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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
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NASA NTRS2000

Star Formation in the Galaxy and the Fluctuating UV Radiation Field

We examine the formation of massive stars in the Galaxy, the resultant fluctuating UV radiation field, and the effect of this Field on the star-forming interstellar medium. Following previous researchers such as Habing (1968), we calculate the average interstellar radiation field at the Solar Circle of the Galaxy. However, our new calculations follow more closely the time dependence of the field at any point. We show that there is a significant difference between the mean field and the median field, and that there are substantial fluctuations of the field (on timescales of order 100 million years) at a given point. Far Ultraviolet Radiation (FUV, photon energies of 6 eV - 13.6 eV) has been recognized as the main source of heating of the neutral interstellar gas. Given the pressure of the interstellar medium (ISM) the FUV field determines whether the thermal balance of the neutral gas results in cold (T approximately 50 - 100 K) clouds (CNM), warm (T about 10,000 K) (WNM), for a combination of the two (the two phase ISM) We present results for the time history of the FUV field for points in the local ISM of the Milky Way Galaxy. The presence of this fluctuating heating rate converts CNM to WNM and vice versa. We show how to calculate the average fractions of the gas in the CNM and WNM when the interstellar gas is subject to this fluctuating FUV field. The knowledge of how these fractions depend on the gas properties (i.e. mean density and composition) and on the FUV-sources (i.e. the star formation rate, or the IMF, or the size distribution of associations) is a basic step in building any detailed model of the large scale behavior of the ISM and the mutual relation between the ISM and the SFR.

Hollenbach, David↗
NASA NTRS1986

The ionization structure of the circumstellar envelope of Alpha Orionis

The physical processes which affect the ionization of the outer circumstellar envelope of Alpha Ori are analyzed and evaluated. The ultraviolet radiation fields of the chromosphere and the interstellar medium dominate the envelope, and the most common forms of all species are neutral atoms and first ions. Hydrogen recombines just outside the chromosphere, where atoms with smaller ionization potential are essentially fully ionized. The heavier ions gradually recombine with increasing distance from the star, until the interstellar radiation field reverses this trend. The electron fraction in the outer envelope is approximately equal to the abundance of all such heavy atoms, i.e., of the order of 0.0001. The analysis is applied to the case of neutral K, whose density in the envelope has been determined by scattering experiments. The theory predicts that the slope of the K I density distribution should decrease from -1.5 to -3.5 in the outer envelope. The mass loss rate of Alpha Ori implied by the K I scattering experiments is 4 x 10 to the -6th solar mass/yr.

Glassgold, A. E.↗
NASA NTRS2010

Detection of the 158 Micrometers[CII] Transition at z=1.3: Evidence for a Galaxy-Wide Starburst

We report the detection of 158 micrometer [C II] fine-structure line emission from MIPS J 142824.0+3526l9, a hyperluminous (L(sub IR) approx. 10(exp 13) Solar Luminosity starburst galaxy at z = 1.3. The line is bright, corresponding to a fraction L[C II]/L(sub FIR) approx. equals 2 x l0(exp -3) of the far-IR(FIR) continuum. The [C II], CO, and FIR continuum emission may be modeled as arising from photodissociation regions (PDRs) that have a characteristic gas density of n approx. 10(exp 4.2)/cu cm., and that are illuminated by a far-UV radiation field approx. 10(exp 3.2) times more intense than the local interstellar radiation field. The mass in these PDRs accounts for approximately half of the molecular gas mass in this galaxy. The L[C II]/L(sub F1R) ratio is higher than observed in local ultraluminous infrared galaxies or in the few high-redshift QSOs detected in [C II], but the L[CII]/L(sub FIR) and L(sub CO)/L(sub FIR) ratios are similar to the values seen in nearby starburst galaxies

Hailey-Dunsheath, S.↗
NASA NTRS2010

Detection of the 158 Micrometers[CII] Transition at z=1.3: Evidence for a Galaxy-Wide Starburst

We report the detection of 158 micron [C II] fine-structure line emission from MIPS J142824.0+352619, a hyperluminous ( L(sub IR) approximates 10(exp 13) L (sub solar)) starburst galaxy at z = 1.3. The line is bright, corresponding to a fraction L(sub [Cu II] L(sub Fir) approximates 2 x 10(exp -3) of the far-IR (FIR) continuum. The [C II], CO, and FIR continuum emission may be modeled as arising from photodissociation regions (PDRs) that have a characteristic gas density of n approximates 10(exp 4.2) /cm(exp 3) , and that are illuminated by a far-UV radiation field approximately 10(exp 3.2) times more intense than the local interstellar radiation field. The mass in these PDRs accounts for approximately half of the molecular gas mass in this galaxy. The L(sub [CII])/L(sub FIR) ratio is higher than observed in local ultralummous infrared galaxies or in the few high-redshift QSOs detected in [C II], but the L(sub [CII])/L(sub FIR) and L(sub CO)/L(sub FIR) ratios are similar to the values seen in nearby starburst galaxies. This suggests that MIPS J142824.0+352619 is a scaled-up version of a starburst nucleus, with the burst extended over several kiloparsecs.

