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At least 1,171 records · Page 65
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NASA NTRS1993

Infrared fine-structure line diagnostics of shrouded active galactic nuclei

Far-infrared spectroscopy of celestial objects will improve dramatically in the coming decade, allowing astronomers to use fine-structure line emission to probe photoionized regions obscured in the optical band by thick clouds of dust. The ultraluminous far-IR galaxies revealed by IRAS, quasar-like in luminosity but smothered in molecular gas, probably conceal either immense starbursts or luminous active nuclei. In both scenarios, these objects ought to produce copious infrared fine-structure emission with several lines comparable to H(beta) in luminosity. This paper shows how these lines, if detected, can be used to determine the electron densities and far-IR obscurations of shrouded photoionized regions and to constrain the shape and ionization parameter of the ionizing spectra. The presence of (Ne V) emission in particular will distinguish shrouded AGN's from shrouded starbursts. Since all active galaxies photoionize at least some surrounding material, these diagnostics can also be applied to active galaxies in general and will aid in studying how an active nucleus interacts with the interstellar medium of its host galaxy.

Voit, G. M.↗
NASA NTRS1993

Powering the superwind in NGC 253

NGC 253 is a prototypical moderate nuclear starburst galaxy. It is a barred SBc spiral galaxy at a distance of approximately 3 Mpc and can be studied on scales down to 15 pc in the optical and near IR. It is a bright IRAS source with a flux of 1000 Jy at 60 microns and a FIR luminosity of 3 x 10(exp 10) solar luminosity. It has a strong Br(gamma) emission line, a signature of ongoing massive star formation and deep CO absorption bands, which are indicative of the dominance of red supergiants in the near IR. It contains a population of compact radio sources, similar to those seen in M82. Optical spectra show that the nucleus is heavily reddened, with a Balmer decrement of approximately 30. NGC 253 possesses a 'superwind,' seen both in x-ray emission and in optical line emission. Nuclear ejection was first suggested to explain the kinematics of the nuclear region. We have obtained J, H, and K images of the entire galaxy at 1.3 arcsec/pixel (18 pc/pixel) using the superconducting quantum interference device (SQUID) on the KPNO 1.3 m. We have constructed a mosaic of 180 s exposures which traces the galaxy over much of its optical extent. The data were shifted, rotated, magnified, and calibrated following normal practice.

Watson, Alan↗
NASA NTRS1992

Limits on detectability of mass loss from cool dwarfs

Recent spectroscopic evidence supports the theoretical expectation that certain cool dwarfs may have stellar winds with M-dot values several orders of magnitude larger than the solar rate. For large enough values of M-dot, the emission from the wind is expected to have a spectrum which, at low enough frequencies, becomes a power law, S(v) about v exp alpha with alpha about 0.7. Data from IRAS and VLA suggest that such a spectrum may in fact occur in certain M dwarfs: a key test of the wind spectrum would be provided if the stars could be detected at lambda about 1 mm. We show that the M-dot required to ensure power-law emission is a few times 10 exp -10 solar mass/yr. With M-dot of this order, fluxes at lambda about 1 mm would be tens of mJy. Using the James Clerk Maxwell Telescope, we have tested this prediction on several stars: the data are suggestive but are near the limits of detection. Confirmation of our estimates will be important for evolution and for interstellar medium (ISM) physics: if even a few percent of all M dwarfs are losing mass at the above rates, the mass balance of the ISM will be dominated by M dwarfs.

Mullan, D. J.↗
NASA NTRS1992

An infrared supershell surrounding the Cygnus OB1 association

New studies are reported of a large, 2 x 5 deg peanut-shaped cavity in the far-infrared emission seen using IRAS data for the Cygnus X region. A more complete and better defined infrared supershell than reported by Lozinskaya and Repin (1990) is found and connected to the Cyg OB1 association. It is shown that the cavity represents the early stages of a superbubble produced by the winds and possible SNe from 10 to 20 massive stars. The locations and properties of these stars are used to estimate the energy deposition rate and to understand the manner in which supershells form and propagate. In Cyg OB1, spatially distributed subclustering appears to have played an important role in determining the nonspherical morphology of the superbubble.

Saken, Jon M.↗
NASA NTRS1992

The compactness of far-infrared bright galaxies

The linear size distribution of 218 galaxies in the IRAS infrared bright galaxy sample is studied by examining new measurements of the 1.49 Hz radio continuum emitting regions. We find that the radio surface brightness varies over a wide range at a given luminosity, but its median scales approximately as luminosity to the power of 6/5. Consequently, the median effective radio size of galaxies in a given luminosity range actually decreases with increasing luminosity, especially among galaxies with LFIR greater than 10 exp 11 solar luminosities. The close correlation between FIR and radio fluxes of these star-forming galaxies then leads to a picture contrary to earlier theoretical predictions that the sizes of FIR emitting regions should increase with luminosity. We propose that such a steep increase of surface brightness in luminous galaxies is a direct result of a high interstellar medium density and an enhanced radiation density of the heating photons driven by more active star formation.

