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Sellgren, K.

Publications and source records attributed to Sellgren, K..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

New Infrared Emission Features and Spectral Variations in Ngc 7023

We observed the reflection nebula NGC 7023, with the Short-High module and the long-slit Short-Low and Long-Low modules of the Infrared Spectrograph on the Spitzer Space Telescope. We also present Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) images of NGC 7023 at 3.6, 4.5, 8.0, and 24 m. We observe the aromatic emission features (AEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 m, plus a wealth of weaker features. We find new unidentified interstellar emission features at 6.7, 10.1, 15.8, 17.4, and 19.0 m. Possible identifications include aromatic hydrocarbons or nanoparticles of unknown mineralogy. We see variations in relative feature strengths, central wavelengths, and feature widths, in the AEFs and weaker emission features, depending on both distance from the star and nebular position (southeast vs. northwest).

molecules↗
NASA NTRS2002

CNO abundances in the quintuplet cluster M supergiant 5-7

We present and analyze infrared spectra of the supergiant VR 5-7, in the Quintuplet cluster 30 pc from the Galactic center. Within the uncertainties, the [C/H],[N/H], and [O/H] abundances in this star are equal of Ori, a star which exhibits mixing of CNO processed elements, but distinct from the abundance patterns in IRS 7.

stars Galactic Center↗
NASA NTRS1999

ISO Mid-Infrared Spectra of Reflection Nebulae

Our goal is to test predictions of models attributing the IEFs to polycyclic aromatic hydrocarbons (PAHs). Interstellar models predict PAHs change from singly ionized to neutral as the UV intensity, Go, decreases.

reflection nebulae dust infrared astronomy↗
NASA NTRS1996

Really Cool Stars at the Galactic Center

New and existing K-band spectra for 19 Galactic center late-type stars have been analyzed along with representative spectra of disk and bulge M giants and supergiants. Absorption strengths for strong atomic and molecular features have been measured. The Galactic center stars generally exhibit stronger absorption features centered near Na I (2.206 microns) and Ca I (2.264 microns) than representative disk M stars at the same CO absorption strength. Based on the absolute K - band magnitudes and CO and H2O absorption strengths for the Galactic center stars and known M supergiants and asymptotic giant branch (AGB) stars, we conclude that only IRS 7 must be a supergiant. Two other bright stars in our Galactic center sample are likely supergiant as well. The remaining bright, cool stars in the Galactic center that we have observed are most consistent with being intermediate mass/age AGB stars. We identify four of the Galactic center stars as long period variables based on their K-band spectral properties and associated photometric variability. Estimates of initial masses and ages for the GC stars suggest multiple epochs of star formation have occurred in the Galactic center over the last 7-100 Myr. 0 1996 American Astronomical Society.

Blum, R. D.↗
NASA NTRS1996

Really Cool Stars at the Galactic Center

New and existing K-band spectra for 19 Galactic center late-type stars have been analyzed along with representative spectra of disk and bulge M giants and supergiants. Absorption strengths for strong atomic and molecular features have been measured. The Galactic center stars generally exhibit stronger absorption features centered near Na I (2.206 microns) and Ca I (2.264 microns) than representative disk M stars at the same CO absorption strength. Based on the absolute K-band magnitudes and CO and H2O absorption strengths for the Galactic center stars and known M supergiants and asymptotic giant branch (AGB) stars, we conclude that only IRS 7 must be a supergiant. Two other bright stars in our Galactic center sample are likely supergiants as well. The remaining bright, cool stars in the Galactic center that we have observed are most consistent with being intermediate mass/age AGB stars. We identify four of the Galactic center stars as long period variables based on their K-band spectral properties and associated photometric variability. Estimates of initial masses and ages for the GC stars suggest multiple epochs of star formation have occurred in the Galactic center over the last 7-100 Myr.

