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 similar to 100 MeV and similar to 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 similar 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 (W-CO) at a 1 degrees x 1 degrees pixel level. The correlation is found to be linear over a W-CO range of similar to 10-fold when divided in three regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The W-CO-to-mass conversion factor, X-CO, is found to be similar to 2.3 x 10(20) cm(-2) (K km s(-1))(-1) for the high-longitude part of Orion A (l > 212 degrees), similar to 1.7 times higher than similar to 1.3 x 10(20) 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 H-2 and CO densities in the diffuse molecular gas. W-CO decreases faster than the H-2 column density in the region making the gas "darker" to W-CO.
GAMMA-RAY OBSERVATIONS OF THE ORION MOLECULAR CLOUDS WITH THE FERMI LARGE AREA TELESCOPE / Ackermann M; Ajello M; Allafort A; Antolini E; Baldini L; Ballet J; Barbiellini G; Bastieri D; Bechtol K; Bellazzini R; Berenji B; Blandford RD; Bloom ED; Bonamente E; Borgland AW; Bottacini E; Brandt TJ; Bregeon J; Brigida M; Bruel P; Buehler R; Buson S; Caliandro GA; Cameron RA; Caraveo PA; Cecchi C; Chekhtman A; Chiang J; Ciprini S; Claus R; Cohen-Tanugi J; Conrad J; DAmmando F; de Angelis A; de Palma F; Dermer CD; Silva EDE; Drell PS; Drlica-Wagner A; Enoto T; Falletti L; Favuzzi C; Fegan SJ; Ferrara EC; Focke WB; 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; Harding AK; Hayashida M; Hayashi K; Horan D; Hou X; Hughes RE; Jackson MS; Johannesson G; Johnson AS; Kamae T; Katagiri H; Kataoka J; Kerr M; Knodlseder J; Kuss M; Lande J; Larsson S; Lee SH; Longo F; Loparco F; Lovellette MN; Lubrano P; Makishima K; Mazziotta MN; Mehault J; Mitthumsiri W; Moiseev AA; Monte C; Monzani ME; Morselli A; Moskalenko IV; Murgia S; Nakamori T; Naumann-Godo M; Nishino S; Norris JP; Nuss E; Ohno M; Ohsugi T; Okumura A; Orienti M; Orlando E; Ormes JF; Ozaki M; Paneque D; Panetta JH; Parent D; Pelassa V; Pesce-Rollins M; Pierbattista M; Piron F; Pivato G; Porter TA; Raino S; Razzano M; Reimer A; Reimer O; Roth M; Sadrozinski HFW; Sgro C; Siskind EJ; Spandre G; Spinelli P; Strong AW; Takahashi H; Takahashi T; Tanaka T; Thayer JG; Thayer JB; Tibolla O; Tinivella M; Torres DF; Tramacere A; Troja E; Uchiyama Y; Usher TL; Vandenbroucke J; Vasileiou V; Vianello G; Vitale V; Waite AP; Wang P; Winer BL; Wood KS; Yang Z; Zimmer S. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 0004-637X. - STAMPA. - 756:1(2012), pp. 4.4-4.19. [10.1088/0004-637X/756/1/4]
GAMMA-RAY OBSERVATIONS OF THE ORION MOLECULAR CLOUDS WITH THE FERMI LARGE AREA TELESCOPE
D'AMMANDO, FILIPPO;
2012
Abstract
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 similar to 100 MeV and similar to 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 similar 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 (W-CO) at a 1 degrees x 1 degrees pixel level. The correlation is found to be linear over a W-CO range of similar to 10-fold when divided in three regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The W-CO-to-mass conversion factor, X-CO, is found to be similar to 2.3 x 10(20) cm(-2) (K km s(-1))(-1) for the high-longitude part of Orion A (l > 212 degrees), similar to 1.7 times higher than similar to 1.3 x 10(20) 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 H-2 and CO densities in the diffuse molecular gas. W-CO decreases faster than the H-2 column density in the region making the gas "darker" to W-CO.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
