THE ISOTOPIC RATIO OF NITROGEN
As seen in the previous chapter the isotopic ratio of nitrogen may be key to disentangling the mystery of the heritage of the protosolar nebula. To be able to understand the measurements that we have of 14N/15N in the ISM today we need to understand the sources of the two isotopes of nitrogen 14N and 15N (Section 2.1). With that knowledge in hand we need to choose our target molecules between the multitude of molecules seen in the ISM (Section 2.2) to carry out the measurements of the nitrogen isotopic ratio (Section 2.3). From these measurements (Section 2.4), a big picture is emerging which will be the framework for this thesis (Section 2.4.5).
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 socalled 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
In the warm clouds observed in the present study (TK > 25 K; see Table 3), this reaction is unlikely to cause significant fractionation. It could be more important in cold dark clouds, on the other hand, such as Barnard-1, where TK < 10 K. Lis et al. (2010), however, found 14N/15N = 334 ± 50 toward Barnard-1, using inversion transitions of NH3—close in value to those derived from CN and HNC at similar DGC. These results suggest that if any fractionation occurs for CN-bearing species through isotope exchange reactions, the effect is minimal.
Another possible fractionation effect is from differential self-shielding of the more abundant 14N isotopologues of CN and HNC, relative to rarer C15N and H15NC, from the ambient UV radiation field. The so-called isotope-selective Terzieva & Herbst (200 gen fractionation for com clouds. These authors con be very limited because topic exchange processes sequently cannot compet which would scramble an Charnley (2008) suggest enhancement in CN, HN regions of molecular clou etary disks. They were p observed in the organic c tional studies of the propo such cold cores.
Of the CNO isotopes, biggest uncertainties rem and subsequent mixing produced as a byproduct intermediate-mass stars a in the ISM during dredg 14N are more massive st (Audouze et al. 1975; Ro contributor to 15N is belie also a secondary process the cold CNO cycle is ve 2003). Observations of CN by Bakker & Lambert (1 14N/15N > 2000, in agree In our Galaxy, the meta of heavy elements, decre center (Maciel & Costa element, produced in sign in binary white dwarfs an should decrease with gal contrast, drops at a lower r As a result, the 14N/15N ra distance.
Various models of GCE hypotheses of the nitroge of the ISM (e.g., Tosi 19 recent model by Romano is produced only as a sec stellar yields adopted for mass stars for 14N produc as a function of DGC from vary in stellar yields chos with a galactic gradient o 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.
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 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).
This method has been usually used to derive the isotopic ratio of nitrogen in species of the nitrile family (species containing the CN group, HCN, CN, HNC, etc). In such a case the abundance of the 13C and the 15N isotopologues is measured and is subsequently mul-tiplied by an assumed 12C/13C:
XCN = X13CN 12C (2.1)
XC15N XC15N 13C
where X stand for any chemical group bound to the CN group. This method has been extensively used to measure the isotopic ratio of ni-trogen (e.g. Adande and Ziurys, 2012; Hily-Blant et al., 2013a; Guzmán et al., 2017), but a lot of uncertainties remain as the 12C/13C is usually unknown and therefore adopted to be the elemental one (12C/13C=70, Milam et al., 2005). This may indeed not be the case, as there is obser-vational evidence that this ratio may be lower than the elemental ratio (HCN/H13CN 30 Daniel et al., 2013), and theoretical expectations that it could be higher than the elemental ratio (HCN/H13CN=90 140 Roueff, Loison, and Hickson, 2015). Due to this incredible discrep-ancies, it is hard to judge the uncertainties in double isotopologue measurements.
review of existing 14 N/15 N measurements
To better comprehend what new information would be brought by new measurements of the isotopic ratio of nitrogen we need to make a review of the measurements that are already available in the literature. This review of the available measurements is divided in 4 subsections: the measurements in the Solar system, the measure-ments in the local ISM; the measurements in protostellar envelopes and the measurements in protoplanetary disks. The measurements are divided in this manner as the main idea of this thesis is to com-prehend what we see in the Solar system in respect to the nitrogen isotopic ratio, with what can be observed during the phases of star formation.
Measurements in the Solar system
In the Solar system today we see a marked difference between the main mass reservoirs (the Sun and Jupiter) and the smaller mass reser-voirs (asteroids, comets, interplanetary dust, inner planets). The Sun as the dominant mass reservoir displays the value of the 14N/15N at the moment and location of the birth of the Sun. All the other reser-voirs that show isotopic ratios that differ from that of the Sun have been fractionated by some unknown physical process.
Material extracted from meteorites presents differing degrees of fractionation. The most refractory substance containing nitrogen, os-bornite (TiN), has an isotopic ratio very similar to the bulk of the Solar system (14N/15N = 424 3 Meibom et al., 2007), while other reservoirs of nitrogen inside meteorites can show incredibly low iso-topic ratios (14N/15N 50, Bonal et al., 2010). On the other hand measurements of 14N/15N on the gases in the coma of comets are remarkably homogeneous, with all the values very close to 150 and a weighted average which gives 14N/15N = 144 3 (see Figure 1.11 Hily-Blant et al., 2017).
