Galactic annihilation emission from nucleosynthesis positrons

The Galaxy hosts a widespread population of low-energy positrons revealed by successive generations of gamma-ray telescopes through a bright annihilation emission from the bulge region, with a fainter contribution from the inner disk. The exact origin of these particles remains currently unknown. We estimate the contribution to the annihilation signal of positrons generated in the decay of radioactive 26Al, 56Ni and 44Ti. We adapted the GALPROP propagation code to simulate the transport and annihilation of radioactivity positrons in a model of our Galaxy. Using plausible source spatial distributions, we explored several possible propagation scenarios to account for the large uncertainties on the transport of ~1MeV positrons in the interstellar medium. We then compared the predicted intensity distributions to the INTEGRAL/SPI observations. We obtain similar intensity distributions with small bulge-to-disk ratios, even for extreme large-scale transport prescriptions. At least half of the positrons annihilate close to their sources, even when they are allowed to travel far away. In the high-diffusion, ballistic case, up to 40% of them escape the Galaxy. In proportion, this affects bulge positrons more than disk positrons because they are injected further off the plane in a tenuous medium, while disk positrons are mostly injected in the dense molecular ring. The predicted intensity distributions are fully consistent with the observed longitudinally-extended disk-like emission, but the transport scenario cannot be strongly constrained by the current data. Nucleosynthesis positrons alone cannot account for the observed annihilation emission in the frame of our model. An additional component is needed to explain the strong bulge contribution, and the latter is very likely concentrated in the central regions if positrons have initial energies in the 100keV-1MeV range.Comment: 16 pages, 7 figures, accepted for publication in A&

Techniques for carrying out radiative transfer calculations for the Martian atmospheric dust

A description is given of the modification of a theory on the reflectance of particulate media so as to apply it to analysis of the infrared spectra obtained by the IRIS instrument on Mariner 9. With the aid of this theory and the optical constants of muscovite mica, quartz, andesite, anorthosite, diopside pyroxenite, and dunite, modeling calculations were made to refine previous estimates of the mineralogical composition of the Martian dust particles. These calculations suggest that a feldspar rich mixture is a very likely composition for the dust particles. The optical constants used for anorthosite and diopside pyroxenite were derived during this program from reflectance measurements. Those for the mica were derived from literature reflectance data. Finally, a computer program was written to invert the measured radiance data so as to obtain the absorption coefficient spectrum which should then be independent of the temperature profile and gaseous component effects

Diffuse continuum gamma rays from the Galaxy

A new study of the diffuse Galactic gamma-ray continuum radiation is presented, using a cosmic-ray propagation model which includes nucleons, antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of the inverse Compton (IC) scattering includes the effect of anisotropic scattering in the Galactic interstellar radiation field (ISRF) and a new evaluation of the ISRF itself. Models based on locally measured electron and nucleon spectra and synchrotron constraints are consistent with gamma-ray measurements in the 30-500 MeV range, but outside this range excesses are apparent. A harder nucleon spectrum is considered but fitting to gamma rays causes it to violate limits from positrons and antiprotons. A harder interstellar electron spectrum allows the gamma-ray spectrum to be fitted above 1 GeV as well, and this can be further improved when combined with a modified nucleon spectrum which still respects the limits imposed by antiprotons and positrons. A large electron/IC halo is proposed which reproduces well the high-latitude variation of gamma-ray emission. The halo contribution of Galactic emission to the high-latitude gamma-ray intensity is large, with implications for the study of the diffuse extragalactic component and signatures of dark matter. The constraints provided by the radio synchrotron spectral index do not allow all of the <30 MeV gamma-ray emission to be explained in terms of a steep electron spectrum unless this takes the form of a sharp upturn below 200 MeV. This leads us to prefer a source population as the origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal (vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49 ps-figures, uses emulateapj.sty. More details can be found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm

Giant Molecular Clouds in M33 - I. BIMA All Disk Survey

We present the first interferometric CO(J=1->0) map of the entire H-alpha disk of M33. The 13" diameter synthesized beam corresponds to a linear resolution of 50 pc, sufficient to distinguish individual giant molecular clouds (GMCs). From these data we generated a catalog of 148 GMCs with an expectation that no more than 15 of the sources are spurious. The catalog is complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of 2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that our survey detects ~50% of the CO flux, hence that the total molecular mass of M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our catalog are confined largely to the central region (R < 4 kpc). They show a remarkable spatial and kinematic correlation with overdense HI filaments; the geometry suggests that the formation of GMCs follows that of the filaments. The GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper than that found in the Milky Way and in the LMC. Combined with the total mass, this steep function implies that the GMCs in M33 form with a characteristic mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions, implying that the GMCs have a short quiescent period. Our results suggest the rapid assembly of molecular clouds from atomic gas, with prompt onset of massive star formation.Comment: 19 pages, Accepted for Publication in the Astrophysical Journal Supplemen

Float zone processing in a weightless environment

Results are given for investigations into: (1) the physical limits which set the maximum practical diameters of Si crystals that can be processed by the float-zone method in a near weightless environment, and (2) the economic impact of large, space-produced Si crystals on the electronics industry. The stability of the melt is evaluated. Heat transfer and fluid flow within the melt as dependent on the crystal size and the degree and type of rotation imparted to the melt are studied. Methods of utilizing the weightless environment for the production of large, stress-free Si crystals of uniform composition are proposed. The economic effect of large size Si crystals, their potential applications, likely utilization and cost advantages in LSI, integrated circuits, and power devices are also evaluated. Foreseeable advantages of larger diameter wafers of good characteristics and the possibilities seen for greater perfection resulting from stress-free growth are discussed

Development of a theory of the spectral reflectance of minerals, part 2

Theory of diffuse reflectance of particulate media including garnet, glass, corundum powders, and mixture

SPI Measurements of the Diffuse Galactic Hard X-ray Continuum

INTEGRAL Spectrometer SPI data from the first year of the Galactic Centre Deep Exposure has been analysed for the diffuse continuum from the Galactic ridge. A new catalogue of sources from the INTEGRAL Imager IBIS has been used to account for their contribution to the celestial signal. Apparently diffuse emission is detected at a level ~10% of the total source flux. A comparison of the spectrum of diffuse emission with that from an analysis of IBIS data alone shows that they are consistent. The question of the contribution of unresolved sources to this ridge emission is still open.Comment: Proceedings of the 5th INTEGRAL Workshop, Munich 16-20 February 2004. ESA SP-552. Reference to Terrier et al. (2004) updated to include astro-ph versio