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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

The chemical composition of interstellar molecular clouds

Quantitative molecular abundances are becoming available for dense interstellar clouds and circumstellar envelopes, revealing both similarities across a wide range of source conditions and significant differences in the chemistries involved. As understanding concerning the processes that lead to particular compositions increases, it may become possible to relate these findings to the evolution of molecular clouds and hence to the chemistry of regions in which stellar and planetary formation is in progress. Attention is given to the results of a recently completed spectral scan of the Orion molecular cloud, as well as the envelope around the evolved star IRC + 10216, published by Johansson et al. (1983).

Irvine, W. M.↗
NASA NTRS1990

Hot interstellar gas and ionization of embedded clouds

Researchers present detailed photoionization calculations for the instellar cloud in which the Sun is embedded. They consider the EUV radiation field with contribution from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 10 to the 6th power k coronal substrate. They establish lower limits to the fractional ionization of hydrogen and helium of 0.17 and 0.29 respectively. The high He ionization fraction results primarily from very strong line emission below 500 A originating in the surrounding coronal substrate while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 10 to the 6th k plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V and O VI through the cloud skin. Calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N (CIII)/N (CII) and N (NII)/N (NI) are dominated by the EUV radiation field in the local interstellar medium. These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. Spacecraft such as Lyman which is designed to obtain high resolution spectral data down to the Lyman limit at 912 A could sample interstellar lines of these ions.

Cheng, K.-P.↗
NASA NTRS1992

Evidence for an old Galactic bulge from RR Lyrae stars in Baade's window - Implications for the formation of the Galaxy and the age of the universe

Recent abundance measurements of RR Lyrae variables in the Baade's window field of the Galactic nuclear bulge are examined, and the observed metallicity distributions of the RR Lyraes in different Galactic radial zones are compared with those predicted for the HB population models. It is shown that the observed systematic variation with Galactocentric distance is only explained if the age of the stellar population increases, in the mean, with decreasing R(G). Thus, the oldest stellar population, the RR Lyraes in the Galactic nuclear bulge, is indeed older than that in the halo.

Lee, Young-Wook↗
NASA NTRS2011

Modeling the Entry of Micrometeoroids into the Atmospheres of Earth-like Planets

The temperature profiles of micrometeors entering the atmospheres of Earth-like planets are calculated to determine the altitude at which exogenous organic compounds may be released. Previous experiments have shown that flash-heated micrometeorite analogs release organic compounds at temperatures from roughly 500 to 1000 K [1]. The altitude of release is of great importance because it determines the fate of the compound. Organic compounds that are released deeper in the atmosphere are more likely to rapidly mix to lower altitudes where they can accumulate to higher abundances or form more complex molecules and/or aerosols. Variables that are explored here are particle size, entry angle, atmospheric density profiles, spectral type of the parent star, and planet mass. The problem reduces to these questions: (1) How much atmosphere does the particle pass through by the time it is heated to 500 K? (2) Is the atmosphere above sufficient to attenuate stellar UV such that the mixing timescale is shorter than the photochemical timescale for a particular compound? We present preliminary results that the effect of the planetary and particle parameters have on the altitude of organic release.

Pevyhouse, A. R.↗
DOE OSTI2021

Synthesis of magnesiowüstite nanocrystallites embedded in an amorphous silicate matrix via low energy multiple ion implantations

The synthesis process is presented for experimentally simulating modifications in cosmic dust grains using sequential ion implantations or irradiations followed by thermal annealing. Cosmic silicate dust analogues were prepared via implantation of 20–80 keV Fe - , Mg - , and O - ions into commercially available p-type silicon (100)wafers. The as-implanted analogues are amorphous with a Mg/(Fe+Mg) ratio of 0.5 tailored to match theoretical abundances in circumstellar dusts. Before the ion implantations were performed, Monte-Carlo-based ion-solid interaction codes were used to model the dynamic redistribution of the implanted atoms in the silicon substrate. 600 keV helium ion irradiation was performed on one of the samples before thermal annealing. Two samples were thermally annealed at a temperature appropriate for an M-class stellar wind, 1000 K, for 8.3 h in a vacuum chamber with a pressure of 1 x 10 -7 torr. The elemental depth profiles were extracted utilizing Rutherford Backscattering Spectrometry (RBS) in the samples before and after thermal annealing. X-ray diffraction (XRD)analysis was employed for the identification of various phases in crystalline minerals in the annealed analogues. Transmission electron microscopy (TEM) analysis was utilized to identify specific crystal structures. RBS analysis shows redistribution of the implanted Fe, Mg, and O after thermal annealing due to incorporation into the crystal structures for each sample type. XRD patterns along with TEM analysis showed nanocrystalline Mg and Fe oxides with possible incorporation of additional silicate minerals.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Stragglers of the thick disc

