L. d'Hendecourt

A 3-5 mu m VLT spectroscopic survey of embedded young low mass stars I. Structure of the CO ice

Medium resolution (λ/Δλ = 5000-10000) VLT-ISAAC M-band spectra are presented of 39 young stellar objects in nearby low-mass star forming clouds showing the 4.67 μm stretching vibration mode of solid CO. By taking advantage of the unprecedentedly large sample, high S/N ratio and high spectral resolution, similarities in the ice profiles from source to source are identified. It is found that excellent fits to all the spectra can be obtained using a phenomenological decomposition of the CO stretching vibration profile at 4.67 μm into 3 components, centered on 2143.7 cm^(-1),2139.9 cm^(-1), and 2136.5 cm^(-1) with fixed widths of 3.0, 3.5 and 10.6 cm ^(-1), respectively. All observed interstellar CO profiles can thus be uniquely described by a model depending on only 3 linear fit parameters, indicating that a maximum of 3 specific molecular environments of solid CO exist under astrophysical conditions. A simple physical model of the CO ice is presented, which shows that the 2139.9 cm^(-1) component is indistinguishable from pure CO ice. It is concluded, that in the majority of the observed lines of sight, 60-90% of the CO is in a nearly pure form. In the same model the 2143.7 cm^(-1) component can possibly be explained by the longitudinal optical (LO) component of the vibrational transition in pure crystalline CO ice which appears when the background source is linearly polarised. The model therefore predicts the polarisation fraction at 4.67 μm, which can be confirmed by imaging polarimetry. The 2152 cm^(-1) feature characteristic of CO on or in an unprocessed water matrix is not detected toward any source and stringent upper limits are given. When this is taken into account, the 2136.5 cm ^(-1) component is not consistent with the available water-rich laboratory mixtures and we suggest that the carrier is not yet fully understood. A shallow absorption band centered between 2165 cm^(-1) and 2180 cn^(1) is detected towards 30 sources. For low-mass stars, this band is correlated with the CO component at 2136.5 cm^(-1), suggesting the presence of a carrier different from XCN at 2175 cm^(-1). Furthermore the absorption band from solid ^(13)CO at 2092 cm^(-1) is detected towards IRS 51 in the ρ Ophiuchi cloud complex and an isotopic ratio of ^(12)CO/^(13)CO = 68 ± 10 is derived. It is shown that all the observed solid ^(12)CO profiles, along with the solid ^(13)CO profile, are consistent with grains with an irregularly shaped CO ice mantle simulated by a Continuous Distribution of Ellipsoids (CDE), but inconsistent with the commonly used models of spherical grains in the Rayleigh limit.

New emission features in the infrared spectra of two IRAS sources

Observations of IRAS 21282 + 5050 and IRAS 03055 + 5819 (AFGL 437) obtained at 7.7-22.5 microns using the IRAS LRS during 1983 and at 3.1-3.8 microns using the 3.75-m UKIRT in September 1985, are reported and analyzed. The data are presented graphically, and the significance of three new nonatomic emission features at 3.46, 3.51, and 3.56 microns; the known emission features at 3.3, 3.40, 7.7, 8.6, and 11.3 microns; and the plateaus at 3.4-3.6 and above 11.5 microns is discussed in detail. It is suggested that the plateau and narrow features between 3.4 and 3.6 microns may be due to polycyclic aromatic hydrocarbons with attached molecular subgroups such as -CH/sub 3/ or -C/sub 2/H/sub 5/, a hypothesis developed by analogy to that proposed by Cohen et al. (1985), and Puget et al. (1985) for the 12-micron plateaus seen in many IRAS spectra. 26 references.

Energetic and thermal processing of interstellar ices

Our current knowledge on the chemical composition of interstellar ices is summarized with respect to the possible contribution of energetic photons and particles to their observed state. We describe the inventory of astronomical ices as determined by infrared observations from the Infrared Space Observatory (ISO) and laboratory spectroscopy. Sources of radiolysis, UV photolysis, and ice heating are then discussed in the context of the chemical state of material in dense molecular clouds and in protostellar cores. Through specific examples we show how energetic processing can explain the observed solid-state characteristics of several key molecules: CH 3 OH, CO 2 , and OCN-. We also discuss the gaseous and solid-state photochemistry of the first organic acid detected in interstellar ices, HCOOH.

Absorption features in the 5-8 micron spectra of protostars

High signal-to-noise ratio 5-8 ..mu..m spectra of four sources embedded in molecular clouds are presented. All four sources show evidence for the presence of absorption features. The shape of these features changes, however, dramatically from source to source. They range from two relatively narrow bands at 6.0 and 6.8 ..mu..m in W33A to a broad, shallow feature, which extends from about 5.2 to 7.8 ..mu..m and shows some structure, in Mon R2-IRS2, BN, and NGC 2264.