Wang, Zhong↗
NASA NTRS1992

SIRTF - Space Infrared Telescope Facility

SIRTF (Space Infrared Telescope Facility), a one meter class cryogenically cooled observatory for infrared astronomy, is briefly discussed. The characteristics of the Infrared Array Camera, Infrared Spectrograph, and Multiband Imaging Photometer on SIRTF are summarized and the SIRTF characteristics are compared with those of IRAS. The key scientific aims and capabilities of SIRTF are outlined.

Erickson, E. F.↗
NASA NTRS1992

Infrared emission and mass loss from evolved stars in elliptical galaxies

Small aperture 10.2-micron measurements of normal elliptical galaxies show that for almost all of these galaxies the 12-micron emission seen by IRAS is extended on the scale of the galaxy. NGC 1052 and NGC 3998 are exceptions to this; much of their 10-12-micron emission comes from the inner regions of the galaxies and may be associated with their active nuclei, as is the case for many radio galaxies. The distribution of the IR light and the IR colors of elliptical galaxies suggest that the most plausible source of the 12-micron emission is photospheric and circumstellear emission from cool evolved red giant stars. The 12-micron emission is well in excess of that expected from photospheric emission alone; about 40 percent of it probably comes from circumstellar dust.

Knapp, G. R.↗
NASA NTRS1992

Infrared fine-structure line diagnostics of shrouded active galactic nuclei

The ultraluminous far-IR galaxies revealed by IRAS, quasar-like in luminosity but smothered in molecular gas, probably conceal either immense starbursts or luminous active nuclei. In both scenarios, these objects ought to produce copious infrared fine-structure emission with several lines comparable to H-beta in luminosity. We show how these lines, if detected, can be used to determine the electron densities and far-IR obscurations of shrouded photoionized regions and to constrain the shape and ionization parameter of the ionizing spectra. The presence of Ne v emission in particular will distinguish shrouded AGNs from shrouded starbursts. Since all active galaxies photoionize at least some surrounding material, these diagnostics can also be applied to active galaxies in general and will aid in studying how an active nucleus interacts with the interstellar medium of its host galaxy.

Voit, G. M.↗
NASA NTRS1992

Circumprotostellar environments. I - Molecular outflows from protostar candidates in Taurus

We report on the search for molecular outflows from a complete, flux-limited, IRAS-selected sample of cold, low-mass protostar candidates located in the Taurus cloud complex. We find that 18 of the 24 sources have outflows associated with them, only four of which have been previously reported, implying that these sources are predominantly protostars in an early stage of evolution. We argue that at least a few of the outflows are very small in extent, indicating that their dynamical ages are less than a few thousand years.

Moriarty-Schieven, G. H.↗
NASA NTRS1992

Infrared emission from X-ray and optically emitting regions in the Cygnus Loop supernova remnant

We present a new technique for analyzing the infrared (IR) emission of a supernova remnant (SNR) and apply it to the Cygnus Loop. The technique consists of a decomposition of the IR emission from the remnant (in all four IRAS bands) into two distinct emission components. The first component is spatially correlated with the X-ray emission from the remnant and arises from collisionally heated dust in the shocked gas. The second component is spatially correlated with the optical filaments and arises from dust which is radiatively or collisionally heated within the filaments, and from IR lines emitted by the shocked gas. The decomposition allows us to determine the characteristics of the dust, examine evidence for grain processing through the shock, constrain various remnant parameters, and demonstrate how the morphology of the environment determines the IR emission from the remnant.

Arendt, Richard G.↗
NASA NTRS1992

Ultraviolet emission in the dark cloud toward HD 62542

We have obtained low resolution IUE short wavelength spectra at several positions within a ridge of molecular gas within the Gunn Nebula. Optical absorption line measurements toward HD 62542 indicate moderately high densities within the ridge. The physical appearance of the ridge, along with H-alpha and IRAS images of the nebula, suggest that winds and radiation from Gamma Vel and Zeta Pup are heating and compressing the ridge. The IUE spectra revealed faint C II and C III emission at one position, while C IV was weakly detected at another. The former are interpreted as arising in photoionized gas while the latter probably arises in a shocked wind striking the ridge. No detections were made at other positions in the ridge.

O'Donnell, James E.↗
NASA NTRS1992

The nearby 2-solar mass Bok globule LBN 11 - Sub-sonic molecular clumps in a magnetic environment

A multiwavelength study of the nearby small Bok globule LBN 11 is performed in order to investigate the relationship between embedded magnetic fields, cloud structure, and star formation. Optical polarimetry of background stars is used to characterize and trace the embedded magnetic field direction. High-dispersion CO isotopic mapping is employed to determine the radial density distribution of the gas, measure cloud rotation, and identify individual gas clumps. CS and SO mapping revealed dense cores within the (C-13)O clumps. Analysis of coadded IRAS images reveal the cloud to be free of current or recent star formation: there are no IR point sources of sufficient flux or proper colors to signify young stars associated with the cloud. The molecular line maps show the cloud to be extremely clumpy. Clumps are found to range in size from 0.2 pc for the largest CO clump to about 0.04 pc for the smallest CS feature. It is concluded that, at least in the envelope of the cloud, the magnetic field and the CO clumps are coupled.

