The Astrophysical Journal, 744:194 (15pp), 2012 January 10
C 2012.
doi:10.1088/0004-637X/744/2/194
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
MILLIMETER-WAVE OBSERVATIONS OF CN AND HNC AND THEIR 15 N ISOTOPOLOGUES: A NEW
EVALUATION OF THE 14 N/15 N RATIO ACROSS THE GALAXY
1
G. R. Adande1,2,3 and L. M. Ziurys1,2,3,4
Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
2 Department of Chemistry, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
3 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
4 Arizona Radio Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
Received 2011 September 3; accepted 2011 October 21; published 2011 December 22
ABSTRACT
The N = 1 → 0 transitions of CN and C15 N (X2 Σ + ), as well as the J = 1 → 0 lines of HN13 C and H15 NC, have been
observed toward 11 molecular clouds using the new 3 mm ALMA-type receiver of the 12 m telescope of the Arizona
Radio Observatory. These sources span a wide range of distances from the Galactic center and are all regions of star
formation. From these observations, 14 N/15 N ratios have been determined using two independent methods. First,
the measurements of C14 N and C15 N were directly compared to establish this ratio, correcting for high opacities
when needed, as indicated by the nitrogen hyperfine intensities. Second, the ratio was calculated from the quantity
[HN13 C]/[H15 NC], determined from the HNC data, and then scaled by 12 C/13 C ratios previously established, i.e.,
the so-called double isotope method. Values from both methods are in reasonable agreement, and fall in the range
∼120–400, somewhat lower than previous 14 N/15 N ratios derived from HCN. The ratios exhibit a distinct positive
gradient with distance from the Galactic center, following the relationship 14 N/15 N = 21.1 (5.2) kpc−1 DGC +
123.8 (37.1). This gradient is consistent with predictions of Galactic chemical evolution models in which 15 N has a
secondary origin in novae, while primary and secondary sources exist for 14 N. The local interstellar medium value
was found to be 4 N/15 N = 290 ± 40, in agreement with the ratio found in nearby diffuse clouds and close to the
value of 272 found in Earth’s atmosphere.
Key words: astrochemistry – Galaxy: abundances – Galaxy: evolution – ISM: clouds – ISM: molecules – local
interstellar matter
of 14 N, i.e., synthesized starting from H and He in a given star,
while the CNO cycle is a secondary source. In contrast, it is
thought that 15 N can only be produced through the hot CNO
cycle, where elevated temperatures create large quantities of
15
O. This nucleus subsequently decays to 15 N, resulting in a far
greater abundance for this isotope than in the cold version of the
cycle (Audouze et al. 1975; Clayton 2003). It is also speculated
that 15 N is additionally made in Type Ia and Type II supernovae
(Romano & Matteucci 2003). Therefore, 15 N is principally a
secondary element.
The differences in primary and secondary origins of the two
nitrogen isotopes are predicted to lead to an increase in the
14
N/15 N ratio with galactocentric distance; such a gradient has
been calculated by some GCE models (Tosi 1982; Romano &
Matteucci 2003). However, measurements of this ratio across
the Galaxy, based on radio and millimeter molecular line
observations, have been somewhat contradictory. Early work
by Wannier et al. (1981) and Güsten & Ungerechts (1985)
found larger ratios near the galactic center (14 N/15 N ∼ 1000)
than in the local ISM, based on HCN and NH3 observations in
molecular clouds; their data showed evidence for a decreasing
14
N/15 N ratio with distance from the Galactic center (DGC ).
In contrast, a more recent study of HCN lines by Dahmen
et al. (1995) found a ratio that clearly increased with increasing
galactocentric distance, with values of ∼300 at DGC = 0. These
authors employed observations of H13 CN and HC15 N to derive
their values, scaled by known 12 C/13 C ratios (Wilson 1999).
Another controversial issue is the interpretation of the terrestrial value of the 14 N/15 N ratio, relative to the other solar system bodies and the local ISM (Anders & Grevesse
1989). In Earth’s atmosphere, the ratio from N2 is known to be
1. INTRODUCTION
The 14 N/15 N isotope ratio in the Galaxy remains relatively
uncertain, although it is critical in understanding Galactic
chemical evolution (GCE) and the origin of the solar system
(Audouze et al. 1975; Wilson 1999). Like other elemental
ratios, 14 N/15 N is believed to be a good indicator of stellar
nucleosynthesis and the mixing that subsequently occurs. It
is thought that both nitrogen isotopes are mainly by-products
from the CNO cycle, which occurs in more massive (M >
1 M ) stars, and converts hydrogen into helium. The details
of 14 N and 15 N production are still debated, however (Clayton
2003; Prantzos 2003; Romano & Matteucci 2003; Wiescher
et al. 2010). It is postulated that 14 N is produced by both
the cold and hot CNO cycles, as well as in so-called Hot
Bottom Burning (HBB) in asymptotic giant branch (AGB) stars
(Wiescher et al. 2010), although massive, rotating stars may
be sources as well (Prantzos 2011). In the cold CNO cycle,
which takes place in main-sequence stars and the H-burning
shells of red giants, 14 N is formed from either 13 C or 17 O.
Its subsequent destruction to 15 O occurs sufficiently slowly
that this isotope has a large equilibrium abundance. Dredgeup on the red giant branch brings the 14 N to the stellar surface,
where it can enter the interstellar medium (ISM) via mass loss.
14
N is produced by similar reactions in the hot CNO cycle,
which occurs principally in novae outbursts and has a more
complex sub-cycle structure (Sackmann et al. 1974; Clayton
2003; Wiescher et al. 2010). In HBB, 14 N is created via the
CNO cycle at the base of the AGB convective envelope from 12 C
that has been dredged-up from the He-burning shell (Sackmann
et al. 1974; Prantzos 2011). HBB is considered a primary source
1
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
N/ N = 272. Furthermore, analysis of N ions from the solar wind has resulted in 14 N/15 N ∼ 200 ± 55 (Kallenbach et al.
1998), while that derived from lunar soil samples, associated
with the solar wind, has indicated an upper limit of 14 N/15 N 205 (Hashizume et al. 2000). Yet, in the atmosphere of Jupiter,
in situ measurements of NH3 have yielded 14 N/15 N ∼ 435
(Owen et al. 2001) and IR observations from ISO-SWS suggest
14
N/15 N ∼ 526 (Fouchet et al. 2000). Carbonaceous chondrites
and interplanetary dust particles (IDPs) also show wide variations in the value of this ratio, ranging from approximately 54 to
500 (Bonal et al. 2010; Shepton et al. 2003; Aleon et al. 2003).
