Engineering topics

Gull, G. E.

Publications and source records attributed to Gull, G. E..

At least 19 records

NASA NTRS1994

10-micrometers images and spectra of T Tauri stars

T Tauri stars are young stars usually surrounded by dusty disks similar to the one from which we believe our own Solar System formed. Most T Tauri stars exhibit a broad emission or absorption band between 7.5 and 13.5 micrometers which is attributed to silicate grains in the circumstellar environment. We imaged three spatially resolved T Tauri binaries through a set of broadband filters which include the spectral region occupied by the silicate band. Two of these objects (T Tauri and Haro 6-10) are 'infrared companion' systems in which one component is optically much fainter but contributes strongly in the infrared. Both infrared companions exhibit a deep silicate absorption which is not present in their primaries, indicating that they suffer very strong local extinction whigh may be due to an edge-on circumstellar disk or to a dense shell. We also took low resolution spectra of the silicate feature of two unresolved T Tauris to look for narrow features in the silicate band which would indicate the presence of specific minerals such as olivine. We observed GK Tau, for which Cohen and Witteborn (1985) reported a narrow emission feature at 9.7 micrometers, but do not find evidence for this feature, and conclude that it is either time-dependent or an artifact of absorption by telluric ozone.

Van Cleve, J. E.↗

NASA NTRS1990

Infrared and radio measurements of the density structure of compact H II regions

The density structure of compact H II regions is studied by comparing the ratio of the S III forbidden 33.47-micron and 18.71-micron emission lines and 5-GHz continuum maps. Electron densities are computed for DR 22, G29.9 - 0.0, G75.84 + 0.4, M8, M42, S158, W3, and W33. For DR 22, M42, and W3, the electron densities determined from the IR data are consistent with those derived from radio observations. The densities for S158 and M8 are quite low, implying that the S III emission is dominated by the diffuse ionized gas observed in the radio continuum. For G29.9 - 0.0, G75.84 + 0.4, and W33, the S III data give higher densities than predicted from radio observations. This implies that either these nebulae are filled with dense clumps of ionized gas which are not resolved in the radio maps, or that the S III zone is confined to the small, dense cores found in the radio maps of all three nebulae.

Megeath, S. T.↗

NASA NTRS1989

Improved Si:As BIBIB (Back-Illuminated Blocked-Impurity-Band) hybrid arrays

Results of a program to increase the short wavelength (less than 10 microns) detective quantum efficiency, eta/beta, of Si:As Impurity Band Conduction arrays are presented. The arrays are epitaxially grown Back-Illuminated Blocked (BIB) Impurity-Band (BIBIB) 10x50 detectors bonded to switched-FET multiplexers. It is shown that the 4.7 microns detective quantum efficiency increases proportionately with the thickness of the infrared active layer. A BIB array with a thick active layer, designed for low dark current, exhibits eta/beta = 7 to 9 percent at 4.7 microns for applied bias voltages between 3 and 5 V. The product of quantum efficiency and photoelectric gain, etaG, increases from 0.3 to 2.5 as the voltage increases from 3 to 5 V. Over this voltage range, the dark current increases from 8 to 120 e(-)s(-1) at a device temperature of 4.2 K and is under 70 e(-)s(-1) for all voltages at 2 K. Because of device gain, the effective dark current (equivalent photon rate) is less than 3 e(-)s(-1) under all operating conditions. The effective read noise (equivalent photon noise) is found to be less than 12 electrons under all operating conditions and for integration times between 0.05 and 100 seconds.

Herter, T.↗

NASA NTRS1989

Si II mapping of the neutral gas ring in the Galactic center - Evidence for dust destruction

The distribution of Si II emission in the Galactic center has been measured and found to extend beyond 4 pc from the center. The observed forbidden Si II 34.8 micron line, which is thermally excited by H I and H2 collisions, originates from the photodissociation region formed in the neutral gas ring surrounding the ionized core. Two peaks in the Si II emission are found, one lying 20 arcsec northeast of Sgr A-asterisk along the Galactic plane and the other located about 80 arcsec north of Sgr A-asterisk. The latter peak is located beyond the northern arm seen in the ionized gas. The ratio of Si II to dust mass density varies in the ring, indicating changes in the gas-to-dust ratio. The mass density of Si II and dust are anticorrelated, indicating that Si II tracks the destruction of dust.

Herter, T.↗

NASA NTRS1989

Electron temperatures in the high-excitation zones of planetary nebulae

Airborne infrared and ground-based UV observations of the forbidden Ne-V 24.3-micron and forbidden Ne-V 3426-A fine-structure lines of 17 planetary nebulae are discussed. Significantly higher Ne-V than O-IV temperatures are found for Hu 1-2 and NGC 2440, while IC 2165, NGC 6818, and NGC 7662 have similar Ne-V and O-IV temperatures. It is suggested that the Ne-V and O-IV temperature similarities may be due to the presence of large abundances of the dominant coolants (oxygen or neon) in the Ne-V zone.

