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# The ALMA-PILS survey: First detection of nitrous acid (HONO) in the interstellar medium

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Nitrogen oxides are thought to play a significant role as a nitrogen reservoir and to potentially participate in the formation of more complex species. Until now, only NO, NO, and HNO have been detected in the interstellar medium. We report the first interstellar detection of nitrous acid (HONO). Twelve lines were identified towards component B of the low-mass protostellar binary IRAS 16293-2422 with the Atacama Large Millimeter/submillimeter Array, at the position where NO and NO have previously been seen. A local thermodynamic equilibrium model was used to derive the column density (∼9 × 1014 cm in a 0 .″5 beam) and excitation temperature (∼100 K) of this molecule. HNO, NO, NO+, and HNO3 were also searched for in the data, but not detected. We simulated the HONO formation using an updated version of the chemical code Nautilus and compared the results with the observations. The chemical model is able to reproduce satisfactorily the HONO, NO, and NO abundances, but not the NO, HNO, and NHOH abundances. This could be due to some thermal desorption mechanisms being destructive and therefore limiting the amount of HNO and NHOH present in the gas phase. Other options are UV photodestruction of these species in ices or missing reactions potentially relevant at protostellar temperatures.
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A&A 623, L13 (2019)
https://doi.org/10.1051/0004-6361/201935040
c
A. Coutens et al. 2019
Astronomy
&
Astrophysics
LETTER TO THE EDITOR
The ALMA-PILS survey: First detection of nitrous acid (HONO)
in the interstellar medium
A. Coutens1, N. F. W. Ligterink2, J.-C. Loison3, V. Wakelam1, H. Calcutt4, M. N. Drozdovskaya2, J. K. Jørgensen5,
H. S. P. Müller6, E. F. van Dishoeck7,8, and S. F. Wampﬂer2
1Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Georoy Saint-Hilaire, 33615 Pessac, France
e-mail: audrey.coutens@u-bordeaux.fr
2Center for Space and Habitability (CSH), University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
3Institut des Sciences Moléculaires (ISM), CNRS, Université Bordeaux, 351 cours de la Libération, 33400 Talence, France
4Department of Space, Earth and Environment, Chalmers University of Technology, 41296 Gothenburg, Sweden
5Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum of Denmark, University of Copenhagen,
Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
6I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
7Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
8Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
Received 10 January 2019 /Accepted 7 March 2019
ABSTRACT
Nitrogen oxides are thought to play a signiﬁcant role as a nitrogen reservoir and to potentially participate in the formation of more
complex species. Until now, only NO, N2O, and HNO have been detected in the interstellar medium. We report the ﬁrst interstellar de-
tection of nitrous acid (HONO). Twelve lines were identiﬁed towards component B of the low-mass protostellar binary IRAS 16293–
2422 with the Atacama Large Millimeter/submillimeter Array, at the position where NO and N2O have previously been seen. A local
thermodynamic equilibrium model was used to derive the column density (9×1014 cm2in a 0.
005 beam) and excitation temperature
(100 K) of this molecule. HNO, NO2, NO+, and HNO3were also searched for in the data, but not detected. We simulated the HONO
formation using an updated version of the chemical code Nautilus and compared the results with the observations. The chemical
model is able to reproduce satisfactorily the HONO, N2O, and NO2abundances, but not the NO, HNO, and NH2OH abundances.
This could be due to some thermal desorption mechanisms being destructive and therefore limiting the amount of HNO and NH2OH
present in the gas phase. Other options are UV photodestruction of these species in ices or missing reactions potentially relevant at
protostellar temperatures.
Key words. astrochemistry – stars: formation – stars: protostars – ISM: molecules – ISM: individual objects: IRAS 16293–2422
1. Introduction
The interstellar medium (ISM) is characterised by a rich and var-
ied chemistry with closely connected groups of species found to
be prominent in regions with diering physics. An example is
the group of nitrogen oxides, i.e. molecules containing nitrogen-
oxygen-hydrogen bonds. Secure interstellar detections have been
made for three molecules: nitric oxide (NO; e.g. Liszt & Turner
1978;McGonagle et al. 1990;Ziurys et al. 1991;Caux et al.
2011;Codella et al. 2018;Ligterink et al. 2018), nitrosyl hydride
(HNO; Snyder et al. 1993) and nitrous oxide (N2O; Ziurys et al.
1994;Ligterink et al. 2018). These species, in particular NO, are
thought to be critical for the overall nitrogen chemistry of the ISM
as they may lock up signiﬁcant amounts of atomic nitrogen, and
are often only second in abundance to molecular nitrogen (e.g.
Herbst & Leung 1986;Nejad et al. 1990;Pineau des Forêts et al.
1990;Visser et al. 2011). Nitrogen oxides can be at the basis
of greater chemical complexity, as demonstrated, for example,
with the solid-state hydrogenation of NO into hydroxylamine
(NH2OH; Congiu et al. 2012;Fedoseev et al. 2012,2016) or ener-
getic processing of N2O ice (de Barros et al. 2017).
Despite the relevance of nitrogen oxides as a nitrogen reser-
voir and as precursors of complex molecules, a number of
important members of this group have not yet been detected
in the ISM. Examples are nitrogen dioxide (NO2), nitrous acid
(HONO), and nitric acid (HNO3), which on Earth play a role in
atmospheric pollution (e.g. Possanzini et al. 1988). In particular,
OH radicals, which in turn engage in various oxidation reactions
and the formation of ground-level ozone (O3;Ren et al. 2003;
Lee et al. 2013;Gligorovski 2016;Zhang et al. 2016). Because
of its relevance in atmospheric chemistry, the formation, destruc-
tion, and characteristics of HONO have been well studied (e.g.
Cox & Derwent 1976;Jenkin et al. 1988;Joshi et al. 2012).
In this work, HONO and other nitrogen oxides were searched
for towards the low-mass protostar IRAS 16293–2422 (here-
after IRAS 16293), located at a distance of 140 pc in the
ρOphiuchus cloud complex (Dzib et al. 2018). This Class 0
object is known for its chemical complexity and is consid-
ered an astrochemical reference among solar-type protostars
(e.g. van Dishoeck et al. 1995;Cazaux et al. 2003;Caux et al.
2011;Jørgensen et al. 2016). A large number of species have
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
L13, page 1 of 9
A&A 623, L13 (2019)
ﬁrst been detected towards a low-mass source in this object.
