The interstellar dust cycle
Interstellar dust is not static in the ISM, but rather in continuous evolution through what is known as the interstellar dust cycle (Greenberg 1982) schematically depicted in Fig. 1.4. Dust particles are sequentially and repeatedly being cycled into and out of diffuse and molecular clouds, with each single cycle lasting about 108 years. During this cycling, dust particles experience various physical and chemical processes that modify their structure and/or composition.
Dust clumps in some portions of molecular clouds collapse under gravity, fragment, and form dense cores. These latter continue to collapse and start to turn into rotating disks. Most of the material in the core forms a star, and a much smaller part forms a planetary system (with comets, asteroids and IDPs as leftovers from planetary formation). At the end of its lifetime, the star replenishes the interstellar medium with dust ejected (e.g. in the AGB phase of red giants and supergiants and in supernova explosions). Dust in the ISM aggregates in diffuse and then in dense clouds under the effect of supernova shocks. The cycle of stellar birth, death and rebirth is replayed and in the meantime the dust grains are reprocessed, consumed by star and planet formation or become part of comets, asteroids and other minor bodies. According to Greenberg’s model the grain may pass through a series of diffuse cloud/molecular cloud cycles. A typical grain anywhere in space undergoes at least 20 cycles but only one time out of 20 it is incorporated into a star and destroyed. The other times, the grain gets ejected, and only the mantle is destroyed (Greenberg 1989).
Interstellar ice mantles
Dust grains are, by far, the primary sites of molecules formation acting as “catalyzers” and favoring chemical reactions at their surfaces. In dense molecular clouds, H atoms from the gas phase collide with the cold grain surfaces (~10 K) and are physically adsorbed; then these atoms diffuse on the surface and form H2 molecules when they encounter each other (Hollenbach & Salpeter 1970, 1971). The formation of H2 is indeed impossible in the gas phase as the molecule does not possess dipolar transitions to get rid of its huge formation energy (energy bond = 4.5 eV) and thus needs a third body to stabilize the newly formed molecule. Other gaseous species, in atomic, radical and molecular form (C, N, O, CO), accrete onto the surface of the refractory dust grains mostly via collisions (Sandford & Allamandola 1993) and weak van der Waals forces to form volatile van der Waals solids (d’Hendecourt & Dartois 2001) better known as ice mantles (Tielens & Hagen 1982; d’Hendecourt et al. 1985; Greenberg 1986; Whittet 1993; Tielens & Whittet 1997; Tielens & Charnley 1997). This is not the case in diffuse clouds where the higher temperatures and radiation field prevent mantle formation or favor constant destruction of growing mantles, as evidenced by the lack of observation of what is called the ice band at 3.1 micron in locally low extinction regions (Williams et al. 1992). In the interior of dense clouds, the accretion rate of the mantles is typically of one species per day, lasting for ~105 years before all the gas condense (Tielens & Allamandola 1987). Species with relatively low volatility accrete more readily than those with high volatility. Once on the grain, reactive species migrate until they encounter species with which they can react, typically by sticking together with the grain absorbing the excess energy (Greenberg 1986; Whittet 1993; Schmitt 1994). At the low temperatures of dense clouds, the chemistry occurs via reactions that are mostly exothermic and that have no potential barriers between reactants and products. Under normal terrestrial conditions the reactive species would be very short-lived, but the low temperatures allow some of them to persist, trapped in a cage of ices. The composition of the first ices is expected to be dominated by hydrogenated atoms (H2O, NH4, CH4…) since hydrogen is orders of magnitude more abundant than any of the heavier atoms. CO forms efficiently in the gas phase via ion-molecule reactions, and thus CO ice is explained by direct freeze-out of the molecular gas. Oxygenation of CO results in CO2, while successive hydrogenations of CO through H2CO produce formaldehyde and methanol (Watanabe et al. 2003). These first ice mantles, resulting of simple atom addition reactions, are thus expected to be dominated by water (H2O), with significant amounts of H2CO, N2, CH4, CO2, H2O2, NH3, and CO (Allen & Robinson 1977; Tielens & Hagen 1982; d’Hendecourt et al. 1985; Brown & Charnley 1990; Hasegawa et al. 1992). A possible path for ice chemistry is illustrated in Fig. 1.5. After their formation, ice mantles can continually be ejected into the ISM after several mechanisms such grain-grain collisions, desorptions induced by cosmic ion bombardment (Baratta et al. 1991) and UV photons (Westley et al. 1995; Leto & Baratta 2003; Öberg et al. 2009), and explosive exothermic reactions (d’Hendecourt et al. 1982; Garrod et al. 2008).
