nova Cassiopeiae 1993

In this paper we describe the evolution of Nova Cas 1993 over the first two months of its outburst. We present an ultraviolet light curve that covers the period from announcement to just after dust began forming in the ejecta (1994 Feb. 15) and International Ultraviolet Explorer (IUE) spacecraft constraints forced us to halt our observations. We have used spherical, expanding, Non-local Thermodynamic Equilibrium (NLTE) stellar atmospheres to compute synthetic spectra and have compared the results to combined ultraviolet (low-resolution 1200-3400 A and high-resolution 2400-3300 A) spectra. Our fits show that the effective temperature of the ejecta increased from approx. 8000 to about approx. 16 000 K between 1993 Dec. 12 and 1993 Dec. 26. The temperature then increased more slowly to approx. 24 000 on 1994 Jan. 28. A preliminary abundance analysis shows evidence for hydrogen depletion, as we also found for Nova V1974 Cygni; however we find a larger enhancement of carbon, nitrogen, and oxygen. We also show that the principal mechanism for mass ejection in this nova is a radiation pressure driven wind and that mechanical driving is not necessary.

Hauschildt, Peter H.↗

NASA NTRS1994

The early bolometric evolution of Nova Cygni 1992

We report on combined ultraviolet and optical observations of early stages of the outburst of Nova Cygni 1992. We show that the initial optical rise and decline were produced by the dramatic increase in UV line opacity and its gradual subsequent lifting as the ejecta expand. The source of the M(sub v) - t(sub 3) relation is easily understood in light of these low-resolution UV spectra. The multiwavelength data confirm the theoretical prediction that a nova evolves at a constant bolometric luminosity in the early stages of outburst. We verify the prediction for this nova for at least the first month of the outburst. The detection of far-UV radiation with the Voyager Ultraviolet Spectrometer and the detection of an increasing X-ray flux with Roentgen Satellite (ROSAT) imply that this phase lasted for more than 6 months.

Shore, Steven N.↗

NASA NTRS1993

The early ultraviolet spectral evolution of Nova Cygni 1992

We report on the high resolution ultraviolet line profile evolution of Nova Cygni 1992 using the International Ultraviolet Explorer (IUE) satellite and the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST). Our observations cover the period from discovery in 1992 February through 1993 April. The initial expansion velocity, derived from P Cyg profiles on Mg II and other resonance lines, was about 4500 km/s. All resonance transitions displayed P Cyg profiles around the time of UV maximum. We first discuss the spectral development as the initially optically thick atomic absorption curtain became optically thin. We then present the interpretation of the nebular spectral stage. The high resolution line profiles show that the shell rapidly developed nearly symmetric knots, or filaments, of emission. The optically thin transitions show an emission weighted expansion velocity of about 1800 km/s, consistent with the velocity inferred from the radio and infrared data. The emission lines display a symmetric set of filaments that appeared as soon as the line profiles became optically thin. The GHRS observations demonstrate that the filaments were located in the inner, slower moving parts of the ejecta. These probably record an initial low wave number instability imposed on the ejecta at the time of outburst. We suggest that the likely cause is a Rayleigh-Taylor instability. The emission at later stages is dominated by the filaments and, since this phenomenon is seen in virtually all novae, nebular line formation codes will have to account for the density inhomgeneities in order to adequately determine abundances for nove ejecta.

Shore, Steven N.↗

NASA NTRS1993

Spectral classification with the International Ultraviolet Explorer: An atlas of B-type spectra

New criteria for the spectral classification of B stars in the ultraviolet show that photospheric absorption lines in the 1200-1900A wavelength region can be used to classify the spectra of B-type dwarfs, subgiants, and giants on a 2-D system consistent with the optical MK system. This atlas illustrates a large number of such spectra at the scale used for classification. These spectra provide a dense matrix of standard stars, and also show the effects of rapid stellar rotation and stellar winds on the spectra and their classification. The observational material consists of high-dispersion spectra from the International Ultraviolet Explorer archives, resampled to a resolution of 0.25 A, uniformly normalized, and plotted at 10 A/cm. The atlas should be useful for the classification of other IUE high-dispersion spectra, especially for stars that have not been observed in the optical.