Young alpha-rich (YAR) stars have been detected in the past as outliers to the local age - [α/Fe] relation. These objects are enhanced in α-elements, but they are apparently younger than typical thick disc stars. Here, we study the global kinematics and chemical properties of YAR giant stars in the APOGEE DR17 survey and show that they have properties similar to those of the standard thick disc stellar population. This leads us to conclude that YAR are rejuvenated thick disc objects, and the most likely explanation is that they are evolved blue stragglers. This is confirmed by their position in the Hertzsprung–Russel diagram (HRD). Extending our selection to dwarfs allowed us to obtain the first general straggler distribution in an HRD of field stars. We also compared the elemental abundances of our sample with those of standard thick disc stars and found that our YAR stars are shifted in oxygen, magnesium, sodium, and the slow neutron-capture element cerium. Although we detected no sign of binarity for most objects, the enhancement in cerium may be a signature of a mass transfer from an asymptotic giant branch companion. The most massive YAR stars suggest that mass transfer from an evolved star may not be the only plausible formation pathway and that other scenarios, such as collision or coalescence, should be considered.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

OAO-2 observations of C III 1909 A line in gamma 2 Velorum

The C III 1909 A emission line in gamma 2 Velorum was observed over a period of 24 hours in an attempt to find short period time variability. Analysis of this data found no periods in the range of 0.5 to 200 minutes, and placed an upper limit of 3% on the amplitude of the stellar pulsations. Improved spectral resolution data for this line was obtained by offsetting the OAO-2 pointing in steps of 15 seconds of arc. The line profile derived by combining this offset data with the slit function shows a definite asymmetry. Although the improved resolution gives results which are barely sufficient for comparison with the prediction from model envelopes, some information concerning the abundance of carbon and the physical conditions of the envelope can be gained.

West, D. K.↗
NASA NTRS1982

The anomalous X-ray absorption spectrum of Vela X-1

The HEAO 2 satellite's Solid State Spectrometer and Monitor Proportional Counter was used to observe one orbit of the massive X-ray binary Vela X-1. Using spectral fits to the data as a function of orbital phase, the column density and state of the material along the line of sight to the X-ray source has been inferred. The spectrum near orbital phase 0.2 compares favorably with absorption by neutral material with a column density corresponding to plausible values of stellar wind velocity law and total primary mass loss rate. Spectra at later orbital phases, which show unexpected strong absorption features near 2.0 and 2.5 keV, are interpreted as due to absorption by material with suppressed opacity below 2.0 keV. The opacity required to produce the observed features implies either the presence of an intense soft X-ray flux, or altered elemental abundances in the gas near Vela X-1.

Kallman, T. R.↗
NASA NTRS1991

Co-57 and Ti-44 production in SN 1987A

A survey of parameterized explosive silicon burning is carried out in order to limit the range of allowed masses of Co-57 produced by SN 1987A. The most likely value is between 0.5 and 2.5 times that implied by a solar ratio for Fe-57/Fe-56. Values more than four and less than one-third times the solar ratio would pose very severe problems for the theory of stellar nucleosynthesis, implying, for example, that Fe-56 was not the dominant iron group isotope or that no electron capture occurred during oxygen burning. Though dependent somewhat on the neutron excess, the Fe-57 yield is most sensitive to the occurrence of an alpha-rich freeze-out. An accurate measurement of the Fe-57 yield, as reflected observationally by the current abundance of radioactive Co-57, is thus an important constraint upon the supernova explosion mechanism. The abundance of Ti-44, another nucleus produced exclusively in the alpha-rich freeze-out, cannot be much greater than that implied by the solar ratio of Ca-44/Fe-56.

Woosley, S. E.↗
NASA NTRS1994

ASCA observation of three bright early-type galaxies: NGC 4472, NGC 4406, and NGC 4636

We report Advanced Satellite for Cosmology and Astrophysics (ASCA) 0.3-10 keV and X-ray observations of three early type galaxies, NGC 4472, NGC 4406, and NGC 4636. The extended mission in these galaxies is well described by thin thermal eimssion from hot gas. The gas temperature is 0.92 +/- 0.02 keV for NGC 4472, 0.79 +/- 0.01 keV for NGC 4406, and 0.73 +/- 0.02 keV for NGC 4636. The metal abundance for NGC 4472, NGC 4406, and NGC 4636 are, under the assumption of solar ratios, 0.63 +/- 0.15, 0.45 +/- 0.10, and 0.38 +/- 0.07, respectively. Detailed analysis has allowed determination of the abundances of oxygen, silicon, sulfur, and iron. The observed abundances are consistent with the solar ratios. For NGC 4472 and NGC 4406 we also determined the mean temperature of the gas producing the Si lines from the ratio of the Si H to He-like lines and find it to be consistent with the continuum temperature. The X-ray temperature is in good agreement with the observed optical velocity dispersion, stellar density profile, and gas density profile. Our data indicates that the supernova rate should be less than one fifth of the nominal rate in early type galaxies. We derive the mass of these systems within fixed angular scales and find that M/L greater than 40, confirming that elliptical galaxies are dark matter dominated at large radii.