Note that, as described later on in this thesis, the grain surfaces do not really act as catalyzers in a chemical sense, their role is to enhance the reaction rates (as a catalyst will do) only because they locally provide a medium of very high density (that of a solid) where a very complex chemistry can take place, contrary to what happens in the gas phase where two body reactions between reactive species and various efficient destruction mechanisms do limit the complexity.
Ices can be directly detected in the infrared from absorption features superimposed to the background of the hot inner shell of dust spectra, according to the following mechanism. A star, situated in or behind a molecular cloud, produces a continuous infrared emission spectrum; as this radiation passes through a cloud, the molecules present in the ices, along the line of sight, absorb at their characteristic frequencies giving rise to a set of absorption features in the spectra. The infrared range between 2 and 30m is particularly suitable for the study of interstellar ices because it covers the portion of the spectrum where vibrational transitions of molecular bonds associated with the most abundant cosmic species fall.
The presence of ice mantles is well known since the advent of infrared spectroscopy in astronomy in the 70’s. It was thanks to the OH broad infrared absorption at 3.1m that solid water was detected in the Orion BN-KL star forming region (Gillet & Forrest 1973). However, it was not before 1979 that laboratory simulations proved the amorphous nature of the interstellar ices by reproducing almost exactly the observed spectrum, a major advance at that time (Léger et al. 1979). Some years later, solid CO was identified from its 4.67m sharp spike and the associated wing in several protostellar sources (Lacy et al. 1984). The CO2 was identified from the 15.2m band in IRAS-LRS spectra of three protostellar sources, an identification entirely based on laboratory data dedicated to reproduce interstellar ice analogs (d’Hendecourt & Jourdain de Muizon 1989). Numerous other molecules were then identified as components of ice mantles, such as CH3OH (Grim 1991), CH4 (Lacy et al. 1991), NH3 (Lacy et al. 1998), OCS (Geballe et al. 1985), H2CO (Schutte et al. 1996), etc. Today about 20 different features have been observed in ice mantles and assigned to their carrier molecule (Dartois 2005). The abundances of several of these molecules, relative to the water molecule, are given in Fig. 1.6. Water ice largely dominates the ices abundances; CO2 and CO are also widely observed and display large abundances variations among different sources. CH3OH can be dominant in some sources, becoming the second most abundant species after water ice, but also can be almost absent in other sources. Species such as e.g. HCOO− (Schutte et al. 1999), HCONH2, and H2NCONH2 (Raunier et al. 2004a) are also observed.
Ices have been widely observed in a number of different infrared sources, from star forming regions to quiescent dense clouds, thanks to the Kuiper Airborne Observatory (KAO), the first infrared astronomical satellite (IRAS), then the Infrared Space Observatory (ISO), and the Spitzer Space Telescope. Note that, although observations are difficult within the Earth’s atmosphere, large ground-based telescopes such as UKIRT (United Kingdom Infra-Red Telescope, located on Mauna Kea, Hawaii) or the VLT (Very Large Telescope, in the Atacama Desert of northern Chile) have also contributed to the understanding of the composition of the interstellar ices. A well-known example of interstellar ice spectra is shown in Fig. 1.7. This is the infrared spectrum towards the dust-embedded high mass young stellar object (YSO) W33A obtained with the ISO satellite from the Short Wavelength Spectrometer (SWS) (Gibb et al. 2000). This typical spectrum shows ice features including H2O, CO, CO2, CH3OH, and CH4, and amorphous silicate features. These latter come from the dust grain core and are always observed in the interstellar dust.