These detections include the small species NO and N2O
(Caux et al. 2011;Ligterink et al. 2018), the simplest “sugar”
glycolaldehyde (HOCH2CHO; Jørgensen et al. 2012,2016), the
peptide-like molecules formamide (NH2CHO) and methyl iso-
cyanate (CH3NCO; Kahane et al. 2013;Coutens et al. 2016;
Ligterink et al. 2017;Martín-Doménech et al. 2017), cyanamide
(NH2CN; Coutens et al. 2018), methyl isocyanide (CH3NC;
Calcutt et al. 2018), ethylene oxide (c-C2H4O, Lykke et al.
2017), and the isomers of acetone (CH3COCH3) and propanal
(C2H5CHO, Lykke et al. 2017). Recently, the ﬁrst interstellar
detection of the organohalogen CH3Cl was also reported towards
this source (Fayolle et al. 2017).
In this Letter, we present the ﬁrst interstellar detection of
HONO. Further constraints on the nitrogen oxide chemistry
towards IRAS 16293 are given, and a ﬁrst attempt is made at
modelling the HONO formation network.
2. Observations and analysis
Data from the Protostellar Interferometric Line Survey (PILS)
of the low-mass protobinary IRAS 16293 were used to search
for nitrogen oxides. This survey, taken with the Atacama Large
Millimeter/submillimeter Array (ALMA), is fully described
in Jørgensen et al. (2016). A short overview is given in this
section. The survey covers part of Band 7 in the spectral range
329.147–362.896 GHz, at a spectral resolution of 0.2 km s1,
and with a sensitivity of 6–10 mJy beam1channel1(i.e.
4–5 mJy beam1km s1). A circular restoring beam of 0.
005 was
used to produce the ﬁnal dataset. IRAS 16293 is a binary.
HONO is identiﬁed towards source B, but not towards source
A. Source B is analysed at a position oset by one beam
with respect to the continuum peak position in the south-west
direction (αJ2000 =16h32m22.
s58, δJ2000 =2428032.800). The
very narrow line widths (1km s1) at this position limit line
blending and facilitate easier identiﬁcation of molecules (e.g.
Lykke et al. 2017).
To analyse the spectra and identify the HONO lines, the
CASSIS line analysis software1, as well as the Jet Propulsion
Laboratory (JPL2) spectroscopic database (Pickett et al. 1998)
and the Cologne Database for Molecular Spectroscopy (CDMS3;
Müller et al. 2001,2005) were used. The spectroscopy of HONO
available in the JPL database was studied by Guilmot et al.
(1993a,b) and Dehayem-Kamadjeu et al. (2005). HONO has two
dierent conformers, trans and cis. The JPL entry assumes that
the isomers are in thermal equilibrium. The trans/cis energy
dierence (130.2 cm1) is from Varma & Curl (1976). Since
the spectra of IRAS 16293 are very line-rich, a careful check
was performed to exclude blended or partially blended lines.
To achieve this, we compared all lines tentatively identiﬁed as
HONO with a template containing the lines of the molecules
previously detected in this source (see Appendix A). Similar to
previous PILS studies (e.g. Ligterink et al. 2018), the observed
spectra were ﬁtted with a synthetic spectrum, assuming local
thermodynamic equilibrium (LTE) conditions, using a source
size of 0.
005 and a VLSR velocity of 2.5 km s1. As the line emis-
sion is coupled with dust emission in IRAS 16293, a correc-
tion to the background temperature (TBG =21 K) was applied
1CASSIS has been developed by IRAP-UPS/CNRS (http://
cassis.irap.omp.eu/)
2http://spec.jpl.nasa.gov
3https://cdms.astro.uni-koeln.de/
imisation routine was employed to ﬁnd the best-ﬁt model to the
observed data and derive the column density (N) and excitation
Ligterink et al. 2018). The grid covers excitation temperatures
between 50 and 300 K with steps of 25 K. After a ﬁrst estimate
of the column density, the grid was reﬁned between 5×1014
and 3 ×1015 cm2with a step of 1 ×1014 cm2. To avoid any
bias in the determination of the best-ﬁt model with the χ2cal-
culation, we included some undetected transitions (333925.02,
348264.91, and 358979.13 MHz) that are predicted to be above
the noise limit for certain models in the grid.
3. Observational results
In total, we found 12 lines that could be identiﬁed as (trans-)
HONO, which are not blended with any known species (see
Fig. 1and Table A.1). The intensities of nine out of these
lines are higher or equal to 5σ. Two lines are 3 or 4σdetec-
tions and one is a marginal (2σ) detection. The best-ﬁt model
is obtained for an excitation temperature of 100 K and a col-
umn density of 9 ×1014 cm2. The column density is not very
sensitive to the excitation temperature. For a ﬁxed excitation
temperature of 300 K, which is derived for several complex
organic molecules (see Jørgensen et al. 2018), the best-ﬁt col-
umn density is 1.4 ×1015 cm2, i.e. only 50% larger. Nev-
ertheless, the model at 300 K overproduces some undetected
lines at 333925.02, 348264.91, and 358979.13 MHz (Table A.2)
and does not properly reproduce the line at 353468.14 MHz.
The model at 300 K however better reproduces the line at
329519.48 MHz than the model at 100 K (see Fig. 1). The best-ﬁt
excitation temperature of 100 K is consistent with the excitation
temperature obtained for the other nitrogen oxides, especially
NO (Ligterink et al. 2018). Three lines (329519.48, 329685.92,
and 355001.15 GHz) have their ﬂuxes underproduced by the
best-ﬁt model and could be blended with unknown species,
although the ﬁrst line is only detected at 3σ. Alternatively,
it could be that for molecules with low-frequency vibrational
modes such as HONO, the excitation does not need to be in LTE,
but there could be infrared pumping for selected lines.
Lines of HONO were also searched towards other high
sensitivity ALMA observations of the low-mass protostar
IRAS 16293. One line is present at 93008.6 MHz in the lower
spatial resolution data of the ALMA-PILS observations carried
out in band 3 (Jørgensen et al. 2016, see Fig. 1). None are present
in the band 6 data. According to our calculations, one HONO
transition at 236131.076 MHz should also be observed in the
ALMA data presented in Taquet et al. (2018) with an intensity
of 7 mJy for a similar spatial resolution of 0.
005. An unidentiﬁed
line is present at the same frequency, but its intensity is a factor 3
higher, which could mean that the observed line is blended with
another species (see Fig. 1).
Maps of HONO (see Fig. 2) show that the emission is
very compact around IRAS 16293 B, similar to the majority
of the molecules detected in this source, especially the com-
plex organic molecules (see e.g. Coutens et al. 2016,2018;
Lykke et al. 2017) and NO (Ligterink et al. 2018).
