EDP Sciences
Free Access
Issue
A&A
Volume 603, July 2017
Article Number A22
Number of page(s) 11
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201630210
Published online 03 July 2017

© ESO, 2017

1. Introduction

For decades, our solar system was believed to have resided in a relatively isolated, low-mass molecular cloud core during its formation. However the detection of short-lived radioactive species in meteorites have suggested a different scenario in which the birthplace of the Sun may have been a massive cluster affected by a supernova event (Adams 2010; Dukes & Krumholz 2012; Pfalzner 2013; Nicholson & Parker 2017). If so, the initial chemical composition of our solar system, and thus of planets, meteorites and comets, may have been affected by the same physical process.

Measurements of the abundance isotopic ratios of the elements can be used to unveil the initial chemical composition of the protosolar nebulae (PSN) from which the solar system formed. The isotopic ratios of carbon (12C/13C) and oxygen (16O/18O) show a remarkable agreement among cometary materials, the local interstellar medium (ISM), and the solar value (Manfroid et al. 2009; Milam et al. 2005; Wilson & Rood 1994). Nitrogen, by contrast, has a peculiar behaviour since its 14N/15N isotopic ratio exhibits discrepancies across various environments within the solar system. The 14N/15N ratio measured in Jupiter’s atmosphere (450 ± 100, Fouchet et al. 2004) is considered as the most representative value of the PSN and matches the present day solar wind value (441 ± 6, Marty et al. 2010). However, the ratios measured in the terrestrial atmosphere (TA; ~272 in Earth, Junk & Svec 1958; 272 ± 54 in Venus, Hoffman et al. 1979; 173 ± 11 in Mars, Wong et al. 2013), comets (147.8 ± 5.7, Manfroid et al. 2009, 139 ± 26 from HCN and 165 ± 40 from CN, Bockelée-Morvan et al. 2008), interplanetary cust particles (or IDPs; values of 180305; Floss et al. 2006) and meteorites (192291, Alexander et al. 2007), are lower than those measured in Jupiter’s atmosphere.

In molecular clouds, the discrepancies in the 14N/15N isotopic ratios spread over a larger range. In contrast to many molecular species (e.g. CO), N-bearing molecules do not suffer significant freeze-out onto grains in the coldest, densest regions of IRDCs, and are therefore reliable tracers of the gas chemistry and kinematics in cores. The nitrogen fractionation mechanisms are either due to chemical fractionation (Terzieva & Herbst 2000; Rodgers & Charnley 2008; Wirström et al. 2012; Hily-Blant et al. 2013) or a selective photodissociation effect (Lyons et al. 2009; Heays et al. 2014). Since IRDCs are dense and highly extinguished (with visual extinctions >10 mag; Kainulainen & Tan 2013), selective photodissociation is not expected to play an important role because this process becomes inefficient at Av ≥ 3 mag (Heays et al. 2014). As for chemical fractionation, these tracers can be categorized into: 1) hydride-bearing molecules with an amine (-NH) functional group believed to have originated from reactions with N2; and 2) nitrile-bearing molecules with a nitrile (-CN) functional group that form via reactions with atomic N (Rodgers & Charnley 2008; Hily-Blant et al. 2013). Numerous measurements of the 14N/15N ratio exist towards low-mass prestellar cores (334 ± 50, 1000 ± 200 and 230 ± 90 from NH3, N2H+ and HCN, respectively; Lis et al. 2010; Bizzocchi et al. 2013; Hily-Blant et al. 2013) and protostars (~160290 from HCN and HNC; Wampfler et al. 2014), but observations of this ratio towards their massive counterparts are lacking.

A re-investigation of the fractionation processes of nitrogen in the ISM by Roueff et al. (2015) showed that nitrogen chemistry depends on the temperature and density of the primordial gas in the parental cloud. Since the Sun may have formed in a massive cluster, and low-mass and high-mass star-forming regions present gas temperatures and densities that differ by ~510 K and by factors of 10 (Pillai et al. 2006; Crapsi et al. 2007; Henshaw et al. 2013), measurements of the 14N/15N ratio in high-mass star-forming regions could provide insight into the initial bulk composition of the PSN.

