Nitrogen Fractionation in Protoplanetary Disks from the H13CN/HC15N Ratio
V. V. Guzmán K. I. Öberg J. Huang R. Loomis C. Qi


















vguzman@cfa.harvard.edu









Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

⋆Currently at Joint Atacama Large Millimeter/submillimeter Array (ALMA) Observatory, Avenida Alonso de Córdova 3107, Vitacura, Santiago, Chile.


Nitrogen fractionation is commonly used to assess the thermal history
of Solar System volatiles. With ALMA it is for the first time possible
to directly measure  ratios in common molecules during
assembly of planetary systems. We present ALMA observations of the
 and  J=3-2 lines at 0”.5 angular resolution,
toward a sample of six protoplanetary disks, selected to span a range
of stellar and disk structure properties. Adopting a typical 
ratio of 70, we find comet-like  ratios of 80-160 in 5/6 of
the disks (3 T Tauri and 2 Herbig Ae disks) and lack constraints for
one of the T Tauri disks (IM Lup). There are no systematic differences
between T Tauri and Herbig Ae disks, or between full and transition
disks within the sample. In addition, no correlation is observed
between disk-averaged D/H and  ratios in the sample. One of
the disks, V4046 Sgr, presents unusually bright HCN isotopologue
emission, enabling us to model the radial profiles of  and
. We find tentative evidence of an increasing  ratio
with radius, indicating that selective photodissociation in the inner
disk is important in setting the  ratio during planet
formation.








 


































































§ INTRODUCTION





The origin of Solar System organics is a fundamental and highly
debated topic. It is unclear whether the organics in the different
Solar System bodies were inherited from the cold and dense molecular
parent cloud of our Sun, or if they are the result of chemical
processing within the Solar nebula protoplanetary disk. The isotopic
composition of present day organics may help to shed light on their
origins, since isotopic fractionation chemistry is highly environment
specific and can leave a permanent imprint. Comets are especially
interesting in this context, since they should preserve the isotopic
compositions in different molecules during the assembly of the Solar
System.

Among the different methods used to trace the origin of molecules, the
 isotopic ratio is one of the most popular ones. Nitrogen
isotopic ratios span at least an order of magnitude between different
Solar System bodies. A low nitrogen fractionation (high ) is
observed toward the Sun and Jupiter <cit.>, while a high fractionation is observed in the rocky
planets, comets and meteorites
<cit.>. The origin of these 
variations is not well understood, but it suggests that different Solar
System bodies obtained their nitrogen from different nitrogen
reservoirs <cit.>. Based on observations of the
Interstellar Medium (ISM), comets and chemical models, there are three
major nitrogen reservoirs in dense interstellar and circumstellar media,
N_2, NH_3 and HCN. These species have different fractionation
pathways <cit.>, and it is thus important to
measure the  ratio in molecules representative of these nitrogen
reservoirs.

This study focuses on HCN. HCN is readily detected in the ISM
<cit.>, comets
<cit.> and protoplanetary disks
<cit.>. While several studies have
been made toward prestellar cores and protostars, the
characterization of isotopic ratios in protoplanetary disks is rather
new due to the intrinsic weak line emission. <cit.>
presented the first detection of  and  in the disk
around Herbig Ae star MWC 480, and provided the first measurement of
the  in a disk. They found an isotopic ratio of 200±100,
which is similar to what is observed in the cold ISM and in comets.

The low  value in the MWC 480 disk implies either inheritance
of HCN from the ISM, or the presence of an active fractionation
chemistry in the disk. There are two potentially active fractionation
channels in disks and in the ISM. The first is through isotope
exchange reactions, such as

    HC^14NH^+ + ^15N→HC^15NH^+ + ^14N + hν

which favor the incorporation of ^15N into molecules at low
temperatures (<20 K). HC^15NH^+ can later recombine with
free electrons to produce . Observations of HCN and HNC
fractionation toward protostars present a tentative trend of the
 with temperature, supporting this scenario
<cit.>. The second mechanism is selective
photo-dissociation of ^14N^15N over ^14N_2, due to
self-shielding of ^14N_2 <cit.>. In the surface layers
of protoplanetary disks, which are directly illuminated by the
radiation field of the central star, the dominant formation pathways
leading to HCN and  are

