ACKERMANN, Markus; AJELLO, Marco; ALLAFORT, A.; ANTOLINI, E.; BALDINI, Luca; BALLET, Jean; BARBIELLINI, Guido; BASTIERI, D.; BECHTOL, K.; BELLAZZINI, R.; BERENJI, B.; D. BLANDFORD, R.; D. BLOOM, E.; BONAMENTE, E.; W. BORGLAND, A.; BOTTACINI, E.; J. BRANDT, T.; BREGEON, J.; BRIGIDA, M.; BRUEL, Pascal; BUEHLER, R.; BUSON, S.; A. CALIANDRO, G.; A. CAMERON, R.; CARAVEO, P. A.; CECCHI, C.; CHEKHTMAN, A.; CHIANG, J.; CIPRINI, S.; CLAUS, R.; COHEN-TANUGI, J.; CONRAD, J.; D'AMMANDO, F.; DE ANGELIS, A.; DE PALMA, F.; D. DERMER, C.; DO COUTO E SILVA, E.; S. DRELL, P.; DRLICA-WAGNER, A.; ENOTO, T.; FALLETTI, L.; FAVUZZI, C.; J. FEGAN, S.; C. FERRARA, E.; B. FOCKE, W.; FUKAZAWA, Y.; FUKUI, Y.; FUSCO, P.; GARGANO, F.; GASPARRINI, D.; GERMANI, S.; GIGLIETTO, N.; GIORDANO, F.; GIROLETTI, M.; GLANZMAN, T.; GODFREY, G.; GUIRIEC, S.; HADASCH, D.; HANABATA, Y.; K. HARDING, A.; HAYASHIDA, M.; HAYASHI, K.; HORAN, D.; HOU, X.; E. HUGHES, R.; S. JACKSON, M.; J 'OHANNESSON, G.; S. JOHNSON, A.; KAMAE, T.; KATAGIRI, H.; KATAOKA, J.; KERR, M.; KNÖDLSEDER, Jürgen; KUSS, M.; LANDE, J.; LARSSON, S.; LEE, S.-H.; LONGO, F.; LOPARCO, F.; N. LOVELLETTE, M.; LUBRANO, P.; MAKISHIMA, K.; N. MAZZIOTTA, M.; MEHAULT, J.; MITTHUMSIRI, W.; A. MOISEEV, A.; MONTE, C.; E. MONZANI, M.; MORSELLI, A.; V. MOSKALENKO, I.; MURGIA, S.; NAKAMORI, T.; NAUMANN-GODO, M.; NISHINO, S.; P. NORRIS, J.; NUSS, E.; OHNO, M.; OHSUGI, T.; OKUMURA, A.; ORIENTI, M.; ORLANDO, E.; F. ORMES, J.; OZAKI, M.; PANEQUE, D.; H. PANETTA, J.; PARENT, D.; PELASSA, V.; PESCE-ROLLINS, M.; PIERBATTISTA, M.; PIRON, F.; PIVATO, G.; PORTER, T.A.; RAIN'O, S.; RAZZANO, M.; REIMER, A.; REIMER, O.; ROTH, M.; F.-W. SADROZINSKI, H.; SGR'O, C.; J. SISKIND, E.; SPANDRE, G.; SPINELLI, P.; W. STRONG, A.; TAKAHASHI, H.; TAKAHASHI, T.; TANAKA, T.; G. THAYER, J.; B. THAYER, J.; TIBOLLA, O.; TINIVELLA, M.; F. TORRES, D.; TRAMACERE, A.; TROJA, E.; UCHIYAMA, Y.; L. USHER, T.; VANDENBROUCKE, J.; VASILEIOU, V.; VIANELLO, G.; VITALE, V.; P. WAITE, A.; WANG, P.; L. WINER, B.; S. WOOD, K.; YANG, Z.; ZIMMER, S.

doi:10.1088/0004-637X/756/1/4

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The Astrophysical Journal. 2012, vol. 756, p. 4

American Astronomical Society

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We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between ∼100 MeV and ∼100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to ∼10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity-integrated CO intensity (WCO) at a 1◦ × 1◦ pixel level. The correlation is found to be linear over a WCO range of ∼10-fold when divided in three regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The WCO-to-mass conversion factor, XCO, is found to be ∼2.3 × 1020 cm−2 (K km s−1)−1 for the high-longitude part of Orion A (l > 212◦), ∼1.7 times higher than ∼1.3 × 1020 found for the rest of Orion A and B. We interpret the apparent high XCO in the high-longitude region of Orion A in the light of recent works proposing a nonlinear relation between H2 and CO densities in the diffuse molecular gas. WCO decreases faster than the H2 column density in the region making the gas "darker" to WCO.< Réduire

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gamma rays: ISM

ISM: clouds

ISM: general

ISM: individual objects (Orion A and Orion B) Online-only material: color figures

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GAMMA-RAY OBSERVATIONS OF THE ORION MOLECULAR CLOUDS WITH THE FERMI LARGE AREA TELESCOPE

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