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Hackwell, John A.

Publications and source records attributed to Hackwell, John A..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

The Link Between UV Extinction and Infrared Cirrus

Low resolution spectra from the International Ultraviolet Explorer satellite were used to derive ultraviolet extinction curves for stars in four clusters away from the galactic plane. The extinction in three of the clusters is very similar to the general interstellar curve defined by Seaton. Stars in the fourth region, near the Rho Ophiuci dark cloud, have extinction curves that are characterized by a small "linear" term component. The star BD +36 deg 781 is unique amongst the 20 stars observed in that it shows evidence for extinction by diamond grains near 1700 angstroms. We used data from the final release of the IRAS Sky Survey Atlas (ISSA) to determine the 60 micron to 100 micron intensity ratio for the infrared cirrus. The ISSA data, which have been corrected for zodiacal light, gave intensity ratios that are more robust and self-consistent than for other data sets that we used. When the infrared and ultraviolet data are combined, we see a general trend for low values of the ultraviolet "linear term" (al) to correlate with high values of 60 micron/100 micron ratio. This implies that, in regions where the average dust temperature is hotter (high 60 micron/100 micron ratio), there is a relative absence of the small silicate grains that are responsible for the ultraviolet linear term. However, the new data do not bear out our earlier contention that the 60 micron and 100 micron emissions are poorly correlated spatially in regions where the 60 micron/100 micron ratio is low. Only NGC 1647 shows this result. It may be that the different dust types are particularly poorly mixed in this area.

Hackwell, John A.↗
NASA NTRS1996

Mid-Infrared Spectra of Comets P/Borrelly, P/Faye, and P/Schaumasse

A 10 micron silicate emission feature has been discovered in the spectra of comets P/BorrelIy and P/Faye at R approximately 1.5 AU. These are the first short period comets in which silicate emission has definitely been detected. The broad emission features are about 25% above the continuum. No emission feature was present in the spectrum of P/Schaumasse; it is possible that the nucleus of P/Schaumasse was directly detected. If all of the observed flux originated from the nucleus, then the effective radius is about 3 km; the observed color temperature is consistent with a rapidly rotating nucleus. We present models that show how the shape of the silicate feature can depend on the way in which silicate and absorbing material are mixed in the grains.

Hanner, Martha S.↗
NASA NTRS1994

Silicate emission feature in the spectrum of comet Mueller 1993a

An 8- to 13-micron spectrum of comet Mueller 1993a, a dynamically new comet, was acquired when the comet was at R = 2 AU. Strong, structured silicate emission is present, closely resembling that seen in Comet P/Halley at smaller R. For the first time in a new comet, the 11.2-micron peak of crystalline olivine was detected, demonstrating that crystalline olivine particles were widespread in the solar nebula. Crystalline olivine particles could have formed in the inner protosolar nebula at temperatures greater than 1200 K; extensive radial mixing would have been required to transport these grains to the region of comet formation. Either there was more radial mixing in the solar nebula than some current theories predict or the olivine grains have a presolar origin.

Hanner, Martha S.↗
NASA NTRS1992

8- to 13-micron spectroscopy of Comet Levy 1990 XX

The results are reported of IR spectroscopy of Comet Levy 1990 XX over a three-day period when the comet was about 1.54 AU from the sun roughly 70 days before perihelion. Comet Levy 1990 XX was bright, and for at least part of its inbound journey toward perihelion, active. At a distance of 1.54 AU from the sun it showed strong structured silicate emission with peaks or shoulders at 9.8 and 11.2 microns. These features resemble those of Comets P/Halley and Bradfield 1987 XXIX. The comet was variable in brightness. Specifically, the contrast of the silicate features changed by a factor of two relative to the continuum level and showed some evidence for a shape change as well.

Lynch, David K.↗
NASA NTRS1992

CO (v = 1-0) emission in the molecular shock regions of OMC-1

Using the new Aerospace spectrometer on the Kuiper Airborne Observatory, we have obtained observations of the molecular shocks associated with OMC-1. Unexpectedly these observations reveal (b = 1-0) emission from CO at 4.6 microns superposed on a strong continuum. Our observations strongly suggest that both the emission feature and the continuum are produced in molecular shocks. Since the (v = 1-0) band of CO is only excited in high-velocity shocks, we may be observing for the first time the primary driving mechanism in these regions. Even if these features are produced by scattering, the characteristics will provide new constraints on the conditions in and the geometry of the shock regions.