A large fraction of interstellar molecules is incorporated in the solid state in refractory grains and ice mantles, however most of the known interstellar molecules are revealed in the gas phase only (Woon 2008). Gas phase molecules are free to rotate; this produces narrow and characteristic bands which fall in the radio and sub-millimeter portions of the spectrum. From these narrow bands a finer and more sensitive identification of molecules is possible with respect to the infrared ones. This explains the much larger number of interstellar molecular identifications in the gas phase.
The first interstellar molecules were discovered in the gas phase in the 1940s with the observation of CH, CH+, and CN at optical wavelengths (McKellar, 1940; Adams 1941; Douglas & Herzberg 1941). At a later stage, many molecular detections such as NH3, H2CO were made at radio frequencies (Cheung et al. 1968; Snyder et al. 1969), but it was in the 1980s, with the development of new millimeter receiver technologies, that the study of interstellar molecules blossomed. As of today, more than 150 different interstellar molecules have been identified in the ISM, most of them in giant molecular clouds such as Sagittarius B2. The sources include circumstellar envelopes around evolved stars, hot cores and corinos, outflows from young stars, etc. A list of the interstellar molecules detected in space in all the spectral domains is presented in Table 1.2. The molecules detected include light hydrides, ions, chain molecules, cyclic molecules, complex molecules, etc., many of them also common on Earth. One may note from this table that, as already discussed in this chapter, the observed gas phase chemistry is essentially made of the most abundant atoms and will favor organic chemistry. In this list the number of molecules decreases with the number of atoms, which is indeed not surprising. By interstellar standards, molecules containing six or more molecules are viewed as complex. With this definition, about one third of the molecules detected in space are complex and the more complex ones are those in the smaller relative concentrations. The largest organic molecule (13 atoms) that has been unambiguously identified is HC11N (Bell et al. 1997). Among these complex molecules, methanol (CH3OH) is the only one detected unambiguously also in the solid state, while all the rest have been detected exclusively in the gas phase. This remark reflects the fact that absorption infrared features from molecules in the solid state are broad and cannot be assigned with as much certainty as narrow gas-phase features. Among the complex molecules detected, some of them are thought to be involved in processes useful for prebiotic chemistry on Earth. Molecules of this category have some structural elements in common with those found in living organisms. Of the interstellar prebiotic molecules, many were first detected toward Sgr B2(N-LMH). For example, the first interstellar sugar glycolaldehyde (Hollis et al. 2004), ethylene glycol (Hollis et al. 2002), acetamide (Hollis et al. 2006), and amino acetonitrile (Belloche et al. 2008), a direct precursor of the simplest amino acid, glycine. Furthermore there is a strong evidence for the presence of PAH (polycyclic aromatic hydrocarbons), large aromatic molecules containing up to 100 carbon atoms, although specific assignments to any peculiar molecule have not yet been possible (Léger & Puget 1984; Allamandola et al. 1989). Conversely, the detection of glycine, claimed by Kuan et al. (2003) remains still controversial.
In summary, many complex molecules are observed in the ISM (mainly in the gas phase) and some of them contain up to 13 atoms. Moreover, it is likely that many additional molecules exist both in the gas and in the ice phase. Unfortunately, in the ice phase the large complex molecules cannot be observed since line broadening and intermolecular reactions caused by the solid environment leads to overlapping features that are dominated by the most abundant ice components. In the next section we will see how these complex molecules can be formed.
