Nitrogen in the ISM and circumstellar environments

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nucleosynthesis of nitrogen

During the big bang nucleosynthesis only the lightest atomic nu-clei were produced: H, He, Li and Be (Alpher, Bethe, and Gamow, 1948). All the other elements in the universe have been created by stars long after the big bang either in their cores (Hoyle, 1946) or in supernova explosions (Hoyle et al., 1956). The first stable atomic nucleus produced in stars is 4He through the proton-proton chain (Eddington, 1920; Bethe, 1939). The next nuclei to be produced are those that can be built directly from helium nuclei (ex: 12C, 16O, 20Ne, etc). Other atomic nuclei that can be build from this elements simply by the addition of D or H are also produced (ex: 14N, 22Na), but in much lower quantities as they require an extra step to be produced. These nuclei are called primary isotopes as they can be built directly from hydrogen and helium in one single star. Other nuclei, such as 13C, 18O, 15N, require that the stars that produce them are born with a non negligible amount of nuclei heavier than 4He to be efficiently produced, and are thus called secondary isotopes. Since their produc-tion depend on the abundance of the elements built previously their abundances are generally 2 to 3 orders of magnitude inferior to the primary isotopes of the same element (ex: 14N/15N = 441 in the Solar system, 12C/13C=70 in the local ISM, Füri, Chaussidon, and Marty, 2015; Milam et al., 2005, respectively).
Nitrogen has two stable isotopes: 14N is a primary isotope, and 15N is a secondary isotope. The details behind the production of these two isotopes are still debated (Clayton, 2003; Prantzos, 2003; Romano and Matteucci, 2003; Wiescher et al., 2010), but the main processes have been identified (Wilson and Rood, 1994). 14N is pro-duced by both the cold and hot CNO cycles, as well as in the so- called hot bottom burning process in asymptotic giant branch (AGB) stars, although massive, rotating stars may be sources as well (Prant-zos, 2011). In contrast, it is thought that 15N can only be produced through the hot CNO cycle, during nova outburst or in Type Ia and Type II supernovae (Romano and Matteucci, 2003). The differences in primary and secondary origins of the two nitrogen isotopes are predicted to lead to an increase in the 14N/15N ratio with distance from the galactic centre. Indeed Adande and Ziurys (2012) have mea-sured a positive gradient with increasing distance from the galactic centre, 14N/15N = 21.1DGC + 123.8, where DGC is the distance from the galactic centre in kpc. As can be seen in Figure 2.1 the observed galactic gradient is compatible with current galactic evolution models for the 14N/15N ratio.

nitrogen in the ism and circumstellar environments

Nitrogen is an atom capable of a very rich chemistry, and being the seventh most abundant element in the universe it is readily available to for a multitude of compounds, specially with hydrogen carbon and oxygen. Nitrogen has been detected in the diffuse ISM in several forms, atomic, mainly through its UV absorption bands (Nieva and Przybilla, 2012), or in molecular carriers (HCN N2 e.g. Liszt and Lucas, 2001; Knauth et al., 2004; Li et al., 2013). CN was among the first molecules to have ever been identified in the diffuse ISM (McKellar, 1940; Swings and Rosenfeld, 1937).
In the dense ISM Nitrogen can be found in a variety of species, from very simple ones such as CN, to big complex ones like HC11N. This rich chemistry is a result of the much denser gas and the presence of ions that allow fast ion-neutral reactions to take place (Terzieva and Herbst, 2000). It is theorised that the main reservoir of nitrogen in the dense ISM should be either atomic N or N2, with some part of it being locked in ices as NH3 (Herbst and Klemperer, 1973). Of these three main reservoirs only NH3 in ices is observable, and is measured to be an important component of interstellar ices NH3/H2O 5 20% Boogert, Gerakines, and Whittet, 2015. The other two reservoirs, N2 and N are not observable in the dense ISM (Herbst, Schubert, and Certain, 1977). Atomic nitrogen by being a single atom presents only fine structure transitions and electronic transitions. Both types of transitions have upper levels that are not excitable in the dense ISM, due to the low temperatures. The possibility remains to use those transitions as absorption probes through the use of background sources of continuum radiation. The problem is that the electronic lines fall in the ultraviolet which is strongly affected by the extinction of dust in the dense ISM, and that stars do not emit sufficient light in the wavelengths of the fine structure lines to be used in as an absorption probe. For N2 the electronic lines are also not available for the same reasons and being a closed shell molecule there are no dipole fine structure transitions allowed. The hope should remain on its rotational or vibrational transitions, but since it is a mono atomic species there is no permanent dipole moment therefore there are no dipolar radiative rotational transitions.
The picture that can be built from this is that the two candidates for the main reservoir of Nitrogen in the dense ISM are unobserv-able. We therefore must rely on the study of the trace species that are detectable in the dense ISM and use their abundances to reverse en-gineer the properties of the main reservoirs of nitrogen in the dense ISM, and also trying to identify which is the dominant one. The re-verse engineering is done through the use of theoretical chemistry simulations using the so-called chemical networks. Chemical net-works are databases of possible chemical reactions and their effective rates.
In circumstellar environments nitrogen is seen in a larger multi-tude of molecules, ranging from simple ones like CN to big organic molecules like CH3CH2CN (Cazaux et al., 2003). This complex chem-istry is still not completely understood but may be related to the sub-limation of ice mantles close to the protostars (van Dishoeck, 2014) or in the case of formamide (NH2CHO) be a product of gas phase reactions only (Barone et al., 2015). In disks fewer species have been detected, but nevertheless nitrogen carriers are commonly observed. Chapillon et al. (2012b) demonstrate that CN and HCN are ubiqui-tous in protoplanetary disks. A few complex molecules have been detected in disks, such as HC3N (Chapillon et al., 2012a) and CH3CN (Öberg et al., 2015).