Blum, R. D.↗
NASA NTRS1995

Near-infrared polarization in the bipolar outflow OH 0739-14

We present linear polarization observations of the bipolar outlfow source OH 0739-14 from 1.2 to 3.6 micrometers. The high levels of polarization (approximatly 47% in the bipolar lobes) and the angles of the vectors in the outflow lobes imply that the 1.2-3.6 micrometer polarization is due to single scattering by dust grains of light from the central source or from its immediate vicinity. Our polarization measurements, combined with phase-lag measurements of variability in the nebula by Kastner et al. (1992), tightly constrain the inclination angle i between the bipolar axis and the plane of the sky to be 35 deg less than or = i less than or = 37 deg. We observe the percentage polarization of the bipolar lobes to be constant with wavelength from 1.2 to 3.6 micrometers, which rules out any significant contribution by unpolarized emission, such as tiny grain emission, to the 3.6 micrometer emission. We propose to explain the K-L' color of the nebula as due to illumination by both the central star and by thermal emission from dust in a surrounding circumstellar shell with a dust temperature of 600-1000 K. Using this model, we find a relatively high minimum scattering optical depth at 3.75 micrometers of tau omega greater than 0.1. This is difficult to reconcile with Rayleigh scattering, which would then imply optically thick scattering at wavelengths of 1.2 and 1.65 micrometers, in constrast to the observations. We also find that the albedo of the grains at 3.75 micrometers and probably at 2.2 micrometers is higher than predicted for normal interstellar grains.

Shure, Mark↗
NASA NTRS1994

Near-infrared absorption spectroscopy of interstellar hydrocarbon grains

We present new 3600 - 2700/cm (2.8 - 3.7 micrometer) spectra of objects whose extinction is dominated by dust in the diffuse interstellar medium. The observations presented here augment an ongoing study of the organic component of the diffuse interstellar medium. These spectra contain a broad feature centered near 3300/cm (3.0 micrometers) and/or a feature with a more complex profile near 2950/cm (3.4 micrometers), the latter of which is attributed to saturated aliphatic hydrocarbons in interstellar grains and is the primary interest of this paper. As in our earlier work, the similarity of the absorption bands near 2950/cm (3.4 micrometers) along different lines of sight and the correlation of these features with interstellar extinction reveal that the carrier of this band lies in the dust in the diffuse interstellar medium (DISM). At least 2.5% of the cosmic carbon in the local interstellar medium and 4% toward the Galactic center is tied up in the carrier of the 2950/cm (3.4 micrometer) band. The spectral structure of the diffuse dust hydrocarbon C-H stretch absorption features is reasonably similar to UV photolyzed laboratory ice residues and is quite similar to the carbonaceous component of the Murchison meteorite. The similarity between the DISM and the meteoritic spectrum suggests that some of the interstellar material originally incorporated into the solar nebula may have survived relatively untouched in primitive solar system bodies. Comparisons of the DISM spectrum to hydrogenated amorphous carbon and quenched carbonaceous composite are also presented. The A(sub V)/tau ratio for the 2950/cm (3.4 micrometer) feature is lower toward the Galactic center than toward sources in the local solar neighborhood (approximately 150 for the Galactic center sources vs. approximately 250 for the local ISM sources). A similar trend has been observed previously for silicates in the diffuse medium by Roche & Aitken, suggesting that (1) the silicate and carbonaceous materials in the DISM may be physically correlated and (2) there is either dust compositional variation in the galaxy or galactic variation in the grain population density distribution. We also note a possible absorption feature near 3050/cm (3.28 micrometers), a wavelength position that is characteristic of polycyclic aromatic hydrocarbons (PAHs).

