Nitrogen isotopes in the interstellar medium: A chemical journey across the Galaxy

. One of the most important tools to investigate the chemical history of our Galaxy and our own Solar System is to measure the isotopic fractionation of chemical elements. This is the process that distributes the less abundant stable isotopes of an element in di ↵ erent molecules. The isotopic ratios are governed by two main processes: 1. chemical evolution of the whole Galaxy due to stellar nucleosynthesis; 2. local fractionation e ↵ ects. In this Proceeding we report some results highlighting both processes towards massive star-forming regions.


Introduction
One of the main tools to understand the heritage that has been received by the Solar System is to study the isotopic variation.In fact, pristine Solar System materials, like comets and carbonaceous chondrites, present enrichment in 15 N and D, with respect to the proto-Solar Nebula (PSN) value (see e.g. the left panel of Fig. 1).This means that there was an alteration of isotopic ratios (e.g. the 14 N/ 15 N ratio) during the Solar formation process, whose causes are still not well understood.
All of these di↵erent 14 N/ 15 N ratios depending on the physical conditions of the sources and on the observed molecular species, raising interest also from a theoretical point of view.First, the so-called low-temperature isotopic-exchange reactions in chemical models have been invoked to explain the observed ratios (e.g.[13]; [14]; [15]), as: which exchange one of the 14 N of N 2 H + with 15 N.These reactions are very efficient in a lowtemperature environment (e.g.5-10 K for pre-stellar cores, [16]) because of their exothermicities.Moreover, such reactions are also very successful in reproducing the D/H ratios in molecular clouds (e.g.[17]).However, this is not so for nitrogen fractionation.This indicates that there are other chemical processes that need to be taken into account.Other mechanisms, such as di↵erent rates for the dissociative recombination of N 2 H + (e.g.[15]), or the isotope-selective photodissociation of N 2 (e.g.[18]) have been also proposed.

The role of the Milky Way chemical evolution
The 14 N/ 15 N ratios are also governed by the chemical evolution of the whole Galaxy due to stellar nucleosynthesis (e.g.[19]).In this respect, we have analysed a sample of 87 massive star-forming regions observed with the IRAM 30m radiotelescope 1 .The sources span galactocentric distances, D GC , from 2 up to 12 kpc allowing to study the 14 N/ 15 N ratio of HCN and HNC as a function of D GC (Fig. 2).For the two datasets we have performed a linear regression fit (light blue solid lines in Fig. 2) and a parabolic analysis (red parabolas in Fig. 2).
From the linear trends we have derived a new local ISM 14 N/ 15 N value of 375±75 ( [9]).Then, the observational results have been compared with the predictions of the Galactic Chemical Evolution (GCE) model [19] (magenta lines in Fig. 2).They predict a linear positive trend up to 8 kpc because of novae outbursts as the main way to produce the 15 N, and a flattening trend above this distance because of the low-level star formation and gas infall on long timescales (inside-out galaxy formation) that reduce the nova e↵ect (see e.g.[19,20]).GCE models can reproduce the observational trend but not the absolute values.However, the shift on the y-axis of the models is defined by the ejected mass in the form of 15 N, M 15 N ejec , which is an assumed parameter considering valid ranges taken from hydrodynamic simulations (see [19] for more details).More recent GCE models [20], which take into account di↵erent rotation velocities of low-metallicity massive stars, updated M 15 N ejec and could better reproduce also the absolute value of observations.
We are now extending this trend towards the outer part of our Galaxy (D GC >12 kpc), also updating GCE models with new stellar yields and approaching distances where CNO isotopes 1 For more information on the observations and the analysis done see [8,9]. in molecular clouds (e.g.[17]).However, this is not so for nitrogen fractionation.This indicates that there are other chemical processes that need to be taken into account.Other mechanisms, such as di↵erent rates for the dissociative recombination of N 2 H + (e.g.[15]), or the isotope-selective photodissociation of N 2 (e.g.[18]) have been also proposed.