Recently, Adande & Ziurys (2012) and Fontani et al. (2015) have measured the 14N/15N ratios towards a sample of high-mass star-forming regions. While in the Adande & Ziurys (2012) sample the 14N/15N ratios measured from CN and HNC lie between ~120–400, in the work of Fontani et al. (2015), these measurements range from ~180 to ~1300 in N2H+ and ~190 to ~450 in CN. In both studies, the 14N/15N ratios obtained from CN are comparable and fall between the TA and PSN values. However, only a few of these objects were prestellar in nature and larger samples of high-mass starless or prestellar cores are needed to measure the 14N/15N isotopic ratio in regions with physical conditions resembling those of the early stages of the solar system formation.

We present measurements of the 14N/15N isotopic ratio in HCN and HNC obtained towards a sample of 22 high-mass cold cores embedded in four IRDCs. These cores are believed to represent the nurseries of high-mass stars and star clusters and have physical properties (densities 104−106cm-3 and temperatures 20 K; Pillai et al. 2006; Butler & Tan 2012) similar to those expected for the initial conditions of the solar system. In Sect. 2, we describe the observations and data analysis. The results are presented in Sect. 3.1 whilst the uncertainties involved in our calculations are discussed in Sect. 3.2. In Sects. 4.14.3 we investigate the correlation of IRDC cores with star formation activity and compare our results with previous measurements of the 14N/15N isotopic ratio in solar system objects along with low-mass and high-mass star-forming regions. In Sects. 4.4 and 4.5, we discuss the effects of 13C depletion on the chemistry of nitrogen fractionation and the systematic trend observed in young and quiescent IRDCs with respect to more evolved, star-forming IRDCs. Our conclusions are presented in Sect. 5.

2. Observations

Observations of the J = 1 → 0 rotational transition of HCN, H13CN, HC15N, HN13C and H15NC were obtained with the IRAM 30 m telescope1 towards 22 massive cores embedded in IRDCs G028.37+00.07, G034.43+00.24, G034.77-00.55, and G035.39-00.33 (hereafter Clouds C, F, G and H respectively, as in Butler & Tan 2012). The frequencies of the transitions and molecular data are included in Table 1 whereas the properties of each IRDC are listed in Table 2. The EMIR receivers were tuned at 87 GHz and the FTS spectrometer provided a spectral resolution of 200 kHz (or ~0.68 km s-1). The HNC(J = 1 → 0) transition was not covered within our frequency range. Typical system temperatures ranged from 106 K to 199 K. The half-power beam width (HPBW) of the telescope was 28′′ at 87 GHz. The spectra were measured in units of antenna temperature, , and converted into main beam temperature, Tmb, using a beam efficiency of 0.81. Data reduction was carried out using the GILDAS/CLASS software package2. The spectra of HCN (J = 1 → 0), H13CN(J = 1 → 0), HC15N(J = 1 → 0), HN13C(J = 1 → 0) and H15NC(J = 1 → 0) were obtained towards all cores reported by Butler & Tan (2012) within Clouds F, G and H. For Cloud C, however, all cores were observed except C7 that laid outside our map. Thus C7 is not be considered in our analysis.

thumbnail Fig. 1

From left to right, top to bottom panels: emission lines of H15NC, HN13C, HC15N, and hyperfine transitions of H13CN observed with IRAM 30 m towards IRDCs C, F, G, and H. Red lines indicate the best Gaussian fit to the lines. The hyperfine components of H13CN were initially fitted but given the bad results of the fit; in a second step only the main F = 2 → 1 component of H13CN was fitted using a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details). See also Figs. A.1A.4 for all spectra taken.

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Table 1

Observed HCN and HNC isotopologue transitions.

Table 2

Properties of IRDCs.