Four other nitrogen-oxides, nitrosyl hydride (HNO), nitrosyl
cation (NO+), nitrogen dioxide (NO2), and nitric acid (HNO3)
were searched for, but not identiﬁed (see Appendix Bfor
details). Table 1gives an overview of the derived column den-
sities of the detected and unidentiﬁed species (upper limits)
towards IRAS 16293 B and includes results on NO, N2O, and
NH2OH from Ligterink et al. (2018).
L13, page 2 of 9
A. Coutens et al.: First detection of HONO in the interstellar medium
329.515 329.520
-0.02
0.00
0.02
0.04
0.06
(Jy/beam)
367 K
329.685 329.690
259 K
329.825 329.830
216 K
330.070 330.075
181 K
330.280 330.285
154 K
353.465 353.470
33 K
353.715 353.720
-0.02
0.00
0.02
0.04
0.06
(Jy/beam)
198 K
353.910 353.915 353.920
171 K
355.000 355.005
171 K
360.675 360.680 360.685
153 K
361.575 361.580
152 K
362.395 362.400
188 K
333.920 333.925
Frequency (GHz)
-0.02
0.00
0.02
0.04
0.06
(Jy/beam)
270 K
348.260 348.265
Frequency (GHz)
443 K
358.975 358.980 358.985
Frequency (GHz)
295 K
93.008 93.010
Frequency (GHz)
x 2
21 K
236.128 236.132
Frequency (GHz)
x 2
27 K
Fig. 1. Lines of HONO observed towards the protostar IRAS 16293 B (in black). The ﬁrst 12 lines are the identiﬁed lines of HONO in the ALMA-
PILS band 7 survey. On the last row, the ﬁrst 3 lines correspond to the undetected transitions that are used to constrain the best-ﬁt model and
the last two lines are those identiﬁed in other ALMA data (see Sect. 3for more details). The 3σlimit is indicated by a dotted line. The best-ﬁt
model with Tex =100 K is shown in blue, while the model in red corresponds to a higher Tex of 300 K. The spectrum at 93 GHz is extracted at
the continuum peak position, given the lower spatial resolution of the data. The column density was multiplied by a factor 2 to take this dierence
into account (Jørgensen et al. 2016). The upper energy level is indicated in green in the bottom left corner of each panel.
329.829 GHz
Eup = 216 K
353.468 GHz
Eup = 33 K
Fig. 2. Integrated intensity maps of two transitions of HONO towards
IRAS 16293 B. The position of the continuum peak is indicated with a
red triangle, while the position analysed for IRAS 16293 B (full-beam
oset) is indicated with a red circle. The beam size is indicated in grey
in the bottom right corner. Left panel: contours are 5, 10, 15, and 20σ.
Right panel: contour levels are 3, 6, and 9σ.
4. Chemical modelling of HONO
To describe HONO and other NxOyHzspecies, we updated the
gas and grain chemical network used in Loison et al. (2019, and
references therein) introducing various species such as HONO,
s-HONO, s-HNO2, s-NH2O, s-HNOH, NH2OH, s-NH2OH,
s-NO3, s-HNO3, and s-H3NO2(s- indicates species on grains).
The reactions involving HONO are summarised in Table C.1 (we
do not present the full network in this Letter). The chemistry of
HONO is well described for Earth atmosphere chemistry where
it is produced through the barrierless three body OH +NO +M
reaction (Forster et al. 1995;Atkinson et al. 2004). However,
this reaction is inecient at the low densities of interstellar
clouds and the radiative rate constant is negligible because of
the small size of the system. All other known gas-phase reac-
Table 1. Column densities at the one beam oset position of
IRAS 16293 B from the ALMA-PILS data.
Molecule Formula N
tot (cm2)Tex (K)
Nitrous acid HONO (9 ±5) ×1014 100
Nitric oxideNO (2.0 ±0.5) ×1016 40–150
Nitrous oxideN2O4.0 ×1016 25–350
HydroxylamineNH2OH 4×1014 [100]
Nitrosyl hydride HNO 3×1014 [100]
Nitrogen dioxide NO22×1016 [100]
Nitrosyl cation NO+2×1014 [100]
Nitric acid HNO35×1014 [100]
Notes. All models assume LTE, FWHM of 1 kms1, peak velocity Vpeak
of 2.5 ±0.2 km s1, and source size of 0.
005. ()The uncertainties are 3σ.
Upper limits are also 3σand determined for an assumed Tex =100 K,
indicated with brackets in the table. ()Results from Ligterink et al.
(2018).
tions producing HONO have negligible rates in the ISM. In our
model, HONO is therefore produced on grains through s-O +
s-HNO, s-H +s-NO2, and s-OH+s-NO surface reactions, all of
which are barrierless in the gas phase (Inomata & Washida 1999;
Du et al. 2004;Michael et al. 1979;Nguyen et al. 1998;Su et al.
2002;Forster et al. 1995;Atkinson et al. 2004).
This network was then used with the Nautilus gas-grain
model (Ruaud et al. 2016), which computes the gas and grain
chemistry. The chemical modelling was carried out in two
steps as in similar previous studies of IRAS 16293 (see for
instance Andron et al. 2018): a cold core phase (a gas and dust
temperature of 10 K, atomic H density of 104cm3, visual
L13, page 3 of 9
A&A 623, L13 (2019)
Table 2. Abundances of HONO derived in IRAS 16293 B and with the chemical model.
[HONO]/[NO] [HONO]/[N2O] [HONO]/[HNO] [HONO]/[NO2] [HONO]/[NH2OH] [HONO]/[CH3OH]
IRAS 16293 B 4.5 ×1022.3 ×102,34.5 ×1022.3 9 ×105
Chemical model 71 2 ×1021×102533 1.7 ×1031.9×104
Notes. ()Upper limit due to a lower limit on N2O.
extinction (AV) of 15, and cosmic-ray ionisation rate of 1.3×
1017 s1) during 106yr followed by a collapse phase. For the
collapse, we used the physical structure derived from a 1D radia-
tive hydrodynamical model (see Aikawa et al. 2008) for parcels
of material collapsing towards the central star. For these simu-
lations, we used these parcels arriving at 62.4 au at the end of
the simulations (see Fig. 5 of Aikawa et al. 2008). The resulting
abundance ratios at this radius at the end of the simulation are
presented in Table 2.