    ^14N + CH_2→HC^14N


    ^15N + CH_2→

Both mechanisms can reduce the HCN/ ratio in protoplanetary
disks <cit.>. Distinguishing between these
different origins of nitrogen fractionation levels in disks and
between inheritance and in situ disk fractionation chemistry (and
further in comets and planets) requires more disk measurements, and
constraints on the radial profiles of the  ratio in disks
with different structures and around stars with different radiation
fields. A constant  ratio across disks would favor a scenario
where disks inherit their organics from the natal cloud, while disk
chemistry should result in a radial gradient, since the disk
environment is dramatically different at different radii.

To begin to address this long-term goal, we present observations of
 and  in a diverse sample of 6 protoplanetary
disks. Because the HCN lines may be optically thick, we use the
 line as a proxy of HCN to derive the  ratio. In
section 2 we present the observations and describe the data reduction
process. The disk-averaged isotopic flux ratios derived from the
observations are presented in Section 3. In Section 4, we model the
disk abundance profiles of  and  in V4046 Sgr, the
source with the highest signal-to-noise ratio detection. In Section 5,
we discuss the results and compare with observations in our Solar
System and in the cold ISM. A summary is presented in Section 6.





§ OBSERVATIONS AND DATA REDUCTION



The  and  J=3-2 lines were observed with ALMA during
Cycle 2 as part of project ADS/JAO.ALMA#2013.1.00226. The Band 6
observations included two spectral settings, at 1.1 and 1.4 mm. The
correlator setup of the 1.1 mm and 1.4 mm settings were configured
with 14 and 13 narrow spectral windows, respectively, targeting
different molecular lines. The main targets of the observations were
deuterated species, which are presented in <cit.>. They include an independent analysis of the  data in
the context of D/H fractionation. A focused study on the DCO^+
emission in IM Lup was presented earlier by <cit.>. The
 data in MWC 480 was also used for the study of CH_3CN by
<cit.>. In addition, <cit.> presented the
 and  data in MWC 480. In this paper, we focus on the
 and  lines for the full sample. For consistency, we
present data reduction and imaged independently of the previous
papers.  For one of the sources, V4046 Sgr, we also use CO J=2-1
isotopologue data from the same survey.

The observations are described in detail by <cit.>. In
short, the Band 6 observations were carried-out between 2014 and 2015
with baseline lengths spanning between 18 and 650 m. The total
on-source time was 20 min, on average. A quasar was observed to
calibrate amplitude and phase temporal variations. To calibrate the
frequency bandpass a quasar was observed. The absolute flux scale was
derived by observing Titan for about half the observations, or a
quasar. The HCN isotopologue lines were covered by two spectral
windows of 59 MHz bandwidth and 61 kHz channel width in the 1.1 mm
spectral setting. The CO isotopologue lines were covered by three
spectral windows in the 1.4 mm spectral setting, with the same
bandwidth and channel width.
 
The data calibration was performed by the ALMA staff using standard
procedures. We took advantage of the bright continuum emission of the
sources to improve the signal-to-noise ratio, by further
self-calibrating the HCN isotopologue data. The self-calibration
solutions were derived on individual spectral windows when possible
(AS 209, LkCa 15, MWC 480 and V4046 Sgr) and on averaged spectral
windows for the weaker sources (IM Lup and HD 163296), and then
applied to each spectral window. The continuum was then subtracted
from the visibilities to produce the spectral line cubes. The clean
images were obtained by deconvolving the visibilities in CASA, using
the CLEAN algorithm with Briggs weighting. The HCN isotopologue data
was regridded to a spectral resolution of 0.5 for the full
sample. The robust parameter was set to 1.0 for , except for
IM Lup where we used a value of 2.0. For  the robust parameter
was set to 2.0 to improve the signal-to-noise ratio, except for
V4046 Sgr and MWC 480 for which a value of 1.0 was used because of the
bright line emission. To help the cleaning process we created
elliptical masks, same for all channels, centered on the dust
continuum centroid, using the disk inclination and position angle
listed in Table <ref>. The mask radius was chosen to cover
the stronger  line emission in all channels, or to cover the
dust continuum emission if the line was not  detected. The
resulting beam, channel rms and moment zero rms values are listed in
Table <ref>. For the CO isotopologues the robust parameter was
set to 0.5, and a Keplerian mask was used to help the cleaning
process, created by selecting emission consistent with the expected
Keplerian rotation of the disk in each channel. 