Awaki, Hisamitsu↗
DOE OSTI2022

Galaxy–halo size relation from Sloan Digital Sky Survey Data Release 7 and the ELUCID simulation

ABSTRACT Based on galaxies in the Sloan Digital Sky Survey Data Release 7 and dark matter haloes in the dark matter only, cosmological, and constrained ELUCID simulation, we investigate the relation between the observed radii of central galaxies with stellar mass $\gtrsim\!{10}^{8} \, h^{-2}\, {\rm M}_\odot$ and the virial radii of their host dark matter haloes with virial mass $\gtrsim\!{10}^{10.5} \, h^{-1}\, {\rm M}_\odot$, and the dependence of galaxy–halo size relation on the halo spin and concentration. Galaxies in observation are matched to dark matter (sub)haloes in the ELUCID simulation using a novel neighbourhood subhalo abundance matching method. For galaxy two-dimensional (2D) half-light radii R50, we find that early- and late-type galaxies have the same power-law index 0.55 with $R_{50} \propto R_{\rm vir}^{0.55}$, although early-type galaxies have smaller 2D half-light radii than late-type galaxies at fixed halo virial radii. When converting the 2D half-light radii R50 to 3D half-mass radii r1/2, both early- and late-type galaxies display similar galaxy–halo size relations with $\log r_{1/2} = 0.55 \log (R_{\rm vir}/210 \, h^{-1}\, {\rm kpc}) + 0.39$. We find that the galaxy–halo size ratio r1/2/Rvir decreases with increasing halo mass. At fixed halo mass, there is no significant dependence of galaxy–halo size ratio on the halo spin or concentration.

Astronomy & Astrophysics↗
NASA NTRS1972

Limits on intergalactic helium from the 3C 273 X-ray spectrum.

An X-ray spectrum of the quasi-stellar object 3C 273 in the interval 0.25-10 keV has been obtained by sounding-rocket observations. The best-fit power-law spectrum has a photon index n = 1.3 with no photoelectric absorption. An upper limit on the X-ray optical depth to 3C 273 has been calculated from the data, permitting upper limits to be set on the absolute abundance of helium in the intergalactic medium.

Margon, B.↗
NASA NTRS1993

Structure and chemistry of Orion S

We present interferometric observations of the SiO J = 2-1, H13CO(+) J = 1-0, HC3N J = 11-10, CH3OH J(K) = 2(0)-1(0), and SO2 J(KpK0) = 8(17)-8(08) transitions along with the 3.1 mm continuum toward the young stellar object Orion S. The HC3N and H13CO(+) emission trace similar spatial and velocity distributions which are extended and follow the Orion molecular ridge. The SiO emission is more spatially confined, peaking to the west of the 3.1 mm continuum source, while the CH3OH emission peaks to the southwest. Weak SO2 emission was detected southeast of the continuum source position. Column densities and fractional abundances are derived for each species at different positions in the region. In general, the molecular abundances near the continuum source are similar to those in the quiescent material near IRc 2, but the abundances decrease toward the continuum source position indicating localized depletions of at least a factor of three. The presence of strong SiO emission with much weaker SO2 emission is interpreted as resulting from high-velocity shock interactions between the outflow from Orion S and the surrounding cloud.

Mcmullin, Joseph P.↗
NASA NTRS1999

In Pursuit of the FIP Effect in Late-Type Stellar Coronae

Spectral line data for several coronally active stars, in addition to EUVE Deep Survey light curves, have been analysed under this program. Much difficulty has been encountered in the study that has resulted in fewer stars being analysed than had been hoped. The difficulties stemmed from the analysis of low X-ray spectra taken with the ASCA satellite that produced results that are strongly discrepant with respect to the EUVE results. There is no obvious explanation for this, though it appears that analysis of ASCA data systematically underestimate metal abundance in hot plasmas. Consequently, the final emphasis in our analyses has been on EUVE data. Observed line profiles have being fitted in order to measure their fluxes using IDL software specially developed under this and parallel efforts.The observed line profiles deviate from pure gaussian forms, but we have found the benefits of using additional functional forms in the fitting process to be of only very small value for the lines with highest S/N. The resulting line fluxes have being processed in terms of the coronal EM using new techniques. Resulting EM distribution models are being used to finalize metallicity and abundance estimates for the stars in the program. Special account of the influence of missing lines in the spectral models has been taken.

