Ice absorption features in the 5-8 μm region toward embedded protostars

We have obtained 5{8 m spectra towards 10 embedded protostars using the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO-SWS) with the aim of studying the composition of interstellar ices. The spectra are dominated by absorption bands at 6.0 m and 6.85 m. The observed peak positions, widths and relative intensities of these bands vary dramatically along the dierent lines of sight. On the basis of comparison with laboratory spectra, the bulk of the 6.0 m absorption band is assigned to amorphous H2O ice. Additional absorption, in this band, is seen toward 5 sources on the short wavelength wing, near 5.8 m, and the long wavelength side near 6.2 m. We attribute the short wavelength absorption to a combination of formic acid (HCOOH) and formaldehyde (H2CO), while the long wavelength absorption has been assigned to the C{C stretching mode of aromatic structures. From an analysis of the 6.85 m band, we conclude that this band is composed of two components: a volatile component centered near 6.75 m and a more refractory component at 6.95 m. From a comparison with various temperature tracers of the thermal history of interstellar ices, we conclude that the two 6.85 m components are related through thermal processing. We explore several possible carriers of the 6.85 m absorption band, but no satisfactory identication can be made at present. Finally, we discuss the possible implications for the origin and evolution of interstellar ices that arise from these new results.

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

[2]  D. Lutz,et al.  The Composition and Distribution of Dust along the Line of Sight toward the Galactic Center , 2000, astro-ph/0002421.

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

[4]  K. Sellgren,et al.  New 3 Micron Spectra of Young Stellar Objects with H2O Ice Bands , 1999, astro-ph/9903487.

[5]  S. Sandford,et al.  Unraveling the 10 Micron “Silicate” Feature of Protostars: The Detection of Frozen Interstellar Ammonia , 1998, The Astrophysical journal.

[6]  M. Maldoni,et al.  A study of the 2.5-25μm spectrum of H2O ice , 1998 .

[7]  K. Hiraoka,et al.  Gas-Grain Processes for the Formation of CH4 and H2O: Reactions of H Atoms with C, O, and CO in the Solid Phase at 12 K , 1998 .

[8]  T. Prusti,et al.  Detection of Abundant CO2 Ice in the Quiescent Dark Cloud Medium toward Elias 16 , 1998 .

[9]  J. Maillard,et al.  The Star-forming Core of Monoceros R2 , 1997 .

[10]  A. Tielens,et al.  Deuterated Methanol in the Orion Compact Ridge , 1997 .

[11]  A. Tielens,et al.  Solid Carbonyl Sulfide (OCS) in Dense Molecular Clouds , 1997 .

[12]  J. Brucato,et al.  Carbonic Acid by Ion Implantation in Water/Carbon Dioxide Ice Mixtures☆ , 1997 .

[13]  M. Wright,et al.  A Multiline Aperture Synthesis Study of Orion-KL , 1996 .

[14]  S. Langhoff.  Theoretical Infrared Spectra for Polycyclic Aromatic Hydrocarbon Neutrals, Cations and Anions , 1996 .

[15]  K. Sellgren,et al.  A Study of Absorption Features in the 3 Micron Spectra of Molecular Cloud Sources with H 2O Ice Bands , 1995, astro-ph/9509067.

[16]  K. Sellgren,et al.  A New 3.25 Micron Absorption Feature toward Monoceros R2/IRS 3 , 1995, astro-ph/9508056.

[17]  D. Hudgins,et al.  Infrared spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. 2. The members of the thermodynamically most favorable series through coronene. , 1995, The Journal of physical chemistry.

[18]  K. Hiraoka,et al.  Formation of formaldehyde and methanol from the reactions of H atoms with solid CO at 10–20 K , 1994 .

[19]  K. Sellgren,et al.  The 3.2-3.6 micron spectra of monoceros R2/IRS-3 and Elias 16 , 1994 .

[20]  S. Sandford,et al.  An experimental study of the organic molecules produced in cometary and interstellar ice analogs by thermal formaldehyde reactions. , 1993, Icarus.

[21]  S. Sandford,et al.  Mid- and far-infrared spectroscopy of ices: optical constants and integrated absorbances. , 1993, The Astrophysical journal. Supplement series.

[22]  E. Herbst,et al.  New gas–grain chemical models of quiescent dense interstellar clouds: the effects of H2 tunnelling reactions and cosmic ray induced desorption , 1993 .

[23]  S. Sandford,et al.  Infrared spectroscopy of dense clouds in the C-H stretch region: methanol and "diamonds." , 1992, The Astrophysical journal.

[24]  S. Sandford,et al.  10 micron spectra of protostars and the solid methanol abundance. , 1991, The Astrophysical journal.

[25]  M. Moore,et al.  Infrared and mass spectral studies of proton irradiated H2O + CO2 ice: Evidence for carbonic acid , 1991 .

[26]  A. Tielens,et al.  Studies of dust grain properties in infrared reflection nebulae. , 1990, The Astrophysical journal.

[27]  E. Herbst,et al.  Cosmic-ray-induced photodissociation and photoionization rates of interstellar molecules , 1989 .

[28]  A. Tielens.  Dust in Dense Clouds , 1989 .

[29]  T. Millar,et al.  A model of the chemistry in hot molecular cores , 1988 .

[30]  A. Dalgarno,et al.  Polycyclic aromatic hydrocarbons in interstellar chemistry , 1988 .

[31]  Y. Maréchal.  IR spectra of carboxylic acids in the gas phase: A quantitative reinvestigation , 1987 .

[32]  A. Tielens,et al.  Infrared Emission from Interstellar PAHs , 1987 .

[33]  S. Sandford.  Laboratory infrared transmission spectra of individual interplanetary dust particles from 2 , 1985 .

[34]  J. Goebel,et al.  Absorption features in the 5-8 micron spectra of protostars , 1984 .

[35]  J. Hecht,et al.  Simulation of Cosmic Dust Spectra , 1984 .

[36]  J. Greenberg,et al.  Visible spectroscopy of matrix isolated HCO: The 2A″(Π)←X 2A′ transition , 1983 .

[37]  S. Prasad,et al.  UV radiation field inside dense clouds: its possible existence and chemical implications , 1983 .

[38]  K. M. Merrill,et al.  Infrared spectra of protostars : composition of the dust shells. , 1982 .

[39]  David A. Williams,et al.  The infrared spectrum of interstellar dust: Surface functional groups on carbon , 1981 .

[40]  D. Huffman.  Interstellar grains The interaction of light with a small-particle system , 1977 .

[41]  K. M. Merrill,et al.  Infrared observations of ices and silicates in molecular clouds , 1976 .

[42]  A. S. Wexler.  Integrated Intensities of Absorption Bands in Infrared Spectroscopy , 1967 .