Spitzer Observations of CO2 Ice Towards Field Stars in the Taurus Molecular Cloud

We present the first Spitzer Infrared Spectrograph observations of the 15.2 micron bending mode of CO2 ice towards field stars behind a quiescent dark cloud. CO2 ice is detected towards 2 field stars (Elias 16, Elias 3) and a single protostar (HL Tau) with anabundance of ~15-20% relative to water ice. CO2 ice is not detected towards the source with lowest extinction in our sample, Tamura 17 (A_V = 3.9m). A comparison of the Elias 16 spectrum with laboratory data demonstrates that the majority of CO2 ice is embedded in a polar H2O-rich ice component, with ~15% of CO2 residing in an apolar H2O-poor mantle. This is the first detection of apolar CO2 towards a field star. We find that the CO2 extinction threshold is A_V = 4m +/- 1m, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO2. Our results confirm CO2 ice forms in tandem with H2O ice along quiescent lines of sight. This argues for CO2 ice formation via a mechanism similar to that responsible for H2O ice formation, viz. simple catalytic reactions on grain surfaces.Comment: Accepted by Astrophysical Journal Letter

A New 3.25 Micron Absorption Feature toward Mon R2/IRS-3

A new 3.2--3.5~$\mu$m spectrum of the protostar Mon~R2/IRS-3 confirms our previous tentative detection of a new absorption feature near 3.25 $\mu$m. The feature in our new spectrum has a central wavelength of 3.256 $\mu$m (3071 cm$^{-1}$) and has a full-width at half maximum of 0.079 $\mu$m (75 cm$^{-1}$). We explore a possible identification with aromatic hydrocarbons at low temperatures, which absorb at a similar wavelength. If the feature is due to aromatics, the derived column density of C--H bonds is $\sim$1.8 $\times$ $10^{18}$ cm$^{-2}$. If the absorbing aromatic molecules are of roughly the same size as those responsible for aromatic emission features in the interstellar medium, then we estimate that $\sim$9\% of the cosmic abundance of carbon along this line of sight would be in aromatic hydrocarbons, in agreement with abundance estimates from emission features.Comment: 12 pages (26 kB), AASTex format. Also 1 Postscript figure (39 kB), tarred, compressed and uuencoded, added with 'figure' command. Postscript file or hardcopy available upon request to [email protected]. ApJL, in pres

Silicon Nanoparticles: Source of Extended Red Emission?

We have reviewed the characteristics of the extended red emission (ERE) as observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. The spectral nature and the photon conversion efficiency of the ERE identify the underlying process as highly efficient photoluminescence by an abundant component of interstellar dust. We have compared the photoluminescence properties of a variety of carbon- and silicon-based materials proposed as sources for the ERE with the observationally established constraints. We found that silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE. If present in interstellar space with an abundance sufficient to explain the intensity of the ERE, silicon nanoparticles will also contribute to the interstellar 9.7 micron Si-O stretch feature in absorption, to the near- and mid-IR nonequilibrium thermal background radiation, and to the continuum extinction in the near- and far-UV. About 36% of the interstellar silicon depleted into the dust phase would be needed in the form of silicon nanoparticles, amounting to less than 5% of the interstellar dust mass. We propose that silicon nanoparticles form through the nucleation of SiO in oxygen-rich stellar mass outflows and that they represent an important small-grain component of the interstellar dust spectrum.Comment: 5 pages; 1 included figure; accepted 1998 May 1, ApJ

Experimental investigation of nitrile formation from VUV photochemistry of interstellar ices analogs: acetonitrile and amino acetonitrile

International audienceContext. The study of the chemical reactivity in interstellar ices in astrophysical environments is an important tool for understanding the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. The laboratory simulations of the reactivity in ice analogs provide important information for understanding the reactivity in these environments. Here, we used these experimental simulations to trace some formation pathways of two nitriles, acetonitrile and amino acetonitrile, which are two potential precursors of amino acids in astrophysical environments. Aims. The purpose of this work is to present the first experimental approach for the formation of acetonitrile and amino acetonitrile in interstellar-like conditions. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy and mass spectrometry to study the formation at 20 K of ace-tonitrile CH 3 CN from VUV irradiation of ethylamine and of amino acetonitrile NH 2 CH 2 CN from VUV irradiation of ammonia: acetonitrile mixture. Isotopic substitutions are used to confirm identifications. Results. We demonstrate that acetonitrile can be formed at 20 K from the VUV irradiation of ethylamine with a yield of 4%. Furthermore, in presence of ammonia, at 20 K and under VUV irradiation, the acetonitrile can lead to the amino acetonitrile for-mation. These results suggest that acetonitrile and amino acetonitrile can be formed in astrophysical environments that are submitted to VUV irradiations