What can ISO tell us about gas–grain chemistry?

Abstract

Recent results of searches for IR absorption lines of gas-phase molecules in the ISO spectra of embedded massive young stars are discussed. Abundant, highly excited gas-phase H₂O is detected in the ‘hot cores’ surrounding the young stellar objects, but not in the colder regions. The abundance of gas-phase CO₂ is low in all sources, in spite of the large observed abundance of solid CO₂. Gas-phase CH₄, C₂H₂ and HCN have been observed as well. The latter two species have high excitation temperatures, of the order of 1000 K, in some sources. The large abundance of CH₄ suggests that the molecule is formed mainly through grain-surface reactions. CO₂ may be produced by grain-surface chemistry as well, athough photochemical production in ices can also play a role. Comparison of gas-phase with solid-state column densities for the same lines of sight allows accurate determination of gas:solid state ratios, which can be analyzed as functions of the temperature of the region. The data indicate that significant evaporation of ices likely occurs in the hotter regions, but that part of the observed gas-phase H₂O may also be produced by high-temperature gas-phase reactions. The results are discussed in the context of the physical evolution of the regions.

References

1. D. R. Bates and L. Spitzer, Astrophys. J., 1951, 113, 441.
2. E. Herbst and W. Klemperer, Astrophys. J., 1973, 185, 505.
3. H. H. Lee, R. P. A. Bettens and E. Herbst, Astron. Astrophys. Suppl., 1996, 119, 111.
4. T. J. Millar, P. R. A. Farquhar and K. Willacy, Astron. Astrophys. Suppl., 1997, 121, 139.
5. W. D. Watson and E. E. Salpeter, Astrophys. J., 1972, 174, 321; 175, 659.
6. M. Allen and G. W. Robinson, Astrophys. J., 1977, 212, 396.
7. A. G. G. M. Tielens and W. Hagen, Astron. Astrophys., 1982, 114, 245.
8. L. B. d'Hendecourt, L. J. Allamandola and J. M. Greenberg, Astron. Astrophys., 1985, 152, 130.
9. D. Hollenbach and E. E. Salpeter, J. Chem. Phys., 1970, 53, 79.
10. Dust and Chemistry in Astronomy, ed. T. J. Millar and D. A. Williams, IOP, Bristol, 1993.
11. E. Herbst, in Dust and Chemistry in Astronomy, eds. T. J. Millar and D. A. Williams, IOP, Bristol, 1993, p. 183.
12. A. G. G. M. Tielens and S. B. Charnley, Origin Life, 1997, 27, 23.
13. D. C. B. Whittet, in Dust and Chemistry in Astronomy, eds. T. J. Millar and D. A. Williams, IOP, Bristol, 1993, p. 9.
14. A. G. G. M. Tielens and D. C. B. Whittet in Molecules in Astrophysics: Probes and Processes, IAU Symposium 178, ed. E. F. van Dishoeck, Kluwer, Dordrecht, 1997, p. 48.
15. C. M. Walmsley and P. Schilke in Dust and Chemistry in Astronomy, eds. T. J. Millar and D. A. Williams, IOP, Bristol, 1993, p. 37.
16. L. E. Snyder, Origin Life, 1997, 27, 115.
17. S. B. Charnley, A. G. G. M. Tielens and T. J. Millar, Astrophys. J., 1992, 399, L71.
18. P. Caselli, T. I. Hasegawa and E. Herbst, Astrophys. J., 1993, 408, 548.
19. Th. de Graauw, L. N. Haser and D. A. Beintema et al., Astron. Astrophys., 1996, 315, L49.
20. M. F. Kessler, J. A. Steinz and M. A. Anderegg et al., Astron. Astrophys., 1996, 315, L27.
21. D. C. B. Whittet, W. A. Schutte, A. G. G. M. Tielens, A. C. A. Boogert, Th. de Graauw, P. Ehrenfreund, P. A. Gerakines, F. P. Helmich, T. Prusti and E. F. van Dishoeck, Astron. Astrophys., 1996, 315, L357.
22. L. B. d'Hendecourt, M. Jourdain de Muizon and E. Dartois et al., Astron. Astrophys., 1996, 315, L365.
23. P. Ehrenfreund, A. C. A. Boogert, P. A. Gerakines and A. G. G. M. Tielens, Faraday Discuss., 1998, 109, 463.
24. F. P. Helmich, E. F. van Dishoeck, J. H. Black, Th. de Graauw, D. A. Beintema, A. M. Heras, F. Lahuis, P. W. Morris and E. A. Valentijn, Astron. Astrophys., 1996, 315, L173.
25. E. F. van Dishoeck, F. P. Helmich and Th. de Graauw et al., Astron. Astrophys., 1996, 315, L349.
26. A. C. A. Boogert, F. P. Helmich, E. F. van Dishoeck, W. A. Schutte, A. G. G. M. Tielens and D. C. B. Whittet, Astron. Astrophys., 1998, in press.
27. E. Dartois, L. d'Hendecourt, F. Boulanger, M. Jourdain de Muizon, M. Breitfellner, J.-L. Puget and H. J. Habing, Astron. Astrophys., 1998, 331, 651.