Large deviations have been observed in comets, from as high as
330 ± 98 (Ziurys et al. 1999) and 323 ± 46 (Jewitt et al. 1997),
to lower values such as 140 ± 30 (Arpigny et al. 2003). Local
ISM ratios are equally diverse. Toward cold dark clouds with
DGC ∼ 8 kpc, for example, observations of NH3 and its isotopologues have yielded 14 N/15 N ∼ 350 ± 50 (Lis et al. 2010)
and ∼360 ± 180 to ∼800 ± 425 (Gerin et al. 2009). Finally,
measurements of absorption lines of HCN and HNC toward local diffuse clouds lead to a ratio of 237 ± 25 (Lucas & Liszt
1998). From the Galactic gradient established by Dahmen et al.,
the local ISM value is ∼450. Based on large deuterium enrichments (Meier & Owen 1999; Messenger 2002; Busemann et al.
2006), the composition of IDPs and carbonaceous chondrites is
believed to at least partially reflect that of the pre-solar nebula.
Comets are also thought to be composed of pristine material
from the nascent molecular cloud that formed the solar system
(Irvine et al. 2000). It might be expected that the 14 N/15 N ratios
found in these objects would reflect that of the local ISM.
Obtaining accurate values of the 14 N/15 N ratio in interstellar
gas is challenging, however. The main difficulty is that the common nitrogen bearing molecules like HCN are usually optically
thick in their lowest rotational transitions; therefore, line intensities are not good indicators of molecular abundance, which
makes determination of the isotope ratio problematic. Furthermore, emission from the 15 N isotopologue is usually quite weak,
requiring very sensitive receivers and long integration times. If
the less abundant 13 C carrier is chosen instead of the 12 C species
(e.g., H13 CN), the method used by both Dahmen et al. and
Wannier et al., the optical depth problem is resolved, but the
determination of the 14 N/15 N ratio then relies on that measured
for 12 C/13 C. This indirect technique is not ideal, especially considering the variations in the carbon isotope ratio with molecular
tracer. Values derived from CN or CO, for example, tend to be
lower than those derived from H2 CO (Milam et al. 2005).
In order to further investigate 14 N/15 N ratios in interstellar
clouds, we have conducted millimeter observations of the N =
1 → 0 transition of C14 N and C15 N at 113 and 110 GHz
using the 12 m telescope of the Arizona Radio Observatory
(ARO) toward 12 dense molecular clouds, located at varying
distance from the galactic center. The hyperfine splitting from
the coupling of the nitrogen nuclear spin with the total angular
momentum of the molecule is observable in CN, which allows
an accurate evaluation of the optical depth (Savage et al. 2002).
Consequently, a more direct estimation of the 14 N/15 N ratio can
be made. For comparison, we also observed the J = 1 → 0
transitions of HNC, H15 NC, and HN13 C at 90, 88, and 87 GHz,
and derived ratios from the latter two isotopologues, using
12
C/13 C ratios previously measured in those sources by Milam
et al. (2005). In this paper, we present our observations, data
analysis, and our best estimates of the 14 N/15 N ratio and its
gradient in the Galaxy. We also compare our values with recent
GCE models.
14
15
+
Figure 1. Energy level diagram of the N = 1 → 0, J = 3/2 → 1/2 transition of
CN (a) near 113.5 GHz and C15 N (b) at 110.024 GHz. The solid arrows show
the hyperfine components observed in this work toward molecular clouds, while
the dashed ones indicate other allowed transitions. The energy level differences
are not shown to scale.
2. QUANTUM MECHANICS OF THE
CN RADICAL (X2 Σ + )
CN is a free radical with one unpaired electron in a sigma
orbital, leading to a 2 Σ + electronic ground state. It is best described by the Hund’s case (b) coupling scheme. In this scenario,
the spin angular momentum S, originating with the unpaired
electron, couples with the rotational angular momentum N to
generate the total angular momentum J, excluding nuclear spin,
i.e., J = N + S. Because S = 1/2, this coupling gives rise to
two fine-structure sub-levels for every rotational level except
N = 0. As a consequence, the fundamental N = 1 → 0 transition consists of two fine structure doublets, J = 1/2 → 1/2
and J = 3/2 → 1/2. This pattern is further split into hyperfine
structure by the interaction of the nitrogen nuclei spin I, where
F = I + J, and I = 1 for 14 N. The selection rules for electric
dipole-allowed rotational transitions are ΔN = ±1, ΔJ = 0, ±1,
ΔF = 0, ±1 (except 0 ↔ 0), such that the N = 1 → 0, J =
3/2 → 1/2 transition gives rise to five hyperfine components,
as shown in Table 1. The components are fairly well separated
in frequency, with one that is particularly strong with 33% of the
total intensity in the N = 1 → 0 transition. The C15 N spectrum
displays a very similar pattern, except the nuclear spin of 15 N is
I = 1/2. Therefore, the N = 1 → 0, J = 3/2 → 1/2 transition
exhibits three hyperfine components instead of five; see Figure 1
and Table 1. The two stronger components have about a 1 MHz
frequency separation, or 2.7 km s−1 . A schematic energy level
diagram of both species is shown in Figure 1.
HNC has a 1 Σ + ground state, and thus the quantum number
J describes the rotational pattern. The rotational levels are
additionally split by a very small amount due to nitrogen
quadrupole interactions (14 N only), and nuclear–spin-rotation
coupling (14 N and 15 N). These interactions generate three
2
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
Table 1
Line Parameters for CN (X2 Σ + ) and HNC(X1 Σ + )
Species
Transition
Hyperfine
Component
Frequency
(MHz)
Hyperfine Relative
Intensity
ηC
CNa
N=1→0
J = 3/2 → 1/2
N=1→0
J = 3/2 → 1/2
113488.14
113490.98
113499.64
113508.93
113520.41
110024.59
110023.54
110004.09
90663.57
87090.82
88865.71
0.12
0.33
0.099
0.099
0.012
0.417
0.165
0.085
0.85
C15 Nb
F = 3/2 → 1/2
5/2 → 3/2
1/2 → 1/2
3/2 → 3/2
1/2 → 3/2
F=2→1
1→0
1→1
H14 N12 Cc
HN13 Cc
H15 NCd
J=1→0
J=1→0
J=1→0
0.85
0.88
0.88
0.88
Notes.
a Rest frequencies from Skatrud et al. (1983).
b Rest frequencies from Saleck et al. (1994).
c Rest frequencies from Creswell et al. (1976).
d Rest frequencies from Brown et al. (1977).
hyperfine components for the J = 1 → 0 transition. For
both H14 NC and H15 NC, however, the splitting is quite small
(∼100 kHz or less; Bechtel et al. 2006) and not resolved in
spectra from warm molecular clouds.
