UV-photoprocessing of interstellar ice analogs: Detection of hexamethylenetetramine-based species

The physical conditions governing the dense cloud environment are reproduced in a high vacuum experimental setup at low temperature T 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated by depositing an ice layer on a cold finger, while it is irradiated by ultraviolet (UV) photons. After irradiation the sample is slowly warmed to room temperature; a residue remains, containing the most refractory products of photo- and thermal processing. In this paper we report on the analysis of the residues performed by means of gas chromatography-mass spectrometry (GC-MS). A number of new molecules based on hexamethylenetetramine (HMT, C 6 H 1 2 N 4 ), the most abundant component of the residues reported here, were detected: methyl-HMT (C 6 H 1 1 N 4 -CH 3 ), hydroxy-HMT (C 6 H 1 1 N 4 -OH), methanyl-HMT (C 6 H 1 1 N 4 -CH 2 OH), amin-aldehyd-HMT (C 6 H 1 1 N 4 -NH-CHO) and methanyl-aldehyd-HMT (C 6 H 1 1 N 4 -CHOH-CHO). To the best of our knowledge, this is the first reported synthesis of these molecules. Currently, these are the heaviest identified components of the residue. These species might also be present in the interstellar medium, given that the ice was submitted to high temperatures, of the order of 300 K, and form part of comets. Our work serves as preparation for the ESA-Rosetta mission, which plans to do in situ analysis of the composition of a comet nucleus with the COSAC instrumentation.

[1]  F. R. Krueger,et al.  The organic component in dust from comet Halley as measured by the PUMA mass spectrometer on board Vega 1 , 1987, Nature.

[2]  A. Yencha,et al.  Exploding Interstellar Grains and Complex Molecules , 1973 .

[3]  J. Greenberg,et al.  Interstellar molecule formation in grain mantles: The laboratory analog experiments, results and implications , 1979 .

[4]  E. Dishoeck,et al.  Stringent upper limits to the solid NH3 abundance towards W 33A from near-IR spectroscopy with the Very Large Telescope , 2003 .

[5]  J. Greenberg,et al.  Ultraviolet Photoprocessing of Interstellar Dust Mantles as a Source of Polycyclic Aromatic Hydrocarbons and Other Conjugated Molecules , 2000, The Astrophysical journal.

[6]  Willem A. Schutte,et al.  UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results , 2003 .

[7]  Scott A. Sandford,et al.  Organic Compounds Produced by Photolysis of Realistic Interstellar and Cometary Ice Analogs Containing Methanol , 1995 .

[8]  A. Brack,et al.  Amino acids from ultraviolet irradiation of interstellar ice analogues , 2002, Nature.

[9]  Alexander G. G. M. Tielens,et al.  An Inventory of Interstellar Ices toward the Embedded Protostar W33A , 2000 .

[10]  T. Prusti,et al.  Observations of Solid Carbon Dioxide in Molecular Clouds with the Infrared Space Observatory , 1999 .

[11]  W. Irvine,et al.  Hydrogenation of interstellar molecules: a survey for methylenimine (CH2NH). , 1997 .

[12]  G. M. Badger,et al.  The chemistry of heterocyclic compounds , 1961 .

[13]  Cyril Szopa,et al.  Production of Hexamethylenetetramine in Photolyzed and Irradiated Interstellar Cometary Ice Analogs , 2001 .

[14]  J. Greenberg,et al.  Can spores survive in interstellar space? , 1985, Nature.

[15]  M. Fomenkova.  On the Organic Refractory Component of Cometary Dust , 1999 .

[16]  J. M. Greenberg,et al.  Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium , 2005, Origins of life and evolution of the biosphere.

[17]  M. Fomenkova,et al.  Carbonaceous components in the comet Halley dust. , 1994, Geochimica et cosmochimica acta.

[18]  U. Meierhenrich,et al.  Pyrolytic methylation assisted enantioseparation of chiral hydroxycarboxylic acids , 2001 .

[19]  Scott A. Sandford,et al.  Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues , 2002, Nature.

[20]  J. Oró,et al.  Comets and the Formation of Biochemical Compounds on the Primitive Earth , 1961, Nature.

[21]  J. M. Greenberg,et al.  Photochemical reactions in interstellar grains photolysis of co, NH3, and H2O , 2005, Origins of life and evolution of the biosphere.

[22]  J. Greenberg,et al.  UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier. II. The case of hydrogenated carbon grains , 2001 .

[23]  M. S. Matthews,et al.  The Galaxy and the solar system , 1986 .

[24]  Scott A. Sandford,et al.  Photochemical and thermal evolution of interstellar/precometary ice analogs , 1988 .

[25]  J. Greenberg,et al.  UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier. I. The case of aliphatic and aromatic molecular species , 2001 .

[26]  J. Chiar,et al.  Searching for Ammonia in Grain Mantles toward Massive Young Stellar Objects , 2001 .