Engineering topics

Sellgren, K.

Publications and source records attributed to Sellgren, K..

At least 37 records · Page 2

NASA NTRS1990

The 3.3 micron feature, H2, and ionized gas in the Orion bar

The results of spectroscopy of the 3.3 micron feature, H2 emission, P-alpha, and Br-alpha, obtained along a line perpendicular to the Orion ionization front, are presented. The intensity of the 3.3 micron feature reaches a maximum between the ionization front and the H2 peak. The 3.33 micron spatial distribution appears to be due to destruction of the emitting material within the H II region and extinction of the exciting radiation between the edge of the H II region and the H2 peak, resulting in a maximum between the ionization front and the H2 peak. The H2 peak is consistent with either being due to a shock front or being due to UV-pumped fluorescence from dense clumps of H2. The width of the 3.3 micron feature observed at high spectral resolution is constant in regions of varying UV flux. It is suggested that the strength of the UV field is not the cause of the variations in the 3.3 micron feature width observed in other sources, but rather that these variations are caused by a compositional change related to the age of the emitting material.

Sellgren, K.↗

NASA NTRS1989

Absorption features in the 3 micron spectra of protostars

Low-resolution spectra have been obtained for a selection of infrared protostars and one object located behind the Taurus dark cloud. Most of the differences in the spectra can be attributed to different H2O ice temperatures combined with additional broad absorption between 3.3 and 3.5 microns plus another absorption in the 2.8-2.9 micron range. An NH3-H2O ice mixture, scattering by H2O ice-coated grains, and hydrated silicates are ruled out as explanations for the last type of absorption. The most plausible explanation is still some form of hydrocarbon in the grain mantles.

Smith, Robert G.↗

NASA NTRS1989

Mapping and spectroscopy of the 3.3 microns feature in Orion

The results of spectroscopy of the 3.3 microns feature, H2 emission, and Br alpha, obtained in a spatial cut across the Orion ionization front are presented. The spatial distribution of the 3.3 microns feature peaks between the ionization front and the H2 peak are observed. The shape of the 3.3 microns feature observed at high spectral resolution shows no changes in regions of varying ultraviolet flux and density. The results are discussed in terms of current models for the 3.3 microns emission feature.

Sellgren, K.↗

NASA NTRS1989

Stellar kinematics in the galactic centre

The observations of the stellar kinematics in the central 8 pc of the Galaxy are presented. It is shown that the spectroscopy of the 2.3 microns CO absorption feature, in late type stars, yields the stellar velocity dispersion and the average stellar radial velocity as a function of galactocentric distance. A sample spectra that illustrates the observed velocity shifrs and velocity dispersion is given. The analysis shows that the velocity dispersion of the stars is large, and that it dominates the stellar kinematics. The use of these kinematic data, to probe the mass distribution in the Galactic center, is considered.

Mcginn, M. T.↗

NASA NTRS1989

Spectroscopic images of NGC 7027 in the near-infrared dust emission features

Isophote maps derived from spatially resolved moderate spectral resolution images of the planetary nebula NGC 7027 in the 3.28-micron and 3.4-micron 'unidentified' emission features, and in the Br-alpha hydrogen recombination line are presented. Comparison of these images clearly shows that the 3.28-micron emission is spatially more extended than the ionized gas. Analysis and modeling of the spatial distribution of the 3.28-micron feature strength suggest that roughly equal amounts of feature emission appear to come from within the H II region and from a shell outside the H II region. The spatial distribution of the 3.4-micron feature emission is found to be similar to that of the 3.28-micron feature emission.

Woodward, C. E.↗

NASA NTRS1988

A study of H2O ice in the 3 micron spectrum of OH 231.8+4.2 (OH 0739-14)

Moderate resolution IR spectra of OH 231.8+4.2 in the 2.0-3.8-micron spectral region are presented. It is shown that the large-scale features of the observed spectra can only be modeled with an unusually large upper limit to the grain-size distribution (0.6-0.7 microns). Two new absorption features have been found in the wings of the 3.08-micron feature, near 2.95 and 3.2 microns. The results suggest that a mixture of amorphous and crystalline ice may have produced the 3.08-micron feature in this object.