4. RESULTS
The spectral measurements of CN, C15 N, HN13 C, and H15 NC
are summarized in Table 2 as well as those of HNC. Individual
line intensities (in TR ∗ (K)), linewidths (FWHM, in km s−1 ), and
LSR velocities are listed for all observed features, including
the individual hyperfine components of the CN isotopologues.
As the table shows, all five species were detected in 10 warm
molecular clouds. Only CN observations were conducted toward
the remaining two sources, SgrB2(OH) and Barnard-1, while
the two HNC isotopologues were measured in SgrB2(NW).
The source structure in SgrB2 is complex, and some species
were more suitable for these measurements than others. The
CN data were taken in Barnard-1 for comparison with previous
NH3 observations (Lis et al. 2010). The linewidths and LSR
velocities measured for both the CN and HNC species are typical
for these sources and agree between isotopologues within the
uncertainties (see Table 2). The spectra for 12 C14 N also agree
well with previous measurements by Savage et al. (2002) and
Milam et al. (2005).
Spectra of CN and C15 N for all sources are presented in
Figures 2(a)–(c), and the HNC data in Figures 2(d)–(g). In
the case of the main isotopologue, 12 C14 N, all five hyperfine
transitions were detected in almost all sources, as is apparent
in the figure. For most sources, the five lines were individually
resolved, except in SgrB2, W31, and W51M, where the F =
3/2 → 1/2 and F = 5/2 → 3/2 components are blended
together (see Figures 2(a) and (c)). The spectra also show that
for more than half the sources, the N = 1 → 0 transition appears
to be optically thick. Under optically thin conditions, the relative
intensities of the C14 N hyperfine lines follow the pattern: F =
5/2 → 3/2: 3/2 → 1/2: 1/2 → 1/2: 3/2 → 3/2:1/2 → 3/2 =
0.33:0.12:0.099:0.099:0.012 (see Table 1 and the LTE patterns
shown under the data). Only spectra from M17-SW, Orion-KL,
and Orion Bar display LTE intensities. In Barnard-1, where
the kinetic temperature of the cloud is about 12 K (Bachiller
et al. 1990), the hyperfine pattern departs completely from LTE,
with the F = 3/2 → 3/2 component being the strongest (see
Figure 2(c)). In SgrB2 (OH), contamination by features from
hot core molecules distorts the expected hyperfine pattern. For
example, the F = 1/2 → 1/2 line is likely blended with the
JKa,Kc = 103,7 → 92,8 transition of C2 H5 CN and the 212,19 →
3. OBSERVATIONS
The measurements were carried out in several observing
sessions between 2009 March and 2011 January. Observations
at 3 mm were performed using the ARO 12 m telescope at Kitt
Peak, Arizona. A dual polarization 3 mm (86–116 GHz) receiver
using ALMA Band 3 sideband separating (SBS) mixers was
employed for the observations. Image rejection was typically
16 dB, inherent in the mixer architecture. At the 12 m,
the temperature scale is determined by the chopper wheel
method, corrected for forward spillover losses, and given as
TR∗ . The radiation temperature is then defined as TR = TR∗ /ηc ,
where ηc is the corrected beam efficiency, estimated to be 0.88
and 0.85 at 87–90 GHz and 110–112 GHz, respectively; see
Table 1. Data were taken in position-switching mode, with
a 30 arcmin OFF position in azimuth. Pointing was checked
every 1.5 hr on a nearby planet or strong continuum source.
Toward most clouds observed, the backends employed were
filter banks with 500 kHz and 1000 kHz resolutions, each
possessing 256 channels operated in parallel mode (2 × 128);
for SgrB2 and S156, the 2 MHz and 250 kHz filter banks were
used, respectively. A millimeter autocorrelator was also used as
an additional backend, with either 390 or 195 kHz resolution.
Local oscillator shifts of 10 or 20 MHz were performed in each
source to ensure no image contamination was present near the
lines of interest.
The J = 1 → 0 transition was measured for HNC, HN13 C, and
15
H NC; see Table 1 for the list of frequencies. The five hyperfine
components of the N = 1 → 0, J = 3/2 → 1/2 transition of
C14 N were observed simultaneously within a given backend,
with the receiver tuned to the central frequency of 113.500 GHz.
Similarly, three hyperfine lines of C15 N were simultaneously
measured, with the receiver tuned to 110.023 GHz. A total of 12
sources were observed, spanning most of the Galaxy out to DGC
∼11 kpc. Table 1 summarizes the relevant molecular transitions,
rest frequencies, and telescope parameters. The list of sources
with their galactic coordinates can be found in Table 2.
3
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
Table 2
Observations of CN, HNC, and Isotopologuesa
Source
α
(B1950.0)
β
(B1950.0)
Transition
Hyperfine
Component
TR∗
(K)
Δv 1/2
(km s−1 )
VLSR
(km s−1 )
SgrB2(OH)b
17:44:11.0
−28:22:30.0
CN
N=1→0
J = 3/2 → 1/2
F = 3/2 → 1/2c
F = 5/2 → 3/2c
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
...
0.96(3)
0.36(3)
0.08(3)
0.06(3)
0.020(8)
...
25.9(2.6)
10.9(2.6)
13.6(2.6)
14.5(2.6)
14.8(2.7)
...
...
...
50.1(5.3)
...
56.8(5.3)
...
0.66(3)
0.42(3)
0.072(5)
0.083(5)
...
10.2(3.4)
13.8(3.4)
13.5(3.4)
10.2(3.4)
...
50.0(3.4)
71.5(3.4)
53.5(3.4)
70.4(3.4)
...
2.82(2)
0.56(2)
1.04(2)
0.19(2)
0.030(5)
...
7.9(2.6)
6.0(2.6)
7.2(2.6)
6.0(2.6)
7.5 (2.6)
...