Hailey-Dunsheath, S.↗
NASA NTRS1994

The photoelectric heating mechanism for very small graphitic grains and polycyclic aromatic hydrocarbons

We have theoretically modeled the gas heating associated with the photoelectric ejection of electrons from a size distribution of interstellar carbon grains which extends into the molecular domain. We have considered a wide range of physical conditions for the interstellar gas (1 less than G(sub 0) less than 10(exp 5), with G(sub 0) being the intensity of the incident far-UV field in units of the Habing interstellar radiation field; 2.5 x 10( exp -3) less than n(sub e) less than 75/cu cm, with n(sub e) being the electron density; 10 less than T less than 10,000 K, with T being the gas temperature). The results show that about half of the heating is due to grains less than 1500 C atoms (less than 15 A). The other half originates in somewhat larger grains (1500-4.5 x 10(exp 5) C atoms; 15 less than 100 A). While grains larger than this do absorb about half of the available far-UV photons, they do not contribute appreciably to the gas heating. This strong dependence of gas heating on size results from the decrease in yield and from the increased grain charge (hence larger Coulomb losses) with increasing grain size. We have determined the net photoelectric heating rate and evaluated a simple analytical expression for the heating efficiency, dependent only on G(sub 0), T, and n(sub e). This expression is accurate to 3% over the whole parameter range and is valid up to gas temperatures of 10(exp 4) K, at which point the dominant gas-dust heat exchange mechanism becomes the recombination of electrons with grains rather than photoelectric ejection. The calculated heating efficiency for neutral grains is in good agreement with that derived from observations of the diffuse interstellar clouds. Our results also agree well with the Far Infrared Absolute Spectrometer (FIRAS) observations on the Cosmic Background Explorer Satellite. Finally, our photoelectric heating efficiency is compared to previous studies.

Bakes, E. L. O.↗
NASA NTRS1995

A far-infrared spectral line survey of 23 infrared-bright Galaxy nuclei

We present results from a KAO survey of fine-structure lines observed in 23 infrared-luminous galaxies. One or more of the following lines was observed and/or detected in each galaxy: (S III) 19, 33 microns, (Ne V) 24 microns, (O IV) 26 microns, (Fe II) 26 microns, (Si II) 35 microns, (O III) 52, 88 microns, (0 I) 63, 146 microns, (N III) 57 micro ns, (N II) 122, 205 microns, (C II) 158 microns. The galaxies span a wide range of morphologies (irregular to grand design), have varying metallicities, and include mergers, AGN's, and starburst systems. The observations were made beginning in 1988 using the facility Cryogenic Grating Spectrometer onboard the KAO at a typical resolution of approximately 60-140 km/s and with a 30-44 deg beam. We interpret the (C II) and (O I) fluxes, along with previous measurements of the IR continuum fluxes, in the context of photo dissociation region (PDR) models (Tielens & Hollenbach 1985; Wolfire et al. 1990). With these models, we obtain estimates of the typical interstellar UV fields incident on the line emitting regions (102-104 times the local interstellar radiation field) and the total masses (10(exp 7)-10(exp 8) Solar Mass), densities (10(exp 3)-10(exp 4)/cu cm), and temperatures (100-250 K) of the warm atomic gas. The (O III) (52/88) and (S III) (33/19) line flux ratios constrain the range of electron densities and pressures found within the ionized regions. The (O III) and (S III) lines also provide estimates of the effective temperature of the ionizing stars and elemental abundances within the ionized regions of these galactic nuclei. Our measurements imply typical gas pressures of nT approximately 5 x 10(exp 6)/cu cm K and typical upper mass cutoffs of 25-35 Solar Mass. The low-metallicity systems show high (C II)/CO and (O I)/CO flux ratios, 3-5 times the Milky Way value, indicating that they contain a larger fraction of photodissociated gas relative to the molecular material.