3. Results

3.1. 14N/15N ratios derived from the 13C isotopologues of HCN and HNC

In Fig. 1, we show a sample of spectra of H13CN, HC15N, HN13C, and H15NC obtained towards one massive core in each IRDC; the rest of spectra are shown in the appendix in Figs. A.1A.4. We assume that the emission from these isotopologues is optically thin and consider LTE conditions when calculating their column densities. For the excitation temperature Tex of the molecular gas in these clouds, we assume a lower limit of 10 K and an upper limit of 20 K based on NH3 measurements obtained towards other IRDCs (Pillai et al. 2006). Because the hyperfine structure of HN13C cannot be resolved, we considered only one velocity component for this species, in the same way as for HC15N and H15NC. These lines were therefore fitted by using a single-component Gaussian profile. For H13CN, the hyperfine structure of the J = 1 → 0 transition could be resolved in the spectra and the HFS line fitting method implemented in CLASS was initially used to obtain the optical depth of the H13CN emission.However, the HFS fits presented large uncertainties (see the optical depth values in Table A.5) and therefore, in a second step, we fitted the main F = 2 → 1 component of H13CN with a single-Gaussian component profile, calculated the column densities of H13CN assuming optically thin emission, and corrected them by the statistical weight of the F = 2 → 1 transition. The measured integrated intensities, radial velocities, line widths and peak intensities of the lines are listed in Tables A.1A.4. We consider that a line is detected when its peak intensity is 3σ with σ the rms noise level measured in the spectra (see Col. 2 in Tables A.1A.4). For the cores with no molecular detections, we used the 3σ noise level as upper limits to their peak intensities. If we take into account the optical depths derived from the HFS method (of ~0.15.8), the resulting H13CN column densities and hence the 14N/15N ratios are increased by factors of 36. Nevertheless, these values consistent with the previously determined results within the uncertainties.

The corresponding 14N/15N ratios were computed from the molecular column densities of the 14N, 15N HCN, and HNC isotopologues after correcting by the 12C/13C ratio for each IRDC. This ratio is calculated with the Galactic 12C/13C gradient as a function of Galactocentric distance derived by Milam et al. (2005) from CN measurements. The 12C/13C ratios for IRDCs C, F, G, and H are 40.2, 46.8, 49.8 and 50.0, respectively (see Table 2). The molecular column densities together with the derived 14N/15N ratios at Tex = 15 K are listed in Table 3.

In our measurements, the uncertainties in the 14N/15N ratios were derived by propagating errors and using the 1σ uncertainties in the line integrated intensities calculated as rms , with Δv the average line width of the line for all cores with emission and with δv the velocity resolution of the spectrum (~0.68 km s-1). The derived uncertainties for the 14N/15N ratios are approximately ~35%. This may due to the weak detection of molecules in some of the cores. For the cores with no detections, the 14N/15N ratios were estimated using the 3σ upper limits to the integrated intensities of H15NC and HC15N (see Col. 3 in Tables A.1A.4), and they should be considered as lower limits.

From Table 3, we find that the 14N/15N ratios obtained towards Clouds C, F, G, and H vary over a large range of values. In particular, for IRDCs C, F, and H, the nitrogen ratios in HCN range between 122–571, 282–763, and 142–458, respectively, while in HNC they range between 161478 for Cloud C, 240–541 for Cloud F, and 234488 for Cloud H. On the other hand, the 14N/15N ratios measured towards the cores in Cloud G are systematically lower ranging between 70–181 in HCN and 206–237 in HNC. We tested the effects of Tex on our results, and found that if we use Tex = 10 K or Tex = 20 K instead of Tex = 15 K, the derived 14N/15N isotopic ratios for both HCN and HNC do not vary significantly, lying within the ~30% uncertainties. Higher Tex (e.g. 50 K and 100 K) also confirm this behaviour, with the 14N/15N ratios changing within a factor of 1.2. Nevertheless, Roueff et al. (2015) have recently pointed out that species such as HN13C and H13CN may suffer significant depletion in molecular clouds, challenging the interpretation of 14N/15N isotopic ratios derived from 13C containing isotopologues. In Sect. 3.2, we explore this possibility by directly measuring the 14N/15N ratios using HCN and its 15N isotopologue towards the IRDCs cores in our sample with optically thin HCN emission.

Table 3

Column densities and nitrogen ratios obtained from the 13C isotopologues of HCN and HNC.

