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NASA NTRS1977

Airborne far-infrared observations of the galactic center region

Maps of a region 10 arcmin in diameter around the galactic center made simultaneously in three wavelength bands at 30, 50, and 100 microns with about 1 arcmin resolution are presented, and the distribution of far-IR luminosity and color temperature across this region is derived. The position of highest far-IR surface brightness coincides with the peak of the late-type stellar distribution and with the H II region Sgr A West. The high spatial and temperature resolution of the data is used to identify features of the far-IR maps with known sources of near-IR, radio continuum, and molecular emission. The emission mechanism and energy sources for the far-IR radiation are analyzed qualitatively, and it is concluded that all of the observed far-IR radiation from the galactic-center region can be attributed to thermal emission from dust heated both by the late-type stars and by the ultraviolet sources which ionize the H II regions. A self-consistent model for the far-IR emission from the galactic-center region is presented. It is found that the visual extinction across the central 10 pc of the Galaxy is only about 3 mag and that the dust density is fairly uniform in this region. An upper limit of 10 million suns is set on the luminosity of any still unidentified source of 0.1- to 1-micron radiation at the galactic center.

Gatley, I.↗
NASA NTRS1998

The Anatomy of the Perseus Spiral Arm: 12 CO and IRAS Imaging Observations of the W3-W4-W5 Cloud Complex

Panoramic images of 12CO J = 1-0 and thermal dust emissions from the W3-W4-W5 region of the outer Galaxy are presented. These data and recently published H I 21 cm line emission images provide an approximate 1' resolution perspective to the dynamics and thermal energy content of the interstellar gas and dust components contained within a 9 deg. arc of the Perseus spiral arm. We tabulate the molecular properties of 1560 clouds identified as closed surfaces within the l-b-v CO data cube at a threshold of 0.9 K T* (sub R). Relative surface densities of the molecular (28:1) and atomic (2.5:1) gas components determined within the arm and interarm velocity intervals demonstrate that the gas component that enters the spiral arm is predominantly atomic. Molecular clouds must necessarily condense from the compressed atomic material that enters the spiral arm and are likely short lived within the interarm regions. From the distribution of centroid velocities of clouds, we determine a random cloud-to-cloud velocity dispersion of 4 km s (exp. -1) over the width of the spiral arm but find no clear evidence within the molecular gas for streaming motions induced by the spiral potential. The far-infrared images are analyzed with the CO J = 1-0 and H I 21 cm line emission. The enhanced UV (Ultraviolet) radiation field from members of the Cas OB6 association and embedded newborn stars provide a significant source of heating to the extended dust component within the Perseus arm relative to the quiescent cirrus regions. Much of the measured far-infrared flux (69% at 60 micrometers and 47% at 100 micrometers) originates from regions associated with star formation rather than the extended, infrared cirrus component.

Heyer, Mark H.↗
NASA NTRS2002

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 (ultraviolet) radiation field, and the effect of this field on the star-forming interstellar medium. There are substantial fluctuations of the UV radiation field in space (scales of 100's of parsecs) and time (time-scales of order 100 million years). The FUV (far ultraviolet) (6 eV less than hv less than 13.6 eV) field and the pressure determines whether the thermal balance of the neutral gas results in cold clouds or warm (T approx. 10(exp 4) K) neutral medium. We show how to calculate the average fractions of the gas in the cold and warm phases when the interstellar gas is subject to this fluctuating FUV field. The knowledge of how these fractions depend on the gas properties and on the FUV sources is a basic step in building a model of the large scale behavior of the ISM (interstellar medium) and the mutual relation between the ISM and the star formation rate.

Hollenbach, David↗
NASA NTRS2022

AU Microscopii in the Far-UV Observations in Quiescence, During Flares, and Implications for AU Mic b and c

High-energy X-ray and ultraviolet(UV)radiation from young stars impacts planetary atmospheric chemistry and mass loss. The active∼22 Myr M dwarf AU Mic hosts two exoplanets orbiting interior to its debris disk. Therefore, this system provides a unique opportunity to quantify the effects of stellar X-ray and UV irradiation on planetary atmospheres as a function of both age and orbital separation. In this paper, we present over 5 hr of far-UV (FUV)observations of AU Mic taken with the Cosmic Origins Spectrograph (COS; 1070-1360Å) on the Hubble Space Telescope (HST). We provide an itemization of 120 emission features in the HST/COS FUV spectrum and quantify the flux contributions from formation temperatures ranging from 104 to 107K. We detect 13 flares in the FUV white-light curve with energies ranging from 1029 to 1031erg s. The majority of the energy in each of these flares is released from the transition region between the chromosphere and the corona. There is a 100×increase influx at continuum wavelengths λ<1100 Å in each flare, which may be caused by thermal Bremsstrahlung emission. We calculate that the baseline atmospheric mass-loss rate for AU Mic b is∼108 g s−1, although this rate can be as high as∼1014 g s−1during flares with Lflare 10-33 erg s−1. Finally, we model the transmission spectra for AU Mic b and c with a new panchromatic spectrum of AU Mic and motivate future JWST observations of these planets.