The role of the Milky Way chemical evolution
The 14 N/ 15 N ratios are also governed by the chemical evolution of the whole Galaxy due to stellar nucleosynthesis (e.g.[19]).In this respect, we have analysed a sample of 87 massive star-forming regions observed with the IRAM 30m radiotelescope 1 .The sources span galactocentric distances, D GC , from 2 up to 12 kpc allowing to study the 14 N/ 15 N ratio of HCN and HNC as a function of D GC (Fig. 2).For the two datasets we have performed a linear regression fit (light blue solid lines in Fig. 2) and a parabolic analysis (red parabolas in Fig. 2).
From the linear trends we have derived a new local ISM 14 N/ 15 N value of 375±75 ( [9]).Then, the observational results have been compared with the predictions of the Galactic Chemical Evolution (GCE) model [19] (magenta lines in Fig. 2).They predict a linear positive trend up to 8 kpc because of novae outbursts as the main way to produce the 15 N, and a flattening trend above this distance because of the low-level star formation and gas infall on long timescales (inside-out galaxy formation) that reduce the nova e↵ect (see e.g.[19,20]).GCE models can reproduce the observational trend but not the absolute values.However, the shift on the y-axis of the models is defined by the ejected mass in the form of 15 N, M 15 N ejec , which is an assumed parameter considering valid ranges taken from hydrodynamic simulations (see [19] for more details).More recent GCE models [20], which take into account di↵erent rotation velocities of low-metallicity massive stars, updated M 15 N ejec and could better reproduce also the absolute value of observations.
We are now extending this trend towards the outer part of our Galaxy (D GC >12 kpc), also updating GCE models with new stellar yields and approaching distances where CNO isotopes  15 N ratios for HNC (left panel) and HCN (right panel) as a function of the galactocentric distance (D GC ).The cyan solid line is the linear regression fit computed for the two data sets, the red parabola is the results from the parabolic analysis described in detail in [9], and the magenta solid line represents the GCE model of [19].
have never been studied (Colzi et al. in prep).In fact, this zone of the Galaxy present lower metallicities, a factor of four lower at D GC =19 kpc with respect to the inner Galaxy, which could also a↵ect the behaviour of the elemental 14 N/ 15 N ratio.To investigate the local e↵ect of chemistry in individual star-forming regions we have studied for the first time N-fractionation of N 2 H + at high angular resolution (⇠3", i.e. ⇠6200 au at a distance of 1.8 kpc).In particular, we have used IRAM NOEMA observations towards the massive star-formig region IRAS 05358+3543 ( [21]).

Local chemical fractionation effects
Figure 3 shows the first interferometric maps of the 15 N-isotopologues of N 2 H + at core scales (about 0.03 pc).From this work it appears that the 14 N/ 15 N ratio of N 2 H + is lower in the inner denser cores (⇠100-200) with respect to the more di↵use gas (>250).These results highlight the importance of local fractionation e↵ects, and in particular they demonstrates that isotope-selective photodissociation of N 2 should be introduced in chemical models to explain 15 N-enrichment in star-forming regions (e.g.[18]; [21]; [22]).
We have also obtained new NOEMA observations to study N-fractionation of the nitriles HCN and HNC towards the same source and with the same spatial resolution.The comparison of the 14 N/ 15 N ratio of N 2 H + with that of nitriles will allow a deeper insight of the local chemical processes a↵ecting isotopic ratios.

Figure 2 .
Figure 2. 14 N/15 N ratios for HNC (left panel) and HCN (right panel) as a function of the galactocentric distance (D GC ).The cyan solid line is the linear regression fit computed for the two data sets, the red parabola is the results from the parabolic analysis described in detail in[9], and the magenta solid line represents the GCE model of[19].

Figure 3 .
Figure 3. First interferometric maps of the J=1-0 transition of 15 NNH + (left panel) and N 15 NH + (right panel).The black contour levels are 3, 5 and 7 times the 1σ of the maps (0.5 and 0.72 mJy beam −1 , respectively).The blue contours correspond to the 5σ of the map, from which the 14 N/ 15 N ratios have been derived.The black and blue squares indicate the positions of the continuum sources and other N 2 H + peak positions, respectively.The dashed circle represents the NOEMA field of view and the synthesized beam is the ellipse indicated in the lower left corner.
Right panel: 14 N/15N ratios obtained for di↵erent molecules.Black points represent low-mass pre-stellar cores, the blue points are low-mass protostellar objects, and the cyan points indicate protoplanetary discs.The horizontal yellow and red solid lines represent the PSN value of 441 and the terrestrial atmosphere value of 272, respectively.The green horizontal line denotes the average value measured in comets.The pink area represents measurements in carbonaceous [1]ndrites, where the lower ones are the so-called "hot-spots".Adapted from[1].
[1]t panel: 15 N/14N vs D/H in comets, chondrites, hot spots in the Insoluble Organic Matter (IOM) of meteorites, Earth and PSN.Right panel: 14 N/15N ratios obtained for di↵erent molecules.Black points represent low-mass pre-stellar cores, the blue points are low-mass protostellar objects, and the cyan points indicate protoplanetary discs.The horizontal yellow and red solid lines represent the PSN value of 441 and the terrestrial atmosphere value of 272, respectively.The green horizontal line denotes the average value measured in comets.The pink area represents measurements in carbonaceous chondrites, where the lower ones are the so-called "hot-spots".Adapted from[1].