−2.0(2.6)
−3.0(2.6)
−3.6(2.6)
−3.5(2.6)
−2.2(2.6)
4.64(3)
0.30(3)
0.045(8)
6.6(1.7)
8.6(1.7)
6.8(1.7)
−2.1(1.7)
−2.6(1.7)
−2.6(1.7)
1.59(4)
3.33(4)
0.62(4)
1.15(4)
0.12(4)
0.050(6)
4.0(1.3)
4.6(1.3)
5.3(1.3)
4.0(1.3)
5.3(1.3)
5.4(1.4)
57.7(1.3)
57.3(1.3)
56.7(1.3)
56.9(1.3)
58.3(1.3)
58.2(1.4)
4.95(5)
0.41(4)
0.08(1)
5.4 (1.7)
5.2 (1.7)
4.3 (1.7)
57.2(1.7)
58.0(1.7)
58.0(1.7)
2.23(2)
5.68(2)
1.20(2)
1.60(2)
0.25(2)
0.055(8)
4.5(1.3)
4.6(1.3)
4.0(1.3)
4.5(1.3)
4.5(1.3)
4.1(1.4)
19.7(1.3)
19.9(1.3)
19.5(1.3)
19.5(1.3)
19.7(1.3)
20.1(1.4)
3.7(5)
0.39(2)
0.11(3)
5.1(3.4)
5.2 (1.7)
5.1 (1.7)
20.3(3.4)
20.3(1.7)
19.6(1.7)
C15 N
N=1→0
SgrB2(NW)b
17:44:6.6
−28: 21:20
HNC
J=1→0
HN13 Cd J = 1 → 0
H15 NCd
W31
18:07:30.3
−19:56:38.0
J=1→0
CNb
N=1→0
J = 3/2 → 1/2
C15 Nb
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
G34.3
18:50:46.4
1:11:14.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
M17-SW
18:17:31.0
−16:13:00.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNCb J = 1 → 0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
W51Mb
19:21:26.2
14:24:43.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
DR-21
20:37:14.3
42:09:00.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
4
F = 3/2 → 1/2c
F = 5/2 → 3/2c
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2c
F = 5/2 → 3/2c
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
...
2.72(3)
0.67(3)
0.92(3)
0.11(3)
0.029(7)
...
3.83 (3)
0.28(2)
0.046 (6)
...
10.6(2.6)
11.9(2.6)
10.6(2.6)
11.9(2.6)
13.6(2.7)
...
11.9 (3.4)
13.6 (3.4)
13.5 (3.4)
...
57.5(2.6)
57.7(2.6)
57.5(2.6)
56.9(2.6)
56.7(2.7)
...
57.7(3.4)
57.8(3.4)
58.7(3.4)
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
1.54(3)
2.63(3)
1.00(3)
1.41(3)
0.34(3)
0.051(7)
4.5(1.3)
5.3(1.3)
4.5(1.3)
4.5(1.3)
4.5(1.3)
5.3 (1.4)
−2.6(1.3)
−2.1(1.3)
−2.4(1.3)
−2.4(1.3)
−2.7(1.3)
−1.9(1.4)
3.55(6)
0.53 (4)
0.12(2)
5.0 (1.7)
4.3 (1.7)
4.2(1.7)
−2.1 (1.7)
−2.2(1.7)
−2.4(1.7)
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
Table 2
(Continued)
Source
α
(B1950.0)
β
(B1950.0)
Transition
Hyperfine
Component
TR∗
(K)
Δv 1/2
(km s−1 )
Orion-KL
05:32:46.8
−5:24:23.0
CN
N=1→0
J = 3/2 → 1/2
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
2.11(4)
6.19(4)
1.36(4)
1.45(4)
0.18(4)
0.040(7)
∼0.020
6.60(5)
0.24(3)
0.065(10)
5.3(1.3)
5.3(1.3)
4.6(1.3)
5.3(1.3)
5.3(1.3)
5.4(1.4)
9.1(1.3)
9.5(1.3)
9.3(1.3)
9.5(1.3)
9.7(1.3)
8.7(1.4)
4.1(1.7)
4.3(1.7)
4.3(1.7)
8.4(1.7)
8.3(1.7)
8.4(1.7)
1.16(3)
3.62(3)
0.80(3)
0.90(3)
0.15(3)
0.015(5)
3.3(1.3)
4.0(1.3)
3.3(1.3)
4.0(1.3)
3.3(1.3)
5.4(1.4)
10.7(1.3)
10.5(1.3)
10.3(1.3)
10.1(1.3)
9.9(1.3)
11.0(1.4)
2.09(5)
0.029(5)
0.006(4)
3.3(1.7)
4.2(1.7)
5.1(1.7)
10.0(1.7)
10.0(1.7)
10.0(1.7)
0.83(6)
1.59(6)
0.54(6)
0.69(6)
0.11(6)
0.023(6)
6.0(1.3)
6.6(1.3)
6.0(1.3)
5.5(1.3)
5.5(1.3)
4.8(1.4)
−47.4(1.3)
−48.0(1.3)
−47.4(1.3)
−48.0(1.3)
−46.7(1.3)
−46.7(1.4)
2.90(5)
0.168 (30)
0.033(8)
5.0(1.7)
4.3(1.7)
5.9 (1.7)
−47.4(1.7)
−46.8(1.7)
−47.4(1.7)
1.05(4)
1.96(4)
0.65(4)
0.69(4)
0.12(4)
0.021(7)
5.3(1.3)
5.3(1.3)
4.5(1.3)
4.7(1.3)
4.0(1.3)
5.4(1.4)
−56.7(1.3)
−57.1(1.3)
−58.0(1.3)
−57.0(1.3)
−57.9(1.3)
−57.4(1.4)
3.62 (4)
0.19(4)
0.027(8)
5.0(1.7)
4.3 (1.7)
5.1(1.7)
−56.8(1.7)
−57.2(1.7)
−56.8(1.7)
0.38(4)
1.00(4)
0.36(4)
0.44(4)
...
0.011(5)
3.9(1.3)
4.6(1.3)
3.9(1.3)
4.6(1.3)
...
4.2(1.4)
−49.5(1.3)
−50.2(1.3)
−50.3(1.3)
−50.1(1.3)
...
