Engineering topics

Sonneborn, George

Publications and source records attributed to Sonneborn, George.

84 records · Page 5

NASA NTRS1990

Nova LMC 1990 no. 2: The first extragalactic recurrent nova

International Ultraviolet Explorer (IUE) observations of Nova LMC (Large Magellanic Cloud) 1990 number 2 are presented. It is the first recurrent nova to be observed outside the Galaxy. Earlier optical outburst of the nova was observed in 1968, when it reached a maximum of about V = 10.2. The first observations caught the nova at UV maximum, and IUE spectra were obtained between 16 Feb. and 26 Mar. 1990, with 8 hour resolution during the first day of observations. No P Cyg profiles developed on the resonance lines, suggesting that very little mass was ejected and that the outburst was not spherical. The integrated UV luminosity was then 3.4 x 10(exp 38) erg/s which is above the Eddington limit for a one solar mass white dwarf (assuming E (B - V) = 0.15 and a distance of 55 kpc to the LMC). This is the first time that the luminosity of a recurrent nova has been directly obtained.

Shore, Steven N.↗

NASA NTRS1990

IUE observations of galactic and extragalactic recurrent novae

A collection of International Ultraviolet Explorer (IUE) light curves and sample spectra for the galactic recurrent novae observed in the ultraviolet are presented. These data are compared to that of the Nova LMC (Large Magellanic Cloud) 1990 number 2 which is shown to also be a recurrent novae. Based on the analysis of the outburst of N LMC 1990 number 2 it is suggested that all of the recurrent novae observed in the ultraviolet have luminosities at maximum light which exceed the Eddington limit. V745 Sco and RS Oph, the two recurrent novae with late type giant companions, show strong Mg II throughout the outburst. Those with low luminosity companions, U Sco and V394 CrA do not show Mg II but do show strong He II 1640 earlier in the outburst. Typical emission line velocities are greater than those observed in many of the classical novae. These characteristics suggest that the white dwarfs in recurrent novae systems are close to the Chandrasekhar limit.

Shore, Steven N.↗

NASA NTRS1990

Late stages in the evolution of classical novae

The long term evolution of novae in outburst is studied in order to determine the means by which they return to quiescence when nuclear burning has ended is described. The project involves both International Ultraviolet Explorer (IUE) and optical observations and theoretical predictions. In the initial observational part of this project, IUE SWP (Short Wavelength Prime) spectra of GQ Mus 1983 and QU Vul 1984 are obtained. Each spectrum was a 16 hour exposure using a combined US1 plus Vilspa shift. No novae were sudied in the UV for as long as QU Vul and GQ Mus and observations of their spectral evolution provides unique data on the turnoff time scale. The spectra of old novae from the IUE archives are used to compare the existing spectra with those of GQ Mus and QU Vul. The theoretical prediction is that a nova should be very hot just before turnoff but x ray observations from EXOSAT do not confirm this prediction.

Starrfield, Sumner G.↗

NASA NTRS1990

Narrow circumstellar emission lines in SN 1987A

Low and high dispersion short wavelength prime (SWP) spectra of SN 1987A show that the supernova's progenitor was in a post red giant branch stage of evolution at the time of its demise in February 1987. The development of narrow high temperature emission lines of N 5, N 4, N 3, C 3, He 2 and O 3 were followed since May 1987. Observations through April 1990 are presented. The lines are interpreted as arising in a low density gas surrounding the supernova which was photoionized by the extreme UV pulse at shock breakout. In this picture, their temporal evolution is determined primarily by light travel time effects and recombination. Comparison of models of circumstellar environment with the International Ultraviolet Explorer (IUE) observations are discussed.

Sonneborn, George↗

NASA NTRS1990

The discovery of magnetically controlled circumstellar matter in the helium-weak stars HD 5737 and HD 79158

The paper reports the discovery, using combined IUE spectroscopy and Zeeman polarimetric magnetic field measurements, of magnetically controlled circumstellar material in two helium-weak stars. HD 5737 = Alpha Sculptoris is, except for its extreme helium deficiency, similar to the He weak sn star HD 21699. A unique period for the magnetic and C IV and Si IV variations of 21.65 days is reported. The effective (longitudinal) field nulls coincide extremely well with C IV line strength maxima. The magnetic field and equatorial trapped plasma are highly oblique to the rotation axis (about 70 deg), and the line variations appear to be stable. Similar magnetospheric variations in HD 79158 = 36 Lyncis have been discovered, for which no period had been previously available. The period is 3.84 days, yet it too displays magnetic-equatorial plasma. The magnetospheric axis is highly oblique to the rotation axis, around 80 deg.