For the inner planets the situation appears to be somewhat mixed with the Earth displaying a nitrogen isotopic ratio that is close to half way between that of comets and of the Sun. Füri, Chaussidon, and Marty (2015) argue that this is an evidence that the Earth sampled a third reservoir of nitrogen during its formation. We on the other hand believe that this isotopic ratio is the result of the mixing of the two reservoirs of nitrogen. This scenario is consistent with the Grand Tack scenario for the late stages of planet that predicts that the Earth has sampled volatile rich (cometary like in 14N/15N) and volatile poor (Sun like in 14N/15N) material (e.g. Morbidelli et al., 2012). Mars on the other hand has no records of its primordial nitrogen isotopic ratio, as its atmospheric loss is isotopic selective (Mahaffy, 2016). In Figure 2.2 we have a compilation of the most relevant measurements of the isotopic ratio of nitrogen in the Solar system.
Measurements in the local ISM
In the local ISM there are several measurements of the Nitrogen Isotopic ratio. Some of these measurements are focused on the dif-fuse interstellar medium. These measurements target the species that exist at low densities (nH 6103 cm 3) and lukewarm tempera-tures (Tkin 100 K). Most of this measurements are carried out us-ing the absorption technique as molecular emission in this physical conditions is very feeble. Figure 2.3 displays the available measure-ments of the nitrogen isotopic ratio in the local diffuse ISM, along with the expected value from the galactic gradient of Adande and Ziurys (2012). All the values in the diffuse ISM are more or less consistent with each other, with the uncertainty weighted average being 14N/15N = 275 16.
In the dense ISM there are a plethora of measurements. Species in the hydride family, NH3, N2H+, presumably chemical descendants of N2 (Hily-Blant et al., 2010; Hily-Blant et al., 2013a), had their nitrogen isotopic ratio measured directly, while species for the nitrile family, CN, HCN, etc have mostly indirect measurements of their isotopic ratios, with few exceptions (HCN, HNC and CN in B1 by Daniel et al., 2013). The weighted average of the direct measurements yields 14N/15N = 251 12, a surprisingly low value. This value is probably skewed by the low isotopic ratios measured in the nitriles which can be underestimated due the enormous optical depths of the main iso-topologues (Daniel et al., 2013). If we do this weighted average only with molecules in the hydride family, which are less plagued by high optical depth issues we get 14N/15N = 340 16. This value is very similar to the value measured directly in CN in the protoplanetary disk of TW Hydra of 14N/15N = 323 30 measured by Hily-Blant et al. (2017). The authors point this similarity as an evidence of this being the current value of the isotopic ratio of nitrogen in the local ISM. Figure 2.4 displays the available measurements of 14N/15N in the dense ISM.(2015).
To date there has been only one published paper regarding iso-topic ratios of nitrogen in the envelopes of protostars, Wampfler et al. (2014). The authors measured the nitrogen isotopic ratio through the double isotopic method (Figure 2.5). Their measurements are based on APEX observations of these protostars. The large beam of APEX at the observed frequencies (24″ at 259 GHz and 18″ at 345 GHz) suggest that these measurements sample both the protostar and its cold envelope. The weighted average of these measurements give a weighted average 14N/15N = 227 14. We have obtained a direct measurement of the nitrogen isotopic ratio in the cold envelope of IRAS 03282 + 3035, NH2D/15NH2D= 250 50. This measurement was obtained with the 11,1 ! 11,0 transition of these species.
In protoplanetary disks isotopic measurements are very difficult due to the minute size of disks (diameters of the order of a few arc seconds). Sch measurements became possible with the advent of the Atacama Large Millimetre Array, due to its large collecting area. With ALMA the nitrogen isotopic ratio in HCN has been mea-sured in 5 disks in the Solar neighbourhood through the double iso-topologue method, assuming HCN/H13CN = 70, and found to be quite low, 14N/15N = 111 19 (Guzmán et al., 2015; Guzmán et al., 2017; Hily-Blant et al., 2017).
Table of contents :
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
1.2.3 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
220.127.116.11 D/H ratio as a tracer of interstellar heritage
18.104.22.168 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
3.2 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
3.3 MCMC toolkit
3.3.1 Alico MCMC
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
22.214.171.124 Rotational spectroscopy of HCN
126.96.36.199 Radiative trapping caused by the overlap of hyperfine components
4.1.3 Testing the possible mechanisms
188.8.131.52 Four slabs Toy model
184.108.40.206 Tests with Alico
4.2 HCN and its isotopologues in L1498
220.127.116.11 Spectral observations
18.104.22.168 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
5.2 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?
a software written and example input files
a.1.1 Source code for Nslab
a.1.2 Nslab input
a.1.3 Nslab output
a.1.4 Nslab help
a.2.1 1DART input files
a.2.1.1 Physical model input file
a.2.1.2 Beam convolution input file
a.2.2 Model cloud input file
a.2.2.1 Model cloud help
a.2.3 ALICO gridding software
a.2.3.1 Source file
a.2.3.2 Molecular file
a.2.3.3 Grid file
a.2.3.4 Gridding software help
a.2.4 ALICO classes
a.2.4.1 Model class
a.2.4.2 Observation class
a.2.4.3 Example code using observation and model classes
a.3 MCMC toolkit
a.3.1 MCMC toolkit code
a.3.2 MCMC toolkit example input file
b data tables
c approved telescope time proposals
15N in l1544