Pendleton, Y. J.↗
NASA NTRS1994

The 3.2-3.6 micron spectra of monoceros R2/IRS-3 and Elias 16

We have obtained 3.2-3.6 micron spectra, with a resolution lambda/delta-lambda approximately 750, of the protostar Mon R2/IRS-3 and of Elias 16, a background K giant behind the Taurus molecular cloud. A feature at 3,482 microns (2872/cm), with a full width at half-maximum of 0.09 microns (76/cm), is clearly seen in Mon R2/IRS-3. This feature is not detected in Elias 16. The 3.482 micron feature in Mon R2/IRS-3 is similar to a feature at 3.466-3.478 microns (2875-2885/cm) detected by Allamandola et al. in four protostars and attributed by these authors to a CH stretch in hydrocarbons dominated by sp3-bonded carbon. Neither Mon R2/IRS-3 nor Elias 16 shows absorption at 3.540 microns (2825/cm), which has been detected in two of the four protostars observed by Allamandola et al. and attributed by them to CH3OH ice. Our limit on CH3OH ice toward Elias 16 is compared to models of gas-grain chemistry in dark clouds. Our results confirm those of Allamandola et al. that at this resolution the 3.4 micron absorption due to dust in molecular clouds has very different spectral structure than that due to dust in the diffuse interstellar medium.

Sellgren, K.↗
NASA NTRS1992

Scattering of infrared radiation by dust in NGC 7023 and NGC 2023

The contribution of scattered light to the total nebular emission is determined on the basis of linear polarization measurements at 1.25, 1.65, and 2.2 microns of the visual reflection nebulae NGC 7023 and NGC 2023. The percentage polarization of NGC 7023 slowly increases from 0.3 to 1 micron, with peak polarizations of up to 26 percent at 1.25 micron, then rapidly decreases, with values of 4-7 percent at 2.2 microns. This is interpreted as implying that scattered starlight contributes most to the SW emission, while unpolarized emission from small grains or large molecules dominates at longer wavelengths. IR polarization and surface brightness measurements are combined to derive the intensity of scattered light, which is then compared with scattering models. While the near-IR emission of both NGC 2023 and NGC 7023 is dominated by small-grain or large-molecule emission, IR scattered light plays a larger role in NGC 2023 than in NGC 7023.

Sellgren, K.↗
NASA NTRS1992

The 3 micron spectrum of the classical Be star Beta Monocerotis A

A 3.1-3.7-micron spectrum of the classical Be star Beta Mon A is presented at a resolution of lambda/Delta-lambda of 700-800. The spectrum shows strong hydrogen recombination lines, including Pf-delta and a series of Humphreys lines from Hu 19 to Hu 28. The relative recombination line strengths suggest that Pf-delta has a large optical depth. The Humphreys lines have relative strengths consistent with case B and may be optically thin. The line widths observed are broader than the Balmer lines and similar in width to Fe II lines, consistent with a disk model in which optically thinner lines arise primarily from a faster rotating inner disk, while optically thicker lines come mainly from a slower rotating outer disk. The apparent lack of Stark broadening of the Humphreys lines is used to place an upper limit on the circumstellar electron density of about 10 exp 12/cu cm.

Sellgren, K.↗
NASA NTRS1991

High-resolution spectra of the 3.29 micron interstellar emission feature - A summary

High spectral resolution observations of the 3.29-micron interstellar emission feature show two types of profiles. Type 1 has a central wavelength of 3.289-micron and is observed in extended objects such as planetary nebulae and H II regions. Type 2 has a central wavelength of 3.296 microns and is observed around a small number of stellar sources. Type 2 has a full width at half-maximum of 0.020 micron; Type 1 has a broader FWHM, perhaps as much as 0.042 micron, but this is uncertain because of contamination by Pf(delta) emission. These profiles are tabulated for comparison to laboratory data. It is found that no proposed identification for the 3.29-micron emission feature definitely matches the observational spectra, although amorphous aromatic materials and heated polycyclic aromatic hydrocarbons tend to fit the best.