§ SAMPLE STATISTICS
   




The stellar and disk properties of the sample are summarized in
Table <ref>. The sample includes 4 T Tauri stars and 2
Herbig Ae stars. The stellar masses range between 0.9 (AS 209) and
2.3 (HD 163296), corresponding to luminosities that span an
order of magnitude. Two of the sources, namely LkCa 15 and V4046 Sgr,
are transitional disks, with inner holes resolved at millimeter
wavelengths. The sample is biased toward large disks, with a known
rich molecular emission. However, given the very different physical
properties, in particular the gas temperature, the source selection
allows us to determine the disk averaged  ratio in HCN in a
diverse sample of disks.

Figure <ref> shows the observations for the full sample of
disks. The dust continuum images are shown in the left column. The
1.1 mm continuum images were produced by averaging 1.1 mm line free
spectral windows. The four middle panels display the  and
 velocity integrated maps, for the full line (color images)
and for two velocity ranges, the blue and red shifted parts of the
line, to demonstrate the Keplerian rotation of the disk (blue and red
contours). The right column of the figure shows the disk integrated
line profiles of the  and  lines. The spectra were
extracted using the same elliptical masks used to clean the
data. Figs. <ref> to <ref> show
channels maps of the  and  lines in each source, with
the elliptical masks overlaid on top.

The lines are classified as detected if emission consistent with the
expected Keplerian rotation of the disk is observed in at least three
channels at a 3σ level. From the inspection of the channel maps
we find that  is clearly detected toward all disks except for
the T Tauri disk IM Lup.  is clearly detected toward
V4046 Sgr, MWC 480 and HD 163296, weakly detected toward AS 209 and
LkCa 15, and not detected toward IM Lup. We note that for the two
disks with weak  line emission, while emission is detected in 3
or more individual channels, the emission is almost washed out in the
integrated intensity maps.

The  emission is generally compact compared to the extent of
the dust disk. It appears centrally peaked toward MWC 480 and V4046
Sgr, and possibly toward HD 163296, but presents clear rings toward
the remaining two sources: LkCa 15 and AS 209. The latter is
unexpected, since AS 209 is not a transition disk.  The  line
shows a similar behavior, although the signal-to-noise ratio is lower.

We extracted the disk integrated fluxes from the unclipped moment zero
maps using the same elliptical mask created to clean the data and
extract the spectra. The uncertainty in the flux was estimated by
simulating integrated flux measurements from signal free regions using
the same elliptical mask but centered at random positions. The
integrated disk fluxes and their associated uncertainties are listed
in Table <ref>. 

We use the extracted line flux ratios to estimate the abundance ratios
of the two isotopologues. This is a reasonable first approximation
when comparing HCN isotopologes, as the  and  line
emission is expected to be optically thin, arise in the same region
and this region is dense enough for the molecular rotational
population to be in LTE <cit.>. We compute the
/ abundance ratio in the LTE case and for
=15 K. For MWC 480, we obtain a lower / ratio
(1.8±0.3) than the value of 2.8±1.4 reported in
<cit.>, although both values are consistent within the
errors. This is due to the different method implemented in this paper
to extract the fluxes. The resulting / abundance ratios
span from 1.2 to 2.2 with an average of 1.8. Given the almost
identical upper energies, Einstein coefficients and the ratio Q/g_u
(partition function over upper state degeneracy) for the J=3-2
transition (see Table <ref>), the / abundance
ratios are almost identical to the flux ratios. We note that a higher
excitation temperature of 100 K changes the inferred abundances by
less than 1%.