−49.6(1.4)
0.95(4)
0.052(10)
0.010 (5)
5.0(1.7)
4.3(1.7)
5.1(1.7)
−52.9(1.7)
−52.4(1.7)
−51.3(1.7)
1.6(0.7)
2.0(0.7)
1.6(0.7)
1.6(0.7)
1.7(0.7)
2.6(1.4)
∼1.6
6.5(0.7)
6.7(0.7)
6.5(0.7)
6.7(0.7)
6.5(0.7)
6.5(1.4)
∼6.5
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
Orion Bar
05:32:53.5
−05:27:10.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
W3(OH)
02:23:17.0
61:38:54.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
NGC 7538
23:11:36.6
61:11:47.0
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
S156
23:03:04.9
59:58:45.4
CNe
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
HNC
J=1→0
HN13 C J = 1 → 0
H15 NC J = 1 → 0
Barnard-1f
03:33:20.8
31:07:34
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
F = 3/2 → 1/2
F = 5/2 → 3/2
F = 1/2 → 1/2
F = 3/2 → 3/2
F = 1/2 → 3/2
F = 2 → 1c
F = 1 → 0c
CN
N=1→0
J = 3/2 → 1/2
C15 N
N=1→0
0.47(9)
0.52(9)
0.41(9)
0.59(9)
0.38(9)
0.023(8)
∼0.015
Notes.
a
Uncertainties are ±1σ ; measured with 500 kHz resolution unless otherwise noted; J = 3/2 → 1/2 for C15 N.
b
Measured with 1 MHz resolution.
c
Blended components.
d
Two velocity components.
e
Measured with 195 kHz resolution (MAC data).
f
Measured with 250 kHz resolution.
5
VLSR
(km s−1 )
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
(a)
Figure 2. (a)–(c). Spectra of the N = 1 → 0, J = 3/2 → 1/2 transition of C14 N (upper half) and C15 N (lower half) observed in this study with the ARO 12 m
telescope. The C14 N spectra exhibit a distinct hyperfine structure, indicated underneath the data with LTE relative intensities. Individual components are labeled by
quantum number F. The C15 N data consist of two blended hyperfine components, F = 2 → 1 and F = 1 → 0, also indicated underneath the spectra, which are not
resolved in most clouds. The filter banks resolution is 500 kHz for all sources except SgrB2, W51M, and W31, where 1 MHz filter banks are used. In Barnard-1, the
250 kHz filter banks were employed. (d)–(f) Spectra of the J = 1 → 0 transition of HNC, HN13 C and H15 NC. The dashed lines mark the LSR velocities established
from the observations, as shown in Table 2. The filter banks resolution is the same as in panels (a), (b), and (c), except for W31 (500 kHz in this case). The spectra in
SgrB2(NW) show two velocity components near 50 and 70 km s−1 . Other lines are identified in the spectra.
temperatures, due to variations in selective collisional excitation
(Stutzki & Winnewisser 1985), as commonly observed in other
molecules such as NH3 (Stutzki et al. 1984) and HCN (Turner
& Thaddeus 1977).
The spectra of C15 N were obtained with good signal-to-noise
ratios in all sources. However, the F = 2 → 1 and F = 1 → 0
lines are completely blended in the relatively warm molecular
211,20 transition of C2 H3 CN. In addition, in sources less than
8 kpc from the galactic center, another effect seems at play.
The F = 1/2 → 1/2 hyperfine component appears anomalously
weak relative to the F = 3/2 → 3/2 component (typically
∼10%–35% less intense). These two components should have
identical intensities of 9.99% in the LTE limit. Such anomalies
can occur if the hyperfine levels have different excitation
6
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
(b)
Figure 2. (Continued)
clouds studied here, as they lie 1 MHz apart in frequency. In the
cold cloud Barnard-1, these two features were barely resolved,
see Figure 2(c). The much weaker F = 1 → 1 line appears to be
contaminated by other features, likely arising from vibrationally
excited SO2 and asymmetry components of dimethyl ether.
As Figure 2 demonstrates, the HN13 C and H15 NC spectra
were also measured with good signal-to-noise ratios. In the case
of SgrB2, HNC was observed toward a position in the envelope,
SgrB2(NW), because the hot core regions produce lines that are
heavily self-absorbed. The H15 NC and HN13 C lines at the NW
position exhibit two velocity components, one near 50 km s−1
and another at 70 km s−1 . Two such components had previously
been observed toward this position in HNCO (Kuan & Snyder
1996). According to these authors, these features arise from two
separate clouds that are colliding. Interestingly, the relative line
intensities in the two velocity components vary and thus the
resulting nitrogen isotope ratio.
5. ANALYSIS
5.1. 14 N/15 N Ratios Derived from CN
As discussed in Savage et al. (2002) and Milam et al. (2005),
the hyperfine components in CN allow for an accurate evaluation
7
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
(c)
Figure 2. (Continued)
of the opacity in the N = 1 → 0 transition. For clouds where
CN is optically thick, the opacity was obtained by performing
a simultaneous least-squares fit to the intensities of the five
hyperfine components observed; see Savage et al. (2002). From
this analysis, both excitation temperatures and opacities are
derived. For the special case where the F = 1/2 → 1/2
component is somewhat weaker than the F = 3/2 → 3/2
line (G34.3 and W31), the average line intensity of the two
components was used for the fit. For the sources where the F =
3/2 → 1/2 and F = 5/3 → 3/2 components are blended (W51M
and W31), the sum of their relative intensities was employed in
the analysis. Results of the modeling for CN can be found in
Table 3. From this analysis, the typical opacities and excitation
temperatures determined for CN in the clouds observed are
τ ∼ 1–5 and Tex ∼ 5–10 K (note that τ = 3τ main , where τ main
is the opacity of the main hyperfine component, F = 5/2 →
3/2). These low excitation temperatures are expected, as CN has
a relatively large dipole moment of 1.45 D. Two sources could
not be analyzed: Barnard-1, because the hyperfine intensity ratio
departs dramatically from LTE; and SgrB2 (OH), where broad
lines and contamination from other molecules made such an
analysis highly uncertain.
All 5 N/14 N ratios were calculated using either the brightness
temperatures of the individual lines, or the line opacity and
8
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
(d)
Adande & Ziurys
6
6
W51M
W31
HNC
J=1 0
4
HNC
J=1 0
4
2
2
0
0
13
HN C
J=1 0
0.2
0.0
0.10
15
CH3OCHO
H NC
J=1 0
HN C
J=1 0
0.3
TR *(K)
TR *(K)
13
0.0
15
0.06
0.05
H NC
J=1 0
0.03
0.00
0.00
-100
-50
0
50
100
150
-80 -60 -40 -20
200
-1
G34.3
40
60
4
M17-SW
HNC
J=1 0
2
2
0
0
13
HN C
J=1 0
13
HN C
J=1 0
0.2
TR *(K)
0.6
TR *(K)
80
V LSR (km s )
HNC
J=1 0
4
20
-1
V LSR (km s )
6
0
0.3
0.0
0.0
15
H NC
J=1 0
0.10
0.08
CH3OCHO
0.05
15
H NC
J=1 0
0.12
0.04
0.00
0.00
-20
0
20
40
60
80 100 120
-40
-1
-20
0
20
40
-1
60
80
100
V LSR (km s )
V LSR (km s )
Figure 2. (Continued)
corresponding excitation temperature. In the optically thin
sources (M17-SW, Orion-KL, and Orion Bar), the ratio was derived directly from the brightness temperatures of the strongest
hyperfine components in CN and C15 N, weighted by the hyperfine relative intensities, i.e.,
14
N
15 N
=
TR (CN)
× Rh .