Adina D Feinstein↗
NASA NTRS1982

Far-infrared observations of Sagittarius A - The luminosity and dust density in the central parsec of the Galaxy

Far-infrared observations of the central 4 arcmin of the Galaxy with 30-arcsec resolution made simultaneously at 30 microns, 50 microns, and 100 microns are presented. The 30-micron radiation peaks strongly at the position of the galactic center, as determined from the 2-micron surface brightness and the density of ionized gas. The 50- and 100-micron emission is much more extended along the plane and shows two emission lobes, one on either side of the 30-micron peak. At the position of the galactic center itself there is a local minimum in the 100-micron surface brightness. It is concluded that the dust density decreases inward over the central few parsecs of the Galaxy and that the dust density in the central parsec is so low that optical and ultraviolet radiation freely traverses this region. The total luminosity of the sources heating the dust which radiates the far-infrared emission from the central few parsecs is deduced to be between 1 x 10 to the 7th and 3 x 10 to the 7th solar luminosities.

Becklin, E. E.↗
NASA NTRS1986

Detection of OH from Comet Halley in the far-infrared

The 2 Pi 3/2 (J = 5/2 yields 3/2 + yields - parity) transition of OH was detected in comet Halley at 119.44 microns. The upper limit to the line intensity of the - yields + parity transition is 119.23 microns. The ratio of these lines is consistent with radiative pumping of the rotational levels through absorption of near ultraviolet solar photons which excite the low lying vibrational levels of OH. The far-infrared lines probe the inner regions of the coma where OH is produced through photodissociation of a parent molecule (presumably H2O). Hence, these lines complement the 18 cm radio measurements which are sensitive to the outer regions of the comet coma. The OH production rate is highly model dependent but is of the order 2 to 4 times 10 to the 29th power molecules/sec.

Stacey, Gordon J.↗
NASA NTRS1984

Far Infrared Continuum Emission from the Galactic Center

High angular resolution observations of the Galactic center from the Kuiper Airborne Observatory are discussed. The results of multi-wavelength mapping reveal that the 30 micron distribution shows a single peak which lies between two emission lobes seen at the longer wavelengths. The 30 micron emission is known from ground-based measurements to peak at the galactic center. The far-infrared color temperature and luminosity also peak at the position of the galactic center, and the observations show a strong temperature gradient away from the galactic center in all directions. Consideration of these data leads to the conclusion that the dust density decreases inward over the central 3 pc of the Galaxy, and that the central parsec of the Galaxy is remarkably devoid of dust, so that optical and ultraviolet photons may traverse it freely. The inferred total luminosity of the sources that heat the dust radiating the far-infrared emission from the central few pc of the Galaxy is 1 to 3 x to the 7th power L(Sun). It is probable that these same sources are responsible for ionizing the plasma at the galactic center.

Werner, M. W.↗
NASA NTRS1978

Exobiology and the origin of life

Research supported wholly or in part by NASA is summarized, Topics covered include the molecular analysis of ultraviolet-photoproduced organic solids synthesized under simulated Jovian conditions; the molecular analysis of organic solids produced by electrical discharge in reducing atmospheres; the organic chemistry of interstellar grains; the spectra of possible organic solids present as aerosols in planetary atmospheres; far infrared studies of organic polymers of possible astrophysical interest; organic dust synthesized in reducing environments by ultraviolet radiation or electric discharge; the diffusion of galactic civilizations; eavesdropping on galactic civilizations; Lander imaging as a detector of life on Mars; and continuing puzzles about Mars. Bibliographic data is included for four additional publications not supported by NASA grant, but related to the objectives of the program.