In our model, HONO is essentially formed during the cold
core phase. The ﬁnal HONO/CH3OH ratio predicted by the
model is close to the observed value within a factor of 2. The
model ratios HONO/N2O and HONO/NO2are also in agreement
with the observed upper and lower limits, respectively. How-
ever, our model produces too little NO and too much HNO at
high temperatures in the gas phase, resulting in a HONO/NO
ratio much larger and a HONO/HNO ratio much smaller than the
respective observed values. In our model, most of the NO reacts
on grains with other radicals such as s-NH, when the temperature
increases and NO becomes mobile. An explanation for the large
NO/HNO ratio observed in IRAS 16923 B could be that ther-
mal desorption of s-HNO mainly results in its destruction to NO,
owing to the weak H-NO bond of HNO (2.02 eV; Dixon 1996),
as the formation of s-HNO is very likely due to the absence
of a barrier for the s-H +s-NO reaction (Tsang & Herron 1991;
Nguyen et al. 2004;Washida et al. 1978;Glarborg et al. 1998).
It should be noted that our model, as well as other published
models, overproduce the abundance of NH2OH, which has so far
not been detected in the ISM (Pulliam et al. 2012;McGuire et al.
2015;Ligterink et al. 2018). It has been suggested that NH2OH
cannot desorb without destruction by Jonusas & Krim (2016),
although this is in contradiction with the laboratory experiments
of Congiu et al. (2012). Despite their dierences, both experi-
mental studies used very similar Temperature Programmed Des-
orption (TPD) set-ups and new experiments are therefore clearly
needed to address these discrepancies. Other processes such
as UV photodestruction of these species in ices could also
explain the discrepancy between the model and observations
(Fedoseev et al. 2016). In addition, the chemical network on
grains and in the gas phase may not be fully relevant at proto-
stellar temperatures. Some reactions with barriers, absent from
the current network, may be signiﬁcant.
5. Conclusions
We report the ﬁrst detection of HONO in the ISM. This
molecule, which is known to play a major role in the atmosphere
of our planet, was found with ALMA towards the well-studied
solar-type protostar IRAS 16293 B. This discovery complements
the recent detection of N2O in the same source (Ligterink et al.
2018) and expands our knowledge of the chemical network of
nitrogen oxides. Our updated model allows the abundances of
HONO, N2O, and NO2to be reproduced satisfactorily, but not
those of NO, HNO, and NH2OH. One reason could be that HNO
and NH2OH are destroyed upon thermal desorption, an occur-
rence which deserves to be experimentally studied in detail.
Other explanations could be that they are destroyed by UV
photons in ices or that some grain surface or gas-phase reac-
tions, potentially relevant at protostellar temperatures, are miss-
ing from the network.
Acknowledgements. This paper makes use of the ALMA data ADS/JAO.
ALMA#2013.1.00278.S. 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. A.C.
postdoctoral grant is funded by the ERC Starting Grant 3DICE (grant agree-
ment 336474). V.W. and J.-C.L. acknowledge the CNRS programme Physique
et Chimie du Milieu Interstellaire (PCMI) co-funded by the Centre National
d’Etudes Spatiales (CNES). M.N.D. acknowledges the ﬁnancial support of
the SNSF Ambizione grant 180079, the Center for Space and Habitability
(CSH) Fellowship and the IAU Gruber Foundation Fellowship. J.K.J. acknowl-
edges support from ERC Consolidator Grant “S4F” (grant agreement 646908).
Research at the Centre for Star and Planet Formation is funded by the Danish
National Research Foundation.
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A&A 623, L13 (2019)
Appendix A: Lines of HONO
with known species are listed in Table A.1. To check the
potential blending of the HONO lines with other species,
we deﬁned a template based on the molecules previously
identiﬁed in the ALMA-PILS survey. This template includes
the following species (ranked by mass): CCH, HCN, HNC,
H13CN, HC15 N, DNC, CO, 13CO, C17 O, H13C15N, CH2NH,
NO, C18O, DCO+, H2CO, HDCO, H13
2CO, H2C17O, D2CO,
H2C18O, CH3OH, CH2DOH, CH3OD, 13 CH3OH, D13
2CO,
H2S, CH18
3OH, HDS, HD34S, c-C3H2, CH3CCH, CH3CN,
CH3NC, NH2CN, H2CCO, 13CH3CN, CH13
3CN, CH3C15N,
CH2DCN, H2C13CO, H13
2CCO, HDCCO, HNCO, CHD2CN,
NH13
2CN, NHDCN, CH3CHO, N2O, DNCO, HN13CO,
CS, c-C2H4O, SiO, CH3CDO, 13CH3CHO, CH13
3CHO,
C33S, NH2CHO, C34 S, t-HCOOH, H2CS, NH13
2CHO, cis-
NHDCHO, trans-NHDCHO, NH2CDO, CH3OCH3, C2H5OH,
DCOOH, HCOOD, t-H13COOH, HDCS, a-CH13
3CH2OH,
a-13CH3CH2OH, a-CH3CH2OD, a-CH3CHDOH, a-a-
CH2DCH2OH, a-s-CH2DCH2OH, 13CH3OCH3, a-CH2DOCH3,
sym-CH2DOCH3, SO, C36S, CH3SH, CH35
3Cl, HC3N, CH37
3Cl,
C2H3CN, C2H5CN, CH3NCO, CH3COCH3, C2H5CHO,
CH3OCHO, CH2(OH)CHO, OCS, CH3COOH, O13CS,
OC33S, CH2(OH)13 CHO, 13CH2(OH)CHO, CH3O13 CHO,
CH2(OD)CHO, CHD(OH)CHO, CH2(OH)CDO, CH3OCDO,
CH2DOCHO, CHD2OCHO, aGg’-(CH2OH)2, gGg’-(CH2OH)2,
OC34S, 18 OCS, CHD2OCHO, SO2, and 34SO2. Figure A.1
presents the HONO lines over a larger spectral range with the
overlaid template model in green and the HONO model in red.
Table A.1. List of the detected and unblended HONO transitions.
Molecule Transition Frequency Eup Aij gup
(MHz) (K) (s1)
trans-HONO 14 8 6–13 8 5 329519.5 367.2 2.57 ×10429
trans-HONO 14 8 7–13 8 6 329519.5 367.2 2.57 ×10429
trans-HONO 14 6 8–13 6 7 329685.9 258.6 3.12 ×10429
trans-HONO 14 6 9–13 6 8 329685.9 258.6 3.12 ×10429
trans-HONO 14 5 10–13 5 9 329828.8 215.9 3.34 ×10429
trans-HONO 14 5 9–13 5 8 329829.2 215.9 3.34 ×10429
trans-HONO 14 4 11–13 4 10 330071.5 181.0 3.52 ×10429
trans-HONO 14 3 12–13 3 11 330281.2 153.8 3.67 ×10429
trans-HONO 5 2 4–4 1 3 353468.1 32.5 1.50 ×10411
trans-HONO 15 4 12–14 4 11 353716.9 198.0 4.40 ×10431
trans-HONO 15 3 13–14 3 12 353915.2 170.8 4.55 ×10431
trans-HONO 15 3 12–14 3 11 355001.2 171.0 4.59 ×10431
trans-HONO 15 2 13–14 2 12 360679.9 152.9 4.93 ×10431
trans-HONO 16 1 16–15 1 15 361578.0 151.9 5.04 ×10433
trans-HONO 17 2 16–17 1 17 362397.6 187.8 2.77 ×10435
Notes. Quantum numbers are given as J0K0
aK0
cJ00 K00
aK00
c.