28. M. Ohishi in Molecules in Astrophysics: Probes and Processes, IAU Symposium 178, ed. E. F. van Dishoeck, Kluwer, Dordrecht, 1997, p. 48.
29. E. F. van Dishoeck in Molecules in Astrophysics: Probes and Processes, IAU Symposium 178, ed. E. F. van Dishoeck, Kluwer, Dordrecht, 1997, p. 537, 539; see http://www.strw.leidenuniv.nl/~iau34/ for links to databases.
30. G. F. Mitchell, J.-P. Maillard, M. Allen, R. Beer and K. Belcourt, Astrophys. J., 363, 554; G. F. Mitchell, J. P. Maillard and T. I. Hasegawa, Astrophys. J., 1991, 371, 342.
31. N. J. Evans, J. H. Lacy and J. S. Carr, Astrophys. J., 1991, 383, 674.
32. J. Carr, N. J. Evans, J. H. Lacy and S. Zhou, Astrophys. J., 1995, 450, 667.
33. A. G. G. M. Tielens and L. J. Allamandola, in Interstellar Processes, eds. D. J. Hollenbach and H. A. Thronson, Reidel, Dordrecht, 1987, p. 397.
34. T. I. Hasegawa and E. Herbst, MNRAS, 1993, 261, 83; 263, 589.
35. W. A. Schutte, in The Cosmic Dust Connection, ed. J. M. Greenberg, Kluwer, Dordrecht, 1996, p. 1.
36. D. A. Williams and S. D. Taylor, QJRAS, 1996, 37, 565.
37. D. A. Williams, in Dust and Chemistry in Astronomy, eds. T. J. Millar and D. A. Williams, IOP, Bristol, 1993, p. 143.
38. P. Caselli, T. I. Hasegawa and E. Herbst, Astrophys. J., 1998, in press.
39. W. A. Schutte and J. M. Greenberg, Astron. Astrophys., 1991, 244, 190.
40. B. Schmitt, in Molecules and Grains in Space, ed. I. Nenner, AIP, New York, 1994, p. 735.
41. S. A. Sandford and L. J. Allamandola, Icarus, 1988, 76, 201.
42. A. Léger, M. Jura and A. Omont, Astron. Astrophys., 1985, 144, 147.
43. A. P. Jones and D. A. Williams, MNRAS, 1994, 209, 955.
44. R. J. A. Grim and L. B. d'Hendecourt, Astron. Astrophys., 1986, 167, 161.
45. M. Spaans, M. R. Hogerheijde, L. G. Mundy and E. F. van Dishoeck, Astrophys. J., 1995, 455, L167.
46. R. Gredel, S. Lepp, A. Dalgarno and E. Herbst, Astrophys. J., 1989, 347, 289.
47. L. B. d'Hendecourt, L. J. Allamandola, R. J. A. Grim and J. M. Greenberg, Astron. Astrophys., 1986, 158, 119.
48. S. B. Charnley, Astrophys. J., 1997, 481, 396.
49. T. J. Millar, G. H. Macdonald and R. J. Habing, Astron. Astrophys., 1997, 325, 1163.
50. F. P. Helmich, T. J. Millar and E. F. van Dishoeck, Astron. Astrophys., 1998, to be submitted.
51. C. Ceccarelli, D. J. Hollenbach and A. G. G. M. Tielens, Astrophys. J., 1996, 471, 400.
52. E. F. van Dishoeck, G. A. Blake, B. T. Draine and J. I. Lunine, in Protostars and Planets III, eds. E. H. Levy and J. I. Lunine, University of Arizona, Tucson, 1993, p. 169.
53. E. F. van Dishoeck and G. A. Blake, Astrophys. Spa. Sci., 1995, 224, 237.
54. E. F. van Dishoeck and G. A. Blake, Annu. Rev. Astron. Astrophys., 1998, 36 in press.
55. T. W. Hartquist, P. Caselli, J. M. C. Rawlings, D. P. Ruffle and D. A. Williams, to appear in Molecular Astrophysics II, eds. T. W. Hartquist and D. A. Williams, Oxford University, Oxford, 1998.
56. E. F. van Dishoeck, F. P. Helmich, W. A. Schutte, P. Ehrenfreund, F. Lahuis, A. C. A. Boogert, A. G. G. M. Tielens, Th. de Graauw, P. A. Gerakines and D. C. B. Whittet, in Star Formation with the ISO Satellite, eds. J. Yun and R. Liseau, ASP, Berkeley, 1998, vol. 132, p. 54.
57. F. Lahuis and E. F. van Dishoeck, in First ISO workshop on Analytical Spectroscopy, ESA–SP, 1998, 419, Estec, Noordwijk, p. 275; and in preparation.
58. P. E. Clegg, P. A. R. Ade and C. Armand et al., Astron. Astrophys., 1996, 315, L38.
59. C. M. Wright, E. F. van Dishoeck, F. P. Helmich, F. Lahuis, A. C. A. Boogert and Th. de Graauw, in First ISO workshop on Analytical Spectroscopy, ESA–SP 1998, 419; Estec, Noordwijk, p. 37.