Tokunaga, A. T.↗
NASA NTRS1991

The interstellar C-H stretching band near 3.4 microns - Constraints on the composition of organic material in the diffuse interstellar medium

The composition and history of dust in the diffuse ISM was studied using 3600-2700/cm absorption spectra of objects which have widely varying amounts of visual extinctions along different lines of sight. The 3300/cm and 2950/cm features are attributed to O-H and C-H stretching vibrations, respectively. The O-H feature in OH 32.8-0.3 is suggestive of circumstellar water ice and is probably not due to material in the diffuse ISM. The features in the 3100-2700/cm region are attributed either to C-H vibrations or to M stars. The spectra of the latter show a series of narrow features in this region that are identified with photospheric OH. Objects in which these bands are seen include OH 01-477, T629-5, and the Galactic center source IRS 7. The C-H stretch feature of diffuse ISM dust has subpeaks which fall within 5/cm of C-H stretching vibrations in the -CH2- and -CH3 groups of saturated aliphatic hydrocarbons.

Sandford, S. A.↗
NASA NTRS1991

An infrared jet in Centaurus A - A link to the extranuclear activity in distant radio galaxies?

High-resolution NIR images of the visually obscured central region of Centaurus A (NGC 5128) were obtained with the University of Texas array camera on the AAT in June 1988, in order to investigate the effect of the active nucleus on the surrounding galaxy. The J (1.25 micron), H (1.65 micron), and K (2.2 micron) images of the central 40 arcsec of the galaxy revealed an emission feature extending about 10 arcsec northeast of the nucleus at the same position angle as the X-ray and radio jets. This jet is most prominent at the 1.25 micron wavelength, where its brightness was comparable to that of the nucleus. The observed properties of the 'infrared jet' were found to be similar to those seen in distant radio sources.

Joy, Marshall↗
NASA NTRS1990

The excitation of 12 micron emission from very small particles

The results of IRAS observations of visual reflection nebulae illuminated by stars with effective temperatures between 3000 and 33,000 K are presented. The ratio of the energy radiated in the IRAS 12-microns band to the total energy radiated at far-infrared wavelengths (60 and 100 microns) is about 0.2, with little or no dependence on the temperature of the illuminating star for stellar temperatures between about 5000 and 33,000 K. This is interpreted as requiring that visual photons as well as ultraviolet photons are capable of exciting 12-microns emission from very small particles.

Sellgren, K.↗
NASA NTRS1990

Velocity dispersion and the stellar population in the central 1.2 parsecs of the Galaxy

New observations of the 2.3 micron absorption band head of CO in integrated stellar light in the central 1.2 pc of the Galaxy are presented. These data confirm that the velocity dispersion of the stars increases toward the Galactic center and totally dominates their systematic rotation at small radii. The velocity dispersion is constant an average value of 125 km/s within about 30 arcsec projected diameter central region. The strength of the CO features decreases in the integrated faint light within the same region. These observations are all consistent with the hypothesis that both the diffuse 2 micron light and the bright 2 micron point sources in the central region are dominated by sources that have little or no CO absorption feature. The observed CO feature at projected distances less than 0.6 pc thus actually arises in stars that are at true radii greater than 0.6 pc. The mass distribution at radius between 3.7 and 0.6 pc from IRS 16 is strong evidence for an increasing mass/2 micron radiation ratio toward the Galactic center.

Sellgren, K.↗
NASA NTRS1990

Very small grains in the Milky Way and external galaxies

These studies of the infrared colors of reflection nebulae, HL HI clouds, HII regions and external galaxies have shown the following results. Different classes of objects locate in different regions on the R vs F sub v (60)/F sub v (100) diagram. This is determined both by differences in dust properties and by differences in the illuminating radiation field. For example, HL clouds and reflection nebulae almost have the same behavior since both are in the diffuse Interstellar Medium (ISM) and can be expected to have similar grain populations; the small difference in their infrared colors can be explained by the difference of the illuminating radiation field. On the other hand, the dramatic difference of R=vF sub v (12)/F sub v (far IR) between HII region and diffuse ISM may be due to the destruction of the Very Small Grain (VSG) component in the HII regions, although radiation transfer effects may play a part as well. The ratio R=vF sub v (12)/F sub v (far IR) is approximately constant in normal spiral galaxies. This implies that the mass ratio b=Mass (VSG)/Mass (dust) does not vary greatly from one galaxy to another.

Luan, L.↗