Energetic processing and thermal evolution of ices
Non-energetic reactions, such simple atom additions on grain surface, are necessary to initially form water-based ice mantles; however, additional energetic mechanisms appear to be necessary to further form molecules and, in particular, complex molecules. These mechanisms are often concomitant and include energetic processing by cosmic ion irradiation and UV photolysis, and thermal processing (Fig. 1.8).
Figure 1.8 – Scheme of the different processes occurring on interstellar grains (from Guss 2013).
UV photons are widely present in space environments, and those with energy greater than ~4 eV can drive ice photochemistry. Indeed, ice photochemistry is now accepted to occur deeper into the dense clouds than thought earlier and at much lower energies than in the gas phase (Gudipati & Allamandola 2003). On the other hand, energetic UV photons produced by newly formed stars do not penetrate deeply into the surrounding molecular cloud and those from external stars affect only the outer edges of these clouds. However, internal young stars can be important sources of UV radiation especially during the T-Tauri phase. A high flux as 1012 photons cm-2 s-1 can penetrate the circumstellar disk and efficiently photolyze the ice. Moreover, there is another internal source of UV photons that can affect the entire cloud, which is the decay process of the H2 molecule after cosmic ion bombardment (Shen et al. 2004).
Cosmic ions are dominated by protons, mainly in the low MeV range, with an estimated flux value of 1 proton cm-2 s-1 in dense quiescent regions (Mennella et al. 2003). Cosmic ions penetrating the cloud lead to internal production of UV photons via a mechanism in which secondary electrons excite H2, which then re-radiate photons when they decay. In dense clouds this internal UV flux is estimated to be 1.4-4.8 x 103 photons cm-2 s-1 with a typical energy of ~10 eV (Prasad & Tarafdar 1983; Mennella et al. 2003), much smaller than that one present in the diffuse ISM but strong enough to drive the photochemistry in the ices. This energy is released to the ice through a single photo-dissociation or photo-excitation event. As a consequence of these events, molecular bonds are broken, formed radicals diffuse through the ice and react with other molecules, giving rise to a rearrangement of the chemical structures and to the formation of new molecular species. In this way, a complex chemistry occurs. The energy dose from this internal UV source is thought to be similar in value to the energy contribution from the incoming cosmic ions. Moreover, this is the right dose of irradiation required to form new species in the ice preserving them from further irradiation that could led to their destruction. Recent studies suggest that the production of molecules, such as C2H5OH, HCOOCH3 and CH3OCH3, relies on UV irradiation of interstellar ices (Öberg et al. 2009, Modica & Palumbo 2010).
Within the cloud, several circumstances such as the formation of a new star, stellar outflow winds, and shock waves can lead to warm-up of the ices. The gradual warm-up can promote the mobility of species, which diffuse through the ices and trigger new reactions. Some proposed reactions studied in the laboratory include polymerization to form polyoxymethylene (Schutte et al. 1993). Moreover, with the further increase of the temperature, the more volatile components of the ice mantle can sublimate while the more refractory organic material remains.