60. E. F. van Dishoeck and F. P. Helmich, Astron. Astrophys., 1996, 315, L177.
61. F. P. Helmich, 1996, PhD thesis, University of Leiden; http://www.strw.leidenunv.nl/~fph/papers.
62. F. van der Tak, E. F. van Dishoeck, N. J. Evans and G. A. Blake, Astrophys. J., 1998, to be submitted.
63. J. H. Lacy, R. Knacke, T. R. Geballe and A. T. Tokunaga, Astrophys. J., 1994, 428, L43.
64. A. M. S. Boonman, F. van der Tak, F. P. Helmich and E. F. van Dishoeck, 1998, in preparation; A. M. S. Boonman, C. M. Wright, E. F. van Dishoeck et al., 1998, in preparation.
65. Y. C. Minh, M. K. Brewer, W. M. Irvine, P. Friberg and L. E. B. Johansson, Astron. Astrophys., 1991, 244, 470.
66. J. H. Lacy, J. S. Carr, N. J. Evans, F. Baas and J. Achtermann, Astrophys. J., 1991, 376, 556.
67. J. H. Lacy, N. J. Evans and J. M. Achtermann et al., Astrophys. J., 1989, 342, L43.
68. P. Ehrenfreund, L. d'Hendecourt, E. Dartois, M. Jourdain de Muizon, M. Breitfellner, J. L. Puget and H. J. Habing, Icarus, 1997, 130, 1.
69. L. J. Allamandola, M. P. Bernstein and S. A. Sandford, in Astronomical and biochemical origins and the search for life in the universe, eds. C. B. Cosmovici, S. Bowyer and D. Werthimer, Ed. Comp., Bologna, 1997, p. 23.
70. Th. de Graauw, D. C. B. Whittet and P. A. Gerakines et al., Astron. Astrophys., 1996, 315, L345.
71. J. Gürtler, Th. Henning, C. Kömpe, W. Pfau, W. Krätschmer and D. Lemke, Astron. Astrophys., 1996, 315, L189.
72. P. A. Gerakines, A. C. A. Boogert, P. Ehrenfreund et al., 1998, in preparation.
73. S. P. Willner, F. C. Gillett and T. L. Herter et al., Astrophys. J., 1982, 253, 174.
74. W. A. Schutte, A. G. G. M. Tielens, D. C. B. Whittet, A. C. A. Boogert, P. Ehrenfreund, Th. de Graauw, T. Prusti, E. F. van Dishoeck and P. R. Wesselius, Astron. Astrophys., 1996, 315, L333.
75. A. G. G. M. Tielens, A. T. Tokunaga, T. R. Geballe and F. Baas, Astrophys. J., 1991, 381, 181.
76. S. A. Sandford, L. J. Allamandola, A. G. G. M. Tielens and G. J. Valero, Astrophys. J., 1988, 329, 498.
77. D. C. B. Whittet, P. A. Gerakines, A. G. G. M. Tielens, A. J. Adamson, A. C. A. Boogert, J. E. Chiar, Th. de Graauw, P. Ehrenfreund, T. Prusti, W. A. Schutte, B. Vandenbussche and E. F. van Dishoeck, Astrophys. J., 1998, 489, L159.
78. A. C. A. Boogert, W. A. Schutte, A. G. G. M. Tielens, D. C. B. Whittet, F. P. Helmich, P. Ehrenfreund, P. R. Wesselius, Th. de Graauw and T. Prusti, Astron. Astrophys., 1996, 315, L377.
79. N. Boudin, W. A. Schutte and J. M. Greenberg, Astron. Astrophys., 1998, in press.
80. M. P. Bernstein, S. A. Sandford and L. J. Allamandola, Astrophys. J., 1997, 476, 932.
81. W. A. Schutte, 1997, personal communication.
82. R. G. Smith, K. Sellgren and A. T. Tokunaga, Astrophys. J., 1989, 344, 413.
83. F. Wyrowski and C. M. Walmsley, Astron. Astrophys., 1996, 314, 265.
84. P. D. Gensheimer, R. Mauersberger and T. L. Wilson, Astron. Astrophys., 1996, 314, 281.
85. J. Harju, C. M. Walmsley and J. G. A. Wouterloot, Astron. Astrophys. Suppl., 1988, 98, 51.
86. I. A. Crawford and D. A. Williams, MNRAS, 1997, 291, L53.
87. L. J. Allamandola, S. A. Sandford, A. G. G. M. Tielens and T. Herbst, Astrophys. J., 1992, 399, 134.
88. W. A. Schutte, P. A. Gerakines, T. R. Geballe, E. F. van Dishoeck and J. M. Greenberg, Astron. Astrophys., 1996, 309, 633.
89. M. E. Palumbo, G. A. Baratta, J. R. Brucato, A. C. Castorina, M. A. Satorre and G. Strazulla, Astron. Astrophys., 1998, in press.
90. S. C. Tegler, D. A. Weintraub and T. W. Rettig et al., Astrophys. J., 1995, 439, 279.
91. S. D. Doty and D. A. Neufeld, Astrophys. J., 1997, 489, 122.