Engineering topics

Hildebrand, R. H.

Publications and source records attributed to Hildebrand, R. H..

At least 19 records

NASA NTRS2000

Submillimeter Polarimetric Observations of the Galactic Center

We report the first detection of polarized submillimeter emission from the Sagittarius A region at the Galactic center. We observed three separate 2' x 2' fields: one centered on the circumnuclear disk, one centered on the peak of the molecular cloud M -0.02-0.07 (also called the "50 kilometer per second cloud"), and one centered on the peak of the molecular cloud M -0.13-0.08 (also called the "20 kilometer per second cloud"). Linear polarization at lambda = 350 micrometers was detected in each of the three regions, at a total of 106 distinct sky positions. In the circumnuclear disk, the projected magnetic field directions that we infer from our measurements are similar to those inferred from previous far-infrared (lambda = 100 micrometers) polarimetry. In the "curved ridge" region of M -0.02-0.07 that has been compressed by the expansion of Sgr A East, our results show clearly the effects of this compression on the magnetic field. In M -0.13-0.08, we observe what appears to be a stretched magnetic field, as expected for this tidally sheared cloud. It has been suggested that a "finger-like extension" or streamer" from M -0.13-0.08 is falling into the circumnuclear disk. We tentatively interpret a flaring of magnetic field lines that we see in M -0.13-0.08 as evidence that the entire cloud has a velocity component in the Galactic eastern direction, i.e., toward the circumnuclear disk. Further observations are needed to test this interpretation. We argue that polarimetry of dust emission provides a promising tool for obtaining new information on the complex dynamics of neutral gas in the Galactic center.

Novak, G.↗

NASA NTRS1999

The Far-Infrared Polarization Spectrum: First Results and Analysis

We present data on the polarization of the thermal emission from Galactic Clouds at 60 micrometers, 100 micrometers, and 350 micrometers. There are examples of rising polarization spectra in dense cloud cores [P(350 micrometers/P(100 micrometers) approximately equal to 2], and falling spectra in cloud envelopes [P(350)/P(100 micrometers) approximately equal to 0.6]. We also present data showing that the relationship, P(tau), between polarization and optical depth in cloud cores is different from that in cloud envelopes. We review the principles governing the far-infrared polarization spectrum and discuss applications to the data on P(lambda) and P(tau). We conclude that the cloud envelopes we have observed must contain two populations of grains that differ in their polarization efficiencies and in their emission spectra. We propose a model for cloud envelopes in which the contrasting populations reside in domains of different mean temperatures where the warmer domains contain the aligned grains.

Hildebrand, R. H.↗

NASA NTRS1991

100-micron array polarimetry from the Kuiper Airborne Observatory - Instrumentation, techniques, and first results

The University of Chicago far-infrared array polarimeter, 'STOKES', is the first multiple-beam polarimeter for far-infrared astronomy. Observations are made from the NASA Kuiper Airborne Observatory. Two orthogonal components of linear polarization are detected simultaneously by corresponding pairs of bolometers in two 32-detector arrays. Novel observing and data-analysis techniques are used to overcome the inherent difficulties of array polarimetry. Results from the first observing flights with the new instrument are reported for the molecular clouds W3 and W51. The measurements show that the magnetic-field structure in both clouds is nonuniform on the scale of 0.5-1.5 pc. This is consistent with molecular line and Zeeman observations that indicate the presence of turbulent velocities and significant small-scale structure. Preliminary results from the second flight series have yielded approximately 40 new measurements in the Sgr A complex. These results indicate that modifications made since the first flights have significantly improved the performance of STOKES.

Platt, S. R.↗

NASA NTRS1990

The magnetic field in the dust ring at the center of the Galaxy

Measurements of the polarization of the far-infrared thermal emission from six points in the dust ring at Sgr A are presented. The position angles are approximately perpendicular to the long axis of the ring as projected on the sky. The inferred magnetic field is therefore approximately in the plane of the ring. The pattern traced by the polarization vectors resembles that expected for a magnetic accretion disk. The measurements indicate a field in which the outward radial component is much greater than the axial component at the surface of the disk. The field thus appears to satisfy the condition proposed by Blandford and Payne (1982) for removing energy and angular momentum through centrifugal acceleration of surface material moving along the field lines.

Hildebrand, R. H.↗

NASA NTRS1990

The far-infrared polarization of the Orion nebula

Polarization of the 100 micron thermal emission from 10 points in the Orion nebula has been measured. At one of the positions the degree, 5.7 percent, is the largest far-infrared polarization yet discovered. Except at a position in the barlike structure to the south, the position angles of the polarization vectors are well-ordered, suggesting that a uniform magnetic field threads the cloud. The magnetic field strength is estimated to be between 0.7 and 4 mG. The relationship of the degrees of polarization to the physical conditions in the cloud is discussed.