Sagan, C.↗
NASA NTRS2013

Interannual Variations of MLS Carbon Monoxide Induced by Solar Cycle

More than eight years (2004-2012) of carbon monoxide (CO) measurements from the Aura Microwave Limb Sounder (MLS) are analyzed. The mesospheric CO, largely produced by the carbon dioxide (CO2) photolysis in the lower thermosphere, is sensitive to the solar irradiance variability. The long-term variation of observed mesospheric MLS CO concentrations at high latitudes is likely driven by the solar-cycle modulated UV forcing. Despite of different CO abundances in the southern and northern hemispheric winter, the solar-cycle dependence appears to be similar. This solar signal is further carried down to the lower altitudes by the dynamical descent in the winter polar vortex. Aura MLS CO is compared with the Solar Radiation and Climate Experiment (SORCE) total solar irradiance (TSI) and also with the spectral irradiance in the far ultraviolet (FUV) region from the SORCE Solar-Stellar Irradiance Comparison Experiment (SOLSTICE). Significant positive correlation (up to 0.6) is found between CO and FUVTSI in a large part of the upper atmosphere. The distribution of this positive correlation in the mesosphere is consistent with the expectation of CO changes induced by the solar irradiance variations.

carbon monoxide↗
NASA NTRS1994

The wind of EG Andromedae is not dust driven

The symbiotic star EG Andromedae has recently been the subject of several studies investigating its wind properties. Late-type giants are usually considered to have winds driven by radiation pressure on dust. Indeed, the derived wind velocity for EG Andromedae is consistent with this model. We point out here that there is no appreciable dust opacity in the wind of EG Andromedae using constraints on extinction limits from International Ultraviolet Explorer (IUE) and far infrared fluxes from Infrared Astronomy Satellite (IRAS). An alternate mechanism must operate in this star. We suggest that the wind can be driven by radiation pressure on molecular lines.

Van Buren, Dave↗
NASA NTRS1998

Solar Irradiance and Thermospheric Airglow Rocket Experiments

This report describes work done in support of the Solar Irradiance and Thermospheric Air-glow Rocket Experiments at the University of Colorado for NASA grant NAG5-5021 under the direction of Dr. Stanley C. Solomon. (The overall rocket program is directed by Dr. Thomas N. Woods, formerly at the National Center for Atmospheric Research, and now also at the University of Colorado, for NASA grant NAG5-5141.) Grant NAG5-5021 provided assistance to the overall program through analysis of airglow and solar data, support of two graduate students, laboratory technical services, and field support. The general goals of the rocket program were to measure the solar extreme ultraviolet spectral irradiance, measure the terrestrial far-ultraviolet airglow, and analyze their relationship at various levels of solar activity, including near solar minimum. These have been met, as shown below. In addition, we have used the attenuation of solar radiation as the rocket descends through the thermosphere to measure density changes. This work demonstrates the maturity of the observational and modeling methods connecting energetic solar photon fluxes and airglow emissions through the processes of photoionization and photoelectron production and loss. Without a simultaneous photoelectron measurement, some aspects of this relationship remain obscure, and there are still questions pertaining to cascade contributions to molecular and atomic airglow emissions. However, by removing the solar irradiance as an "adjustable parameter" in the analysis, significant progress has been made toward understanding the relationship of far-ultraviolet airglow emissions to the solar and atmospheric conditions that control them.

Solomon, Stanley C.↗
NASA NTRS1990

The possible presence of interstellar PAHs in meteorites and interplanetary dust particles

The Raman spectra of interplanetary dust particles (IDPs) collected in the stratosphere show two bands at about 1350 and 1600 delta cm(-1) and a broader feature between 2200 and 3300 delta cm(-1) that are characteristic of aromatic molecular units with ordered domains smaller than 25A in diameter. This suggests that the carbonaceous material in IDPs may be similar to the polymeric component seen in meteorites, where this material is thought to consist of aromatic molecular units that are randomly inter-linked by short aliphatic bridges. The features in the Raman spectra of IDPs are similar in position and relative strength to interstellar infrared emission features that have been attributed to vibrational transitions in free molecular polycyclic aromatic hydrocarbons (PAHs). The Raman spectra of some IDPs also showed photoluminescence that is similar to the excess red emission from some astronomical objects and that has also been attributed to PAHs and PAH-related materials. Moreover, a part of the carbonaceous phase in IDPs contains deuterium to hydrogen ratios that are far greater than those found in terrestrial samples. Deuterium enrichment is expected in small free PAHs that are exposed to ultraviolet radiation in the interstellar medium. Taken together, these observations suggest that some fraction of the carbonaceous material in IDPs may have been produced in circumstellar dust shells and only slightly modified in interstellar space. Since many, if not most, IDPs come from comets, this supports the view that cometary material contains primitive components which can provide clues about early solar system (and perhaps even interstellar and circumstellar) processes.

Allamandola, L. J.↗
NASA NTRS2003

Finding the Onset of Convection in Main Sequence Stars

The primary goal of the work performed under this grant was to locate, if possible, the onset of subphotospheric convection zones in normal main sequence stars by using the presence of emission in high temperature lines in far ultraviolet spectra from the FUSE spacecraft as a proxy for convection. The change in stellar structure represented by this boundary between radiative and convective stars has always been difficult to find by other empirical means. A search was conducted through observations of a sample of A-type stars, which were somewhat hotter and more massive than the Sun, and which were carefully chosen to bridge the theoretically expected radiative/convective boundary line along the main sequence.