TR (C15 N)
optically thick case (all other sources), TR (CN) in Equation (1) is
replaced by τ main ∗Tex (see Table 3). Because the main hyperfine
line F = 2 → 1 of C15 N is partly blended with the weaker
F = 1 → 0 component (relative intensities: 0.417:0.165), the
observed line was modeled to account for the contribution of the
weaker feature and corrected accordingly. Typical corrections
were on the order of 10%–20%, depending on the linewidth
of the source. It was also assumed in all ratio calculations that
the two isotopologues have the same linewidths, as verified by
the observations (see Table 2), and occupy the same volume
of the cloud. The 14 N/15 N ratios thus derived are presented in
(1)
Here Rh is the ratio of relative intensities of the C14 N: F =
5/2 → 3/2 line and the C15 N: F = 2 → 1 feature. In the
9
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
(e)
Figure 2. (Continued)
5.2. 14 N/15 N Ratios from the HNC Double Isotopes
Table 4, along with uncertainties, propagated from the errors in
line intensity, opacity, and excitation temperature, as applicable.
Also given in the table are gas kinetic temperatures, TK , of
the sources, derived from the literature, and the CN total
column densities, calculated by the methods given in Savage
et al. (2002). The opacities were considered in the derivation
of column densities, and it was assumed that the rotational
temperature, Trot, was approximately equal to TK . The dipole
moments of CN and C15 N were assumed to be 1.45 D (Thomson
& Dalby 1968).
The nitrogen isotope ratios were also calculated by a second, independent method, using the data for the J = 1 → 0
transition H15 NC and HN13 C. As described by Dahmen et al.
(1995), measurement of the [HN13 C]/[H15 NC] ratio provides
the product (13 C/12 C) × (14 N/15 N). Knowledge of the carbon isotope ratio leads to 14 N/15 N. The carbon isotope ratio
12
C/13 C has been measured using H2 CO (Henkel et al. 1983),
CO (Langer & Penzias 1990), and CN (Savage et al. 2002;
10
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
(f)
4
NGC7538
Adande & Ziurys
HNC
J=1 0
0.6
2
HN C
J=1 0
TR *(K)
0.3
0
13
HN C
J=1 0
0.2
TR *(K)
13
SgrB2(NW)
0.0
CH3OCHO
15
0.10
H NC
J=1 0
0.05
0.00
0.0
0.04
-100
15
H NC
J=1 0
-50
0
50
100
150
200
-1
V LSR (km s )
0.02
Figure 2. (Continued)
Table 3
Opacities, Excitation Temperatures, and Column Density of CN
0.00
Source
-120 -100
-80
-60
-40
-20
0
-1
V LSR (km s )
S156
W31
G34.3
M17-SW
W51M
DR-21
Orion-KL
OrionBar
W3(OH)
NGC 7538
S156
HNC
J=1 0
1
τa
Tex
(K)
TK
(K)
Ntot
(cm−2 )
2.2 ± 0.2
0.52 ± 0.05
...
2.4 ± 0.3
4.9 ± 0.7
...
...
3.0 ± 0.5
2.4 ± 0.3
1.9 ± 0.2
7.2 ± 0.7
27.3 ± 1.0
...
7.6 ± 0.8
6.0 ± 0.6
...
...
5.7 ± 0.6
6.9 ± 0.6
5.2 ± 0.1
50b
30c
50d
32e
78f
80g
85h
35i
25j
27k
1.4 × 1015
5.2 × 1014
1.2 × 1015
1.4 × 1015
2.7 × 1015
2.5 × 1015
1.0 × 1015
9.6 × 1014
5.0 × 1014
2.8 × 1014
20
0
TR *(K)
13
HN C
J=1 0
0.05
0.00
0.02
Notes.
a τ = 3τ
main , where τ main is the opacity of the main hyperfine component,
F = 5/2 → 3/2.
b Beuther et al. (2011).
c Matthews et al. (1987) and Campbell et al. (2004).
d Stutzki & Guesten (1990).
e Sollins et al. (2004).
f White et al. (2010).
g Sempere et al. (2000).
h Hogerheijde et al. (1995).
i Kim et al. (2006).
j Kameya et al. (1986) and Hasegawa & Mitchell (1995).
k Joy et al. (1984).
15
H NC
J=1 0
0.01
0.00
-150
-100
-50
0
Ratios thus derived are listed in Table 4. The main sources of
uncertainties in these ratios are the errors in antenna temperature
and the 12 C/13 C values.
The CN carbon isotope ratios were used in these calculations
because HNC is chemically more similar to CN than H2 CO or
CO, with related formation processes. Therefore, any chemical
fractionation effects should influence both molecules similarly:
see Milam et al. (2005). At early cloud times, neutral–neutral
reactions are thought to produce most of the CN and HNC, via
the following pathways (Nejad et al. 1990):
50
-1
V LSR (km s )
Figure 2. (Continued)
Milam et al. 2005). The estimated 13 C/12 C ratio appears to vary
somewhat, depending on the molecule considered, perhaps due
to chemical fractionation effects. Ratios derived from formaldehyde are typically higher than those from CN and CO (Wilson
1999; Milam et al. 2005). In this work, the brightness temperatures of the J = 1 → 0 transitions of H15 NC and HN13 C were
used to derive values of [HN13 C]/[H15 NC] (see Table 4), from
which 14 N/15 N ratios were determined using the 12 C/13 C ratios
established from CN in each source, from Milam et al. (2005).
11
N + CH → CN + H
(2)
C + NH → CN + H
(3)
C + NH2 → HNC + H.
(4)
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
Table 4
Isotope Ratios
14 N/15 N
Source
CN
HN13 C/H15 NC
HNCa
SgrB2(NW)
...