In order to derive the nitrogen fractionation in HCN, we adopt an
isotopic ratio of =70. Because the C isotopic ratio depends
on the physical conditions of the gas <cit.> we
include a 30% uncertainty in this value to convert the
/ ratio into a  ratio. The inferred
 ratios span from 83 to 156 with an average of 124. All disks
present low cometary-like  ratios. The resulting
/ abundance ratios and the inferred  flux
ratios are listed in Table <ref>. Fig. <ref>
shows the nitrogen fractionation ratios for the full sample and
compares it with DCN/HCN ratios derived by <cit.>. There
is no indication of a correlation between the disk averaged nitrogen
and hydrogen fractionation in these disks, as might have been expected
if both originated from a cold fractionation pathway (see also
section <ref>). We note that the spread in
 is small compared to the errors and we cannot rule out that
there is a trend that is washed out by the noise.



§ THE / PROFILE IN V4046 SGR


HCN isotopologue emission observed toward V4046 Sgr is sufficiently
bright to provide constraints on the  and  abundance
profiles. In this section, we model the emission profile of the
observed lines in order to retrieve the underlying  abundance
ratio across the disk.



 §.§ Disk physical structure


In order to investigate possible variations of the abundance ratio
across the disk a detailed model of the line emission is needed. We
build a parametric model to describe the physical structure of the
disk based on the model described in <cit.>, which was
constructed to reproduce the emission of the dust continuum and the CO
isotopologues. We first parametrize the dust surface density as

    Σ_dust(r) = 
    Σ_c  (r/r_c)^-γexp [ - ( r/r_c )^2-γ ]     r ≥ r_cav
    Σ_cav     0.2AU<r<r_cav

where Σ_c is a normalization factor, r_c is a characteristic
radius, γ=1 is the power-law index of the viscosity, and
r_cav is the radius of the inner cavity. We include two dust
populations, one for the atmosphere and another for the midplane grains.
The dust volume density is computed assuming a vertical Gaussian distribution
of each dust grain population:

    ρ_dust = ∑_i=0,1Σ_i(r)/√(2π) H_i(r)exp
      (-z^2/H_i(r)^2),

The midplane grains, which are larger than the atmospheric ones,
comprise the bulk of the dust mass (Σ_mid = 0.9
Σ_dust, Σ_atm=0.1 Σ_dust). The atmospheric dust
grains are vertically Gaussian distributed with a scale height: 

    H_atm(r) = H_10 ( r/100  AU )^h

and the midplane dust grains are concentrated closer to the midplane,
with a scale height that is half of that of the atmospheric grains.

    H_mid(r) = 1/2 H_atm(r)

With the dust density described above, the radiative transfer code
RADMC-3D <cit.> was then used to compute the dust
temperature throughout the disk. The dust absorption and scattering
opacities, which are needed to solve the thermal balance, were
computed using the pacityTool[https://dianaproject.wp.st-andrews.ac.uk/data-results-downloads/fortran-package/]
from the DIANA project <cit.>. The code assumes a mixture
of amorphous laboratory silicates with amorphous carbon and 25%
porosity for the grain composition. The grain size distribution
follows a power-law of index -3.5. The minimum grain size was set to
5 nm, and the maximum size was set to 10 μm and 1 cm, for the
atmosphere and midplane grain populations, respectively.



The gas temperature is parametrized as

    T_gas(r,z) =
      
      T_a + (T_m-T_a)  ( cos (π z/2 z_q)  )^2δ    z<z_q
    
      T_a     z≥ z_q

following <cit.>. Here, the atmospheric temperature is
given by a power-law (T_a=T_a,0 (r/10)^q_atm), and the
midplane temperature is fixed to the dust temperature
(T_m=(z=0)). The fiducial scale height at which the gas
temperature is allowed to vary vertically, z_q, is fixed to z_q=2
H_gas, where H_gas=2 c_s/Ω is the hydrostatic gas scale
height evaluated at the midplane, z=0. 

