33 research outputs found
Discovery of interstellar isocyanogen (CNCN): further evidence that dicyanopolyynes are abundant in space
It is thought that dicyanopolyynes could be potentially abundant interstellar
molecules, although their lack of dipole moment makes it impossible to detect
them through radioastronomical techniques. Recently, the simplest member of
this chemical family, cyanogen (NCCN), was indirectly probed for the first time
in interstellar space through the detection of its protonated form toward the
dense clouds L483 and TMC-1. Here we present a second firm evidence of the
presence of NCCN in interstellar space, namely the detection of the metastable
and polar isomer isocyanogen (CNCN). This species has been identified in L483
and tentatively in TMC-1 by observing various rotational transitions in the 3
mm band with the IRAM 30m telescope. We derive beam-averaged column densities
for CNCN of 1.6e12 cm-2 in L483 and 9e11 cm-2 in TMC-1, which imply fractional
abundances relative to H2 in the range (5-9)e-11. While the presence of NCCN in
interstellar clouds seems out of doubt owing to the detection of NCCNH+ and
CNCN, putting tight constraints on its abundance is still hampered by the poor
knowledge of the chemistry that links NCCN with NCCNH+ and especially with
CNCN. We estimate that NCCN could be fairly abundant, in the range 1e-9 - 1e-7
relative to H2, as other abundant nitriles like HCN and HC3N.Comment: Accepted for publication in ApJ Letter
Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry
We present the first identification in interstellar space of the thioformyl
radical (HCS) and its metastable isomer HSC. These species were detected toward
the molecular cloud L483 thanks to observations carried out with the IRAM 30m
telescope in the 3 mm band. We derive beam-averaged column densities of 7e12
cm-2 for HCS and 1.8e11 cm-2 for HSC, which translate to fractional abundances
relative to H2 of 2e-10 and 6e-12, respectively. Although the amount of sulfur
locked by these radicals is low, their detection allows to put interesting
constraints on the chemistry of sulfur in dark clouds. Interestingly, the
H2CS/HCS abundance ratio is found to be quite low, around 1, in contrast with
the oxygen analogue case, in which the H2CO/HCO abundance ratio is around 10 in
dark clouds. Moreover, the radical HCS is found to be more abundant than its
oxygen analogue, HCO. The metastable species HOC, the oxygen analogue of HSC,
has not been yet observed in space. These observational constraints are
confronted with the outcome of a recent model of the chemistry of sulfur in
dark clouds. The model underestimates the fractional abundance of HCS by at
least one order of magnitude, overestimates the H2CS/HCS abundance ratio, and
does not provide an abundance prediction for the metastable isomer HSC. These
observations should prompt a revision of the chemistry of sulfur in
interstellar clouds.Comment: Accepted for publication in A&A Letter
The atmospheric chemistry of the warm Neptune GJ 3470b: influence of metallicity and temperature on the CH4/CO ratio
Current observation techniques are able to probe the atmosphere of some giant
exoplanets and get some clues about their atmospheric composition. However, the
chemical compositions derived from observations are not fully understood, as
for instance in the case of the CH4/CO abundance ratio, which is often inferred
different from what has been predicted by chemical models. Recently, the warm
Neptune GJ3470b has been discovered and because of its close distance from us
and high transit depth, it is a very promising candidate for follow up
characterisation of its atmosphere. We study the atmospheric composition of
GJ3470b in order to compare with the current observations of this planet, to
prepare the future ones, but also as a typical case study to understand the
chemical composition of warm (sub-)Neptunes. The metallicity of such
atmospheres is totally uncertain, and vary probably to values up to 100x solar.
We explore the space of unknown parameters to predict the range of possible
atmospheric compositions. Within the parameter space explored we find that in
most cases methane is the major carbon-bearing species. We however find that in
some cases, typically for high metallicities with a sufficiently high
temperature the CH4/CO abundance ratio can become lower than unity, as
suggested by some multiwavelength photometric observations of other warm
(sub-)Neptunes, such as GJ1214b and GJ436b. As for the emission spectrum of
GJ3470b, brightness temperatures at infrared wavelengths may vary between 400
and 800K depending on the thermal profile and metallicity. Combined with a hot
temperature profile, a substantial enrichment in heavy elements by a factor of
100 with respect to the solar composition can shift the carbon balance in
favour of carbon monoxide at the expense of CH4. Nevertheless, current
observations of this planet do not allow yet to determine which model is more
accurate.Comment: 12 pages, 8 figures, accepted in Astronomy & Astrophysic
Aromatic cycles are widespread in cold clouds
We report the detection of large hydrocarbon cycles toward several cold dense
clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q
band (31-50 GHz) using the Yebes 40m radiotelescope. Using the line stack
technique, we find statistically significant evidence of benzonitrile
(CHCN) in L1495B, Lupus-1A, and L483 at levels of 31.8,
15.0, and 17.2, respectively, while there is no hint of
CHCN in the fourth source, L1527. The column densities derived are in
the range (1.7-3.8) cm, which is somewhat below the
value derived toward the cold dense cloud TMC-1. When we simultaneously analyze
all the benzonitrile abundances derived toward cold clouds in this study and in
the literature, a clear trend emerges in that the higher the abundance of