Lord, Steven D.↗
NASA NTRS1998

On the Stability and Evolution of Isolated Molecular Clouds

We present the results of three dimensional hydrodynamic models of evolving, isolated, low mass, quiescent clouds and Bok gobules, where the interstellar radiation field plays an important role in the thermal and chemical evolution, and thermal pressure provides dominant support against gravitational collapse.

pressure chemical evolution interstellar radiation↗
NASA NTRS1993

The nature of unidentified far-infrared point sources

Sources discovered by the Infrared Astronomical Satellite with spectra that rise steeply from 60 to 100 microns are a mixed sample of external galaxies, condensations in the interstellar medium, and potentially unknown astronomical objects. Sources in regions of low total H I column density (as determined with the all-sky 21-cm line surveys), and with no nearby cataloged external galaxy, were observed for presence of 20-cm continuum emission (with the NRAO VLA), enhanced 21-cm H I line emission (with the NAIC Arecibo 305 m), and molecular line emission (with the NRAO 12 m, HCRO 6 m, and FCRAO 14 m). Based on the CO detection rate, 70 percent of our sample are small interstellar molecular clouds. The remainder are mostly faint external galaxies. The dust column densities of the interstellar condensations were determined from the IR flux, using a spherical-cloud model for the radiative transfer of the interstellar radiation field (ISRF). The CO column densities were determined using the large-velocity-gradient model for the excitation and radiative transfer of the rotational transitions. About half of the interstellar sources have normal dust-to-gas ratios and ISRF heating. Sources with excess infrared emission, compared to the molecular column density, are protostellar candidates.

Reach, William T.↗
NASA NTRS1989

Photoabsorption and photodissociation of molecules important in the interstellar medium

Photoabsorption and fluorescence cross sections of molecules important in the interstellar medium were measured in the 90 to 200 nm region using synchrotron radiation, excimer laser, and condensed discharge lamps as light sources. The quantitative spectroscopic data are currently needed for the modeling of formation and destruction rates of molecules by the interstellar radiation field. Fluorescences from excited photofragments produced by vacuum ultraviolet radiation of molecules are dispersed to identify the emitting species. The fluorescence data are useful for the identification of emission sources in interstellar clouds.

Lee, Long C.↗
NASA NTRS1976

Thermal-chemical instabilities in CO clouds

The stability of interstellar clouds containing CO is analyzed taking account of formation processes for CO. Two such processes are examined: O(+) charge exchange and C(+) radiative association. It is found that the C(+) radiative-association chemistry leads to low-temperature instabilities which influence the evolution of clouds. It is also found that instability may set in if CO production increases sufficiently with density, that the O(+) charge-exchange chemistry leads to instability associated with attenuation of the interstellar radiation field by grains, and that thermal instabilities will also result if grain formation, rather than ion-molecule chemistry, dominates CO production. It is suggested that such instabilities play a role in the fragmentation of interstellar clouds and in the formation of protostellar objects.

Glassgold, A. E.↗
NASA NTRS1989

Does the far-infrared/radio correlation in spiral galaxies extend to the spatial domain

A comparison is made between the spatial distribution of the thermal far-infrared and non-thermal radio emission of nearby spiral galaxies. This is done in an attempt to improve our understanding of the well known correlation between the integrated Infrared Astronomy Satellite (IRAS) far-infrared and radio emission of spiral galaxies, e.g., de Jong et al., 1985, Helou et al., 1986. A physical explanation for this correlation is not straight forward due to the ambiguous nature of the origin of the far-infrared and radio, and the dependence of the non-thermal radio on each galaxies' magnetic field. It is now widely believed that the infrared emission detected in the longer wavelength IRAS wavebands (less than 50 microns) arises from at least two distinct sources, e.g., Cox et al., 1986, Persson and Helou, 1987: (1) a warm (T approx. 40 K) component associated with dense dust clouds heated by embedded O and B type stars; and (2) a cooler (T approx. 20 K) component associated with diffuse dust distributed throughout the interstellar matter (ISM) heated by the interstellar radiation field. A link between the warm component and the radio via electrons originating in Type II supernovae (the ultimate fate of many of the O and B type stars responsible for the warm component) has been suggested by numerous authors. The supporting evidence is scarce and inconclusive. Researchers have attempted to provide some insight into the problem by looking at the spatial distribution of the different components in some nearby spiral galaxies, starting with the face-on spiral M51. The source of the far-infrared data is the IRAS chopped photometric channel (CPC) instrument. Warm and cold far-infrared fluxes integrated over all wavelengths and the radio intensity at two frequencies are plotted against radius. All plots are to a common resolution of 100 arcsec, the radio data originating from the Cambridge Low Frequency Synthesis Telescope (151 MHz) and the VLA (1490 MHz, from Condon, 1987). The warm and cold regions are assumed to be representedby a single galactic wide temperatures of 50 K and 20 K respectively. A dust emissivity of 1 has been assumed. The form of the plots is little effected by varying these assumptions. The radio and cold component curves appear to follow each other most closely, in contradiction to the simple OB star/type II supernovae hypothesis.