3.2. 14N/15N ratios derived from HCN and its 15N isotopologue

In this section, we test whether the 14N/15N ratios derived in Sect. 3.1 are significantly affected by 13C depletion as proposed by the modelling of Roueff et al. (2015). We thus carried out direct measurements of the 14N/15N ratios with the J = 1 → 0 rotational transitions of HCN and HC15N, which were observed simultaneously within our frequency setup. HCN (J = 1 → 0) is optically thick in IRDC star-forming cores such as the cores in Clouds C and F, or core H1 in Cloud H. Therefore for this test we only use the IRDC cores within our sample that show optically thin or moderately optically thick emission (i.e. with τ ≲ 1−2). These cores are G1 and G3 in Cloud G, and H2, H3, H4, and H5 in Cloud H. The rms noise level, integrated intensity, central radial velocity, line width, peak intensity, and derived optical depth of the HCN (J = 1 → 0) lines, are shown in Table A.5 in the Appendix.

Following the same analysis procedures as for H13CN in Sect. 3.1, the 14N/15N ratios were calculated from the column densities of HCN and HC15N assuming optically thin emission, Tex = 15 K, and LTE conditions (see Table 4). For the HC15N non-detections, the upper limits to the column density of this molecule were estimated from the 3σ rms noise level in the HC15N spectra. The derived 14N/15N ratios range from 67 to 282. If we compare these values with those from column 6 in Table 3, we find that the 14N/15N ratios inferred from HCN are systematically lower (by factors 1.22.7) with respect to those obtained from H13CN. This is in contrast with the results from Roueff et al. (2015) since, from their models, the 12C/13C isotopic ratio measured from HCN should be a factor of ~2 higher than that derived from CN (i.e. at timescales 1 Myr for the typical densities of IRDC cores of ~105 cm-3; see Fig. 4 in their paper). This also holds if the 14N/15N ratios of the moderately optically thick cores are corrected by their HCN optical depths (with τ(HCN) = 0.711.84, which corresponds to correction factors of ~1.42.2). Except for core H5, the corrected values of the 14N/15N ratios for cores H2, H3, and H4 are 503, 445 and 168, respectively, which are consistent with those inferred from H13CN and lie within the uncertainties. Although our subsample of optically thin and moderately optically thick IRDC cores is small, this test suggests that the 14N/15N ratios obtained with the 13C containing isotopologues are not strongly affected by 13C depletion as proposed by the models of Roueff et al. (2015). In Sect. 4.5, we discuss the possible reasons for this.

Table 4

Column densities and nitrogen ratios obtained in HCN and its 12C isotopologue.

4. Discussion

4.1. Correlation with star formation activity

The chemistry of HCN and HNC is known to be temperature dependent (Pineau des Forets et al. 1990) and any star formation activity in the core could locally heat the molecular gas enhancing the abundance of HCN (and its isotopologues) over HNC. Therefore, it is important to investigate whether the measured 14N/15N isotopic ratios in HCN and HNC show any correlation with the level of star formation activity in the observed IRDC cores. For this purpose, we adopted the classification of the embedded cores in IRDCs C, F, G and H proposed by Chambers et al. (2009) and Rathborne et al. (2010). Each of the cores is classified as quiescent, intermediate or active based upon their colour in Spitzer/IRAC 38 μm images as well as the presence or absence of 24 μm point source emission. The summary of the classification of each core is listed in Table 5.

We report the column densities of the 15N isotopologues against those of the 14N species in relation to their star formation classification in Fig. 2. This figure shows that there is no correlation between the column densities of the HCN or HNC isotopologues with the level of star formation activity in the IRDC cores. Such a conclusion is also confirmed by plotting the column densities of HC15N against that of H15NC. In other words, the measurements of 14N/15N ratio toward IRDCs C, F, G, and H from the J = 1 → 0 transitions of HCN and HNC indeed probe the chemical composition of the envelope of these IRDCs cores. As such, it is in general not affected by local star formation feedback, although the highest 15N/14N ratio is found towards one of the active cores. Therefore, we cannot rule out that higher-J transitions and higher angular resolution observations give 15N/14N ratios that are correlated with star formation activity.