Clemens, Dan P.↗
NASA NTRS1992

The low filling factor of dust in the Galaxy

The neighborhood of 745 luminous stars in the IRAS Skyflux plates was examined for the presence of dust heated by the nearby star. One-hundred twenty-three dust clouds were found around only 106 of the stars with a volume filling factor of 0.006 and an intercloud separation of 46 pc. Nowhere was a region found where the dust is smoothly distributed through the volume of space heated by the star; hence an upper limit of 0.06/cu cm is placed on the equivalent gas density in the intercloud regions. Due to the lack of IR emission near the star, it is found that less than 1 percent of the stellar luminosity is reprocessed within 10 pc of the star. The clouds have an average density of 0.22/cu cm and a radius of 1.9 pc, albeit with wide variations in their properties. Two different scale heights of 140 and 540 pc were found for the number of clouds around different groups of stars, which are interpreted as evidence for different distributions of dust in and out of the Galactic disk.

Murthy, Jayant↗
NASA NTRS1993

CO observations of candidates for carbon-rich asymptotic giant branch and post-asymptotic giant branch stars

Circumstellar CO emission has been detected in a number of featureless or weak SiC emission, low color temperature IRAS sources. The CO detections confirm the suggestion that these are either extreme carbon stars, carbon-rich proto-planetary nebulae (PPNs), or carbon-rich planetary nebulae (PNs). We find that the CO emission is relatively stronger for a given luminosity in post-AGB stars than AGB stars, suggesting a more efficient excitation mechanism is at work in the post-AGB phase. One probable post-AGB star was detected in HCN for the first time. The available HCN data suggest a rapid decline in HCN emission after the AGB. Molecular emission is shown to be a useful tracer of the evolution from asymptotic giant branch to the planetary nebula phase.

Volk, Kevin↗
NASA NTRS1991

The X-ray flux dipole of AGNs as an indicator of the local gravitational field

We have investigated the dipole anisotropy of the sky X-ray flux from nearby AGNs to compare with the peculiar motion of the Local Group (LG) using a more extensive sample than that of Miyaji and Boldt (1990). We have sampled 56 low redshift emission-line AGNs (z less than 0.06) from the HEAO 1 A-2 experiment with a flux limit lower than that of Piccinotti et al. (1982) and with a lower galactic latitude cutoff. Our sample shows a significant dipole anisotropy whose apex is only about 30 deg away from the direction of the Local Group's peculiar motion for the objects with z less than 0.015. The dipole growth vs. redshift shows a sharp rise between z = 0.006 and z = 0.015; the amplitude of the dipole is 40 +/- 10 percent of the corresponding monopole at z = 0.015. The outer redshift cutoff is consistent with the results obtained from analyses using optical and IRAS galaxies. The present sample strengthens our previous conclusion that X-ray emission from AGNs traces the underlying mass distribution as strongly as optical and IR emission from galaxies.

Miyaji, Takamitsu↗
NASA NTRS1993

Young low mass stars in the vicinity of Sigma Scorpii

The region near Sigma Scorpii, a member of the Sco-Cen OB association, is examined for signs of recent star formation. Thirteen candidate young stellar objects are identified over an 80 x 80 arcmin region centered on Sigma Sco using the Point source Catalog and a recent survey for H-alpha emission-line stars. Near-infrared photometry, improved IRAS fluxes, and optical spectra are used to determine the nature of these objects. Four definite young stars are revealed, as well as one additional such object that fell just outside of the present target region. These stars, all of spectral type K or M, are argued to have formed in the vicinity of the B1 giant star Sigma Scorpii and to represent a subsample of the low-mass members of the association.

Meyer, Michael R.↗
NASA NTRS1993

Infrared emission from hydrogenated amorphous carbon and amorphous carbon grains in the interstellar medium

The correlations deduced by Boulanger et al. (1990) from IRAS maps of the Chamaeleon, Taurus and Ursa Major molecular cloud complexes are interpreted in terms of the evolutionary hydrogenated amorphous carbon model of interstellar dust. In particular, regions of relatively strong 12-micron emission may be regions where recently accreted carbon is being converted by ambient UV to small PAHs in situ. Regions of weak 12-micron emission are probably quiescent regions where carbon has been annealed to amorphous carbon. Observational consequences of these inferences are briefly described.

Duley, W. W.↗
NASA NTRS1993

Far-infrared emission from the intracluster medium

IRAS images of 56 clusters of galaxies are examined for evidence of diffuse, extended FIR emission at 60 and 100 microns. Of these 56 clusters, five show evidence of FIR excess, but at low levels of significance. For A496 and MKW 1, this excess is attributable to the presence of a strong point source at or near the position of the central cD galaxy. In A2344, the observed 100-micron excess is the result of the presence of a foreground cirrus concentration which coincides with the cluster. Examination of the sample as a whole indicates that clusters exhibit excess FIR emission at the 2-sigma level for apertures 4 and 10 arcmin in diameter. FIR luminosities range from about 1-0 exp 9 solar luminosities for the smallest aperture to about 10 exp 14 solar luminosities for the entire cluster.

Wise, Michael W.↗