Gamma-ray observations of the orion molecular clouds with the fermi large area telescope

M. Ackermann, M. Ajello, A. Allafort, E. Antolini, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, R. D. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, E. Bottacini, T. J. Brandt, J. Bregeon, M. Brigida, P. Bruel & 125 others R. Buehler, S. Buson, G. A. Caliandro, R. A. Cameron, P. A. Caraveo, C. Cecchi, A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, J. Conrad, F. D'Ammando, A. De Angelis, F. De Palma, C. D. Dermer, E. Do Couto E Silva, P. S. Drell, A. Drlica-Wagner, T. Enoto, L. Falletti, C. Favuzzi, S. J. Fegan, E. C. Ferrara, W. B. Focke, Y. Fukazawa, Y. Fukui, P. Fusco, F. Gargano, D. Gasparrini, S. Germani, N. Giglietto, F. Giordano, M. Giroletti, T. Glanzman, G. Godfrey, S. Guiriec, D. Hadasch, Y. Hanabata, A. K. Harding, M. Hayashida, K. Hayashi, D. Horan, X. Hou, R. E. Hughes, M. S. Jackson, G. Jóhannesson, A. S. Johnson, T. Kamae, H. Katagiri, Jun Kataoka, M. Kerr, J. Knödlseder, M. Kuss, J. Lande, S. Larsson, S. H. Lee, F. Longo, F. Loparco, M. N. Lovellette, P. Lubrano, K. Makishima, M. N. Mazziotta, J. Mehault, W. Mitthumsiri, A. A. Moiseev, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, T. Nakamori, M. Naumann-Godo, S. Nishino, J. P. Norris, E. Nuss, M. Ohno, T. Ohsugi, A. Okumura, M. Orienti, E. Orlando, J. F. Ormes, M. Ozaki, D. Paneque, J. H. Panetta, D. Parent, V. Pelassa, M. Pesce-Rollins, M. Pierbattista, F. Piron, G. Pivato, T. A. Porter, S. Rainò, M. Razzano, A. Reimer, O. Reimer, M. Roth, H. F W Sadrozinski, C. Sgrò, E. J. Siskind, G. Spandre, P. Spinelli, A. W. Strong, H. Takahashi, T. Takahashi, T. Tanaka, J. G. Thayer, J. B. Thayer, O. Tibolla, M. Tinivella, D. F. Torres, A. Tramacere, E. Troja, Y. Uchiyama, T. L. Usher, J. Vandenbroucke, V. Vasileiou, G. Vianello, V. Vitale, A. P. Waite, P. Wang, B. L. Winer, K. S. Wood, Z. Yang, S. Zimmer

    Research output: Contribution to journalArticle

    30 Citations (Scopus)

    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 100MeV and 100GeV 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 (W CO) at a 1° × 1°pixel level. The correlation is found to be linear over a W CO range of ∼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 ∼2.3 × 1020 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > ∼212°), 1.7times higher than ∼1.3 × 10 20 found for the rest of Orion A and B. We interpret the apparent high X CO 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. W CO decreases faster than the H2 column density in the region making the gas "darker" to W CO.

    Original languageEnglish
    Article number4
    JournalAstrophysical Journal
    Volume756
    Issue number1
    DOIs
    Publication statusPublished - 2012

    Fingerprint

    molecular clouds
    gamma rays
    telescopes
    cosmic ray
    gas
    nuclear interactions
    longitude
    cosmic rays
    gases
    interstellar gas
    Fermi Gamma-ray Space Telescope
    molecular gases
    mass distribution
    energy bands
    density distribution
    penetration
    cross section
    fold
    cross sections
    energy

    Keywords

    • gamma rays: ISM
    • ISM: clouds
    • ISM: general
    • ISM: individual objects (Orion A and Orion B)

    ASJC Scopus subject areas

    • Space and Planetary Science
    • Astronomy and Astrophysics

    Cite this

    Ackermann, M., Ajello, M., Allafort, A., Antolini, E., Baldini, L., Ballet, J., ... Zimmer, S. (2012). Gamma-ray observations of the orion molecular clouds with the fermi large area telescope. Astrophysical Journal, 756(1), [4]. https://doi.org/10.1088/0004-637X/756/1/4