Once the gas temperature is obtained, the hydrostatic equation is
solved to derive the gas density across the disk. For this we assume a
vertically integrated gas-to-dust ratio at each radii of 100. The
adopted parameters for the model are listed in
Table <ref>. The resulting gas density and temperature
structures are shown in Figure <ref>. This model
reproduces the main features of the ^12CO, ^13CO and C^18O
emission (see Fig. <ref>) well enough for the purpose of
this study, assuming standard isotopic ratios. In this model, the CO
abundance is kept constant throughout the disk, except in the cold
midplane (<19 K) where the abundance is reduced by a factor of
10^3 due to freeze-out onto dust grains, and in the disk atmosphere
where the CO abundance is reduced by a factor 10^8 due to
photodissociation.






 §.§ Abundance fitting


The molecular abundances for  and  were defined as
power-laws,

    X=X_0(r/R_0)^α

where X_0 is the abundance with respect to total hydrogen at
R_0=100 AU and α is a power-law index. We also include an
outer cut-off radius . This parametrization is a common
approach when modeling molecular abundances in disks
<cit.>.

In order to find the model that best reproduces the HCN isotopologue
observations and the associated uncertainties, we use a Bayesian
approach. In short, we first create a synthetic observation of the
line emission for each species separately. Taking advantage of the
bright HCN isotopologue emission in V4046 Sgr, we produced observed
visibilities and cleaned cubes at a higher spectral resolution of
0.2 for the line modeling and include include 60 channels. We
use the is_sample Python
package[https://pypi.python.org/pypi/vis_sample] to compute
the Fourier Transform of the synthetic model and obtain visibilities
that are correctly re-projected on the u-v points of the
observations. The likelihood function is then computed in the u-v
plane, by computing the weighted difference between model and
observations, for the real and imaginary parts of the complex
visibility. We sample the posterior distribution with the MCMC method
implemented in the mcee package by <cit.>.

We include two free parameters in the line modeling, that is X_0,
α, which are associated with the molecular abundances of 
and . The outer radius, R_out, was fixed to 100 AU (chosen
by the extension of the emission in the moment-zero map), but we
checked that a larger radius of 200 AU gave the same result. The disk
physical structure, that is the gas density and temperature, are fixed
in the line fitting. We adopt the disk inclination, position angle,
stellar mass (including both stars) and systemic velocity listed in
Table <ref>. The level populations were computed using
RADMC and assuming the gas is under LTE. We checked that non-LTE
effects are not important for these lines using the non-LTE radiative
transfer code LIME <cit.> to re-calculate the level
populations for the best-fit model. When generating a new sample, we
included a flat prior for the power-law index -3<α<2, and for
the molecular abundance 10^-20<X_100<10^-8.






The best-fit model corresponds to X_0=8.94±0.30×10^-13 and
α=-0.69±0.03 for , and
X_0=3.37±0.19×10^-13 and α=-1.08±0.04 for
. Our model suggests an increasing / ratio as a
function of radius,  higher fractionation in the inner disk
compared to the outer disk. Fig. <ref> shows the
deprojected radial profiles of the dust continuum and HCN isotopologue
emission in V4046 Sgr (left panel) as well as the observed and modeled
/ flux ratio (right panel). We note that beyond 60 AU,
the ratio is highly uncertain because the signal-to-noise ratio
becomes too low, in particular for . Figure <ref>
shows the posterior probability distribution for the fitted
parameters. Fig. <ref> shows the residuals between our
best-fit model and the data for selected channels. Our simple model
is able to well reproduce the observations.