Gonatas, D. P.↗

NASA NTRS1989

Polarization of far-infrared radiation from molecular clouds

The paper reports measurements of the polarization of far-infrared emission from dust in nine molecular clouds. Detections were obtained in Mon R2, in the Kleinmann-Low (KL) nebula in Orion, and in Sgr A. Upper limits were set for six other clouds. A comparison of the 100 micron polarization of KL with that previously measured at 270 microns provides new evidence that the polarization is due to emission from magnetically aligned dust grains. Comparing the results for Orion with measurements at optical wavelengths, it is inferred that the magnetic field direction in the outer parts of the Orion cloud is the same as that in the dense core. This direction is nearly perpendicular to the ridge of molecular emission and is parallel to both the molecular outflow in KL and the axis of rotation of the cloud core. In Mon R2, the field direction which the measurements imply does not agree withthat derived from 0.9-2.2 micron polarimetry. The discrepancy is attributed to scattering in the near-infrared. In Orion and Sgr A, where comparisons are possible, the measurements are in good agreement with 10 micron polarization measurements.

Novak, G.↗

NASA NTRS1989

A 100-micron polarimeter for the Kuiper Airborne Observatory

Consideration is given to the design and performance of the 100-micron polarimeter proposed for use on the NASA Kuiper Airborne Observatory. The polarimeter specifications are listed. The polarimeter design and data reduction techniques are based on the work of Hildebrand et al. (1984) and Dragovan (1986). The polarimeter has an improved signal-to-noise ratio and systematic measurement errors below 0.2 percent.

Novak, G.↗

NASA NTRS1986

Focal plane optics in far-infrared and submillimeter astronomy

The construction of airborne observatories, high mountain-top observatories, and space observatories designed especially for infrared and submillimeter astronomy has opened fields of research requiring new optical techniques. A typical far-IR photometric study involves measurement of a continuum spectrum in several passbands between approx 30 microns and 1000 microns and diffraction-limited mapping of the source. At these wavelengths, diffraction effects strongly influence the design of the field optics systems which couple the incoming flux to the radiation sensors (cold bolometers). The Airy diffraction disk for a typical telescope at submillimeter wavelengths approx 100 microns-1000 microns is many millimeters in diameter; the size of the field stop must be comparable. The dilute radiation at the stop is fed through a Winston nonimaging concentrator to a small cavity containing the bolometer. The purpose of this paper is to review the principles and techniques of infrared field optics systems, including spectral filters, concentrators, cavities, and bolometers (as optical elements), with emphasis on photometric systems for wavelengths longer than 60 microns.

Hildebrand, R. H.↗

NASA NTRS1985

Far infrared and submillimeter brightness temperatures of the giant planets

The brightness temperatures of Jupiter, Saturn, Uranus, and Neptune in the range 35 to 1000 micron. The effective temperatures derived from the measurements, supplemented by shorter wavelength Voyager data for Jupiter and Saturn, are 126.8 + or - 4.5 K, 93.4 + or - 3.3 K, 58.3 + or - 2.0 K, and 60.3 + or - 2.0 K, respectively. The implications of the measurements for bolometric output and for atmospheric structure and composition are discussed. The temperature spectrum of Jupiter shows a strong peak at approx. 350 microns followed by a deep valley at approx. 450 to 500 microns. Spectra derived from model atmospheres qualitatively reproduced these features but do not fit the data closely.

Hildebrand, R. H.↗

NASA NTRS1985

Focal plane optics in far-infrared and submillimeter astronomy

Hildebrand, R. H.↗

NASA NTRS1985

Far-infrared and submillimeter brightness temperatures of the giant planets

The brightness temperatures of Jupiter, Saturn, Uranus, and Neptune were measured in the 35-1000 micron range with the 3-m NASA Infrared Telescope Facility (at wavelengths greater than 350 microns) and with the Kuiper Airborne Observatory (at wavelengths less than 350 microns). The data indicate the presence in Jupiter's spectrum of excess radiation (compared to theoretical models) at 300-400 microns. In addition, slightly less flux was observed from Saturn at 200 microns than predicted by atmospheric models, which suggests the possible presence of an unmodeled absorber. The submillimeter fluxes from Uranus and Neptune appear to be most consistent with low mixing ratios (less than 1 percent) of CH4 in their deep atmospheres.

Hildebrand, R. H.↗

NASA NTRS1984

Far Infrared and Submillimeter Observations of the Giant Planets

Far infrared measurements of the effective temperatures of Jupiter, Saturn, Uranus and Neptune were made. The measurements presented here cover the range from 35-1000 micrometers in relatively narrow bands. The observations at lambda 350 micrometers were made at the 3m NASA Infrared Telescope Facility (IRTF) of the Mauna Kea Observatory; those at lambda 350 micrometer were made on the Kuiper Airborne Observatory (KAO). All observations of Saturn were made when the ring inclination to Earth was 1.7 deg assuring an unambiguous measurement of the flux from the disk itself. Mars was used as the calibration reference. The results represent a consistent set of calibration standards. In these measurements, it is assumed that sub b(lambda = 350 micrometers) = T sub (lambda 350 micrometers). Measurements have been made of roughly 50% of the total flux emitted by Jupiter, 65% by Saturn, and 92% by Uranus and Neptune. These measurements therefore permit a considerable reduction in the uncertainties associated with the bolometric thermal outputs of the planets. The effective temperatures (T sub e) and the ratios of emitted to absorbed solar radiation were calculated.