    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, R. D.; Bloom, E. D.; Bonamente, E.; Borgland, A. W.; Bottacini, E.; Brandt, T. J.; Bregeon, J.; Brigida, M.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; 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.; Dermer, C. D.; Do Couto E Silva, E.; Drell, P. S.; Drlica-Wagner, A.; Enoto, T.; Falletti, L.; Favuzzi, C.; Fegan, S. J.; Ferrara, E. C.; Focke, W. B.; 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, A. K.; Hayashida, M.; Hayashi, K.; Horan, D.; Hou, X.; Hughes, R. E.; Jackson, M. S.; Jóhannesson, G.; Johnson, A. S.; Kamae, T.; Katagiri, H.; Kataoka, Jun; Kerr, M.; Knödlseder, J.; Kuss, M.; Lande, J.; Larsson, S.; Lee, S. H.; Longo, F.; Loparco, F.; Lovellette, M. N.; Lubrano, P.; Makishima, K.; Mazziotta, M. N.; Mehault, J.; Mitthumsiri, W.; Moiseev, A. A.; Monte, C.; Monzani, M. E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Nakamori, T.; Naumann-Godo, M.; Nishino, S.; Norris, J. P.; Nuss, E.; Ohno, M.; Ohsugi, T.; Okumura, A.; Orienti, M.; Orlando, E.; Ormes, J. F.; Ozaki, M.; Paneque, D.; Panetta, J. H.; Parent, D.; Pelassa, V.; Pesce-Rollins, M.; Pierbattista, M.; Piron, F.; Pivato, G.; Porter, T. A.; Rainò, S.; Razzano, M.; Reimer, A.; Reimer, O.; Roth, M.; Sadrozinski, H. F W; Sgrò, C.; Siskind, E. J.; Spandre, G.; Spinelli, P.; Strong, A. W.; Takahashi, H.; Takahashi, T.; Tanaka, T.; Thayer, J. G.; Thayer, J. B.; Tibolla, O.; Tinivella, M.; Torres, D. F.; Tramacere, A.; Troja, E.; Uchiyama, Y.; Usher, T. L.; Vandenbroucke, J.; Vasileiou, V.; Vianello, G.; Vitale, V.; Waite, A. P.; Wang, P.; Winer, B. L.; Wood, K. S.; Yang, Z.; Zimmer, S.

    In: Astrophysical Journal, Vol. 756, No. 1, 4, 2012.

