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Gehrz, R. D.

Publications and source records attributed to Gehrz, R. D..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

SOFIA FORCAST Far-IR Photometry of Comet ISON and Constraints on the Coma Grain Size Distribution

Comet C/2012 S1 (ISON) was unique in that it was a dynamically new comet derived from the nearly isotropic Oort cloud reservoir of comets with a sun-grazing orbit. Infrared (IR) observations were executed on NASA's Stratospheric Observatory For Infrared Astronomy (SOFIA) by the FORCAST instrument on 2013 October 25 UT (r(sub h)=1.18 AU, Delta=1.5AU). Photometry was obtained in FORCAST filters centered at 11.1, 19.7, and 31.5 micron. The observations compliment a large world-wide effort to observe and characterize comet ISON.

Comet dust properties↗
NASA NTRS2002

Charter and activities of the SIRTF in-orbit checkout focus integrated products team and optical performance of the CTA

We describe a strategy to determine the initial mirror spacing by quantitatively evaluating the shapes of the images formed by the telescope using the Infrared Array Camera and other science instruments. We show that this information can be used to predict the direction and magnitude of the secondary mirror move that will result in the telescope best focus. The tools used to evaluate focus position and optical quality of the in orbit CTA have been qualified during the BRUTUS test are here described.

Integrated↗
NASA NTRS1995

RY Scuti: Infrared and radio observations of the mass-loss wind of a massive binary star system

We report infrared (IR) imaging, IR photometry, IR spectroscopy, optical/IR photopolarimetry, and Very Large Array (VLA) radio observations of the peculiar binary star RY Scuti. These observations provide an unprecedented view of the detailed spatial structure of the equatorial mass-loss wind of a massive, luminous, 'overcontact' binary system. The binary star (0.43 AU separation) is surrounded by a flattened equatorial disk with an outer radius of approximately = 3 x 10(exp 16) cm (2000 AU) that emits strongly in the IR and radio. The inside of the disk is ionized and emits free-free radiation from hydrogen and 12.8 micrometers forbidden-line emission from (Ne II); the outside of the disk emits thermal radiation from silicate dust. Radio continuum emission is also produced in a compact H II region surrounding the binary. The dust may have a polycyclic aromatic hydrocarbon (PAH) component. We use a rudimentary geometric model in which the thermal IR and radio emission from the disk are assumed to arise in a pair of concentric toroidal rings to estimate the physical properties of the disk. The mean radius of the ionized gas toroid is approximately = 1.3 x 10(exp 16) cm (870 AU), and the mean radius of the dust toroid is approximately = 2.2 x 10(exp 16) cm (1470 AU). RY Scuti has a small intrinsic polarization, with the electric vector perpendicular to the equatorial disk, that is probably caused by electron scattering from hot gas close to the central binary. We conclude that neon in the nebula is overabundant with respect to hydrogen and helium by a factor of between 1.6 and 10. Our IR/radio image data suggest that the circumstellar disk is part of an extensive radiation driven mass-loss outflow that is strongly confined to the equatorial plane of the binary system. The sharp spatial separation of the outer dust torous from the inner ionized gas torus confirms earlier suggestions that dust formation in the circumstellar ejecta of very hot stars must occur in regions that are shielded from the hard radiation field.

Gehrz, R. D.↗
NASA NTRS1995

The temporal evolution of the 1-5 micron spectrum of V1974 CYGNI (Nova Cygni 1992)

We present 1-5 micrometer moderate- and high-resolution infrared spectra of the ONeMg nova V1974 (Nova Cygni 1992) obtained at multiple epochs during an approximately 500 day period after outburst. During the first 80 days, the spectra exhibited continuum emission from thermal bremsstrahlung (free-free radiation) with prominent recombination line emission from hydrogen, helium, nitrogen, and oxygen. The measurement Full Width at Half Maximum (FWHM) of these recombination lines suggests that the initial velocity of the ejecta was approximately equals 2400-3400 km/s. We estimate from the hydrogen recombination line ratios that the density of the ionized shell at this epoch was approximately equals 10(exp 9), with an effective electron temperature of approximately equals 5 x 10(exp 3) K. As the temporal evolution of the ejecta progressed, the hydrogen and helium lines diminished in intensity, and coronal lines of aluminum, calcium, magnesium, neon, and sulfur appeared by approximately day 80. The coronal line emission phase persisted for over 400 days. During this epoch, no significant dust formation occurred. We find that the ejecta of V1974 Cyg were overabundant in aluminum with respect to silicon by a factor of approximately equals 5 and in magnesium with respect to silicon by a factor of greater than or equal to 3 relative to the solar photosphere. Comparison of our observed overabundances with recent model predictions of elemental synthesis in ONeMg outbursts suggests that the accreted envelope on V1974 Cyg was close to solar composition and that the precursor ONeMg white dwarf had a mass of approximately equals 1 solar mass.