Smith, Robert G.↗

NASA NTRS1988

High spectral resolution observations of HD 44179 at 3.2-3.7 microns

New high-resolution spectra of HD 44179 (the 'Red rectangle') in the 3.2-3.7 micron range are presented. Contrary to previous observations, no evidence is found for the 3.4 micron feature which is supposed to arise from aromatic hydrocarbons. The strong 3.29 micron feature is found to be different from that seen in NGC 7027 and IRAS 21282+5050.

Tokunaga, A. T.↗

NASA NTRS1988

Search for polarization of the 3.3 and 11.3-micron interstellar emission features

Results are reported from a search for linear polarization of the 3.3 and 11.3-micron emission features in the Orion ionization front, NGC 7027, HD 44179 (the Red Rectangle), NGC 2023, BD + 30 deg 3639, and Elias 1. Recent theoretical calculations have predicted that if these features are due to PAHs they may be linearly polarized in many astronomical situations. The most sensitive measurements of and upper limits on the feature polarization are 1 percent at 3.3 microns and 3 percent at 11.3 microns.

Sellgren, K.↗

NASA NTRS1988

Infrared images and spectroscopy of the bipolar source IRAS 09371 + 1212

High-resolution images of IRAS 09371 + 1212 in the J, H, and K bands show that this source is a small bipolar nebula viewed in the equatorial plane. A spectrum of this object in the 2.0-3.4 micron region shows a very narrow ice feature, interpreted as absorption by primarily pure crystalline water ice.

Hodapp, Klaus-Werner↗

NASA NTRS1988

High-resolution spectroscopy of the 3 micron emission features in NGC 7027 and IRAS 21282+5050

High-resolution 3 micron spectra of NGC 7027 and IRAS 21282+5050 are presented. The well-known 3.29 micron emission feature has very similar shapes in the two objects, but a small shift is present between them. The emission features at 3.46 and 3.52 microns recently found in IRAS 21282+5050 are also present in the spectrum of NGC 7027. In addition, there may be two other weak features at 3.55 and 3.58 microns in NGC 7027. These emission features might be related to saturated hydrocarbons. The spectra provide further constraints on the origin of the 'unidentified' emission features.

Nagata, Tetsuya↗

NASA NTRS1987

Radial velocities of late-type stars in the galactic center

The 2.0-2.4 micron spectra, with 120 km/s resolution, obtained for six late-type stars within 2 pc of the center of the Galaxy were used to derive spectral types and reddenings for these stars. The supergiant density in the Galactic center is lower than in previous determinations. The radial velocities of the Galactic center stars are measured to an uncertainty of 10 km/s using a cross-correlation technique. No correspondence is found between the stellar velocities and the Ne II velocities or other gas velocities observed along the same line of sight. No systematic rotation or ordered motion is seen in the stellar velocity distribution. The total mass distribution derived from the stellar velocity dispersion is compared to the stellar mass distribution derived from the 2 micron light and the total mass distribution derived from gas velocities.

Sellgren, K.↗

NASA NTRS1987

IRAS surface brightness maps of reflection nebulae in the Pleiades

Surface brightness maps at 12, 25, 60, and 100 microns were made of a 2.5 deg x 2.5 deg area of the reflection nebulae in the Pleiades by coadding IRAS scans of this region. Emission is seen surrounding 17 Tau, 20 Tau, 23 Tau, and 25 Tau in all four bands, coextensive with the visible reflection nebulosity, and extending as far as 30 arcminutes from the illuminating stars. The infrared energy distributions of the nebulae peak in the 100 micron band, but up to 40 percent of the total infrared power lies in the 12 and 25 micron bands. The brightness of the 12 and 25 micron emission and the absence of temperature gradients at these wavelengths are inconsistent with the predictions of equilibrium thermal emission models. The emission at these wavelengths appears to be the result of micron nonequilibrium emission from very small grains, or from molecules consisting of 10-100 carbon atoms, which have been excited by ultraviolet radiation from the illuminating stars.