measuring the isotopic ratio of nitrogen in the ism

The methods to measure the 14N/15N ratio can be divided in two broad categories: direct methods and indirect methods. The direct methods are called so because 14N/15Nis computed is taken as the ratio of the column densities of the main and rare isotopologue mea-sured directly. Whereas the indirect methods use the ratio between the rare isotopologue and a third species to estimate the ratio between the main and the rare isotopologue. The most used method in this category is called the double isotopologue method Section 2.3.2.

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Direct methods

To measure the isotopic ratio directly it is necessary to have a reliable way to measure the column density of the main isotopo-logue. The simplest case would be that in which the emission of the main isotopologue is optically thin, but this usually means that the rare isotopologue is undetectable. Yet due to the hyperfine structure displayed by molecules containing nitrogen, sometimes one of the hyperfine components of the main isotopologue is indeed opti-cally thin. One example of such a measurement is the measurement of the 14N/15N in CN in the protoplanetary disk around TW Hydra by Hily-Blant et al. (2017) (14N/15N = 323 30).
Another way of directly measuring the column density of the main isotopologue is through hyperfine fits to the rotational spectra to ob-tain the total opacity of the rotational transition. The caveat of this method is that the hyperfine fit assumes that the hyperfine compo-nents of rotational transition have the same excitation temperature, which may not be true. One example of such a measurement is the measurement of the 14N/15N in NH2D towards a few prestellar cores by Gerin et al. (2009) (14N/15N = 350 850).
The column density may also be estimated through a local ther-modynamical equilibrium analysis which assumes that the different radiative transitions observed can be described by a single excita-tion temperature. This approach is better suited for molecules which have several transitions available, like the cyanopolyynes or organic molecules like CH3CN. The problem with this method is that this as-sumption is rarely valid. Despite this problem this method has been used to compute the isotopic ratio of nitrogen in cyanopolyynes, one example of such an endeavour is Taniguchi and Saito (2017). The au-thors have measured the isotopic ratio of nitrogen in HC5N in TMC1 through this method to obtain 14N/15N = 323 80.
The isotopic ratio of nitrogen can also be measured in the diffuse ISM by measuring the opacity of absorption lines of molecules con-taining nitrogen against a strong background source. This has been carried out for example for HCN which had its 14N/15N ratio mea-sured in the line of sight towards B0415 + 379 (Lucas and Liszt, 1998). The main caveat here is that emission in the line of sight might ham-per the measurements by diminishing the perceived absorption.
The remaining method to measure the column density of the main isotopologue directly is through the usage of complex radiative trans-fer models. In this case a complex source model is assumed and line profiles are computed in order to compare them to observations in search for the best fit. The main difficulty in this method resides in the necessity of an accurate source model which is prone to have correlations between its parameters (Keto et al., 2004). Despite this difficulty there are some examples of such measurements such as in Daniel et al. (2013). These authors have measured the abundances of several species along with their isotopologues in the B1 dark cloud. These abundances were then used to compute the isotopic ratio of nitrogen in several molecules.
28 the isotopic ratio of nitrogen