Howarth, Neil A.↗
NASA NTRS1990

Evidence against a simple two-component model for the far-infrared emission from galaxies

Two of the first Infrared Astronomy Satellite (IRAS) results were that galaxies have a wide range of values for the ratio of 60 micron to 100 micron flux density (0.2 less than or equal to S sub 60/S sub 100 less than or equal to 1.0) and that this ratio is correlated with L sub fir, L sub b, L sub fir being the total far-infrared luminosity and L sub b being the luminosity at visible wavelengths (de Jong et al. 1984; Soifer et al. 1984). From these results arose the following simple model for the far-infrared emission from galaxies (de Jong et al. 1984), which has remained the standard model ever since. In this model, the far-infrared emission comes from two dust components: warm dust (T approx. equals 50 K) intermingled with, and heated by, young massive OB stars in molecular clouds and HII regions, and colder dust (T approx. equals 20 K) associated with the diffuse atomic hydrogen in the interstellar medium and heated by the general interstellar radiation field. As the number of young stars in a galaxy increases, S sub 60/S sub 100 increases, because there is a greater proportion of warm dust, and so does L sub fir/L sub b, because most of the radiation from the young stars is absorbed by the dust, leading to a swifter increase in far-infrared emission than in visible light. Although this model explains the basic IRAS results, it is inelegant. It uses two free parameters to fit two data (the 60 and 100 micron flux densities)-and there are now several observations that contradict it. Despite these major problems with the two-component model, it is not clear what should be put in its place. When considering possible models for the far-infrared emission from galaxies, the observational evidence for our own galaxy must be considered. Researchers suspect that the study by Boulanger and Perault (1988) of the far-infrared properties of the local interstellar medium may be particularly relevant. They showed that molecular clouds are leaky - that most of the light from OB stars in molecular clouds does not heat the dust in the clouds, but instead leaks out. The consequence of this is that that while most of the far-infrared emission from the solar neighborhood is from dust associated with diffuse HI, this dust is mostly heated by young stars.

Eales, Stephen A.↗
NASA NTRS1974

Model calculations for diffuse molecular clouds

A local steady-state isobaric model of the interstellar clouds is developed and applied to the OAO-3 observations. The model allows for cooling by C II fine-structure transitions and for inhomogeneities resulting from attenuations in the external radiations which heat the cloud constituents. Factors affecting the thermal properties of the clouds are emphasized; these include H and H2 balance, interstellar grain heating, electron density, and calcium equilibrium. The thermodynamic consequences of the attenuation of the ultraviolet interstellar radiation field are studied. The effect of varying the most important parameters and their allowed ranges is considered. Results indicate that the model is capable of reproducing the H2 fractional abundances and kinetic temperatures observed by OAO-3.

Glassgold, A. E.↗
NASA NTRS1980

Far-infrared observations of the globule B335

Far-infrared and submillimeter continuum emission from the dark globule B335 was observed. The spectrum is that of optically thin thermal emission from dust with temperature 13-16 K. The estimated dust mass is 0.07-0.17 solar masses. The observed luminosity, 5 solar luminosities, can be supplied by the interstellar radiation field.

Keene, J.↗
NASA NTRS1981

Far-infrared observations of globules

Observations of far-infrared emission from nine globules are presented. The intensity and uniformity of the emission confirm that the heat source is the interstellar radiation field. Spectra of B133 and B335 are presented; they are consistent with optically thin thermal emission from dust with temperature 13-16 K. The temperature and intensity of B335 are used to calculate the ratio of visual extinction to far-infrared emission frequency.

Keene, J.↗
NASA NTRS1983

Far-infrared detection of low-luminosity star formation in the Bok globule B335

Measurements of far-infrared intensities in the Bok globule B335, made with higher spatial and spectral resolution than two earlier studies, are described. The NASA 3-m Infrared Telescope Facility was used in February 1981 and the Kuiper Airborne Observatory 0.9-m telescope was used in October 1981 and August 1982 for the observations. The far-infrared source in B335 was revealed to be more compact than previously assumed, ruling out the interstellar radiation field as the dominant heat source. The new far-infrared size, luminosity, and temperature derived suggest that these may be the first observations of low-mass star formation embedded deeply within a Bok globule. The far-infrared source does not have a near-infrared or radio continuum counterpart. High angular resolution at submillimeter wavelengths will be critical in further studies of low-mass star formation.

Davidson, J. A.↗
NASA NTRS1984

The formation of solar type stars - IRAS observations of the dark cloud Barnard 5

Observations of the dark cloud Barnard 5 show two compact sources of radiation within the dense core. IRS 1 is associated with 30-800 K dust, has a total luminosity of about 10 solar luminosities, and is presumably a newly formed star of roughly solar mass. IRS 2 has a much cooler color temperature, approximately 25 K, and emits only 1.3 solar luminosities. Its status is unclear, but IRS 2 may be at a very early stage of gravitational collapse or a density enhancement within the cloud heated by the interstellar radiation field. Also within the confines of the cloud are two point sources, which, if associated with the cloud, each emit about 0.5 solar luminosities in the IRAS bands. These may be T Tauri stars, separated from the cloud but still enshrouded in dust shells.

Beichman, C. A.↗