Spectroscopy
of the
15
NH
2
radical
Determination of the nitrogen isotopic
ratios in different bodies of the solar system
provides important information regarding the
solar system’s origin. For comets such a ratio
can now be measured for ammonia with
NH
2
emission lines thanks to new laboratory
spectra obtained at the AILES beamline.
These spectra have permitted to measure
accurately the
15
NH
2
line wavelengths. This
radical is produced by the photodissociation
of
15
NH
3
in comets. Thanks to a large dataset
of high-resolution spectra obtained with
the 8-m Very Large Telescope on different
comets it has been possible to identify
for the first time 7 different lines of
15
NH
2
.
Our data analysis has permitted to measure
the
14
N/
15
N isotopic ratio in comets for a
molecule carrying the amine (-NH) functional
group. This ratio, within the error, appears
similar to that measured in comets in the
HCN molecule and the CN radical, and lower
than the protosolar value, suggesting that
N
2
and NH
3
result from the separation of
nitrogen into two distinct reservoirs in the
solar nebula. This ratio also appears similar
to that measured in Titan’s atmospheric N
2
,
supporting the hypothesis that, if the latter
is representative of its primordial value in NH
3
,
these bodies were assembled from building
blocks sharing a common formation location.
The determination of nitrogen isotopic
ratios in solar system objects is of primary
importance for a good understanding of
their origin. The measurements of
14
N/
15
N
isotopic ratio done so far in different
solar system objects and molecules
have revealed a great diversity. This
ratio ranges from 441 for the present-
day Sun, considered as representative
of the protosolar nebula, to 50 in some
organic materials of chondrite and
Interplanetary Dust Particules (IDPs).
Any object of the solar system (except
Jupiter) is actually enriched in
15
N
compared to the protosolar nebula.
Different explanations have been proposed
to explain this enrichment that depends
of the type of molecules considered.
Some authors suggest, for example, that
the molecules carrying the nitrile- (-CN)
functional group would be more enriched
in
15
N than the molecules carrying the
amine (-NH) functional group.
Comets are interesting targets to test
this theory because they contain both HCN
and NH
3
molecules (leading to CN and NH
2
radicals after photodissociation by the
solar radiations). Up to recently the
14
N/
15
N
ratio had only been measured from HCN
and CN in comets. This ratio had been
measured in bright comets through optical
observations of the CN radical [1]
and millimeter observations of HCN [2].
These measurements give for both
species the same non-terrestrial isotopic
composition (
14
N/
15
N ≈ 150 in comets
versus 272 in the Earth atmosphere).
The difficulty for measuring
14
N/
15
N
in cometary ammonia was to identify
unambigously
15
NH
2
emission lines.
This identification implies accurate
knowledge of their wavelengths, poorly
known so far. An experiment conducted
with the AILES beamline spectrometer
has permitted to determine the
15
NH
2
wavelengths by Fourier transform
spectroscopy. The analysis of this
spectrum, recorded at the NH
2
Doppler
width resolution (0.05 cm
-1
, 0.017 Å),
had permitted to extract the
15
NH
2
emission lines wavelengths from the
mixture of
15
N
2
, H
2
and
15
NH
2
emission
lines. From this list of wavelengths
it had been possible to search for
15
NH
2
cometary emission lines not blended
with any other bright emission lines.
Thanks to a unique collection of cometary
high-resolution spectra collected on 12
different comets from 2002 to 2011 with
the UVES spectrometer at the VLT ESO
8-m telescope it has been possible
to search for
15
NH
2
emission lines with
a high sensitivity. Based on a single
averaged cometary spectrum it was
possible to identify seven different
15
NH
2
emission lines belonging to the (0,10,0)-
(0,0,0) band around 5700 Å. These lines
have permitted to compute an average
14
N/
15
N ratio for NH
2
(and, consequently,
cometary ammonia) similar to the one
already measured with CN/HCN, i.e.
lower than the protosolar nebula.
This scientific result, when compared
to other
14
N/
15
N measurements in solar
system objects, suggests that N
2
and NH
3
result from the separation of nitrogen into
two distinct reservoirs in the protosolar
nebula: one with a high
14
N/
15
N ratio
in N
2
(e.g. Jupiter) and another one with
a lower ratio in ammonia (Titan, comets...).
The ratio measured in comets also
appears similar to that measured
in Titan’s atmospheric N
2
(assumed
to come from ammonia), supporting
the hypothesis that, if the latter is
representative of its primordial value
in NH
3
, these bodies were assembled
from building blocks sharing a common
formation location.
ATOMIC AND MOLECULAR PHYSICS, DILUTE MATTER, UNIVERSE SCIENCE
78
SOLEIL
HIGHLIGHTS
2013