Simon, Theodore↗
NASA NTRS1986

XUV radiometric standards at NBS

The far-UV and XUV detector calibration services at NBS, the properties of the detectors, and the spectrometer calibration facilities at the Synchrotron Ultraviolet Radiation Facility (SURF II) at NBS are described. The method used to calibrate the quantum efficiency of windowless photodiode detectors in the ionization chamber, and the facilities for calibrating photodiodes at various wavelengths are discussed. The cathode and anode of the windowless photodiodes and their operation are examined. Consideration is given to the calculation of spectral irradiance from SURF II; the two stations of the spectrometer facility; the large spectrometer calibration chamber; and sources of error in flux determinations for SURF II.

Ott, W. R.↗
NASA NTRS1979

Observations of the cosmic background radiation at 1440 angstroms

A survey of the summer night sky in the 1350-1550-A band at 2.5-deg angular resolution was carried out with a far-ultraviolet channel of the Berkeley Extreme Ultraviolet (EUV) telescope on the Apollo-Soyuz mission. The large collecting area and small field of view of this telescope permitted the identification and removal from the data of individual stars of spectral type A2 or earlier with visual magnitude brighter than about 6.5. A residual signal significantly above background remains which is not of terrestrial or interplanetary origin and varies with view direction between approximately 30 and 2000 counts/s. The source of the emission is observed to be highly concentrated to the galactic plane with a distribution of half-width about 10 deg centered on the plane and a roughly constant-intensity tail extending out to both galactic poles. The minimum signal detected at high and moderate latitudes corresponds to 300 + or - 60 photons/sq cm-s-sr-A. This minimum flux may be due to scattering from interstellar dust, or it may be extragalactic in origin.

Paresco, F.↗
NASA NTRS1980

Galactic and extragalactic contributions to the far-ultraviolet background

Data from the 1350-1550 A bandpass channel of the 2.5-deg angular resolution extreme-ultraviolet telescope on the Apollo-Soyuz mission have been analyzed for stellar and airglow radiation contributions for the galactic latitude range above 30 deg. At intermediate latitudes, the resulting diffuse background is found to be correlated with the neutral hydrogen column density as determined by 21 cm radio data. This result, together with the observed anisotropy and mean intensity of the radiation field, is consistent with the hypothesis that scattering of galactic-plane starlight off dust grains having an albedo of 0.5 and a phase function coefficient of 0.5 is the primary source of the observed radiation field at the intermediate latitudes. The background intensity near the poles is, however, higher than expected from the H I column density and extinction measurements.

Paresce, F.↗
NASA NTRS2001

Mid- and far-infrared spectroscopic studies of the influence of temperature, ultraviolet photolysis and ion irradiation on cosmic-type ices

Infrared (IR) studies of laboratory ices can provide information on the evolution of cosmic-type ices as a function of different simulated space environments involving thermal, ultraviolet (UV), or ion processing. Laboratory radiation experiments can lead to the formation of complex organic molecules. However, because of our lack of knowledge about UV photon and ion fluxes, and exposure lifetimes, it is not certain how well our simulations represent space conditions. Appropriate laboratory experiments are also limited by the absence of knowledge about the composition, density, and temperature of ices in different regions of space. Our current understanding of expected doses due to UV photons and cosmic rays is summarized here, along with an inventory of condensed-phase molecules identified on outer solar system surfaces, comets and interstellar grains. Far-IR spectra of thermally cycled H2O are discussed since these results reflect the dramatic difference between the amorphous and crystalline phases of H2O ice, the most dominant condensed-phase molecule in cosmic ices. A comparison of mid-IR spectra of products in proton-irradiated and UV-photolyzed ices shows that few differences are observed for these two forms of processing for the simple binary mixtures studied to date. IR identification of radiation products and experiments to determine production rates of new molecules in ices during processing are discussed. A new technique for measuring intrinsic IR band strengths of several unstable molecules is presented. An example of our laboratory results applied to Europa observations is included.

Spectrophotometry, Infrared/methods↗
NASA NTRS1976

Mass flow in the O7f binary UW Canis Majoris. II

The far-ultraviolet spectrum of UW CMa was obtained from the telescope spectrometer on the Copernicus satellite at about phase 0.25 in order to supplement earlier observations of the same kind made near phase 0.75. Analysis of the data shows that the high-temperature gas giving rise to the P Cygni features in the far-ultraviolet spectrum is located primarily around the entire binary system and does not share in the motion of either component. The effects of radiation pressure on the Roche (Jacobian) equipotential surfaces and in generating a stellar wind are discussed.

Mccluskey, G. E., Jr.↗