    Research output: Contribution to journalArticle

    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, D'Ammando, F, De Angelis, A, De Palma, F, Dermer, CD, Do Couto E Silva, E, 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, Jóhannesson, G, Johnson, AS, Kamae, T, Katagiri, H, Kataoka, J, Kerr, M, Knödlseder, 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, Rainò, S, Razzano, M, Reimer, A, Reimer, O, Roth, M, Sadrozinski, HFW, Sgrò, 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 2012, 'Gamma-ray observations of the orion molecular clouds with the fermi large area telescope', Astrophysical Journal, vol. 756, no. 1, 4. https://doi.org/10.1088/0004-637X/756/1/4
    Ackermann, M. ; Ajello, M. ; Allafort, A. ; Antolini, E. ; Baldini, L. ; Ballet, J. ; Barbiellini, G. ; Bastieri, D. ; Bechtol, K. ; Bellazzini, R. ; Berenji, B. ; Blandford, R. D. ; Bloom, E. D. ; Bonamente, E. ; Borgland, A. W. ; Bottacini, E. ; Brandt, T. J. ; Bregeon, J. ; Brigida, M. ; Bruel, P. ; Buehler, R. ; Buson, S. ; Caliandro, G. A. ; Cameron, R. A. ; 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. ; Dermer, C. D. ; Do Couto E Silva, E. ; Drell, P. S. ; Drlica-Wagner, A. ; Enoto, T. ; Falletti, L. ; Favuzzi, C. ; Fegan, S. J. ; Ferrara, E. C. ; Focke, W. B. ; 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, A. K. ; Hayashida, M. ; Hayashi, K. ; Horan, D. ; Hou, X. ; Hughes, R. E. ; Jackson, M. S. ; Jóhannesson, G. ; Johnson, A. S. ; Kamae, T. ; Katagiri, H. ; Kataoka, Jun ; Kerr, M. ; Knödlseder, J. ; Kuss, M. ; Lande, J. ; Larsson, S. ; Lee, S. H. ; Longo, F. ; Loparco, F. ; Lovellette, M. N. ; Lubrano, P. ; Makishima, K. ; Mazziotta, M. N. ; Mehault, J. ; Mitthumsiri, W. ; Moiseev, A. A. ; Monte, C. ; Monzani, M. E. ; Morselli, A. ; Moskalenko, I. V. ; Murgia, S. ; Nakamori, T. ; Naumann-Godo, M. ; Nishino, S. ; Norris, J. P. ; Nuss, E. ; Ohno, M. ; Ohsugi, T. ; Okumura, A. ; Orienti, M. ; Orlando, E. ; Ormes, J. F. ; Ozaki, M. ; Paneque, D. ; Panetta, J. H. ; Parent, D. ; Pelassa, V. ; Pesce-Rollins, M. ; Pierbattista, M. ; Piron, F. ; Pivato, G. ; Porter, T. A. ; Rainò, S. ; Razzano, M. ; Reimer, A. ; Reimer, O. ; Roth, M. ; Sadrozinski, H. F W ; Sgrò, C. ; Siskind, E. J. ; Spandre, G. ; Spinelli, P. ; Strong, A. W. ; Takahashi, H. ; Takahashi, T. ; Tanaka, T. ; Thayer, J. G. ; Thayer, J. B. ; Tibolla, O. ; Tinivella, M. ; Torres, D. F. ; Tramacere, A. ; Troja, E. ; Uchiyama, Y. ; Usher, T. L. ; Vandenbroucke, J. ; Vasileiou, V. ; Vianello, G. ; Vitale, V. ; Waite, A. P. ; Wang, P. ; Winer, B. L. ; Wood, K. S. ; Yang, Z. ; Zimmer, S. / Gamma-ray observations of the orion molecular clouds with the fermi large area telescope. In: Astrophysical Journal. 2012 ; Vol. 756, No. 1.
    @article{2f791fe03d194ddcb2154a688f82d81b,
    title = "Gamma-ray observations of the orion molecular clouds with the fermi large area telescope",
    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 100MeV and 100GeV 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 (W CO) at a 1° × 1°pixel level. The correlation is found to be linear over a W CO range of ∼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 ∼2.3 × 1020 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > ∼212°), 1.7times higher than ∼1.3 × 10 20 found for the rest of Orion A and B. We interpret the apparent high X CO 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. W CO decreases faster than the H2 column density in the region making the gas {"}darker{"} to W CO.",
    keywords = "gamma rays: ISM, ISM: clouds, ISM: general, ISM: individual objects (Orion A and Orion B)",
    author = "M. Ackermann and M. Ajello and A. Allafort and E. Antolini and L. Baldini and J. Ballet and G. Barbiellini and D. Bastieri and K. Bechtol and R. Bellazzini and B. Berenji and Blandford, {R. D.} and Bloom, {E. D.} and E. Bonamente and Borgland, {A. W.} and E. Bottacini and Brandt, {T. J.} and J. Bregeon and M. Brigida and P. Bruel and R. Buehler and S. Buson and Caliandro, {G. A.} and Cameron, {R. A.} and Caraveo, {P. A.} and C. Cecchi and A. Chekhtman and J. Chiang and S. Ciprini and R. Claus and J. Cohen-Tanugi and J. Conrad and F. D'Ammando and {De Angelis}, A. and {De Palma}, F. and Dermer, {C. D.} and {Do Couto E Silva}, E. and Drell, {P. S.} and A. Drlica-Wagner and T. Enoto and L. Falletti and C. Favuzzi and Fegan, {S. J.} and Ferrara, {E. C.} and Focke, {W. B.} and Y. Fukazawa and Y. Fukui and P. Fusco and F. Gargano and D. Gasparrini and S. Germani and N. Giglietto and F. Giordano and M. Giroletti and T. Glanzman and G. Godfrey and S. Guiriec and D. Hadasch and Y. Hanabata and Harding, {A. K.} and M. Hayashida and K. Hayashi and D. Horan and X. Hou and Hughes, {R. E.} and Jackson, {M. S.} and G. J{\'o}hannesson and Johnson, {A. S.} and T. Kamae and H. Katagiri and Jun Kataoka and M. Kerr and J. Kn{\"o}dlseder and M. Kuss and J. Lande and S. Larsson and Lee, {S. H.} and F. Longo and F. Loparco and Lovellette, {M. N.} and P. Lubrano and K. Makishima and Mazziotta, {M. N.} and J. Mehault and W. Mitthumsiri and Moiseev, {A. A.} and C. Monte and Monzani, {M. E.} and A. Morselli and Moskalenko, {I. V.} and S. Murgia and T. Nakamori and M. Naumann-Godo and S. Nishino and Norris, {J. P.} and E. Nuss and M. Ohno and T. Ohsugi and A. Okumura and M. Orienti and E. Orlando and Ormes, {J. F.} and M. Ozaki and D. Paneque and Panetta, {J. H.} and D. Parent and V. Pelassa and M. Pesce-Rollins and M. Pierbattista and F. Piron and G. Pivato and Porter, {T. A.} and S. Rain{\`o} and M. Razzano and A. Reimer and O. Reimer and M. Roth and Sadrozinski, {H. F W} and C. Sgr{\`o} and Siskind, {E. J.} and G. Spandre and P. Spinelli and Strong, {A. W.} and H. Takahashi and T. Takahashi and T. Tanaka and Thayer, {J. G.} and Thayer, {J. B.} and O. Tibolla and M. Tinivella and Torres, {D. F.} and A. Tramacere and E. Troja and Y. Uchiyama and Usher, {T. L.} and J. Vandenbroucke and V. Vasileiou and G. Vianello and V. Vitale and Waite, {A. P.} and P. Wang and Winer, {B. L.} and Wood, {K. S.} and Z. Yang and S. Zimmer",
    year = "2012",
    doi = "10.1088/0004-637X/756/1/4",
    language = "English",
    volume = "756",
    journal = "Astrophysical Journal",
    issn = "0004-637X",
    publisher = "IOP Publishing Ltd.",
    number = "1",

