Chemical complexity evolution
Molecular complexity increases during the formation of low mass stars. It can be de-scribed by five stages, as suggested by Caselli & Ceccarelli (2012) (see Figure 1.2).
1. Pre-stellar cores: In the gas phase of a primordial cloud, atoms and simple molecules freeze out onto dust grains. At this stage, formation of molecules like water (H2O), formaldehyde (H2CO), methanol (CH3OH) begins from surface hydro-genation of O and CO by the H atoms.
2. Protostellar envelopes: As the collapse continues, the temperature of the future star increases until it reached the mantle sublimation value ( 100 K) in the so-called hot corino regions.
3. Protoplanetary disks: In time, a protoplanetary disk is formed and the envelope disappears. New and more complex molecules are synthesised in the hot regions close to the central object. In contrast molecules inside the cold equatorial plan freeze out again into grain mantle surfaces. At this stage, legacy and preservation starts.
4. Planetesimal formation: As dust grains coagulate into larger masses, namely planetesimals, icy grain mantles are also condensed within. Thus, these plan-etesimals, which will form future Solar System bodies, carry the history of their formation.
5. Planet formation: In the Solar System, during the formation of the early Earth, the planet is subjected to showers of comets and asteroids that contain ices trapped in the planetesimals. With the formation of oceans and atmosphere, life emerged about 2 billion years ago.
Census of molecules in the interstellar medium
The nature of molecules and estimation of their abundance in space has been ob-tained from spectroscopic observations. Many molecules have been discovered over several years of research. At first, simple molecules such as CH (Swings & Rosen-feld 1937), CN (McKellar 1940) and CH+ (Douglas & Herzberg 1941) were identified. Much later, following the development of radioastronomy, more complex molecules like NH3 were also discovered (Cheung et al. 1968). As detection technics improved, so did the number of identified molecules as well as their complexity. So far, this amounts to about 200 molecules according to CDMS databases ( https://www.astro.uni-koeln.de/cdms/molecules). The number of identified molecules, sorted by increasing num-ber of constituting atoms, are given in Figure 1.3a, while Figure 1.3b shows the numbers of molecules detected in the Orion nebula, and Figure 1.3c shows those detected in the solar type protostar IRAS16293-2422. As it is clear from these three figures, the most numerous molecules are the simplest ones, i.e., those with two or three atoms.
In Figure 1.4, the number of the detected molecules is plotted as a function of the molecular weight for five regions: ISM (Figure 1.4a), the Orion nebula (Figure 1.4b), IRAS16293-2422 (Figure 1.4c), OMC-2 FIR4 (Figure 1.4d) and extragalactic sources (Fig-ure 1.4e). Interestingly, a similar peak at molecular weight around 40 to 49 is evidenced in these graphs, probably because the most abundant species in the universe are hy-drogen, carbon, nitrogen and oxygen. Indeed, the simple combination of a few of these atoms would easily result in species averaging 40-49 in molecular mass. Amongst the several combinational possibilities, we find for example HNCO and NH2CHO which are important in the formation of more complex prebiotic species. This could indicate that the greater molecular abundance in the universe is formed from these atoms which being also the most abundant maybe not be surprising: Nature uses what is available. The majority of detected molecules are relatively simple, but several more complex polyatomic molecules have also been identified, albeit in low abundance. Many more are likely to be discovered in the near future. Thus, questions arised concerning their provenance: How did these molecules formed and can they be linked to the origin of terrestrial life?
Aims and structure of this thesis
This thesis aims to a better understanding of the chemistry in the early phases of low-mass star-forming regions. The thesis is separated into seven Chapters, as follows:
Chapter 1 presents a general brief review of the current knowledge on low-mass star formation and the chemical evolution.
Chapter 2 describes the two sources studied in detail in this thesis. For IRAS16293-2422, I describe in details the physical and chemical structure, along with outflow sys-tem. OMC-2 FIR 4 much less is known, so that the description is more concise.
Table of contents :
1.2 Low-mass star formation
1.3 Chemical complexity evolution
1.4 Census of molecules in the interstellar medium
1.5 Aims and structure of this thesis
2 Description of IRAS16293-2422 and OMC-2 FIR 4
2.1.1 The physical structure
2.1.2 The outflow system in the IRAS16293
2.1.3 The chemical structure
2.1.4 Deuteration in IRAS16293
2.2 OMC-2 FIR4
3 Used Tools
3.2 Spectral surveys
3.3 Lines identification
3.3.1 Criteria for identification
3.3.2 Tool: CASSIS
3.4 Lines parameters
3.4.1 Gaussian fit
3.4.2 LTE Modeling for upper limits
3.5 SLED Modeling
3.5.2 General description of the package
3.5.3 Method of work
4 COMs in IRAS16293-2422
4.3 Source description
4.4 The data set
4.4.2 Species identification
4.5 Analysis and results
4.5.1 Model description
5 Cyanopolyynes in IRAS16293-2422
5.3 Source description
5.4 The data set
5.4.2 Species identification
5.5 Line modeling
5.5.1 Model description
Undetected species and conclusive remarks
5.6 The chemical origin of HC3N
5.6.1 Cold envelope
5.6.2 Hot corino
5.7.1 General remarks on cyanopolyynes in different environments
5.7.2 The present and past history of IRAS16293
5.7.3 The HC3N deuteration
6 Formamide in Low- and Intermediate-Mass Objects
6.3 Source sample
6.4 Observations and data reduction
6.5.1 Line spectra
6.5.2 Derivation of physical properties
Rotational diagram analysis
Radiative transfer analysis taking into account the source structure
6.6.1 Formation routes of NH2CHO
6.6.2 Correlation between HNCO and NH2CHO
7 Conclusions and FutureWork