9.1 ± 1.1
5.1 ± 0.6
164
92
236 ± 55
6.7 ± 1.3
134 ± 48
122 ± 23
5.1 ± 0.8
143 ± 31
146 ± 26
252 ± 89
3.5 ± 0.7
6.0 ± 0.7
173 ± 54
210 ± 67
243 ± 66
234 ± 47
361 ± 141
341 ± 143
4.4 ± 0.6
3.7 ± 0.7
4.8 ± 4.5
7.0 ± 2.5
159 ± 31
159 ± 40
338 ± 311
394 ± 159
306 ± 119
325 ± 100
5.1 ± 1.3
5.2 ± 2.9
321± 126
406 ± 328
IRAS 17160–3707
W31
IRAS 17220–3609
IRAS 15567–5236
G34.3
IRAS 16065–5158
IRAS 16172–5028
M17-SW
W51M
IRAS 15520–5234
IRAS 17059–4132
IRAS 17009–4042
IRAS 16562–3959
IRAS 17233–3606
DR-21
Orion-KL
OrionBar
NGC 7538
IRAS 08303–4303
W3(OH)
S156
HCNb
DGC
(kpc)
0.1
244 ± 81
194 ± 60
262 ± 74
252 ± 65
328 ± 82
371 ± 91
320 ± 79
353 ± 84
343 ± 80
276 ± 62
391 ± 89
3.0
3.1
3.6
4.3
5.1
5.4
5.6
6.1
6.1
6.2
6.2
6.5
7.0
7.7
8.0
8.3
8.3
8.9
9.1
9.6
10.9
Notes.
a 12 C/13 C ratio from measurements of 12 CN/13 CN by Milam et al. (2005).
b Ratio obtained by applying the 12 CN/13 CN ratio from the linear fit of Milam et al. (2005) to the H13 CN/HC15 N
data of Dahmen et al. (1995).
At later times, both molecular species are created via electron
dissociative recombination of HNCH + (Nejad et al. 1990).
CO. If the HCN data of Dahmen et al. (1995) are scaled by
the 12 C/13 C ratios from CN, instead of H2 CO, the resulting
14
N/15 N ratios agree with the HNC values established here
within the uncertainties. These ratios are also listed in
Table 4.
In Figure 3, nitrogen isotope ratios obtained in this study from
CN and HNC are plotted relative to galactocentric distance.
Also included on the plot are the revised values from Dahmen
et al. The plot shows an obvious trend of increasing 14 N/15 N
ratios with galactocentric distance, in agreement with Dahmen
et al. (1995). A linear fit of the 14 N/15 N ratio relative to the
galactocentric distance, using all three data sets, results in the
relationship:
6. DISCUSSION
6.1. The
14
N/15 N Ratio Across the Galaxy
As can be seen in Table 4, the nitrogen isotope ratios
derived from measurement of HNC and CN are in reasonable
agreement, within the uncertainties. (The propagated errors
typically amount to about 20%–30% of the calculated ratio
values). The nitrogen isotope ratios derived from both methods,
however, are lower than those established by Dahmen et al.
(1995). These authors typically found 14 N/15 N ∼ 250–650,
while our values are ∼120–400. One obvious cause for this
difference is that Dahmen et al. (1995) used the 12 C/13 C ratios
derived from H2 CO in their calculations. These ratios are higher
than the numbers obtained from both CO and CN, and thus lead
to larger 14 N/15 N ratios.
The carbon ratios from H2 CO may be exhibiting fractionation
effects, because this molecule has more complex formation
pathways than CO and CN (Millar et al. 1979; Roueff et al.
2006). Based on zero-point energy differences, the following
ion-exchange reaction would tend to enhance 13 C in CO (Sakai
et al. 2010):
13
C + CO → CO + C
+
12
13
12
+
ΔE ≈ 35 K.
14
N/15 N = 21.1(5.2) kpc−1 × DGC + 123.8(37.1).
(6)
The previous gradient of Dahmen et al. had a slope of 19.7
(8.9) kpc−1 , revised by Wilson (1999) to 19.0 (8.5) kpc−1 , both
of which agree with the new value, within the uncertainties.
However, in both these cases the y-intercept was 288.6 (65.1),
significantly different from 123.8 (37.1) established here. This
new gradient implies a local ISM value of 14 N/15 N = 290 ± 40,
at ∼7.9 kpc.
6.2. Fractionation in Nitrogen
The amount of fractionation in interstellar cyanides is not
currently known. Differences in zero-point energies, however,
have been calculated by Bakker & Lambert (1998) for the CN
ion–molecule isotope exchange:
(5)
Since CO is the second most abundant molecule, this process
could in turn deplete 13 C in other species that have CO as a
precursor, including H2 CO. Such depletions do not seem to apply to CN, which has 12 C/13 C ratios very similar to that of
15
12
N+ + CN → C15 N + N+
ΔE ≈ 23 K.
(7)
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
Terzieva & Herbst (2000) examined the possibility of nitrogen fractionation for common N-bearing species in molecular
clouds. These authors concluded that 15 N enhancement should
be very limited because equilibrium coefficients of most isotopic exchange processes are predicted to be small. They consequently cannot compete with faster ion–molecule reactions,
which would scramble any isotopic enhancements. Rodgers &
Charnley (2008) suggest several scenarios leading to high 15 N
enhancement in CN, HNC, and HCN in very dense and cold
regions of molecular clouds that further evolve into protoplanetary disks. They were proposed to explain the 15 N excesses
observed in the organic content of some meteorites. Observational studies of the proposed mechanisms are clearly needed in
such cold cores.
Galactic nitrogen isotope ratio
800
14N/15N
600
400
200
6.3. Implications for Galactic Chemical Evolution
0
0
2
4
6
8
10
12
Of the CNO isotopes, nitrogen is the element where the
biggest uncertainties remain with regard to its nucleosynthesis
and subsequent mixing in the ISM. As discussed, 14 N is
produced as a byproduct of the cold CNO cycle in low- and
intermediate-mass stars as a secondary element, and released
in the ISM during dredge-up phases. Primary contributors to
14
N are more massive stars, principally in He-shell burning
(Audouze et al. 1975; Romano & Matteucci 2003). The main
contributor to 15 N is believed to be the hot CNO cycle in novae,
also a secondary process. Note that the equilibrium ratio in
the cold CNO cycle is very high (14 N/15 N ∼ 30,000: Clayton
2003). Observations of CN and C15 N in the AGB star HD 56126
by Bakker & Lambert (1998) has produced a lower limit of
14
N/15 N > 2000, in agreement with theory.