Table 5

Summary of IRDC cores classification.

thumbnail Fig. 2

Column densities of HCN (top panel) and HNC (bottom panel) 15N isotopologues plotted against those of the 14N species for all the cores in the sample. The cores are classified as active (red), intermediate (green), and quiescent (blue). Black indicates cores with no known classification. Different symbols are used to denote different clouds: IRDC C (circle), IRDC F(star), IRDC G (square), and IRDC H (triangle). The straight lines indicate the lowest and highest value of the corresponding 14N/15N ratio for each molecule.

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4.2. Comparison with solar system objects and low-mass star-forming regions

To understand whether IRDC cores have a nitrogen chemical composition that is consistent with that of the birthplace of the solar system, it is essential to compare the results obtained towards our sample of IRDC cores with those measured in solar system objects (see Fig. 3). For completeness, Fig. 3 also reports the 14N/15N ratios obtained towards low-mass prestellar or starless and star-forming cores. Overall, the 14N/15N ratios from Clouds C, F, and H show values that are similar to those observed in the Sun, planets, and prestellar or star-forming regions and consistent with the TA and the PSN values. On the other hand, the 14N/15N ratios measured towards Cloud G lie mostly at the level of the TA value and are significantly lower than the PSN value in Fig. 3. The results in Cloud G are also in agreement with the measurements obtained in comets, IDPs, meteorites, and especially protoplanetary disks (80160) as recently measured by Guzmán et al. (2017). Furthermore, they marginally agree with the lower end of the ratios derived in starless or prestellar and star-forming cores. We also caution that half of the 14N/15N ratios in Cloud G are lower limits; HC15N has not been detected in cores G2 and G3 and H15NC has not been detected in core G1 (see Tables A.2 and A.4). Therefore we may be lacking enough statistics to draw a firm conclusion.

4.3. Comparison with high-mass star-forming regions

The comparison with the measurements from Adande & Ziurys (2012) and Fontani et al. (2015) towards high-mass star-forming regions, shows that our measurements are consistent with their results as a whole. Especially, the 14N/15N ratios from HNC in IRDCs almost lie in the same range as those measured by Adande & Ziurys (2012) in CN and HNC and by Fontani et al. (2015) in CN. The 14N/15N ratios from HCN are also compatible with the results measured by Fontani et al. (2015) in N2H+ emission. Since Fontani et al. (2015) also measured the 14N/15N ratio towards cores C1, F1, F2, and G2 included in our sample, we compared the results between each individual core and they show some discrepancies as a result. Indeed, the 14N/15N ratios obtained by Fontani et al. (2015) in N15NH+ and 15NNH+ (CN was not detected towards these cores) are ~1445 and ~1217 for C1, ~672 and ~566 for F1, and ~872 and ~856 for G2, respectively, i.e. overall significantly higher than those measured in this work. In contrast, F2 shows a lower value of ~232 and 195 in 15NNH+ and N15NH+, respectively. These large discrepancies have also been found in low-mass prestellar cores and could be associated with the different chemistries involved in the formation of N2H+ and HNC/HCN (Wirström et al. 2012; Hily-Blant et al. 2013; Bizzocchi et al. 2013). More recently, cores C1, C3, F1, F2 and G2 have been studied independently by Colzi et al. (2017) using isotopologues of HCN and HNC. A similar range for the 14N/15N ratios has been found in HCN (150748) whilst results in HNC lie in a slightly higher range (263813). In both samples, core G2 shows one of the smallest ratios in HCN and HNC.

4.4. 13C depletion and its effects on nitrogen fractionation

We evaluated whether the depletion of 13C for species such (as HCN and HNC as predicted by the models of Roueff et al. 2015) could affect our derived values of the 14N/15N ratios in Sect. 3.2. Our test revealed that the 14N/15N values inferred from HCN are either consistent, or lower, than those measured from the 13C isotopologue, in contrast to the modelling predictions. This could be due to two reasons: first, the timescales (age) of IRDC cores and, second the kinetic temperature of the gas within them.

Regarding the timescales, Kong et al. (2017) have modelled the chemistry of deuterated species such as N2D+ in IRDC cores to provide constraints to the dynamical age of these cores. Their modelling shows that the enhanced D/H ratio in these objects can be reproduced for timescales of ~105 yr; these authors model the N2D+ emission arising from the C1 core in Cloud C. On the other hand, Roueff et al. (2015) predict similar 12C/13C ratios associated with CN and with HCN/HNC at these timescales (of ~105 yr) for the typical H2 gas densities of IRDC cores (of ~105 cm-3; see Butler & Tan 2012). The large differences in 12C/13C ratios associated with HCN/HNC, such as those discussed in Sect. 3.2, would therefore not be expected.