Rowlands, N.↗

NASA NTRS1988

Distribution of Si II in the Galactic center

A map of the Galactic center region in the forbidden Si II 34.8-micron line is presented. The line emission arises from within the photodissociation region (PDR) associated with the neutral gas ring surrounding an ionized gas core confined within 2 pc of the Galactic center. Si II is a useful probe of the inner regions of the ring since it is always optically thin. The Si II data, when analyzed in conjunction with O I, C II, and molecular measurements, outlines the transition region between the PDR and the surrounding molecular cloud. The Si II emission is found to extend beyond that of the O II into the neutral gas ring. Although the interpretation is not unique, the data are consistent with a constant gas-phase abundance of silicon within the inner part of the PDR while the gaseous silicon is depleted by molecule formation in the transition region.

Graf, P.↗

NASA NTRS1986

Airborne spectrophotometry of P/Halley from 16 to 30 microns

Comet Halley was observed in the 16 to 30 micron region using the Cornell University 7-channel spectrometer (resolution = 0.02) on board the Kuiper Airborne Observatory on 1985 Dec. 14.2. A 30-arcsec aperture (FWHM) was used. Measurements centered on the nuclear condensation micron indicate that if present, the 20 micron silicate feature is very weak, and that a relatively narrow strong feature centered at 28.4 microns possibly exists. However, this feature may be an artifact of incomplete correction for telluric water vapor absorption.

Herter, T.↗

NASA NTRS1986

Detection of silicon in the Galactic center

The detection of the forbidden Si II 34.815 micron line in the Galactic center region is reported. It is demonstrated that, in the case of the Galactic center, the emission arises mainly from the photodissociation region and not the shocked molecular component seen via H2, CO, and OH lines. It is also shown that significant Si II emission is not expected from the ionized gas even though the Galactic center is a rather low excitation H II region.

Hertier, T.↗

NASA NTRS1984

Mid-infrared Spectroscopy of Planetary Nebulae

The application of infrared spectral lines observed in part of the broadened spectral range (18 to 36 microns) to planetary nebula studies is discussed. Infrared spectral lines in the mid-infrared supply information about planetary nebulae which both supplement and complement optical results. Their utility lies mainly in the insensitivity of the line intensities to temperature variations or uncertainties, in contrast to the behavior of optical emission. This in turn leads to more accurate ionic abundances than presently available from optical lines. Although densities from optical line ratios are not very sensitive to temperature, the lines are weighted toward regions of higher temperatures. A comparison with infrared densities from the same ions therefore provide clues to the density and/or temperature variations within nebulae. Temperatures from OIV and NeV ions show a part of high-excitation nebulae previously inaccessible. They verify the picture of physical processes in the HeIII zone.

Shure, M. A.↗

NASA NTRS1984

The electron density in M82 from the S III mid-infrared line ratio

Strong detections have been made of both the 18.7 microns and 33.4 microns lines of S III in the irregular galaxy M82. This represents the first detection of the 33.4 microns line in an extragalactic object. These measurements sample the ionized gas component which dominates the observed infrared forbidden line and infrared hydrogen recombination line emission. From the ratio of the line fluxes an electron density of 120 (+ 280, - 120) per cu cm is deduced. This result is in agreement with previous estimates of the density derived from high-frequency radio continuum measurements and an assumed emitting volume. They impose further constraints on possible starburst models.

Houck, J. R.↗

NASA NTRS1984

The electron density in the galactic center as derived from the S III 18.71/33.47 micron line ratio

High signal-to-noise ratio measurements of the S III lines 18.71 and 33.47 microns from the galactic center, Sgr A West, are reported. The average electron density is calculated for the S III-emitting region and is found to be 1800 + or - 500 per cu cm. This value is much less than the rms electron density of the clumps as derived from radio observations. The implications for various models are discussed. The S III result is consistent with the electron density previously derived from low signal-to-noise ratio measurements of the O III lines at 88 and 52 microns, if newer values are used for the O III atomic constants.

Herter, T.↗

NASA NTRS1984

Detection of the Ne III 36 micron forbidden line in the planetary nebula NGC 6543

The first observation of the Ne III 36.02 micron forbidden line in a planetary nebula, NGC 6543, is presented. Since the dominant form of neon in medium-excitation to high-excitation planetary nebulae is Ne III, the abundance of this ion is important in determining the total neon abundance. Use of the 36 micron line for an abundance determination has the advantage of insensitivity to temperature uncertainties. However, current atomic parameters lead to a Ne III abundance in NGC 6543 which is 4.5 times the cosmic neon abundance and 2.6 times the abundance from optical line studies. Although such a high abundance cannot be ruled out immediately, inaccuracies in the infrared level collision strengths are suspected because resonances were neglected in their calculation. The 36 micron line is also useful as a temperature probe when combined with the Ne III 3868-A forbidden line. When compared to Ne III 15.56 micron forbidden line fluxes, a temperature-insensitive density estimate may be obtained. The utility of these line ratios depends upon the actual infrared level collision strengths, which will affect the density range over which they are sensitive.