The inferred abundance / ratio at 10 and 50 AU are
1.08±0.14 and 2.02±0.27, respectively. Assuming
=70±21, we infer an abundance  ratio of 76±25
and 142±47, at 10 and 50 AU respectively.  
We note that the derived disk integrated HCN/ flux ratio of
115±43 falls in between the inferred abundance ratios at 10 and
50 AU. Given the consistent nitrogen fractionation levels inferred from the
observations and line modeling in V4046 Sgr, we expect the observed
 ratio in the other sources to be representative of their
abundance ratio in the comet forming region.





§ DISCUSSION





 §.§ Disk-averaged nitrogen fractionation in protoplanetary disks















We have shown that HCN isotopologues are abundant in disks. Both
 and  are detected toward 5/6 disks in our sample –
the one exception being the disk around T Tauri star IM Lup. The disk
around IM Lup is very massive (=0.1), very cold and also
very young <cit.> compared to the rest of the
disks in the sample. The non-detection of  was surprising
considering that IM Lup is quite bright in the main HCN isotopologue
<cit.>. Given the observed HCN flux density of
3.5 Jy , we could expect a  flux density of
50 mJy  if HCN is optically thin and =70. This is
consistent with the observed 3σ upper limit of 51 mJy .

The inferred disk-averaged nitrogen fractionation ratios range from 83±37
to 156±78. Despite the different physical conditions of the disks
in the sample, the observed / ratios are consistent with
sampling a constant disk-averaged fractionation level. In particular,
we find no difference in the nitrogen fractionation level between disks
around T Tauri and Herbig Ae stars, which have an order of magnitude
difference in the stellar radiation field. No difference in the
disk-averaged  is observed between full and transitional
disks, either. The age of the star does not seem to play an important
role either – we target young (∼1 Myr) and old (>10 Myr)
sources – suggesting either that the  is inherited from the
parent cloud and is not modified in the disk, or the disk chemistry
sets the global, disk averaged nitrogen fractionation level early
(≲1 Myr) in the protoplanetary disk life. 

Although we do not observe differences in the nitrogen fractionation ratio
between the sources, the data suggest that there may be a difference
in the nitrogen abundance between the disks around stars V4046 Sgr,
MWC 480 and HD 163206, and the disks around T Tauri stars AS 209 and
LkCa 15. The old disk around the binary T Tauri stars V4046 Sgr and
the two Herbig Ae stars in the sample are enriched in HCN compared to
the young T Tauri stars. Disk models have shown that dust migration
and carbon and oxygen depletion (mainly due to CO and H_2O
freeze-out) can increase the column density of cyanides by up to two
orders of magnitude in the outer disk <cit.>. Future
observations toward a larger sample of disks will show if this
corresponds to an evolutionary trend.



 §.§ Comparison between nitrogen fractionation in disks, Solar System and ISM


Fig. <ref> shows the observed  ratios in
different Solar System bodies, the cold ISM and in the diffuse medium.
There are large variations in the  ratio among different
Solar system bodies, in particular between the rocky and gaseous
bodies. The Sun has the highest  value in this comparison
(441±5), measured by the Genesis mission that sampled solar wind
ions, N^+ among them <cit.>. An almost identical value
was found in the atmosphere of Jupiter through the NH_3 observations
carried-out by the Cassini spacecraft <cit.>. Both
measurements are expected to trace the lack of nitrogen fractionation
in N_2, the main nitrogen reservoir of the protosolar nebula. The
^15N-depleted Solar value is thus considered to be representative
of the conditions of the gas when the Sun formed. All the other Solar
system bodies are enriched in ^15N compared to the Sun. A value of
=272 is found in the Earth's atmosphere, measured in
N_2. The  has also been measured in several comets. An
isotopic ratio of ∼150 was found in C/1995 O1 (Hale-Bopp) and
17P/Holmes, value which was consistent for both HCN and CN
<cit.>. Observations of 18 comets from both the Oort
cloud and the Kuiper Belt all show consistently low
HCN/≃100-250 ratios <cit.>.



