The Synthesis of Benzene in the Proto-planetary Nebula CRL 618

We show that the physical conditions in CRL 618 are such that efficient formation of benzene, C6H6, occurs. A combination of high temperatures, high densities, and high ionization rates drives an efficient ion-molecule chemistry involving condensation reactions of acetylene and its derivatives, rather than reactions involving atomic hydrogen, as was suggested for the interstellar synthesis of benzene. We find a column density of benzene within a factor of 2 of that observed providing that the material is trapped in a long-lived reservoir of gas in the disk around CRL 618. We note that the chemistry can give rise to other carbon chain molecules as well as a large abundance of benzonitrile, C6H5CN.

[1]  New H and H2 Reactions with Small Hydrocarbon Ions and Their Roles in Benzene Synthesis in Dense Interstellar Clouds , 1999 .

[2]  Gary J. Melnick,et al.  Discovery of water vapour around IRC+10216 as evidence for comets orbiting another star , 2001, Nature.

[3]  Ana Heras,et al.  Infrared Space Observatory's Discovery of C4H2, C6H2, and Benzene in CRL 618 , 2001 .

[4]  A. Cameron,et al.  Massive Supernovae, Orion Gamma Rays, and the Formation of the Solar System , 1995 .

[5]  M. McEwan,et al.  CmHn+ Reactions with H and H2: An Experimental Study , 1997 .

[6]  M. Livio,et al.  X-ray sources and planetary nebulae , 1975, Nature.

[7]  T. Millar,et al.  Large molecules in the envelope surrounding IRC+10216 , 2000 .

[8]  N. Kaifu,et al.  Molecular envelope of the planetary nebula NGC 7027 , 1990 .

[9]  H. Gail,et al.  Radial mixing in protoplanetary accretion disks II. Time dependent disk models with annealing and carbon combustion , 2002 .

[10]  L. Nittler,et al.  Presolar SiC Grains of Type A and B: Their Isotopic Compositions and Stellar Origins , 2001 .

[11]  Alexander G. G. M. Tielens,et al.  Methylpolyynes and Small Hydrocarbons in CRL 618 , 2001 .

[12]  A. Tielens,et al.  Polycyclic aromatic hydrocarbon formation in carbon-rich stellar envelopes , 1992 .

[13]  E. Herbst,et al.  Two-dimensional distributions and column densities of gaseous molecules , 1999, astro-ph/0202062.

[14]  E. Feigelson,et al.  Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes , 1989 .

[15]  A. Castro-Carrizo,et al.  Mass, linear momentum and kinetic energy of bipolar flows in protoplanetary nebulae , 2001 .

[16]  A. Tielens,et al.  Near-Infrared Spectroscopy of the Proto-Planetary Nebula CRL 618 and the Origin of the Hydrocarbon Dust Component in the Interstellar Medium , 1998, The Astrophysical journal.

[17]  K. Johnston,et al.  The Disruption of the Dense Disk in CRL 618 , 1995 .

[18]  M. Jura,et al.  A Long-lived Disk around the Red Rectangle? , 1995 .