    }

    TY - JOUR

    T1 - Gamma-ray observations of the orion molecular clouds with the fermi large area telescope

    AU - Ackermann, M.

    AU - Ajello, M.

    AU - Allafort, A.

    AU - Antolini, E.

    AU - Baldini, L.

    AU - Ballet, J.

    AU - Barbiellini, G.

    AU - Bastieri, D.

    AU - Bechtol, K.

    AU - Bellazzini, R.

    AU - Berenji, B.

    AU - Blandford, R. D.

    AU - Bloom, E. D.

    AU - Bonamente, E.

    AU - Borgland, A. W.

    AU - Bottacini, E.

    AU - Brandt, T. J.

    AU - Bregeon, J.

    AU - Brigida, M.

    AU - Bruel, P.

    AU - Buehler, R.

    AU - Buson, S.

    AU - Caliandro, G. A.

    AU - Cameron, R. A.

    AU - Caraveo, P. A.

    AU - Cecchi, C.

    AU - Chekhtman, A.

    AU - Chiang, J.

    AU - Ciprini, S.

    AU - Claus, R.

    AU - Cohen-Tanugi, J.

    AU - Conrad, J.

    AU - D'Ammando, F.

    AU - De Angelis, A.

    AU - De Palma, F.

    AU - Dermer, C. D.

    AU - Do Couto E Silva, E.

    AU - Drell, P. S.

    AU - Drlica-Wagner, A.

    AU - Enoto, T.

    AU - Falletti, L.

    AU - Favuzzi, C.

    AU - Fegan, S. J.

    AU - Ferrara, E. C.

    AU - Focke, W. B.

    AU - Fukazawa, Y.

    AU - Fukui, Y.

    AU - Fusco, P.

    AU - Gargano, F.

    AU - Gasparrini, D.

    AU - Germani, S.

    AU - Giglietto, N.

    AU - Giordano, F.

    AU - Giroletti, M.

    AU - Glanzman, T.

    AU - Godfrey, G.

    AU - Guiriec, S.

    AU - Hadasch, D.

    AU - Hanabata, Y.

    AU - Harding, A. K.

    AU - Hayashida, M.

    AU - Hayashi, K.