The cold interstellar medium is also enriched in ^15N.
<cit.> measured the / toward two
prestellar cores, L183 and L1544, and found  values of
140-250 and 140-360, respectively. The  in CN was later
measured toward L1544 resulting in a surprisingly high CN/C^15N
ratio of 500±75 <cit.>. The authors were able to
reproduce the observed difference in CN and HCN with chemical models
of cold gas. The fact that CN and HCN present similar fractionation
levels in comets, could be explained if CN is produced in the coma
from photo-dissociation of HCN
<cit.>. <cit.> also found a low
HCN/ ratio of 151±16 toward the prestellar core
L1521E.

Finally, the averaged ^15N enrichment observed for HCN in
protoplanetary disks are similar to comets and the cold ISM. While the
similar nitrogen fractionation ratios found in the cold ISM, comets
and disks is consistent with an inheritance scenario for the origin of
organics in the Solar System, as we discuss in the next section, the
increasing  ratio in the disk of V4046 Sgr suggests that in
situ disk chemistry also contributes to the observed fractionation
patterns.








 §.§ Resolved nitrogen fractionation chemistry in disks



The bright emission of the HCN isotopologues in V4046 Sgr allows us to
trace the  profile across a disk for the first time. In
general, there are three possibilities for what could be observed: a
flat, decreasing or increasing  as a function of disk
radius. If  and  are inherited from the prestellar gas
and no further chemical processing occurs in the disk, then a constant
 ratio is expected across the disk. On the other hand, if the
nitrogen chemistry is altered by in-situ chemical fractionation in the
disk then a varying  is expected. In this case, if chemical
fractionation dominates the nitrogen fractionation then a low
 is expected to occur in the outer disk, where the gas
temperature is low (, a decreasing  with radius). In
contrast, if selective photodissociation is the dominant pathway to
fractionate HCN, then an increasing  profile would be
observed because this pathway is most important in regions exposed to
UV photons,  the inner disk which is illuminated by the central
star.

The observations toward V4046 Sgr show that both  and
 are best reproduced by a decreasing abundance
profile. However, the inferred  emission profile is slightly
steeper than that of , pointing to a higher fractionation in
the inner disk than in the outer disk. The varying  observed
in V4046 Sgr shows that there is an active nitrogen fractionation in
the disk, that changes the original fractionation pattern. The fact
that  is lower in the inner disk suggests that selective
photodissociation is indeed an important pathway to fractionate HCN in
the inner disk. Higher signal-to-noise ratio observations are needed
to determine if there is also an active N fractionation chemistry in
the outer disk.

There is additional evidence that cold ion-molecule fractionation does
not alone regulate the / ratio in
disks. <cit.> measured the D/H isotopic ratio in DCO^+
and HCN toward the same sample of disks. They found enhanced D/H
ratios compared to the elemental ratio in the local ISM
(∼2×10^-5) in all disks, with DCN/HCN ratios ranging from
0.005-0.08. If the cold pathway dominates the fractionation for both
species in these disks we could expect a correlation between the D/H
and  ratios. However, we do not see such correlation in the
sample (see Fig. <ref>).



 §.§ Future directions


We have shown that the  ratio increases with radius in the
disk around T Tauri stars V4046 Sgr. However, the angular resolution
of the current data (0”.5 corresponding to ∼36 AU at 73 pc)
prevents us from resolving  variations at smaller scales and
the low signal-to-noise ratio prevents us from constraining the
chemistry beyond 60 AU. Future observations at higher angular
resolution should allow us to measure the radial dependence of
 at Solar System scales, and determine how the fractionation
ratio changes from the inner (<15 AU) to the outer (>30 AU) disk.

High-angular resolution observations toward more disks are needed to
determine whether an increasing  is a general characteristic
of disks or unique to V4046 Sgr. A larger sample is also needed to
draw more general conclusions on the dominant fractionation pathways
in disks, and how the  pattern depends on the physical
conditions of the disk. In this respect, new chemical models that
include all nitrogen fractionation pathways (selective
photodissociation and cold isotope exchange reactions) as well as
inheritance from the parent cloud are key to interpret the
observations. In addition, it is desirable to include the
fractionation of both carbon and nitrogen in the models, since most
 measurements in both disks and cold ISM rely on observations
of the ^13C isotopologues to measure the contribution of the main
isotopologues.