Table A.2. List of the undetected HONO transitions used in the χ2calculation.
Species Transition Frequency Eup Aij gup
(MHz) (K) (s1)
trans-HONO 21 1 20–21 0 21 333925.0 270.3 5.42 ×10443
cis-HONO 20 1 19–20 0 20 348264.9 443.2 7.87 ×10441
trans-HONO 22 1 21–22 0 22 358979.1 295.3 6.82 ×10445
Notes. Quantum numbers are given as J0K0
aK0
cJ00 K00
aK00
c.
L13, page 6 of 9
A. Coutens et al.: First detection of HONO in the interstellar medium
329.5 329.6 329.7 329.8
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
329.9 330.0 330.1 330.2
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
353.2 353.3 353.4 353.5
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
353.6 353.7 353.8 353.9
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
354.8 354.9 355.0 355.1
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
360.5 360.6 360.7 360.8
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
361.4 361.5 361.6 361.7
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
362.2 362.3 362.4 362.5
Frequency (GHz)
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
(Jy/beam)
Fig. A.1. Lines of HONO observed towards the protostar IRAS 16293 B over a larger spectral range. The HONO best-ﬁt model is shown in red,
while the template model used to check the potential blending of the lines is overlaid in green.
L13, page 7 of 9
A&A 623, L13 (2019)
Appendix B: Upper limit determination of HNO,
NO+, NO2, and HNO3
The molecules HNO, NO+, NO2, and HNO3were searched for
in the PILS data without success. Upper limit column densities
of 3σwere determined for Tex =100 K. When possible, line-
free regions of the observed data, where transitions are expected,
were used. For HNO, the 41,3–31,2transition at 332106.6 MHz,
which is close to the transition of an unidentiﬁed species, was
used. For NO+, only the 3–2 transition at 357.564 GHz is cov-
ered in the PILS range. It is found to be blended with a line of
ethylene glycol. We determined a conservative upper limit based
on the total ﬂux of this line. For NO2, the undetected transitions
at 348062.5 and 348820.7 MHz were used. For HNO3, many
undetected lines are covered in the spectral range of the PILS
survey. The upper limit was derived based on the brightest lines
predicted in line-free regions. At Tex =100 K, this yields 3σ
upper limit column densities of 3 ×1014, 2 ×1014, 2 ×1016, and
5×1014 cm2for HNO, NO+, NO2, and HNO3, respectively.
Appendix C: Chemical reactions for HONO
The reactions involving HONO are listed in Table C.1.
L13, page 8 of 9
A. Coutens et al.: First detection of HONO in the interstellar medium
Table C.1. Summary of the reactions involving HONO
Reaction E
kJ mol1α β γ F0gReference
1. H++HONO
-
HONO++H
H2O+NO+
235
693
0
1.0 2.0×1095.27 3 0 Ionpol1, capture rate theory. We avoid introducing HONO+.
2. He++HONO HONO++He
HO +NO++He
0
1.0 1.0×1095.27 3 0 Ionpol1, capture rate theory. We avoid introducing HONO+.
3. C +HONO CO +NO +H459 3.0×1010 0 0 2 0 Capture rate theory, approximate branching ratio.
4. C++HONO CO +NO++H
HCO++NO
635
817
2.0×109
0
0.4
0.4
0
0
3
3
0
0Capture rate theory, approximate branching ratio. (HCO++NO is likely a non-
negligible exit channel).
5. OH +HONO NO2+H2O21 7.0×1012 0.6 0 1.6 10 This reaction has been studied between 278 and 373K. The results from
Jenkin & Cox (1987) show large uncertainty due to complicated secondary
reactions. The results from Burkholder et al. (1992) show no barrier and a neg-
ative temperature dependency. We use an expression compatible with experi-
mental data and leading to reasonable rate constant at 10K.
6.
HONO +H+
3H2ONO++H2
HONOH++H2
HONHO++H2
358
232
196
1.0
0
0
2.38×1095.27 2 0 Ionpol1. The others isomers are also likely to be produced but we avoid intro-
ducing too many species without a notably dierent chemical behaviour.
7. HONO +HCO+H2ONO++CO 210 1.0 9.45×1010 5.27 2 0 Ionpol1.
8.
H2ONO++eHONO +H
H2O+NO
H+OH +NO
H2O+N+O
524
825
339
207
2.0×107
1.0×107
2.0×107
1.0×107
0.5
0.5
0.5
0.5
0
0
0
0
3
3
3
3
0
0
0
0
Rate by comparison with similar DR (Fournier et al. 2013;
Florescu-Mitchell & Mitchell 2006;Geppert et al. 2004) and branching
ratios deduced roughly from similar reactions using Plessis et al. (2012). The
important fact is that HONO is likely a non-negligible product.
9.
s-H +s-NO2s-HONO
HNO2
s-NO +s-OH
317
281
132
0.7
0.3
0
0
0
Nguyen et al. 1998;Su et al. 2002). Both approach towards N and O atoms
are attractive.
10.
s-H +s-HONO s-H2NO2
s-H2+s-NO2
s-NO +s-H2O
s-HON +s-OH
163
109
301
+164
1
0
0
0
2600 We use the theoretical work from Hsu et al. (1997). H2NO2=(HN(O)OH).
Some NO +H2O are likely to be produced but this exit channel involves a
transition state close to the H +HONO entrance level.
11.
s-H +s-H2NO2s-H3NO2
s-HON +s-H2O
s-HNO +s-H2O
302
159
327
1
0
0
0M06-2X/AVTZ calculations (this work). H3NO2=HO-NH-OH. Some HNO
may be produced.
12. s-OH +s-NO s-HONO 185 1 0 The OH +NO reaction is a radical-radical barrierless reaction (Forster et al.
1995).