Nevertheless, the definition of a core formation timescale is somewhat ambiguous, for example Barnes et al. (2016) found that the D/H fraction within Cloud F would have taken several 106 yr to form. In light of this, a more self-consistent comparison between timescales inferred by chemical models is required.

Concerning the gas temperature of IRDC cores, measurements of the emission of NH3 towards these cores give kinetic temperatures of the gas of 1520 K (Pillai et al. 2006), which are higher than those assumed in the models of Roueff et al. (2015, of 10 K). Since carbon depletion is strongly dependent on gas/dust temperature, it is unclear whether these results can be compared directly to IRDC cores; there are no models are provided for temperatures higher than 10 K. Therefore, additional modelling is needed to test the effects of 13C depletion in the chemistry of nitrogen fractionation at slightly higher temperatures similar to those found in IRDCs.

4.5. Systematic trend of 14N/15N ratio between IRDCs

Table 3 and Fig. 3 show that the 14N/15N ratios observed in Cloud G are systematically lower than those measured in Clouds C, F and H. This may be due to the properties of Cloud G itself. As discussed in Sect. 4.1, cores G1, G2 and G3 do not show any trace of star formation activity, whilst the other three IRDCs show several cores that are actively forming stars (see e.g. cores C2 or H2). In addition, Cloud G is the least massive, the most diffuse (it has the weakest emission in high-density tracers; Cosentino et al., in prep.), and has the lowest peak H2 mass surface density amongst the four targeted IRDCs (see Table 2 and Butler & Tan 2012). Given that the kinetic temperature of the gas is similar across IRDCs (~1520 K), we propose that density could be one of the important parameters that is responsible for the discrepancies found between Cloud G and the other IRDCs, although the models do not agree with this scenario. Therefore, we speculate that the PSN may have formed in an IRDC with properties similar to those of cloud G. However, the properties of this sample of IRDC cores need to be further investigated along with relevant chemical models to confirm the proposed idea.

thumbnail Fig. 3

Nitrogen isotope ratio variations measured in different sources starting from small solar system bodies (in red; Busemann et al. 2006; Floss et al. 2006; Alexander et al. 2007; Bockelée-Morvan et al. 2008; Manfroid et al. 2009; Mumma & Charnley 2011; Guzmán et al. 2017) to planets (in black; Junk & Svec 1958; Hoffman et al. 1979; Fouchet et al. 2004; Niemann et al. 2005; Marty et al. 2010; Wong et al. 2013; Fletcher et al. 2014), low-mass prestellar cores (in yellow; Gerin et al. 2009; Bizzocchi et al. 2010; Lis et al. 2010; Bizzocchi et al. 2013; Hily-Blant et al. 2013), low-mass star-forming cores (in light grey; Wampfler et al. 2014) and high-mass star-forming cores (in blue; Adande & Ziurys 2012; Fontani et al. 2015). Our measurements in IRDCs are shown on the right. On the upper right corner, we show a representative error bar for our measurements of the 14N/15N ratio. Black arrows indicate the lower limits in our measurements. The 14N/15N ratio measured in HCN from core G2 is shown as zero with no lower limit indication owing to both of the column densities of H13CN and HC15N are only given as upper limits.

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5. Conclusions

We have measured the nitrogen isotopic ratio 14N/15N towards a sample of cold IRDC cores. This ratio ranges between ~70 and 763 in HCN and between ~161 and ~541 in HNC. In particular, Cloud G systematically shows lower nitrogen isotopic ratios than the other three clouds, where values are consistent with the ratio measured towards small solar system bodies such as comets, IDPs, meteorites and also proto-planetary disks. Since Cloud G shows lower overall gas densities, and since it likely is at an earliest stage of evolution, we propose that gas density is the key parameter in nitrogen fractionation in IRDCs. Higher angular resolution observations, as well as chemical modelling of the nitrogen fractionation of HCN and HNC at temperatures similar to those found in IRDCs, are needed to establish the origin of the discrepancies in the measured 14N/15N ratios found in Cloud G with respect to Clouds C, F and H. The comparison between the modeling predictions and our IRDC measurements may allow us to constrain the main chemical reactions involved in the fractionation process of nitrogen in the protosolar nebula.