Woodward, Charles E.↗
NASA NTRS1995

Infrared observations of an outburst of small dust grains from the nucleus of Comet P/Halley 1986 III at perihelion

A close examination of the 0.7- to 23-micron infrared data base acquired by Gehrz and Ney (1992), suggests that the nucleus of Comet P/Halley 1986 III emitted a burst of small dust grains during a 3-day period commencing within hours of perihelion passage on 1986 February 9.46 UT. The outburst was characterized by significant increases in the coma's grain color temperature T(sub obs), temperature excess (superheat: S = T(sub obs)/T(sub BB)), infrared luminosity, albedo, and 10-micron silicate emission feature strength. These changes are all consistent with the sudden ejection from the nucleus of a cloud of grains with radii of approximately 0.5 micron. This outburst may have produced the dust that was responsible for some of the tail streamers photographed on 1986 February 22 UT. The peak of the dust outburst occurred about 3 days before a pronounced increase in the water production rate measured by the Pioneer Venus Orbiter Ultraviolet Spectrometer. We suggest that jets that release large quantities of small particles may be largely responsible for some of the variable infrared behavior that has been reported for P/Halley and other comets during the past two decades. Such jets may also account for some of the differences IR Type I and IR Type II comets.

Gehrz, R. D.↗
NASA NTRS1994

The temporal evolution of the 4-14 micron spectrum of V1974 Cygni (Nova Cygni 1992)

We present 4-14 micron spectra of the ONeMg nova V1974 Cygni (Nova Cygni 1992) obtained during 1992 May on the NASA Kuiper Airborne Observatory (KAO) and from the NASA 1.52 m infrared telescope at Mount Lemmon, Arizona in 1992 October/November with the HIFOGS mid-infrared spectrometer. The spectra at both epochs showed continuum emission from thermal bremstrahlung (free-free radiation) with emission lines from hydrogen and (Ne II), (Ar III), and Ne VI). During May, approximately = 80 days after outburst, the dominant emission lines in the mid-infrared spectra were a blend of three lines (Pf-alpha, Hu-beta, H11-7) near 7.5 microns and (Ne II) at 12.8 microns. By October (approximately 160 days later), the hydrogen emission had virtually disappeared, the (Ne II) 12.8 micron line had weakened considerably, and a pronounced (Ne VI) emission line had appeared at 7.6 microns. This behavior confirms that V1974 Cyg is similar to the prototypical slow ONeMg 'neon nova', Nova QU Vulpeculae (1984 No. 2). The remarkable evolution of the spectrum suggests that the ionization conditions changed drastically between 1992 May and 1992 October. We find that the ejecta of V1974 Cyg were overabundant in neon with respect to silicon by a factor of approximately 10 relative to the solar photosphere. We present new model calculations of infrared sodium forbidden line emission from (Na III) 7.319 microns (Na IV) 9.039 microns, and (Na IV) 21.29 microns that can be compared with recent model predictions of sodium synthesis in ONeMg nova outbursts. We conclude that sodium abundances in ONeMg novae can be determined by observations of infrared coronal lines of sodium that are accssible to the Infrared Space Observatory (ISO) and instruments at the NASA Infrared Telescope Facility (IRTF).

Gehrz, R. D.↗
NASA NTRS1992

Another neon nova - Early infrared photometry and spectroscopy of Nova Cygni 1992

Infrared photometry and spectrophotometry of Nova Cygni 1992 taken within 54 days of its eruption show a strong 12.8-micron Ne II forbidden emission line as well as hydrogen recombination lines. Spectra with lambda/Delta lambda of about 2000 resolve the Ne II forbidden and 12.37-micron Hu-alpha lines with about 2200 km/s (FWHM). The Ne II forbidden line shows multiple velocity components. The amount of forbidden Ne II required to produce the observed emission feature exceeds the solar abundance of neon by at least a factor of 4.

Hayward, T. L.↗
NASA NTRS1992

The peculiar infrared temporal development of Nova Vulpeculae 1987 (QV Vulpeculae)

The paper reports 1.25-19.5-micron IR photometry and optical/IR spectroscopy of Nova QV Vul (1987) from November 1987 through September 1989. The measurements show that an optically thick carbon dust shell formed within 83 d of the outburst, and that the spectral signatures of four types of astrophysical grains appeared at various times during a 2-yr period following the eruption. Carbon, SiC, and hydrocarbons formed first; oxygen-rich silicates formed later. It is suggested that the carbon dust components formed in fast-moving polar flumes, and that the silicates formed in a slow-moving equatorial ring. Mass estimates from the IR photometry and optical spectroscopy confirm that grain condensation in both the slow and fast ejecta of QV Vul is consistent with constraints established by previous observations of other dusty novae. It is concluded that the condensible elements in these grains were present in approximately solar abundance.