Rountree, Janet↗

NASA NTRS1992

Analysis of the photospheric epoch spectra of type 1a supernovae SN 1990N and SN 1991T

This paper presents an LTE analysis of the photospheric epoch optical and IUE UV spectra of the recent Type Ia supernovae SN 1990N and SN 1991T. It is found that SN 1990N has matter moving as fast as 40,000 km/s, while SN 1991T has matter moving as fast as 20,000 km/s. The exponential density profile with e-folding velocity of 3160 km/s suggested by deflation and delayed/late-detonation explosion models is adequate for calculating fits to the observed spectra. Findings on the iron peak elements suggest that in both SNe some nuclear burning continues into the outer ejecta or the newly synthesized elements are mixed into the outer ejecta. The observed spectra of SN 1990N are consistent with a composition of the inner envelope like that of model W7. Intermediate-mass elements with abundances higher than solar are probably in the outer ejecta of both SNe, while silicon, sulfur, and calcium are underabundance in SN 1991T relative to SN 1990N by factors of order 3, 3, and 120, respectively.

Jeffery, David J.↗

NASA NTRS1992

A prediction of the gamma-ray flux from Nova Herculis 1991

UV observations made with the IUE satellite of Nova Her 1991 are presented and discussed. A distance of 3.4 kpc and reddening of 0.6 E(B-V) to Nova Her 1991 are determined by comparing its UV and optical outburst behavior with those of the extragalactic ONeMg nova LMC 1990 No. 1. Her 1991 was also detected by Rosat five days into its outburst, and it is argued that a small fraction of the X-rays may be the product of Comptonized 1.2 and 0.5 MeV emission from the decay of Na-22. If the mass of ejected shell is indeed as high as was estimated by Woodward et al. (1992), namely 0.0001 solar mass, then the present results predicted that Na-22 gamma-ray emission from the ejecta of Nova Her 1991 would still be detectable by GRO in the spring of 1992. The detection level was expected to be about 5 sigma in a two-week observation. Such a detection would strongly guide and constrain the hydrodynamic and nucleosynthesis simulations of nova outbursts on massive white dwarfs.

Starrfield, Sumner↗

NASA NTRS1992

Ultraviolet studies of Novae in the Large Magellanic Cloud

We report on the ultraviolet observations of five novae in the Large Magellanic Cloud (LMC). They are LMC 1988 #1, which was a CO nove similar to OS And; LMC 1988 #2, which was an ONeMg nova similar to Nova Her 1991; LNC 1990 #1, which was a very fast ONeMG nova similar to V693 CrA 1981; LMC 1990 #2, which was a recurrent nova similar to U Sco; and LMC 1991 which reached V approximately 9.

Starrfield, Sumner↗

NASA NTRS1991

Premaximum observations of the type Ia SN 1990N

Spectroscopic and photometric observations of SN 1990N were obtained at ultraviolet and optical wavelengths, beginning 14 days before maximum light. The early observations reveal important differences from spectra of SN Ia's around maximum light. Photometry and spectroscopy obtained after maximum show that SN 1990N is a typical SN Ia and that most of the observed differences are due to the early epoch of the observations. The most significant characteristics are (1) the high velocities of Ca and Si up to 22,000 km/s; (2) the presence of Co and Fe 2 weeks before maximum; and (3) the more rapid increase in the UV flux compared to the optical. The most popular models for white dwarf deflagration that have provided the standard interpretation for SN Ia's at maximum light do not reproduce the high velocities of Ca II and Si II lines observed in SN 1990N.

Leibundgut, Bruno↗

NASA NTRS1991

Spectral transformation of the unusual variable star MWC560 to resemble a nova

A dramatic change has occurred in the ultraviolet spectrum of the emission-line star MWC560, so that it now closely resembles the spectrum of a nova shortly after outburst. This event may signal a major mass-ejection episode such as presumably occurred in past centuries in the symbiotic star R Aquarii to produce the well-known bipolar nebula, and it may herald the emergence of a standard symbiotic-star emission-line spectrum in MWC560, corresponding to a change in evolutionary state.

Maran, Stephen P.↗

NASA NTRS1991

Multiwavelength observations of Nova LMC 1990 Number 2 - The first extragalactic recurrent nova

The paper presents IUE and optical observations of Nova LMC 1990 Number 2, the first recurrent nova to be observed outside the Galaxy. The ultraviolet observations reported here caught the nova at UV maximum; no P Cygni profiles developed on the resonance lines, suggesting that very little mass was ejected and the outburst was not spherical. The integrated UV luminosity at UV maximum was 2.9 x 10 to the 38th ergs/s. Thus the luminosity exceeded the Eddington limit for a 1-solar-mass white dwarf.