Evidences for energetic processing of ices arise from the analysis of the infrared spectra of observed sources. The interstellar 4.62 µm absorption band was discovered in the ground based spectrum of W33A (Lacy et al. 1984). Commonly referred to as the “XCN” feature (Gibb et al. 2000), this band is observed in a large variety of objects (Soifer et al. 1979; Tegler et al. 1993, 1995; Demyk et al. 1998; Pendleton et al. 1999; Gibb et al. 2000; Keane et al. 2001; Whittet et al. 2001; Chiar et al. 2002; Spoon et al. 2003). The 4.62 µm absorption is now unambiguously attributed to OCN- formed by UV photolysis or cosmic ion irradiation of NH3 bearing ices, as supported by laboratory works (Palumbo et al. 2000; Bernstein et al. 2000; Hudson et al. 2001) and is considered as a key indicator of such processing. However, besides energetic processing, thermal evolution is also a possible formation alternative for the OCN-(van Broekhuizen et al. 2004). The most probable counter ion of OCN- is NH4+ (thus forming the ammonium cyanate salt) which is properly observed in evolved processed ices (Schutte & Khanna 2003). Another indication for energetic processing of interstellar ices is that of CO2, a molecule detected for the first time by d’Hendecourt & Jourdain de Muizon (1989) by its 15.2 µm absorption in three distinct infrared sources with the IRAS telescope. Interestingly, this molecule was been predicted from laboratory experiments on UV photochemistry of ice mixtures of water and carbon monoxide, revealing the potentiality of this laboratory approach (d’Hendecourt et al. 1985, 1986; Allamandola 1988). Several attempts have been performed to detect the effects of UV photochemistry in space with some tentative successes, such as in NGC 7538-IRS9 where the 5 to 8 micron region may be interpreted by the presence of complex molecules such as urea, formamide and glycerol (Raunier et al. 2004). Some features, observed in the astronomical spectra, still remain not fully explained such as the 6.8 µm feature, attributed to NH4+ formed by UV photolysis of ice analogs (Shutte & Khanna 2003) but more likely due to multiple contributors (Boogert et al. 2008). However, the most emblematic spectroscopic signature which is considered the sign of interstellar ice processing is the series of absorption near 3.4m in the diffuse interstellar medium. These features have been attributed to the C-H stretching in aliphatic hydrocarbon chains carrying the CH2 and CH3 functional groups (Sandford et al. 1991; Pendleton et al. 1994). The correlation of these absorption features with those from different interstellar analogs (hydrogenated amorphous carbon, quenched carbonaceous condensate, and various organic residues including the ones described in this thesis) is remarkable. However, the best candidate material for the 3.4m feature is likely to be hydrogenated amorphous carbon (Chiar et al. 2000; Pendleton & Allamandola 2002; Mennella et al. 2002).
Finally, the presence of organic refractory residues has long been postulated as a candidate for the carbon-rich components observed in interstellar dust at 3.1, 3.4, and 6 µm (Greenberg et al. 1995; Greenberg & Hage 1990; Pendleton et al. 1994; Whittet et al. 2001). This residue, that is expected to be rich in complex organics, is thought to be the result of ice energetic processing. As suggested by Gibb & Whittet (2002) this residue, which is not widespread in the diffuse interstellar medium, could in part contribute to the 6 µm band observed in YSOs. This hypothesis, well documented in the laboratory, is supported only by one observation of evolved ices in Mon R2 IRS3, the only observational evidence for organic refractory residues formation (Shutte & Khanna 2003). It is interesting to note how much these observations are biased because evolved ices can be tentatively observed only in evolved objects when the surrounding molecular cloud starts to dissipate. Finally, observations are quite limited in number as well as in performance. At this stage it is clear that a much more sensitive spaceborn telescope such as the JWST is needed.