Gehrz, R. D.↗
NASA NTRS1992

0.7- to 23-micron photometric observations of P/Halley 2986 III and six recent bright comets

Infrared photometry over 0.7-23 micron range is presented for P/Halley 1986 III and six other recent bright comets. It is concluded that comets can be classified by their thermal infrared energy distributions. IR Type I comets have low continuum superheat and the 10-micron silicate emission is muted or absent. IR Type II comets have large continuum superheat and strong silicate emission features. Simultaneous measurements of P/Halley and Bradfield 1980 XV are generally consistent with the steady-state model for nuclear ablation. P/Halley's dust coma had an average albedo of 0.20 at a scattering angle of 130 deg. The correlation between superheat and 20-micron silicate excess, the flattened forward-scattering peak of the albedo curve, and the relatively low backscattering albedo are all consistent with laboratory and theoretical results for nonspherical and fluffy grains. The results appear to be especially consistent with core-mantle grain models such as those proposed by Greenberg and Hage (1989).

Gehrz, R. D.↗
NASA NTRS1992

Observations of the 2.2-micron emission from the halo of NGC 4244 at large galactocentric distances

Observations of 2.2-micron emission from the halo of the edge-on spiral galaxy NGC 4244 are reported. These observations were made at larger radii from the center of the galaxy and with a smaller beam than reported in previous studies. The emission detected is consistent with the de Vaucouleurs r exp 1/4 law, and the mass-to-light ratio given by the present measurements at 2.2 microns is in good agreement with previous results. The present data reveal no evidence for a massive halo of cool objects.

Bergstrom, J. W.↗
NASA NTRS1992

IRAS observations of novae. III - The related objects

Results are presented on an analysis of IRAS observations of a selected sample of 116 objects that show properties similar to those of classical novae detected and modeled by Harrison and Gehrz (1988, 1991). These objects include dwarf novae, novalike variables, symbiotic stars, and planetary nebulae with close binary central-star systems. A high detection rate of 80 percent was found for this sample, as compared with about 50 percent detection rate found in the earlier examined sample. A reexamination of data showed that, at low galactic latitudes, source confusion can account for many of the detections of novae. The seemingly high detection rates of other objects and the many high-latitude novae are difficult to explain.

Harrison, Tom↗
NASA NTRS1991

IRAS observations of classical novae. II - Modeling the detections

The analysis of IRAS data on 64 classical novae extracted since an initial study of 41 novae (Harrison and Gehrz, 1988) is reported, and it is concluded that 45 of the 105 novae investigated were detected by IRAS. A model is presented that compares the IRAS detections of novae with predictions of the strength of the far-infrared forbidden line emission from the gaseous ejecta. The distances are known for 23 of the 45 detected novae so that this sample of 23 can be used to compare the fluxes observed at earth with fluxes predicted by the line emission model. The fluxes from 9 of these 23 novae can be explained by line emission given relatively conservative assumptions about the physical conditions in their shells. Accepting more extreme values for these parameters makes it possible to attribute three more detections to possible line emission. There are 11 novae whose IRAS detections remain difficult to explain, even when dust emission and free-free emission are considered as alternatives to line emission.

Harrison, T. E.↗
NASA NTRS1990

An infrared study of Orion Molecular Cloud-2 (OMC-2)

This paper reports 1.2-23 micron photometry for 11 discrete sources in Orion Molecular Cloud-2 (OMC-2). These data, combined with H and K photometric and K polarimetric images, are used to model the cluster sources. Most appear to be young stars of roughly solar mass. Some have circumstellar dust reradiation or reflection nebulosity. A model based on single scattering of light from an exciting star explains some features of the IRS 1 nebula, the largest reflection nebula in OMC-2. However, the red colors and high surface brightness of the IRS 1 nebula require a cool excitation source that is more luminous than far-infrared observations would indicate.

Johnson, J. J.↗
NASA NTRS1990

The infrared coronal lines of recent novae

IR coronal line emissions are reported in the novae V1819 Cyg and V827 Her; high-resolution near-IR spectra of coronal line emission in the nova QU Vul have also been obtained, increasing the number of known IR coronal line novae from two to four. Each of the four is found to be characterized by approximately the same Si VI/Si VII line-intensity ratio, which indicates a common coronal-zone electron temperature of 300,000 K. The coronal-line novae are noted to possess a remarkably similar near-IR spectrum, suggesting that their coronal line emission will only occasionally be observed at optical wavelengths.

Greenhouse, Matthew A.↗
NASA NTRS1990

The infrared spectrum of Comet Bradfield (1987s) and the silicate emission feature

Infrared (1-20 microns) observations of Comet Bradfield (1987s) from three observatories are reported. Silicate emission is prominent in all the data, from heliocentric distance r = 0.87 to 1.45 AU. A CVF spectrum at r = 1.45 AU shows a peak at 11.3 microns identified as crystalline olivine; the spectral shape is similar to that in Halley. Dust optical properties are similar to those of the grains in Halley's jets. Dust production rate near perihelion was about 10 to the 6th g/s and varied as N exp-2. It is suggested that some differences in grain properties among comets could result from difference in the thermal history of the nuclear surface and the relative fraction of the dust particles originating in the subsurface icy layer versus the devolatilized mantle.

Hanner, M. S.↗