Loewenstein, R. F.↗

NASA NTRS1984

First Results of Submillimeter Polarimetry

Airborne results of submillimeter polarization at one wavelength, 270 micro m for just three points in the sky are presented. Polarizations of 1.7% at each of two points in Orion are shown. A null result at 400 micro m from ground-based observations of Mars at opposition is also presented. A null result for W3(OH) is given. The Kleinmann-Low Nebula (KL) was chosen for one of the measurements because it is bright and polarization had been observed 10 micro m. Airborne results of submillimeter polarimetry indicate that: (1) Cool, dense interstellar clouds can emit polarized submillimeter radiation; (2) The direction of the magnetic field, averaged over the 90 beams, is the same for the Kleinmann-Low Nebula and the 400 micro m peak 1.5 south of the Nebula; and (3) The effectiveness of the grain alignment mechanism, averaged over the 90 beams, is the same for the Kleinmann-Low Nebula and the 400 micro m peak.

Hildebrand, R. H.↗

NASA NTRS1984

Detection of submillimeter polarization in the Orion Nebula

Linear polarization of the submillimeter (270 micron) continuum radiation from two regions of Orion was observed: one centered on the Kleinmann-Low Nebula and one centered on the 400 micron peak 1.5' south of the nebula. The polarizations measured for these regions are P = (1.7 +/-0.4)% at phi = 23 deg +/-7 deg and P=(1.7 +/- 0.5)% at phi = 27 deg +/- 7 deg respectively. A 2(sigma) upper limit, P or = 1.6%, was found for the nebular W3(OH). The position angle at KL is orthogonal to that measured at 11 microns by Dyck and Beichman and at 11 and 20 microns by Knacke and Capps. The far-IR values for KL reported by Gull et. al. (approx 2%) and by Cudlip et al. (1 to 2% level) are consistent with the submillimeter results.

Hildebrand, R. H.↗

NASA NTRS1984

Far-infrared and submillimeter observations of the multiple cores in S255, W3, and OMC-1 - Evidence for fragmentation?

The two cores of the S255 cloud were mapped at 60, 100, 185, and 400 microns and 40-400 microns maps were made of the cores and S255, OMC-1 and W3. Absolute flux densities were determined with an accuracy of 30 percent. The luminosities and masses of the S255 cores were equivalent to those of the other objects, including the concentration of a compact H II region around young objects. The densified regions are regarded as typical of areas where massive objects are forming. The observed 1.4-4 km/sec velocity differences between two cores in any one cloud indicates that the angular momentum of the original clouds, which drove the collapse of the cores, is transferred to the massive young stars.

Jaffe, D. T.↗

NASA NTRS1984

Detection of submillimeter polarization in the Orion nebula

Linear polarization has been observed in the submillimeter radiation (270 microns) from two regions of Orion: one centered in the Kleinmann-Low nebula and one centered 1.5 arcmin south of the nebula. The observations were performed in September of 1983 and January of 1984 with the NASA Kuiper Airborne Observatory (KAO). The polarizations measured for the two regions were both 1.7 percent, plus or minus 0.4 and 0.5 percent, respectively. The angle of the outflow from both sources was 27 degrees, plus or minus seven degrees. An upper limit for polarization in the submillimeter radiation from the nebula W3(OH) was established at 1.6 percent. The observational data are compared with results from several other recent polarimetric observations of Orion, and some of their implications are discussed.

Hildebrand, R. H.↗

NASA NTRS1984

Far-infrared selected star formation regions

Detailed far-IR observations and complementary submillimeter, 5 GHz continuum and C(O-18) observations of a sample of eight far-IR selected luminous regions of star formation are presented. The observations show that the sources of luminosity coincide with density peaks in the molecular clouds and that the exciting stars lie deep within these condensations. The far-IR sources have diversely shaped 40-180 micron spectra even though their 60-100 micron color temperatures are similar. The radio and far-IR results together show that the exciting stars are in clusters containing either zero-age main-sequence and pre-main-sequence stars or consisting entirely of pre-main-sequence objects. C(O-18) and submillimeter observations imply gas densities approximately 100,000-high enough to make T(dust) approximately T(gas).

Harper, D. A.↗

NASA NTRS1984

Far-IR selected star formation regions

Detailed far-IR observations and complemenary submillimeter, 5 GHz continuum and c(18)0 observations of a sample of far-IR selected luminous regions of star formation. The clouds and that the exciting stars lie deep within these condensations. The far-IR sources have diversely shaped 40 micron to 180 micron spectra even through their 60 micron to 100 micron color temperatures are similar. The radio and far-IR results together show that the exciting stars are in clusters containing either zero-age main sequence and pre-main sequence stars or consisting entirely of premain sequence objects. C(18)0 and submillimeter observations imply gas densities approximately .00005 - high enough to make t(sub dust) approximately t(sub gas).

Jaffe, D. T.↗