Shure, M. A.↗

NASA NTRS1983

Determinations of S III, O IV, and Ne V abundances in planetary nebulae from infrared lines

Twelve planetary nebulae have been subjected to airborne observations of the S III (18.71 microns), Ne V (24.28 microns) and O IV (25.87 microns) IR forbidden lines, of which one or more lines were detected in seven of the nebulae. These results are insensitive to nebula temperatures, in contrast to optical or UV lines, although density estimates from the latter were required to obtain abundances. The Ne V electron temperature of 11,200 + 2000 or - 1100 K temperature obtained overlaps O III temperature measurements. Since the ionization potential of Ne IV is much greater than that of O II, the electron temperature of Ne V should be much greater than that of O III. This discrepancy may indicate inaccuracies in currently available atomic parameters for Ne V.

Shure, M. A.↗

NASA NTRS1983

Detection of sulfur in the galactic center

A strong detection at the S III forbidden 18.71 micron line is reported for the galactic center region, Sgr A West. A line flux of 1.7 + or - 0.2 x 10 to the -17th W/sq cm is found for a 20 inch beam size measurement centered on IRS 1. A preliminary analysis indicates that the S III abundance relative to hydrogen is consistent with the cosmic abundance of sulfur, 0.000016, if a filling factor of unity within the known clumps is assumed. However, the sulfur abundance in the galactic center may be as much as a factor of 3 overabundant if a filling factor of 0.03 is adopted, a value found to hold for some galactic H II regions.

Herter, T.↗

NASA NTRS1983

Detection of sulphur in the galactic center

A strong detection at the (SIII) 18.71 micron line is reported for the Galactic Center region, Sgr A West. A line flux of 1.7 + or - 0.2x10 to the -17th power W cm(-2) is found for a 20-arc second beam-size measurement centered on IRS 1. A preliminary analysis indicates that the SIII abundance relative to hydrogen is consistent with the cosmic abundance of sulfur, 1.6x10 to the -5th power, if a filling factor of unity within the known clumps is assumed. However, the sulfur abundance in the Galactic Center may be as much as a factor of 3 overabundant if a filling factor of 0.03 is adopted, a value found to hold for some galactic HII regions.

Herter, T.↗

NASA NTRS1983

Determinations of SIII, OIV and NeV abundances in planetary nebulae from IR lines

Airborne observations of the infrared forbidden lines (SIII) 18.71 microns, (NeV) 24.28 microns and (OIV) 25.87 microns were made for twelve planetary nebulae. One or more of the lines was detected in seven of these nebulae and ionic abundances were calculated. These results are insensitive to nebula temperatures, in contrast to the case for optical or UV lines. However, density estimates from optical and UV forbidden lines were required to obtain abundances. The NeV infrared line flux from NGC 7662 was combined with the 3426A flux to obtain a NeV electron temperature of 11,200 (+2000, - 1100) K, which overlaps OIII temperature measurements. Since the ionization potential of NeIV is much greater than that of OII, T sub e (NeV) would be expected to be much greater than T sub e (OIII). In fact, numerical models predict T sub e (NeV) (16-20) x 1000 K. This discrepancy may indicate inaccuracies in currently available atomic parameters for NeV.

Shure, M. A.↗

NASA NTRS1983

Cooled grating infrared spectrometer for astronomical observations

A liquid helium-cooled infrared spectrometer for the 16 to 50 micron range is described. The instrument has six detectors, three each of Si:Sb and Ge:Ga and two diffraction gratings mounted back-to-back. Cold preoptics are used to match the spectrometer to the telescope. In its nominal configuration the system resolution is 0.03 micron from 16 to 30 microns and 0.07 micron from 28 to 50 microns. A cooled filter wheel is used to change order sorting filters. The gratings are driven by a steel band and gear train operating at 4 K. The detector outputs are amplified by a TIA, employing a matched pair of JFETs operating at 70 K inside the dewar. The external warm electronics include a gain stage for the TIA and dc-coupled gating circuit to remove charged-particle (cosmic-ray secondary)-induced noise spikes. The gating circuit reduces the overall system noise by a factor of two when the spectrometer is used on NASA's Kuiper Airborne Observatory. Sample spectra are presented and the deglitcher performance is illustrated.

Houck, J. R.↗

NASA NTRS1982

Observations of the infrared fine-structure lines of S III at 18.71 and 33.47 microns in four H II regions

Infrared fine-structure lines provide a particularly useful probe of ionized nebulae. The present investigation is concerned with measurements of the forbidden S III lines at 18.71 and 33.47 micrometers for four H II regions, S158A, S158G, G75.84+0.4, and W3 IRS 1. These lines are used to estimate densities, and comparisons are made with rms densities determined from radio observations to evaluate the importance of clumping. For the case of the optical H II region S158A, comparisons are made with both optical and forbidden O III line determinations of the density. The reported observations were made using a dual-grating, liquid-helium-cooled spectrometer containing a three-element Si:Sb detector array and a three-element Ge:Ga detector array. It is found that clumping is important in the cases of G75.84+0.4, W3 IRS 1, and M42. These three H II regions have filling factors of 0.024, 0.09, and 0.03, respectively.

Herter, T.↗