1

Based on observations carried out under projects number 134-12 and 027-13 with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

Acknowledgments

We would like to thank Audrey Coutens and the anonymous referee for valuable comments to a previous version of the manuscript. I.J.-S. acknowledges the financial support received from the STFC through an Ernest Rutherford Fellowship (proposal number ST/L004801/2). P.C. acknowledges financial support of the European Research Council (ERC; project PALs 320620). The research leading to these results has also received funding from the European Commission (FP/20072013) under grant agreement No. 283393 (RadioNet3).

References

Appendix A: Spectra and fitting parameters of isotopologues of HCN and HNC

thumbnail Fig. A.1

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC C. The red line indicates the best Gaussian fit. The hyperfine components of H13CN were initially fitted but given the bad results of the fit, in a second step only the main F = 2 → 1 component of H13CN was fitted with a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details).

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thumbnail Fig. A.2

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC F. The red line indicates the best Gaussian fit. The hyperfine components of H13CN were initially fitted but given the bad results of the fit, in a second step only the main F = 2 → 1 component of H13CN was fitted with a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details).

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thumbnail Fig. A.3

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC G. Only the main component of H13CN was fitted with a single-Gaussian component profile, which was then used to calculate the column densities. The red line indicates the best Gaussian fit and these spectra were scaled to fit into each individual panel.

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thumbnail Fig. A.4

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC H. The red line indicates the best Gaussian fit. The hyperfine components of H13CN were initially fitted but given the bad results of the fit, in a second step only the main F = 2 → 1 component of H13CN was fitted with a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details).

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Table A.1

Root mean square noise in the spectra rms, integrated intensities Tmbdυ, radial velocity υ, line widths Δυ, and peak temperature Tpeak of HC15N(1 → 0) in cores within Clouds C, F, G, and H.

Table A.2

Root mean square noise in the spectra rms, integrated intensities Tmbdυ, radial velocity υ, line widths Δυ, and peak temperature Tpeak of HN13C(10) in cores within Clouds C, F, G, and H.

Table A.3

Root mean square noise in the spectra rms, integrated intensities Tmbdυ, radial velocity υ, line widths Δυ, and peak temperature Tpeak of H15NC(1 0) in cores within Clouds C, F, G, and H.

Table A.4

Root mean square noise in the spectra rms, integrated intensities , radial velocity υ, line widths Δυ, peak temperature Tpeak of the H13CN (1 → 0, F = 2 → 1) hyperfine component and optical depth τ of the H13CN(1 0) emission in cores within Clouds C, F, G, and H.

Table A.5

Root mean square noise in the spectra rms, integrated intensities , radial velocity υ, line widths Δυ, peak temperature Tpeak of the HCN (1 0, F = 2 → 1) hyperfine component and optical depth τ of the HCN(10) emission in cores within Clouds G and H.

All Tables

Table 1

Observed HCN and HNC isotopologue transitions.

Table 2

Properties of IRDCs.

Table 3

Column densities and nitrogen ratios obtained from the 13C isotopologues of HCN and HNC.

Table 4

Column densities and nitrogen ratios obtained in HCN and its 12C isotopologue.

Table 5

Summary of IRDC cores classification.

Table A.1

Root mean square noise in the spectra rms, integrated intensities Tmbdυ, radial velocity υ, line widths Δυ, and peak temperature Tpeak of HC15N(1 → 0) in cores within Clouds C, F, G, and H.

Table A.2

Root mean square noise in the spectra rms, integrated intensities Tmbdυ, radial velocity υ, line widths Δυ, and peak temperature Tpeak of HN13C(10) in cores within Clouds C, F, G, and H.

Table A.3

Root mean square noise in the spectra rms, integrated intensities Tmbdυ, radial velocity υ, line widths Δυ, and peak temperature Tpeak of H15NC(1 0) in cores within Clouds C, F, G, and H.