Castelaz, Michael W.↗

NASA NTRS1986

3 micron spectroscopy of comet Halley (1982i)

Near-infrared spectroscopy and photometry was obtained of comet Halley in December 1985 when its heliocentric distance was 1.19 AU. No water ice absorption or molecular band emission were observed. The interstellar 3.3-micron emission band was also not observed. The 3-micron spectrum of comet Halley consisted of scattered sunlight at wavelengths less than 3.1 microns and of thermal emission by dust at longer wavelengths.

Tokunaga, A. T.↗

NASA NTRS1986

More interstellar emission features at 3.3-3.6 micrometers!

The present data set consists of 3.20 to 3.55 micron spectra of HD44179, NGC 7027, BD+30 3639, and Elias 1 obtained with a cooled-grating array spectrometer (CGAS) at the NASA Infrared Telescope Facility. Emission features and details of the emission feature profiles are presented for high resolution spectra. Greater complexity is shown than might be expected. It is significant that the 3.29 micron feature has an invariant central wavelength, even at high resolution, and this strongly supports the case for a very specific substance or mixture of substances which is giving rise to this feature.

Tokunaga, A. T.↗

NASA NTRS1986

IRAS surface brightness maps of visible reflection nebulae: Evidence for non-equilibrium infrared emission

Surface brightness maps at 12, 25, 60, and 100 microns of 16 visible reflection nebulae were extracted from the Infrared Astronomy Satellite (IRAS) database. The maps were produced by coadding IRAS survey scans over areas centered on the illuminating stars, and have spatial resolutions of 0.9' x 4' at 12 and 25 microns, 1.8' x 4.5' at 60 microns, and 3.6' x 5' at 100 microns. Extended emission in the four IRAS bandpasses was detected in fourteen of the reflection nebulae. The IRAS data were used to measure the flux of the infrared emission associated with each source. The energy distributions show that the 12 micron flux is greater than the 25 micron flux in 11 of the nebulae, and the peak flux occurs in the 60 or 100 micron bandpass in all 16 nebular. The 60 and 100 micron flux can be approximated by blackbodies with temperatures between 30 and 50 K, consistent with temperatures expected from extrapolation of greybody fits to the 60 and 100 micron data. The excess 12 and 25 micron emission is attributed to a nonequilibrium process such as emission from thermal fluctuations of very small grains excited by single ultraviolet photons, or emission from polycyclic aromatic hydrocarbons (PAHs) excited by ultraviolet radiation. The common features of the energy distributions of the 16 reflection nebulae, also seen in the reflection nebulae associated with the Pleiades, suggest that PAHs or very small grains may be found in most reflection nebulae.

Castelaz, M. W.↗

NASA NTRS1986

Moderate spectral resolution observations of 3 micron absorption features in highly obscured objects

The 3 micron absorption spectra of sources seen in or behind molecular clouds generally show a variety of absorption features. Three separate absorptions are used to explain these features. The cooled-grating array spectrometer (CGAS) at the NASA Infrared Telescope Facility was used to obtain spectra of the late-type mass-loss star OH 0739-12 and the protostars MonR2 IRS-2 and IRS-3 (solid circles). The differences between the spectra are discussed.

Smith, R. G.↗

NASA NTRS1986

Ultraviolet-pumped infrared fluorescent molecular hydrogen emission in reflection nebulae

Strong molecular hydrogen emission at 2.41 microns has been observed in three out of six reflection nebulae surveyed. A spectrum of one nebula, Parsamyan 18, shows several H2 lines whose intensity ratios have values agreeing with those predicted if the excitation is due to UV-pumped fluoresence and disagreeing with those predicted for shock and X-ray excitations.

Sellgren, K.↗

NASA NTRS1986

Nature of very small grains - PAH molecules or silicates?

The predictions of the model of Puget et al. (1985) for the emission from Very Small Grains (VSGs) including both graphitic and silicate components are compared with published 8-13-micron observations of astronomical sources. The VSGs are found to be mainly graphitic and an upper limit is placed on the relative mass of silicates based on lack of the 9.7-micron silicate emission feature on M 82 and NGC 2023. This dissymetry in the composition of VSGs supports the suggestion that they are formed in grain-grain collisions where the behaviors of graphite and silicate grains are expected to be quite different.

Desert, F. X.↗