Notes. Exothermicities of the reactions (E in kJ/mol) are calculated at M06-2X/AVTZ level using Gaussian 2009 software. Deﬁnitions of α,β,γ, F0, g, Ionpol1 and Ionpol2 can been found in
Wakelam et al. (2010,2012): k =α×(T/300)β×exp(-γ/T) cm3molecule1s1, T range is 10-300K except in some cases (noted). Ionpol1: k =αβ(0.62+0.4767γ(300/T)0.5) cm3molecule1s1,
Ionpol2: k =αβ(1+0.0967γ(300/T)0.5+(γ2/10.526) (300/T)) cm3molecule1s1, F0=exp(k/k0) (1+(k/k0) and F(T)=F0exp(g|1/T-1/T0|).
L13, page 9 of 9
... fitting and noise attenuation using Locally Weighted Scatterplot Smoothing (LOWESS) and Savitzky-Golay (S-G) filter. Confirming previous research, the results presented here suggest that the relative absence of N-O bearing species in interstellar objects may be due to unfavourable formation mechanisms rather than to detection constraints from space or Earth-based telescopes (McGonagle 1995;Ponciano et al. 2008;Pulliam, McGuire & Remijan 2012;Coutens et al. 2019). The results show that only five molecular species containing an N-O bond were found (including two species that have also been detected in the ISM), but none of the reported species bear at least one carbon atom in addition to the N-O bond. ...
... In conclusion, even though the Cosmo-chemistry of simple molecules, such as HNO, HONO, NO, and N2O, have been extensively studied (e.g. Ziurys et al. 1994;Jamieson et al. 2005;Congiu et al. 2012;Fedoseev et al. 2016;Coutens et al. 2019), the feasibility of the existence of larger N-O species in ISM objects is far from being certain. In fact, Jamieson et al. (2006) and Duarte et al. (2010) concluded that ozone and oxygen atoms did not play a significant role in the formation routes larger oxocarbons upon cosmic-ray processing of CO ices. ...
Article
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Preprint
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Context. Several sugar-like molecules have been found in the interstellar medium (ISM). The molecule studied in this work, 2-hydroxyprop-2-enal, is among the candidates to be searched for, as it is a dehydration product of C 3 sugars and contains structural motifs that are typical for some interstellar molecules. Furthermore, it has recently been predicted that it is more abundant in the ISM than its tentatively detected isomer 3-hydroxypropenal. Aims. So far, only low-frequency microwave data of 2-hydroxyprop-2-enal have been published. The aim of this work is to deepen our knowledge about the millimetre-wave spectrum of 2-hydroxyprop-2-enal, enabling its detailed search towards astronomical objects. In particular, we target the solar-type protostar IRAS 16293-2422 and the star-forming region Sagittarius (Sgr) B2(N). Methods. The rotational spectrum of 2-hydroxyprop-2-enal was measured and analysed in the frequency regions of 128-166 GHz and 285-329 GHz. The interstellar exploration towards IRAS 16293-2422 was based on the Atacama Large Millimeter/submillimeter Array (ALMA) data of the Protostellar Interferometric Line Survey (PILS). We also used the imaging spectral line survey ReMoCA performed with ALMA towards Sgr B2(N) to search for 2-hydroxyprop-2-enal in the ISM. We modelled the astronomical spectra under the assumption of local thermodynamic equilibrium (LTE). Results. We provide laboratory analysis of hundreds of rotational transitions of 2-hydroxyprop-2-enal in the ground state and the lowest lying excited vibrational state. We report its non-detection towards IRAS 16293 B. The 2-hydroxyprop-2-enal/3-hydroxypropenal abundance ratio is estimated to be ≲0.9–1.3, in agreement with the predicted value of ~1.4. We report the non-detection of 2-hydroxyprop-2-enal towards the hot molecular core Sgr B2(N1), and we did not detect the related aldehydes 2-hydroxypropanal and 3-hydroxypropenal either. We find that these three molecules are at least nine, four, and ten times less abundant than acetaldehyde in this source, respectively. Conclusions. Despite the non-detections of 2-hydroxyprop-2-enal, the results of this work represent a significant improvement on previous investigations in the microwave region and meet the requirements for further searches for this molecule in the ISM.
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To date, 241 individual molecular species, composed of 19 different elements, have been detected in the interstellar and circumstellar medium by astronomical observations. These molecules range in size from two atoms to 70 and have been detected across the electromagnetic spectrum from centimeter wavelengths to the ultraviolet. This census presents a summary of the first detection of each molecular species, including the observational facility, wavelength range, transitions, and enabling laboratory spectroscopic work, as well as listing tentative and disputed detections. Tables of molecules detected in interstellar ices, external galaxies, protoplanetary disks, and exoplanetary atmospheres are provided. A number of visual representations of these aggregate data are presented and briefly discussed in context.
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Methyl cyanide (CH3CN) and propyne (CH3CCH) are two molecules commonly used as gas thermometers for interstellar gas. They are detected in several astrophysical environments and in particular towards protostars. Using data of the low-mass protostar IRAS 16293-2422 obtained with the IRAM 30m single-dish telescope, we constrained the origin of these two molecules in the envelope of the source. The line shape comparison and the results of a radiative transfer analysis both indicate that the emission of CH3CN arises from a warmer and inner region of the envelope than the CH3CCH emission. We compare the observational results with the predictions of a gas-grain chemical model. Our model predicts a peak abundance of CH3CCH in the gas-phase in the outer part of the envelope, at around 2000 au from the central star, which is relatively close to the emission size derived from the observations. The predicted CH3CN abundance only rises at the radius where the grain mantle ices evaporate, with an abundance similar to the one derived from the observations.
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IRAS 16293-2422 is a very well studied young stellar system seen in projection towards the L1689N cloud in the Ophiuchus complex. However, its distance is still uncertain with a range of values from 120 pc to 180 pc. Our goal is to measure the trigonometric parallax of this young star by means of H$_2$O maser emission. We use archival data from 15 epochs of VLBA observations of the 22.2 GHz water maser line. By modeling the displacement on the sky of the H$_2$O maser spots, we derived a trigonometric parallax of $7.1\pm1.3$ mas, corresponding to a distance of $141_{-21}^{+30}$ pc. This new distance is in good agreement with recent values obtained for other magnetically active young stars in the L1689 cloud. We relate the kinematics of these masers with the outflows and the recent ejections powered by source A in the system.