IR spectroscopic study of olivine, enstatite and diopside irradiated with low energy H + and He + ions

In this article we investigate the interaction of silicate grains with light atoms ionized and accelerated during the propagation of shock waves in the diffuse interstellar medium (ISM). Such an interaction which is equivalent to the irradiation of the grains with accelerated ions, is a potentially important process for silicate grain evolution in the ISM. We present the results of irradiation experiments aimed at simulating this process. The same crystalline silicates as those identified around evolved stars before their injection in the ISM, forsterite, enstatite and diopside, were irradiated with light ions (H + ,H e + )a t low energies (10-50 keV). The IR spectroscopic characteristics of the samples were analyzed before and after the irradiation in the 2-35 µm range allowing us to study the structural modifications in the irradiated minerals. The experiments show that low energy H + (10 keV) and He + (≤50 keV) ions efficiently amorphize crystalline silicates with fluence ≤10 18 ions/cm 2 .S ince these experimental conditions are compatible with interstellar environments, the interaction of grains with high velocity shock waves may be responsible for the absence of crystalline silicates in the ISM. The comparison of the IR spectra of the irradiated silicates with observations of the Galactic Center is presented. This comparison calls into question the classical assignment of the interstellar amorphous silicate bands.

A 'dry' condensation origin for circumstellar carbonates.

The signature of carbonate minerals has long been suspected in the mid-infrared spectra of various astrophysical environments such as protostars. Abiogenic carbonates are considered as indicators of aqueous mineral alteration in the presence of CO2-rich liquid water. The recent claimed detection of calcite associated with amorphous silicates in two planetary nebulae and protostars devoid of planetary bodies questions the relevance of this indicator; but in the absence of an alternative mode of formation under circumstellar conditions, this detection remains controversial. The main dust component observed in circumstellar envelopes is amorphous silicates, which are thought to have formed by non-equilibrium condensation. Here we report experiments demonstrating that carbonates can be formed with amorphous silicates during the non-equilibrium condensation of a silicate gas in a H2O-CO2-rich vapour. We propose that the observed astrophysical carbonates have condensed in H2O(g)-CO2(g)-rich, high-temperature and high-density regions such as evolved stellar winds, or those induced by grain sputtering upon shocks in protostellar outflows.

Detection of the overtone of the 3.3 micron emission feature in IRAS 21282+5050

The 1.6-1.8 micron spectrum of the planetary nebula, IRAS 21282+5050, a strong emitter of the unidentified interstellar bands, contains a 0.02 micron wide eimission feature centered at 1.680 micron, which is well matched by laboratory spectra of the 0-2 CH stretching mode in polycyclic aromatic hydrocarbons (PAHs). We identify the new feature as the overtone of the well-known 3.3 micron band. In view of the high excitation required for emission in this band, the identification indicates that the emission is by free molecules rather than molecular moieties in solid dust grains. Modeling of the intensity ratio of the 2-0 to 1-0 band implied that the PAHs emitting in these bands contain about 60 carbon atoms. It is inferred that the nu = 2-1 hot band of the CH stretching mode occurs at about 3.43 micron and contributes to the long-wavelength shoulder of the 3.40 micron feature. The main 3.40 micron feature probably is due to aliphatic sidegroups on PAH molecules.

Similarity of the infrared spectrum of an Orgueil organic polymer with interstellar organic compounds in the line of sight towards IRS 7.

We present a comparison between the IR spectrum of the galactic center source IRS 7 and the spectrum of a carbonaceous polymer from the Orgueil meteorite. We have obtained an almost perfect match between the two spectra in the region between 3020-2790 cm-1, which suggests that the chemical composition of the interstellar organic matter and that of the meteorite polymer are similar or that the meteoritic polymer could be a well preserved interstellar organic molecule. Assuming that the meteoritic polymer has the same C/H ratio as these interstellar molecules, we find that 45 % of the total abundance of carbon in the line of sight toward IRS 7 is trapped in such an interstellar organic grain material.

ISO observations of interstellar ices

The first spectroscopic results from ISO (Infrared Space Observatory, Kessler et al. 1996) have revealed a wealth of interesting features and in particular absorption signatures of a wide variety of solid state molecular species. We present here some new ISO data obtained with SWS (Short Wavelength Spectrometer) toward the young deeply embedded object associated with RAFGL 7009S. Signatures of H2O, CO, CO2 ices can be readily identified but also some much less abundant species such as 13CO2, H2CO and CH4, all occurring in the near and mid-infrared region between 3 and 16 μm. The detection of CO2 at 4.27 and 15.2 μm confirms its presence in the IRAS-LRS spectra of several heavily absorbed sources. The very high extinction toward RAFGL 7009S makes it an excellent case to study other weak solid state absorption features, commonly measured in laboratory experiments.

Present situation in the identification of polycyclic aromatic hydrocarbons

Abstract Recently, the interpretation given to the so-called “unidentified” infrared emission bands has drastically improved. These bands are attributed to the fundamental vibrations of large molecules known in organic chemistry as Polycyclic Aromatic Hydrocarbons (PAHs). In this paper, we review the main arguments which led to this conclusion and focus our attention to some recent IR and UV spectroscopic studies which allow us to interpret further the main characteristics of astronomical spectra.