Indirect methods

Indirect methods to measure the nitrogen isotopic ratio consist in measuring the column density of the 15N isotopologue and a third species. This third species is a species for which there is an expected ratio between it and the 14N isotopologue. Usually this third species is taken as a rare isotopologue on another element such as carbon. In such cases this method is called the double isotopologue method (Dahmen, Wilson, and Matteucci, 1995).

Table of contents :

1 introduction 
1.1 The process of low mass star formation
1.1.1 The prestellar phase
1.1.2 The protostellar phase
1.1.3 The Protoplanetary disk phase
1.2 The results of the star formation process
1.2.1 Stars
1.2.2 Planets
1.2.3 Asteroids and comets
1.3 The interstellar heritage of planetary systems
1.3.1 Whats is a reservoir?
1.3.2 What are the different reservoirs that we see in the Solar system today
1.3.3 What are the different reservoirs that we see in the dense interstellar medium
1.4 How do we trace the heritage of reservoirs?
1.4.1 Isotopic ratios as a tool to follow reservoirs
1.4.1.1 D/H ratio as a tracer of interstellar heritage
1.4.1.2 Using Carbon Isotopic ratio to elucidate formation mechanisms of carbon chains
1.4.2 The puzzle of the isotopic ratio of nitrogen in the Solar System
1.4.3 The PhD project
2 the isotopic ratio of nitrogen 
2.1 Nucleosynthesis of nitrogen
2.2 Nitrogen in the ISM and circumstellar environments
2.3 Measuring the isotopic ratio of nitrogen in the ISM
2.3.1 Direct methods
2.3.2 Indirect methods
2.4 Review of existing 14N/15N measurements
2.4.1 Measurements in the Solar system
2.4.2 Measurements in the local ISM
2.4.3 Protostellar envelopes
2.4.4 Protoplanetary disks
2.4.5 The big picture
3 software tools 
3.1 Nslab
3.1.1 Implementation
3.1.2 Benchmarking
3.1.3 Perspectives
3.2 Accelerated Lambda Iteration COde, Alico
3.2.1 Model cloud
3.2.2 Alico griding software
3.2.3 The Alico model and observation classes
3.2.4 Perspectives
3.3 MCMC toolkit
3.3.1 Alico MCMC
3.3.2 Perspectives
3.4 Summary of radiative transfer tools
4 measuring 14N/15N in prestellar cores 
4.1 The hyperfine anomalies of HCN
4.1.1 What are Hyperfine Anomalies?
4.1.2 Possible mechanisms for the origins of the hyperfine anomalies
4.1.2.1 Rotational spectroscopy of HCN
4.1.2.2 Self-absorption
4.1.2.3 Radiative trapping caused by the overlap of hyperfine components
4.1.3 Testing the possible mechanisms
4.1.3.1 Four slabs Toy model
4.1.3.2 Tests with Alico
4.1.4 Conclusions
4.2 HCN and its isotopologues in L1498
4.2.1 Observations
4.2.1.1 Spectral observations
4.2.1.2 Continuum observations
4.2.2 HCN/HC15N under the single excitation temperature hypothesis
4.2.3 Measuring the isotopic ratio of HCN with the escape probability formalism
4.2.4 Density profile of L1498
4.2.5 Alico/MCMC fitting of HCN, H13CN and HC15N spectra
4.2.6 Article on HCN/HC15N in L1498
5 the 14N/15N ratio in protosolar nebula analogs 
5.1 Interferometric observations
5.1.1 Self-calibration
5.2 Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue
6 14N/15N in protostellar environments 
6.1 NH2Din IRAS03282
6.2 The nitrogen isotopic ratio in VLA1623 and IRAS16293
7.1 HCN hyperfine anomalies
7.2 Accurate measurements of isotopic ratios
7.3 HCN isotopic ratios in L1498
7.4 Where when and how are the two reservoirs of nitrogen seen in protoplanetary disks separated?
i appendix
bibliography 

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