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Searches for the prebiotically-relevant cyanamide (NH$_2$CN) towards solar-type protostars have not been reported in the literature. We here present the first detection of this species in the warm gas surrounding two solar-type protostars, using data from the Atacama Large Millimeter/Submillimeter Array Protostellar Interferometric Line Survey (PILS) of IRAS 16293-2422 B and observations from the IRAM Plateau de Bure Interferometer of NGC1333 IRAS2A. We furthermore detect the deuterated and $^{13}$C isotopologues of NH$_2$CN towards IRAS 16293-2422 B. This is the first detection of NHDCN in the interstellar medium. Based on a local thermodynamic equilibrium analysis, we find that the deuteration of cyanamide ($\sim$ 1.7%) is similar to that of formamide (NH$_2$CHO), which may suggest that these two molecules share NH$_2$ as a common precursor. The NH$_2$CN/NH$_2$CHO abundance ratio is about 0.2 for IRAS 16293-2422 B and 0.02 for IRAS2A, which is comparable to the range of values found for Sgr B2. We explored the possible formation of NH$_2$CN on grains through the NH$_2$ + CN reaction using the chemical model MAGICKAL. Grain-surface chemistry appears capable of reproducing the gas-phase abundance of NH$_2$CN with the correct choice of physical parameters.
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The high-sensitivity of the IRAM 30-m ASAI unbiased spectral survey in the mm-window allows us to detect NO emission towards both the Class I object SVS13-A and the protostellar outflow shock L1157-B1. We detect the hyperfine components of the $^2\Pi_{\rm 1/2}$ $J$ = 3/2 $\to$ 1/2 (at 151 GHz) and the $^2\Pi_{\rm 1/2}$ $J$ = 5/2 $\to$ 3/2 (250 GHz) spectral pattern. The two objects show different NO profiles: (i) SVS13-A emits through narrow (1.5 km s$^{-1}$) lines at the systemic velocity, while (ii) L1157-B1 shows broad ($\sim$ 5 km s$^{-1}$) blue-shifted emission. For SVS13-A the analysis leads to $T_{\rm ex}$ $\geq$ 4 K, $N(\rm NO)$ $\leq$ 3 $\times$ 10$^{15}$ cm$^{-2}$, and indicates the association of NO with the protostellar envelope. In L1157-B1, NO is tracing the extended outflow cavity: $T_{\rm ex}$ $\simeq$ 4--5 K, and $N(\rm NO)$ = 5.5$\pm$1.5 $\times$ 10$^{15}$ cm$^{-2}$. Using C$^{18}$O, $^{13}$C$^{18}$O, C$^{17}$O, and $^{13}$C$^{17}$O ASAI observations we derive an NO fractional abundance less than $\sim$ 10$^{-7}$ for the SVS13-A envelope, in agreement with previous measurements towards extended PDRs and prestellar objects. Conversely, a definite $X(NO)$ enhancement is measured towards L1157-B1, $\sim$ 6 $\times$ 10$^{-6}$, showing that the NO production increases in shocks. The public code UCLCHEM was used to interpret the NO observations, confirming that the abundance observed in SVS13-A can be attained in an envelope with a gas density of 10$^5$ cm$^{-3}$ and a kinetic temperature of 40 K. The NO abundance in L1157-B1 is reproduced with pre-shock densities of 10$^5$ cm$^{-3}$ subjected to a $\sim$ 45 km s$^{-1}$ shock.
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Organohalogens, a class of molecules that contain at least one halogen atom bonded to carbon, are abundant on the Earth where they are mainly produced through industrial and biological processes ¹. Consequently, they have been proposed as biomarkers in the search for life on exoplanets ². Simple halogen hydrides have been detected in interstellar sources and in comets, but the presence and possible incorporation of more complex halogen-containing molecules such as organohalogens into planet-forming regions is uncertain 3,4. Here we report the interstellar detection of two isotopologues of the organohalogen CH3Cl and put some constraints on CH3F in the gas surrounding the low-mass protostar IRAS 16293-2422, using the Atacama Large Millimeter/submillimeter Array (ALMA). We also find CH3Cl in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) by using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument. The detections reveal an efficient pre-planetary formation pathway of organohalogens. Cometary impacts may deliver these species to young planets and should thus be included as a potential abiotical production source when interpreting future organohalogen detections in atmospheres of rocky planets.
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Recent measurements carried out at comet 67P/Churyumov–Gerasimenko (67P) with the Rosetta probe revealed that molecular oxygen, O₂, is the fourth most abundant molecule in comets. Models show that O₂ is likely of primordial nature, coming from the interstellar cloud from which our solar system was formed. However, gaseous O₂ is an elusive molecule in the interstellar medium with only one detection towards quiescent molecular clouds, in the ρ Oph A core. We perform a deep search for molecular oxygen, through the 2₁−0₁ rotational transition at 234 GHz of its ¹⁶O¹⁸O isotopologue, towards the warm compact gas surrounding the nearby Class 0 protostar IRAS 16293–2422 B with the ALMA interferometer. We also look for the chemical daughters of O₂, HO₂, and H₂O₂. Unfortunately, the H₂O₂ rotational transition is dominated by ethylene oxide c-C₂H₄O while HO₂ is not detected. The targeted ¹⁶O¹⁸O transition is surrounded by two brighter transitions at ± 1 km s⁻¹ relative to the expected ¹⁶O¹⁸O transition frequency. After subtraction of these two transitions, residual emission at a 3σ level remains, but with a velocity offset of 0.3−0.5 km s⁻¹ relative to the source velocity, rendering the detection “tentative”. We derive the O₂ column density for two excitation temperatures Tₑₓ of 125 and 300 K, as indicated by other molecules, in order to compare the O₂ abundance between IRAS 16293 and comet 67P. Assuming that ¹⁶O¹⁸O is not detected and using methanol CH₃OH as a reference species, we obtain a [O₂]/[CH₃OH] abundance ratio lower than 2−5, depending on the assumed Tₑₓ, a three to four times lower abundance than the [O₂]/[CH₃OH] ratio of 5−15 found in comet 67P. Such a low O₂ abundance could be explained by the lower temperature of the dense cloud precursor of IRAS 16293 with respect to the one at the origin of our solar system that prevented efficient formation of O₂ in interstellar ices.