Grasdalen, G. L.↗
NASA NTRS1991

Observations that link infrared cirrus and ultraviolet extinction

Results are presented of UV extinction measurements of 17 stars in the open cluster IC 4665 and of four stars in the open cluster NGC 1647, showing that the UV extinctions of the two clusters stars have different properties. Whereas the stars in NGC 1647 cluster are typical of general interstellar space, the stars in IC 4665 have a small 'linear' term which is typical of stars found near regions of active star formation. Using data from the IRAS satellite to study the emission by the 'infrared cirrus' for each of the regions and to measure the I(60-micron)/I(100-micron) ratio for the two regions, it was found that, when the linear term component is present, it dominates the 100-micron cirrus emission. When it is absent, the 100-micron emission is from the relatively hot particles that are responsible for the 60-micron emission. It is suggested that the 60-micron emitters are small carbonaceous grains that are major contributors to all parts of the UV extinction except the linear term.

Hackwell, John A.↗
NASA NTRS1990

High spatial-resolution IRAS images of M51

High spatial-resolution (approx. equal to 30 seconds) images of M51 in the four Infrared Astronomy Satellite (IRAS) bands (12, 25, 60, and 100 microns) have been obtained. The spatial variation in flux in all four bands is coincident with the spiral features seen in H alpha and 6 cm with a few exceptions. In the nuclear region (4 minutes) the position of the peak of maximum intensity shifts in relation to the visual nucleus: it is coincident with the nucleus at 12 microns, shifts approximately 45 seconds to south-southwest, and is 45 seconds northwest of the nucleus at 60 and 100 microns.

Canterna, R.↗
NASA NTRS1990

High spatial resolution restoration of IRAS images

A general technique to improve the spatial resolution of the IRAS AO data was developed at The Aerospace Corporation using the Maximum Entropy algorithm of Skilling and Gull. The technique has been applied to a variety of fields and several individual AO MACROS. With this general technique, resolutions of 15 arcsec were achieved in 12 and 25 micron images and 30 arcsec in 60 and 100 micron images. Results on galactic plane fields show that both photometric and positional accuracy achieved in the general IRAS survey are also achieved in the reconstructed images.

Grasdalen, Gary L.↗
NASA NTRS1989

High-resolution H and K mapping of W51

The H-II region W51 has been mapped in the near-infrared H and K wavebands with a resolution of about 6 arcsec over an area of about 14 sq arcsec. The W51 IRS 1 source has been resolved into six components, and an arc-like extension of small H-II regions is found that seems to parallel the edge of the molecular cloud. Measurements of the fluxes and extinctions of these sources are presented. A close correspondence is noted between the present K maps and the radio maps of Scott (1978).

Little, Stephen J.↗
NASA NTRS1987

Airborne spectrophotometry of Eta Carinae from 4.5 to 7.5 microns and a model for source morphology

Spectrophotometric observations of Eta Car between 4.5 and 7.5 microns show a featureless thermal-like spectrum with no fine-structure lines or broad emission or absorption features. The color temperature of the spectrum is approximately 375 K. High spatial resolution maps at 3.5, 4.8, and 10 microns obtained from the ground are used to discuss the dust distribution and temperature structure, and to present a model for general source morphology. The upper limit to the brightness of the forbidden Ar II fine-structure emission line at 6.98 microns is less than 7 x 10 to the -16th W/sq cm, which still allows for a significant overabundance of argon and is consistent with the evolved nature of the source.

Russell, Ray W.↗
NASA NTRS1987

The discovery of an infrared reflection nebula surrounding R Coronae Australis IRS 5

The infrared source R CrA IRS 5 in the R CrA dark cloud has been mapped, and it is found that it is extended at H (1.6 micron) and at K (2.2 micron). The geometry of the data suggest that this object consists of a disk and polar reflection lobe surrounding a star, similar to GSS 30 in the Ophiuchus dark cloud and to SGS 1 in the NGC 1333 dark cloud. The illuminating star has an effective temperature below 8000 K, a luminosity less than 9 solar luminosities, and lies above the main sequence. The luminosity of this star is smaller than the luminosity of any other known star which illuminates an infrared reflection nebula.

Castelaz, Michael W.↗