In contrast to HCN, hydrides (, N_2H^+ and NH_3) are found
to be ^15N-depleted in the ISM. Toward L1544,
<cit.> found a nitrogen isotopic ratio in N_2H^+ of
1000. Toward the class 0 protostar B1b, <cit.> found
 ratios of 260-355 for NH_3 and an upper limit of >600
for N_2H^+. <cit.> proposed that the difference in
nitrogen fractionation between cyanides and hydrides in the cold ISM
is the result of their different chemical origins: HCN derives from
atomic N, while NH_3 and N_2H^+ derive from molecular
nitrogen. This hypothesis, however, is challenged by the recent
measurement of the  in NH_2, a photodissociation product of
NH_3 in cometary coma, toward comet C/2012 S1 (ISON), where a
fractionation ratio of 139±38 was found <cit.>. A
similarly low value (∼130) was found previously by
<cit.> based on the averaged spectrum of 12
comets. Toward comet ISON, CN and HCN were also found to be highly
enriched in ^15N (∼150). As explained by
<cit.>, one possibility to obtain similar HCN and NH_3
fractionation levels in comets, despite very different fractionation
levels in the cold ISM, is through grain surface chemistry. Indeed,
the ISM values represent the  in the gas-phase, while
cometary values represent the  in the ices. It is also
possible that the measured cometary values is not representative of
the composition of the nucleus. Measurements of the isotopic ratio in
NH_3 and N_2H^+ in disks would provide additional clues to
answer these questions. In particular, if the cyanide/hydride
dichotomy observed in the cold ISM holds for disks as well.

Finally, more observations toward comets targeting different
molecular parent species (, species tracing the cometary
nucleus and not daughter species which are produced in the coma) will
be important to compare with observations in the ISM and disks, and to
elucidate the origins of ^15N enhancements, and ultimately the
origins of organics, across the Solar System.



§ CONCLUSIONS



We have presented ALMA observations at ∼0”.5 angular resolution
of the HCN isotopologues in a diverse sample of six protoplanetary
disks. The sample contains 4 T Tauri and 2 Herbig Ae stars, which
sample an order of magnitude in radiation fields. Both  and
 are detected toward all the sources, except for IM Lup,
which is the most massive and coldest disk, and likely the youngest,
in the sample. Adopting a standard  ratio of 70 (with a 30%
uncertainty), we infer disk-averaged  ratios of 80-160 for
the sources. Despite the different physical conditions of the disks,
 in HCN is similar for all sources. No differences are
observed between T Tauri and Herbig Ae stars, or between the full
disks and the transitional disks, which feature large dust
cavities. Also, no correlation is observed between disk-averaged D/H
and  ratios in the sample. The observed disk-averaged
 ratios are similar to what is observed in comets and in the
cold ISM, which is consistent with the inheritance scenario for the
origin of organics in the Solar System. However, chemical processing
within the protoplanetary disk phase based on these ratios alone
cannot be ruled out. Indeed, in the one disk where we could resolve
the / ratio as a function of radius we find a slightly
steeper profile for ,  an increasing  ratio with
radius. The higher nitrogen fractionation level in the inner disk
compared to the outer disk suggest that selective photodissociation is
an important fractionation pathway in the inner disk.


This paper makes use of ALMA data, project code:
ADS/JAO.ALMA#2013.1.00226. ALMA is a partnership of ESO (representing
its member states), NSF (USA) and NINS (Japan), together with NRC
(Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic
of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and
NAOJ. The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative agreement by
Associated Universities, Inc. VVG thanks support from the Chilean
Government through the Becas Chile program. KIÖ also acknowledges
funding from the Packard Foundation and an investigator award from
Simons Collaboration on the Origins of Life (SCOL). JH and RL
acknowledge support from the National Science Foundation (Grant
No. DGE-1144152).




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figuresection





§ CHANNEL MAPS