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Context. One of the important questions of astrochemistry is how complex organic molecules, including potential prebiotic species, are formed in the envelopes around embedded protostars. The abundances of minor isotopologues of a molecule, in particular the D- and ¹³ C-bearing variants, are sensitive to the densities, temperatures and timescales characteristic of the environment in which they form, and can therefore provide important constraints on the formation routes and conditions of individual species. Aims. The aim of this paper is to systematically survey the deuteration and the ¹³ C content of a variety of oxygen-bearing complex organic molecules on solar system scales toward the “B component” of the protostellar binary IRAS16293–2422. Methods. We have used the data from an unbiased molecular line survey of the protostellar binary IRAS16293−2422 between 329 and 363 GHz from the Atacama Large Millimeter/submillimeter Array (ALMA). The data probe scales of 60 AU (diameter) where most of the organic molecules are expected to have sublimated off dust grains and be present in the gas phase. The deuterated and ¹³ C isotopic species of ketene, acetaldehyde and formic acid, as well as deuterated ethanol, are detected unambiguously for the first time in the interstellar medium. These species are analysed together with the ¹³ C isotopic species of ethanol, dimethyl ether and methyl formate along with mono-deuterated methanol, dimethyl ether and methyl formate. Results. The complex organic molecules can be divided into two groups with one group, the simpler species, showing a D/H ratio of ≈2% and the other, the more complex species, D/H ratios of 4–8%. This division may reflect the formation time of each species in the ices before or during warm-up/infall of material through the protostellar envelope. No significant differences are seen in the deuteration of different functional groups for individual species, possibly a result of the short timescale for infall through the innermost warm regions where exchange reactions between different species may be taking place. The species show differences in excitation temperatures between 125 and 300 K. This likely reflects the binding energies of the individual species, in good agreement with what has previously been found for high-mass sources. For dimethyl ether, the ¹² C/ ¹³ C ratio is found to be lower by up to a factor of 2 compared to typical ISM values similar to what has previously been inferred for glycolaldehyde. Tentative identifications suggest that the same may apply for ¹³ C isotopologues of methyl formate and ethanol. If confirmed, this may be a clue to their formation at the late prestellar or early protostellar phases with an enhancement of the available ¹³ C relative to ¹² C related to small differences in binding energies for CO isotopologues or the impact of FUV irradiation by the central protostar. Conclusions. The results point to the importance of ice surface chemistry for the formation of these complex organic molecules at different stages in the evolution of embedded protostars and demonstrate the use of accurate isotope measurements for understanding the history of individual species.
Article
Context. Hydroxylamine (NH 2 OH) and methylamine (CH 3 NH 2 ) have both been suggested as precursors to the formation of amino acids and are therefore, of interest to prebiotic chemistry. Their presence in interstellar space and formation mechanisms, however, are not well established. Aims. We aim to detect both amines and their potential precursor molecules NO, N 2 O, and CH 2 NH towards the low-mass protostellar binary IRAS 16293–2422, in order to investigate their presence and constrain their interstellar formation mechanisms around a young Sun-like protostar. Methods. ALMA observations from the unbiased, high-angular resolution and sensitivity Protostellar Interferometric Line Survey (PILS) are used. Spectral transitions of the molecules under investigation are searched for with the CASSIS line analysis software. Results. CH 2 NH and N 2 O are detected for the first time, towards a low-mass source, the latter molecule through confirmation with the single-dish TIMASSS survey. NO is also detected. CH 3 NH 2 and NH 2 OH are not detected and stringent upper limit column densities are determined. Conclusions. The non-detection of CH 3 NH 2 and NH 2 OH limits the importance of formation routes to amino acids involving these species. The detection of CH 2 NH makes amino acid formation routes starting from this molecule plausible. The low abundances of CH 2 NH and CH 3 NH 2 compared to Sgr B2 indicate that different physical conditions influence their formation in low- and high-mass sources.
Article
Context. Methyl isocyanide (CH 3 NC) is the isocyanide with the largest number of atoms confirmed in the interstellar medium (ISM), but it is not an abundant molecule, having only been detected towards a handful of objects. Conversely, its isomer, methyl cyanide (CH 3 CN), is one of the most abundant complex organic molecules detected in the ISM, with detections in a variety of low- and high-mass sources. Aims. The aims of this work are to determine the abundances of methyl isocyanide in the solar-type protostellar binary IRAS 16293–2422 and to understand the stark abundance differences observed between methyl isocyanide and methyl cyanide in the ISM. Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) to search for methyl isocyanide and compare its abundance with that of its isomer methyl cyanide. We use a new line catalogue from the Cologne Database for Molecular Spectroscopy (CDMS) to identify methyl isocyanide lines. We also model the chemistry with an updated version of the three-phase chemical kinetics model MAGICKAL, presenting the first chemical modelling of methyl isocyanide to date. Results. We detect methyl isocyanide for the first time in a solar-type protostar, IRAS 16293–2422 B, and present upper limits for its companion protostar, IRAS 16293–2422 A. Methyl isocyanide is found to be at least 20 times more abundant in source B compared to source A, with a CH 3 CN/CH 3 NC abundance ratio of 200 in IRAS 16293–2422 B and >5517 in IRAS 16293–2422 A. We also present the results of a chemical model of methyl isocyanide chemistry in both sources, and discuss the implications for methyl isocyanide formation mechanisms and the relative evolutionary stages of both sources. The chemical modelling is unable to match the observed CH 3 CN/CH 3 NC abundance ratio towards the B source at densities representative of that source. The modelling, however, is consistent with the upper limits for the A source. There are many uncertainties in the formation and destruction pathways of methyl isocyanide, and it is therefore not surprising that the initial modelling attempts do not reproduce observations. In particular, it is clear that some destruction mechanism of methyl isocyanide that does not destroy methyl cyanide is needed. Furthermore, these initial model results suggest that the final density plays a key role in setting the abundance ratio. The next steps are therefore to obtain further detections of methyl isocyanide in more objects, as well as undertaking more detailed physico-chemical modelling of sources such as IRAS16293.
Article
The radiolysis of pure N2O ice at 11 and 75 K by 90 MeV ¹³⁶Xe23 + ion irradiation has been studied by infrared spectroscopy (FTIR). Six daughter molecular species have been observed: NO2, (NO)2, N2O3, N2O4, N2O5 and O3. The chemical evolution of the new molecules formed in the sample was followed by the measurement of the column densities of the precursor and products as a function of the beam fluence. This procedure allows the determination of their formation and dissociation cross sections. Other processes monitored by FTIR were sublimation (inexistent at 11 K, but present at 75 K) and ice compaction by the ion beam. Comparison between results obtained for the 11 and 75 K ices shows that formation and destruction cross sections are higher (for light products) or much higher (for heavy products) at 75 K. This enhancement of chemical activity at higher temperature should not be attributed to higher projectile ionization rate but rather to a higher mobility of the radiolysis products in an ice undergoing slow sublimation. Although N2O ice has not yet been observed in space, it is reasonable to expect its occurrence since N and O are very abundant and reactive. Furthermore, if this ice is actually absent, the knowledge of the chemical-physical processes induced by ion irradiation on N2O ice at low temperature is necessary to explain its depletion.