In our Galaxy, the metallicity, as measured by the abundance
of heavy elements, decreases with distance from the galactic
center (Maciel & Costa 2009). If 15 N is mainly a secondary
element, produced in significant amounts only by nova events
in binary white dwarfs and perhaps supernovae, its abundance
should decrease with galactocentric distance; that of 14 N, in
contrast, drops at a lower rate because of its primary component.
As a result, the 14 N/15 N ratio should increase with galactocentric
distance.
Various models of GCE have been formulated to test different
hypotheses of the nitrogen isotope formation and enrichment
of the ISM (e.g., Tosi 1982; Matteucci & D’Antona 1991). A
recent model by Romano & Matteucci (2003) assumes that 15 N
is produced only as a secondary element in novae, but adjusts
stellar yields adopted for massive and low- and intermediatemass stars for 14 N production. Predictions of the 14 N/15 N ratio
as a function of DGC from their models 2s, 3s, and 3n, which
vary in stellar yields chosen, agree well with our observations,
with a galactic gradient of 14 N/15 N of about 20 dex kpc−1 . The
results from their 3n model are plotted in Figure 3. Previously,
these authors had also offset their model in order to match the
observed 14 N/15 N ratios of Dahmen et al. (1995); our CN and
HNC data reflect their actual model predictions.
DGC (kpc)
Obtained from CN data
Obtained from HNC data
Dahmen et al, 1995
Figure 3. Plot of 14 N/15 N isotope ratios as a function of Galactocentric distance,
DGC (kpc). The filled circles indicate direct measurement of the ratios using the
hyperfine components of the CN isotopologues. The open circles represent the
ratios calculated using HN13 C/H15 NC and the 12 C/13 C ratio measured in CN
by Milam et al. (2005). The triangles are the ratios obtained from the HCN
data of Dahmen et al. (1995), combined with the 12 C/13 C ratios of Milam
et al. (2005). The dashed line is the gradient from the model 3n of Romano &
Matteucci (2003), where 15 N is treated as a secondary element and produced
in novae. The rectangular box represents the range of values found in comet
measurements of 14 N/15 N, and the star is the value in Earth’s atmosphere. A
linear fit of these three sets of data results in the expression: 14 N/15 N = 21.1
(5.2) kpc−1 × DGC + 123.8 (37.1).
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 14 N/15 N = 334 ± 50 toward Barnard1, 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 14 N isotopologues of
CN and HNC, relative to rarer C15 N and H15 NC, from the
ambient UV radiation field. The so-called isotope-selective
photodissociation effect has been observed in CN and 13 CN
(Milam et al. 2005) and leads to increased 12 C/13 C ratios in
PDR regions. Within the uncertainties, we find no evidence
of such effect in our 14 N/15 N data. The ratio in the PDR
Orion Bar is somewhat higher but essentially the same as in
the star-forming region Orion-KL (361 ± 141 versus 234 ±
47). Moreover, Lucas & Liszt (1998) determined the nitrogen
isotope ratio in absorption toward the diffuse molecular cloud
B0415 + 379 using HCN. Their ratio 14 N/15 N = 237 ± 25 is
similar to our value for the local ISM of 290 ± 40. The effect
of UV radiation is typically stronger in diffuse as opposed
to dense clouds (Snow & McCall 2006), which suggests that
isotope-selective photodissociation does not severely influence
the nitrogen isotope ratio.
6.4. Implications for Nitrogen Isotope Ratios
in the Solar System
The current best estimate for the 14 N/15 N ratio in the local
dense ISM of 290 ± 40 (7.9 kpc), based on CN, HNC, and HCN
observations, is very close to that found in Earth’s atmosphere
of 272. This ratio is indicated by a star in Figure 3. It is also
closer to average values reported for nitrogen species in comets,
which typically range from 130 to 330 (e.g., Hutsemekers et al.
13
The Astrophysical Journal, 744:194 (15pp), 2012 January 10
Adande & Ziurys
2008). The comet ratios are indicated in Figure 3 by a box. The
scatter of values represented by the box seem to match that in
local ISM values.
As shown in Table 4, ratios from different clouds at the
same DGC also display a range of values. M17-SW has
14
N/15 N ratios of 146 ± 26 (CN) and 173 ± 54 (HNC), while
W51M exhibits 252 ± 89 and 210 ± 67; both sources are at
6.1 kpc. Also, in SgrB2 (2N), where there are two velocity components at ∼50 and ∼70 km s−1 within the same beam, the
HN13 C/H15 NC ratios differ (9.1 and 5.1: see Table 4). This diversity may simply reflect cloud-to-cloud variations. Different
types of stars produce 14 N and 15 N, and it is not clear that the
mixing of nitrogen isotopes after they are released into the ISM
either through the red giant/AGB phases or novae and supernovae is homogeneous (Romano & Matteucci 2003). In addition, as proposed by Rodgers & Charnley (2008), there may be
other fractionation effects that enhance 15 N-bearing species in
the cold, dense core regions.
Recent measurements of 60 Ni in meteorites suggest that the
solar system was born in a high-mass star-forming cluster
(Hester et al. 2004). 60 Ni, a daughter nucleus of short-lived 60 Fe,
can only be produced by supernova (Tachibana & Huss 2003).
The solar protoplanetary disk may then have been contaminated
by sporadic ejecta from nearby supernova explosions (Throop
& Bally 2010; Ouellette et al. 2010). Isotopic variations within
the solar system may partly reflect inhomogeneous mixing of
material expelled from stars, among many other possible effects
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7. CONCLUSIONS
Accurate measurements of the 14 N/15 N isotope ratio have
been carried out for warm molecular clouds at various galactocentric distances, using both CN and HNC and their isotopologues. Both direct (CN) and indirect (HNC) methods yield
results consistent within the uncertainties. The 14 N/15 N gradient derived from these observations, along with the previous
HCN data, is in agreement with recent galactic chemical models
that assume secondary production of 15 N from novae outbursts.
The ratio determined for the local ISM agrees well with that
measured in local diffuse clouds and Earth’s atmosphere, and
is reasonably close to the average value found in comets. Although some source-to-source variation is observed, there is
little evidence for chemical fractionation effects in CN or HNC
in the warm dense clouds studied. These measurements should
help further refine and constrain models of GCE and shed some
light on the large deviations of the 14 N/15 N ratio found in solar
system bodies.
This work has been supported by NSF grant AST-09-06534
and NASA Grant NNX10AR83G.
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