Shore, Steven N.↗

NASA NTRS1989

Highly ionized stellar winds in Be stars. II - Winds in B6-B9.5e stars

The results of a UV survey of stellar winds and circumstellar shells in 40 B6-B9.5e stars covering luminosity classes V-III are presented. A graph is presented of the region from 1520-1560 A, which includes both the Si II UV multiplet 2 transitions and the C IV resonance transition for selected Be stars in the sample. The detection of shortward-shifted discrete component absorption features in nine of the program stars suggests that the material is produced in a stellar wind, which attains velocities of at least a few hundred km/s.

Grady, C. A.↗

NASA NTRS1988

Mass loss in a B2 IIIe star - Omega Orionis 1978-1984

A combination of long-term optical polarimetric, photometric, spectrometric, and UV spectral data has confirmed that all the observed portions of Omega Ori's outer atmosphere are variable on a variety of time scales. The shortest confirmed variations are detectable over a few hours, and the longest changes are observed on time scales of years. A straightforward interpretation of the available data is that the continuum polarization, continuum-magnitude changes, and color changes produced are in a presumably equatorial disk close to the stellar photosphere.

Sonneborn, George↗

NASA NTRS1987

The progenitor of SN 1987A - Spatially resolved ultraviolet spectroscopy of the supernova field

Careful deconvolution of spatially resolved IUE ultraviolet spectra shows that only two of the three stars detected in plate material before 1987 are now present near the site of SN 1987A. The separation, magnitudes, and spectra of these two are consistent with their identification as star 2 and star 3 of the Sanduleak -69 deg 202 trio. The clear implication is that star 1, the 12th mag B3 I star, has disappeared, and it may be identified as the progenitor of SN 1987A.

Sonneborn, George↗

NASA NTRS1987

The ultraviolet variability of AY Ceti

AY Cet is a single-line binary comprised of a spotted G5 III primary and a white dwarf secondary. A series of UV spectra have been obtained with the IUE satellite on five different dates covering a substantial part of the optical cycle of the primary star. No evidence that the continuum or the Ly-alpha absorption line of the secondary star varied is found. There were significant changes in the strengths of the UV emission lines, but the variations were only weakly correlated with either the orbital phase of the binary or the rotational phase of the primary. The UV emission lines were especially strong near maximum visual brightness at a time when the starspot(s) on the primary was least visible. The enhanced line emission is attributed to a flare event on the primary, most likely at a high-latitude site close to the pole of this star. The UV radiative losses of this flare were comparable with those of flares previously observed on the RC CVn variables Lambda And and HR 1099.

Simon, Theodore↗

NASA NTRS1987

Recurrent episodic mass loss in a B2e star - 66 Ophiuchi, 1982-1985

The results of three intensive ultraviolet monitoring campaigns for 66 Oph in September-October 1982, September 1983, and September 1984, supplemented by the available IUE archival data from April, 1982 to June, 1985 are reported. The stellar wind went through at least two and probably three episodes of weak-wind absorption. At wind minimum, the resonance profiles and equivalent widths of C IV, Si IV, Si III, and Al III are similar to those observed in normal B stars of comparable luminosity, effective temperature, and v sin i. The strong-wind absorption profiles show one or more shortward-shifted discrete components superposed upon the weak-wind absorption profiles. Significant profile variations are seen on time scales spanning over two orders of magnitude, ranging from small-amplitude changes detected on time scales of hours to the transitions from wind minimum to maximum, which take months.

Grady, C. A.↗

NASA NTRS1987

Ultraviolet observations of SN 1987A

UV observations of the supernova in the LMC, SN 1987A, were carried out with the IUE satellite. The earliest data show that the UV flux from the supernova was already declining while the optical flux was still rising. The UV spectrum at these epochs consists of broad features associated with the supernova atmosphere punctuated by sharp interstellar absorptions. The rapid decline of the supernova in the short-wavelength ultraviolet allows a glimpse of the stars which remain. One of these is star 2, a neighbor of Sanduleak -69 deg 202, while the other appears to be star 3, a fainter close neighbor of the Sanduleak star. If this is correct, then the star which exploded is Sanduleak -69 deg 202.

Kirshner, Robert P.↗

NASA NTRS1987

The sn stars - Magnetically controlled stellar winds among the helium-weak stars

The paper reports observations of magnetically controlled stellar mass outflows in three helium-weak sn stars: HD 21699 = HR 1063; HD 5737 = Alpha Scl; and HD 79158 = 36 Lyn. IUE observations show that the C IV resonance doublet is variable on the rotational timescale but that there are no other strong-spectrum variations in the UV. Magnetic fields, which reverse sign on the rotational timescale, are present in all three stars. This phenomenology is interpreted in terms of jetlike mass loss above the magnetic poles, and these objects are discussed in the context of a general survey of the C IV and Si IV profiles of other more typical helium-weak stars.

Shore, Steven N.↗