Table A.4

Root mean square noise in the spectra rms, integrated intensities , radial velocity υ, line widths Δυ, peak temperature Tpeak of the H13CN (1 → 0, F = 2 → 1) hyperfine component and optical depth τ of the H13CN(1 0) emission in cores within Clouds C, F, G, and H.

Table A.5

Root mean square noise in the spectra rms, integrated intensities , radial velocity υ, line widths Δυ, peak temperature Tpeak of the HCN (1 0, F = 2 → 1) hyperfine component and optical depth τ of the HCN(10) emission in cores within Clouds G and H.

All Figures

thumbnail Fig. 1

From left to right, top to bottom panels: emission lines of H15NC, HN13C, HC15N, and hyperfine transitions of H13CN observed with IRAM 30 m towards IRDCs C, F, G, and H. Red lines indicate the best Gaussian fit to the lines. The hyperfine components of H13CN were initially fitted but given the bad results of the fit; in a second step only the main F = 2 → 1 component of H13CN was fitted using a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details). See also Figs. A.1A.4 for all spectra taken.

Open with DEXTER
In the text
thumbnail Fig. 2

Column densities of HCN (top panel) and HNC (bottom panel) 15N isotopologues plotted against those of the 14N species for all the cores in the sample. The cores are classified as active (red), intermediate (green), and quiescent (blue). Black indicates cores with no known classification. Different symbols are used to denote different clouds: IRDC C (circle), IRDC F(star), IRDC G (square), and IRDC H (triangle). The straight lines indicate the lowest and highest value of the corresponding 14N/15N ratio for each molecule.

Open with DEXTER
In the text
thumbnail Fig. 3

Nitrogen isotope ratio variations measured in different sources starting from small solar system bodies (in red; Busemann et al. 2006; Floss et al. 2006; Alexander et al. 2007; Bockelée-Morvan et al. 2008; Manfroid et al. 2009; Mumma & Charnley 2011; Guzmán et al. 2017) to planets (in black; Junk & Svec 1958; Hoffman et al. 1979; Fouchet et al. 2004; Niemann et al. 2005; Marty et al. 2010; Wong et al. 2013; Fletcher et al. 2014), low-mass prestellar cores (in yellow; Gerin et al. 2009; Bizzocchi et al. 2010; Lis et al. 2010; Bizzocchi et al. 2013; Hily-Blant et al. 2013), low-mass star-forming cores (in light grey; Wampfler et al. 2014) and high-mass star-forming cores (in blue; Adande & Ziurys 2012; Fontani et al. 2015). Our measurements in IRDCs are shown on the right. On the upper right corner, we show a representative error bar for our measurements of the 14N/15N ratio. Black arrows indicate the lower limits in our measurements. The 14N/15N ratio measured in HCN from core G2 is shown as zero with no lower limit indication owing to both of the column densities of H13CN and HC15N are only given as upper limits.

Open with DEXTER
In the text
thumbnail Fig. A.1

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC C. The red line indicates the best Gaussian fit. The hyperfine components of H13CN were initially fitted but given the bad results of the fit, in a second step only the main F = 2 → 1 component of H13CN was fitted with a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details).

Open with DEXTER
In the text
thumbnail Fig. A.2

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC F. The red line indicates the best Gaussian fit. The hyperfine components of H13CN were initially fitted but given the bad results of the fit, in a second step only the main F = 2 → 1 component of H13CN was fitted with a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details).

Open with DEXTER
In the text
thumbnail Fig. A.3

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC G. Only the main component of H13CN was fitted with a single-Gaussian component profile, which was then used to calculate the column densities. The red line indicates the best Gaussian fit and these spectra were scaled to fit into each individual panel.

Open with DEXTER
In the text
thumbnail Fig. A.4

Spectra of HC15N, HN13C, H15NC, and H13CN observed with IRAM 30 m towards IRDC H. The red line indicates the best Gaussian fit. The hyperfine components of H13CN were initially fitted but given the bad results of the fit, in a second step only the main F = 2 → 1 component of H13CN was fitted with a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details).

Open with DEXTER
In the text

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