Drake, Jeremy↗
DOE OSTI2021

22 Ne Phase Separation as a Solution to the Ultramassive White Dwarf Cooling Anomaly

The precise astrometric measurements of the Gaia Data Release 2 have opened the door to detailed tests of the predictions of white dwarf cooling models. Significant discrepancies between theory and observations have been identified, the most striking affecting ultramassive white dwarfs. Cheng et al. found that a small fraction of white dwarfs on the so-called Q branch must experience an extra cooling delay of ~8 Gyr not predicted by current models. 22 Ne phase separation in a crystallizing C/O white dwarf can lead to a distillation process that efficiently transports 22 Ne toward its center, thereby releasing a considerable amount of gravitational energy. Here, using state-of-the-art Monte Carlo simulations, we show that this mechanism can largely resolve the ultramassive cooling anomaly if the delayed population consists of white dwarfs with moderately above-average 22 Ne abundances. We also argue that 22 Ne phase separation can account for the smaller cooling delay currently missing for models of white dwarfs with more standard compositions.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1999

The Second-Parameter Effect in Metal-Rich Globular Clusters

Recent Hubble Space Telescope (HST) observations have shown that the metal-rich globular clusters (GCs) NGC 6388 and NGC 6441 exhibit a pronounced 2nd parameter effect. Ordinarily metal-rich GCs have only a red horizontal-branch (HB) clump. However, NGC 6388 and NGC 6441 also possess an unexpected population of blue HB stars, indicating that some 2nd parameter is operating in these clusters. Quite remarkably, the HBs in both clusters slope upward with decreasing B -V from the red clump to the top of the blue tail. We review the results of ongoing stellar evolution calculations which indicate (1) that NGC 6388 and NGC 6441 might provide a crucial diagnostic for understanding the origin of the 2nd parameter effect, (2) that differences in age or mass loss along the red-giant branch (RGB) - the two most prominent 2nd parameter candidates - cannot explain the HB morphology of these GCs, and (3) that noncanonical effects involving an enhanced helium abundance or rotation can produce upward sloping HBs. Finally we suggest a new metal-depletion scenario which might help to resolve a baffling conundrum concerning the surface gravities of the blue HB stars in these clusters.

Sweigart, Allen V.↗
NASA NTRS1989

Non-LTE analysis of extremely helium-rich stars. The hot sdO stars LSE 153, 259 and 263

Results of a non-LTE fine analysis based mainly on high-resolution CASPEC spectra for three extremely helium-rich sdO stars are discussed in order to explain hydrogen deficiency in single stars. High temperature (Teff = 70,000 to 75,000 K) and a position in the log Teff - log g diagram were found close to the Eddington limit. Various abundance estimates are derived for hydrogen (upper limits only), carbon, nitrogen, and magnesium. Hydrogen is reduced to less than 10 percent by number in LSE 153 and LSE 263, and to less than 5 percent in LSE 259. The hydrogen deficiency is accompanied by nitrogen- and carbon-enrichment in LSE 153 and LSE 259 only. In LSE 263, carbon is depleted by about 1 dex. Stellar masses obtained by assuming that a core mass-luminosity relation holds for these stars, were found to be in the range 0.6-0.9 solar mass, yielding luminosities log L/L:solar = 3.7-4.5. Two of the program stars (LSE 153 and 259) appear to be possible successors of the R CrB and helium B stars, whereas the third star (LSE 263) displays a much lower carbon content in its photosphere making it an exceptional case among the known hydrogen deficient stars.

Husfeld, D.↗
NASA NTRS2006

NASA's Future X-ray Missions: From Constellation-X to Generation-X

Among the most important topics in modern astrophysics are the formation and evolution of supermassive black holes in concert with galaxy bulges, the nature of the dark energy equation of state, and the self-regulating symmetry imposed by both stellar and AGN feedback. All of these topics are readily addressed with observations at X-ray wavelengths. NASA's next major X-ray observatory is Constellation-X, which is being developed to perform spatially resolved high-resolution X-ray spectroscopy. Con-X will directly measure the physical properties of material near black holes' last stable orbits and the absolute element abundances and velocities of hot gas in clusters of galaxies. The Con-X mission will be described, as well as its successor, Generation-X (anticipated to fly approx.1 decade after Con-X). After describing these missions and their driving science areas, the talk will focus on areas in which Chandra observing programs may enable science with future X-ray observatories. These areas include a possible ultra-deep Chandra imaging survey as an early Universe pathfinder, a large program to spatially resolve the hot intracluster medium of massive clusters to aid dark energy measurements, and possible deep spectroscopic observations to aid in preparatory theoretical atomic physics work needed for interpreting Con-X spectra.

Hornschemeier, A.↗