Complex molecules in hot molecular cores
The formation of a new star has many effects on the chemistry of the surrounding material. One of the most extensively studied regions of star formation is the Orion molecular cloud (OMC-1), which contains a number of bright mid-infrared sources, thought to hold embedded protostars (Wynn-Williams et al. 1984). These sources, called hot molecular cores due to their high temperature (T > 100 K), are compact (< 0.1 pc) and massive (10-103 Msun) condensations of gas and dust surrounding a protostar in the early phases of its evolution (Brown et al. 1988). The Orion hot core is one of the most studied objects of this type (Genzel & Stutzki 1989). The low-mass versions of the hot cores are known as hot corinos (Ceccarelli 2004). Hot cores and corinos are natural laboratories in which the most complex molecules of the interstellar medium can be observed and studied. In these regions grains experience thermal processing that lead to molecular diffusion, structural changes, and subsequently to the complete sublimation of ices. Sublimation allows the molecular species previously formed in the solid state to be released into the gas phase, where they enrich the inventory of detected species and participate in new gas phase reactions. Because of the high temperature of the hot cores, many reactions can occur efficiently, including endothermic processes and exothermic processes with barriers. Hot cores are, in fact, particularly rich in complex organic molecules such as HCOOCH3, HCOOH, CH3OCH3, and CH2CHOH (Snyder et al. 2002; Hollis et al. 2000, 2002; Remijan et al. 2003; Kuan et al. 2003; Cazaux et al. 2003; Kuan et al. 2004; Bottinelli et al. 2004a, 2004b, 2007).The origin of these complex molecules was initially thought to be the rich gas phase chemistry of evaporated or sputtered species (e.g., Millar et al. 1991; Millar & Hatchell 1998; Herbst et al. 1977). However, recent experimental works have shown the inefficiency of gas phase reactions (Horn et al. 2004; Geppert et al. 2006) and that, according to models many of the observed molecules must be primarily produced in ices prior to evaporation (Charnley et al. 1995; Garrod & Herbst 2006; Wakelam et al. 2010). In particular, methyl formate (HCOOCH3), a complex organic molecule widely observed toward numerous different sources, has been suggested to form in the solid state after ion irradiation of ice mantles containing methanol and carbon monoxide (Modica & Palumbo 2010), and released after sublimation into the gas phase, where it can be observed.
Table of contents :
1 Overview on the evolution of ices in space
1.1 The Interstellar Medium
1.1.1 Dust grains
1.1.2 Interstellar ice mantles
1.1.3 Interstellar molecules
1.2 Energetic processing and thermal evolution of ices
1.2.1 Complex molecules in hot molecular cores
1.3 Small Solar System bodies
1.3.2 The exogenous organic delivery to Earth
1.4 Laboratory simulations
2 The MICMOC experiment at IAS 33
2.1 The experimental set-up MICMOC
2.2 Infrared spectroscopy
2.3 The standard experiment
2.3.1 Preparation of the gaseous mixture
2.3.2 Deposition and irradiation
2.4 Infrared analysis of interstellar/circumstellar analogs
2.4.1 Infrared ice spectra before irradiation
2.4.2 Infrared spectra of an irradiated thin ice film
2.4.3 Infrared analysis of a typical residue
3 Asymmetric UV photochemistry of cosmic ice analogs: induction of enantiomeric excesses in amino acids
3.2.1 Experimental campaigns
3.2.2 Preparation of residue-enlarged samples
3.2.3 Analytical procedure
3.3.1 Distribution of identified amino acids
3.3.2 Enantiomeric excesses in alanine
3.3.3 Enantiomeric excesses in five different amino acids
3.4 Astrophysical implications
4 Analysis of laboratory organic residues by FT ICR mass spectrometry
4.1 FT ICR mass spectrometer
4.1.1 Instrument description
4.1.2 Basic principles
4.2 Mass spectra description
4.2.1 Analysis at low and medium mass-to-ratio range
4.2.2 Analysis in the high mass-to-charge ratio range
4.3 Mass defect versus mass diagrams
5 The amino acid content in the Paris meteorite and in laboratory residues
5.1 Carbonaceous chondrites
5.1.2 Organic matter content
5.1.3 The Paris meteorite
5.2 Analytical procedures
5.2.1 Amino acid extraction and derivatization
5.3 Results and discussion
5.3.1 Amino acid content in the Paris meteorite
5.3.2 Enantiomeric measurements in chiral amino acids
5.3.3 Amino acid content in laboratory organic residues
6 Laboratory simulations using silicate surfaces
6.1 Astronomical silicates
6.2 Experiments and discussion
6.2.1 Laboratory make-up of icy silicate grain analogs
6.2.2 Spectral comparison with the Paris meteorite
6.2.3 Hot water extraction test on laboratory samples
6.2.4 The search for HMT in the Paris meteorite
Conclusions and perspectives