Shore, Steven N.↗

NASA NTRS1991

Criteria for the spectral classification of B stars in the ultraviolet

A set of criteria for the classification of G stars from UV spectra alone, using standards drawn form the optical region, is developed. About 100 stars having normal MK spectral types in the range B0-B8, III-V, have been classified. The UV spectral types are found to be very consistent with the optical MK types, implying that it is possible to do two-dimensional spectral classification in the UV without any knowledge of the optical spectrum.

Rountree, Janet↗

NASA NTRS1990

The Lyman far ultraviolet spectroscopic explorer mission

The Lyman-FUSE (Far Ultraviolet Spectroscopic Explorer) mission selected in 1989 for definition and development for flight is described. FUSE will obtain spectra in the 912 to 1250 angstrom region with unprecedented sensitivity. This spectral interval is extraordinarily rich in strong atomic and molecular transitions, including the Lyman series of hydrogen and deuterium, molecular hydrogen (H2 and HD) and the strong transitions of important ions such as N I to III, C I to IV, P II to V, O VI and S III to VI. FUSE will bridge the spectral gap between Hubble Space Telescope and the x ray regime with an additional moderate resolution spectroscopic capability covering 100 to 912 angstroms.

Sonneborn, George↗

NASA NTRS1990

Spectral classification of B stars in the ultraviolet

A set of criteria developed for the classification of B stars from ultraviolet spectra alone, using MK standards drawn from the optical region is described. The observational material consists of 100 archival high dispersion spectra obtained by the SWP (Short Wavelength Prime) camera on the IUE (International Ultraviolet Explorer) spacecraft. Only photospheric absorption lines are used as classification criteria. The C IV, SI IV and N V lines, which in early B stars originate in the stellar wind, are not used in the two dimensional spectra type luminosity classification, but the appearance of these lines is compared with the standard stars, and some anomalies noted. On the whole, the ultraviolet spectral types are very consistent with the optical MK types, implying that it is possible to do a two dimensional spectral classification in the ultraviolet without any knowledge of the optical spectrum.

Rountree, Janet↗

NASA NTRS1990

Ultraviolet light curves of galactic and extra-galactic classical CNO novae: PW Vul, OS, and LMC 1988 no. 1 and no. 2

In order to determine the comparative energy budgets of novae in outburst, the ultraviolet light curves for a number of novae are determined. The low dispersion IUE (International Ultraviolet Explorer) spectra of PW Vul 1984, OS And 1986, LMC 1988 number 1 and number 2 and obtained values for the integrated energy emitted in the wavelength interval from 1200 to 3400 A, not including the geocoronal Lyman-alpha emission line are re-examined. The light curves show that the brightness of the nova declines much more slowly in the ultraviolet than the optical, that the maximum brightness of the nova occurs much later in the ultraviolet than in the optical, and the peak luminosity is super Eddington for LMC 1988 number 2.

Austin, S.↗

NASA NTRS1990

PW Vul: A classical nova with nearly solar abundances

Ultraviolet and optical spectrophotometric data are combined in order to determine the physical parameters in the expanding shell of Nova PW Vul 1984. Ultraviolet data obtained with the International Ultraviolet Explorer (IUE) satellite from August 2, 1984 (a few days after discovery) until it became too faint to study with the satellite is presented. The last IUE exposure was obtained in June 1988 but it was continually followed with ground based optical telescopes. A plot showing the variation of electron density and temperature as a function of time is presented. Analysis of the emission line intensities show that the abundances of this very slow nova are solar for helium, carbon, and oxygen while nitrogen is 50 times solar. The CLOUDY code of Ferland is used to model the emission line intensities and find good agreement with the observations. Plots representative of IUE and optical spectra are displayed.

Saizar, P.↗

NASA NTRS1990

Nova LMC 1990 no. 1: The first extragalactic neon nova

International Ultraviolet Explorer (IUE) observations of nova LMC (Large Magellanic Cloud) 1990 No. 1, the first neon (or ONeMg) nova observed outside the Galaxy are presented. The observations were obtained from 17 Jan. to Mar. 1990, with especially dense coverage during the first 25 days of the outburst. (The neon nova categorization is based on the detection of forbidden Ne 3-4 lines in optical spectra; the ultraviolet neon lines were not detected.) During the first 30 days of the outburst, the radiative losses were dominated by the N 5 delta 1240 and C 4 delta 1550 lines. The maximum ejection velocity was approximately 8000 km/s, based on the blue absorption edge of the C 4 P-Cygni profile. Early in the outburst of Nova LMC 1990 No. 1 the UV luminosity alone was approximately 3 times 10 to the 38th power erg/sec, implying that the bolometric luminosity was well in excess of the Eddington luminosity for a one solar mass object.

Sonneborn, George↗