    AU - Horan, D.

    AU - Hou, X.

    AU - Hughes, R. E.

    AU - Jackson, M. S.

    AU - Jóhannesson, G.

    AU - Johnson, A. S.

    AU - Kamae, T.

    AU - Katagiri, H.

    AU - Kataoka, Jun

    AU - Kerr, M.

    AU - Knödlseder, J.

    AU - Kuss, M.

    AU - Lande, J.

    AU - Larsson, S.

    AU - Lee, S. H.

    AU - Longo, F.

    AU - Loparco, F.

    AU - Lovellette, M. N.

    AU - Lubrano, P.

    AU - Makishima, K.

    AU - Mazziotta, M. N.

    AU - Mehault, J.

    AU - Mitthumsiri, W.

    AU - Moiseev, A. A.

    AU - Monte, C.

    AU - Monzani, M. E.

    AU - Morselli, A.

    AU - Moskalenko, I. V.

    AU - Murgia, S.

    AU - Nakamori, T.

    AU - Naumann-Godo, M.

    AU - Nishino, S.

    AU - Norris, J. P.

    AU - Nuss, E.

    AU - Ohno, M.

    AU - Ohsugi, T.

    AU - Okumura, A.

    AU - Orienti, M.

    AU - Orlando, E.

    AU - Ormes, J. F.

    AU - Ozaki, M.

    AU - Paneque, D.

    AU - Panetta, J. H.

    AU - Parent, D.

    AU - Pelassa, V.

    AU - Pesce-Rollins, M.

    AU - Pierbattista, M.

    AU - Piron, F.

    AU - Pivato, G.

    AU - Porter, T. A.

    AU - Rainò, S.

    AU - Razzano, M.

    AU - Reimer, A.

    AU - Reimer, O.

    AU - Roth, M.

    AU - Sadrozinski, H. F W

    AU - Sgrò, C.

    AU - Siskind, E. J.

    AU - Spandre, G.

    AU - Spinelli, P.

    AU - Strong, A. W.

    AU - Takahashi, H.

    AU - Takahashi, T.

    AU - Tanaka, T.

    AU - Thayer, J. G.

    AU - Thayer, J. B.

    AU - Tibolla, O.

    AU - Tinivella, M.

    AU - Torres, D. F.

    AU - Tramacere, A.

    AU - Troja, E.

    AU - Uchiyama, Y.

    AU - Usher, T. L.

    AU - Vandenbroucke, J.

    AU - Vasileiou, V.

    AU - Vianello, G.

    AU - Vitale, V.

    AU - Waite, A. P.

    AU - Wang, P.

    AU - Winer, B. L.

    AU - Wood, K. S.

    AU - Yang, Z.

    AU - Zimmer, S.

    PY - 2012

    Y1 - 2012

    N2 - 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 100MeV and 100GeV 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 (W CO) at a 1° × 1°pixel level. The correlation is found to be linear over a W CO range of ∼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 ∼2.3 × 1020 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > ∼212°), 1.7times higher than ∼1.3 × 10 20 found for the rest of Orion A and B. We interpret the apparent high X CO 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. W CO decreases faster than the H2 column density in the region making the gas "darker" to W CO.

    AB - 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 100MeV and 100GeV 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 (W CO) at a 1° × 1°pixel level. The correlation is found to be linear over a W CO range of ∼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 ∼2.3 × 1020 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > ∼212°), 1.7times higher than ∼1.3 × 10 20 found for the rest of Orion A and B. We interpret the apparent high X CO 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. W CO decreases faster than the H2 column density in the region making the gas "darker" to W CO.

    KW - gamma rays: ISM

    KW - ISM: clouds

    KW - ISM: general

    KW - ISM: individual objects (Orion A and Orion B)

    UR - http://www.scopus.com/inward/record.url?scp=84865184972&partnerID=8YFLogxK

    UR - http://www.scopus.com/inward/citedby.url?scp=84865184972&partnerID=8YFLogxK

    U2 - 10.1088/0004-637X/756/1/4

    DO - 10.1088/0004-637X/756/1/4

    M3 - Article

    VL - 756

    JO - Astrophysical Journal

    JF - Astrophysical Journal

    SN - 0004-637X

    IS - 1

    M1 - 4

    ER -