HCN, the more abundant CHCN is. This indicates that aromatic cycles
are especially favored in those interstellar clouds where long carbon chains
are abundant, which suggests that the chemical processes that are responsible
for the formation of linear carbon chains are also behind the synthesis of
aromatic rings. We also searched for cycles other than benzonitrile, and found
evidence of indene (CH), cyclopentadiene (CH), and 1-cyano
cyclopentadiene (1-CHCN) at levels of 9.3, 7.5, and
8.4, respectively, toward L1495B, which shows the strongest signal
from CHCN. The relative abundances between the various cycles detected
in L1495B are consistent -- within a factor of three -- with those previously
found in TMC-1. It is therefore likely that not only CHCN but also
other large aromatic cycles are abundant in clouds rich in carbon chains.Comment: Accepted for publication in A&A Letters. Changes with respect to
previous version: language edited, error in abstract corrected, and title
change
The complex dust formation zone of the AGB star IRC+10216 probed with CARMA 0.25 arcsec angular resolution molecular observations
We present low spectral resolution molecular interferometric observations at
1.2 mm obtained with the Combined Array for Research in Millimetre-wave
Astronomy (CARMA) towards the C-rich AGB star IRC+10216. We have mapped the
emission of several lines of SiS, H13CN, SiO, and SiC2 in the ground and first
excited vibrational states with a high angular resolution of 0.25 arcsec. These
observations have allowed us to partially resolve the emission of the envelope
at distances from the star <50 stellar radii (R*), where the stellar wind is
mainly accelerated. The structure of the molecular emission has been modelled
with a 3D radiation transfer code. The emission of line SiS(v=0,J=14-13) is
best reproduced with a set of maser emitting arcs arranged between 5 and 20 R*.
The abundance of H13CN with respect to H2 decreases from 8e-7 at 1-5 R* to 3e-7
at 20 R*. The SiO observations are explained with an abundance <2e-8 in the
shell-like region between 1 and 5 R*. At this point, the SiO abundance sharply
increases up to (2-3)e-7. The vibrational temperature of SiO increases by a
factor of 2 due North-East between 20 and 50 R*. SiC2 is formed at the stellar
surface with an abundance of 8e-7 decreasing down to 8e-8 at 20 R* probably due
to depletion on to dust grains. Several asymmetries are found in the abundance
distributions of H13CN, SiO, and SiC2 which define three remarkable directions
(North-East, South-Southwest, and South-East) in the explored region of the
envelope. There are some differences between the red- and blue-shifted
emissions of these molecules suggesting the existence of additional asymmetries
in their abundance distributions along the line-of-sight.Comment: 22 pages, 16 figures, 9 tables, accepted for publication in MNRA
The abundance and excitation of molecular anions in interstellar clouds
We report new observations of molecular anions with the Yebes 40m and IRAM
30m telescopes toward the cold dense clouds TMC-1 CP, Lupus-1A, L1527, L483,
L1495B, and L1544. We detected for the first time C3N- and C5N- in Lupus-1A and
C4H- and C6H- in L483. In addition, we report new lines of C6H- toward the six
targeted sources, of C4H- toward TMC-1 CP, Lupus-1A, and L1527, and of C8H- and
C3N- in TMC-1 CP. Excitation calculations indicate that the lines of anions
accessible to radiotelescopes run from subthermally excited to thermalized as
the size of the anion increases, with the degree of departure from
thermalization depending on the H2 volume density and the line frequency. We
noticed that the collision rate coefficients available for the radical C6H
cannot explain various observational facts, which advises for a revisitation of
the collision data for this species. The observations presented here, together
with observational data from the literature, are used to model the excitation
of interstellar anions and to constrain their abundances. In general, the
anion-to-neutral ratios derived here agree within 50 % (a factor of two at
most) with literature values, when available, except for the C4H-/C4H ratio,
which shows higher differences due to a revision of the dipole moment of C4H.
From the set of anion-to-neutral abundance ratios derived two conclusions can
be drawn. First, the C6H-/C6H ratio shows a tentative trend in which it
increases with increasing H2 density, as expected from theoretical grounds. And
second, it is incontestable that the higher the molecular size the higher the
anion-to-neutral ratio, which supports a formation mechanism based on radiative
electron attachment. Nonetheless, calculated rate coefficients for electron
attachment to the medium size species C4H and C3N are probably too high and too
low, respectively, by more than one order of magnitude.Comment: Accepted for publication in A&
The spatial distribution of an aromatic molecule, C6H5CN, in the cold dark cloud TMC-1
We present a highly sensitive 2D line survey of TMC-1 obtained with the Yebes
40m radio telescope in the Q-band (31.13-49.53 GHz). These maps cover a region
of 320 arcsec x 320 arcsec centred on the position of the QUIJOTE line survey
with a spatial sampling of 20 arcsec. The region covering 240 arcsec x 240
arcsec, where a longer integration time was used, shows a homogenous
sensitivity of 2-4 mK across the band. We present in this work the first
determination of the spatial extent of benzonitrile (C6H5CN), which follows
that of cyanopolyynes rather well, but differs significantly from that of the
radicals CnH and CnN. We definitively conclude that aromatic species in TMC-1
are formed from chemical reactions involving smaller species in the densest
zones of the cloud.Comment: Accepted for publication in A&A Letter
The chemistry of H2NC in the interstellar medium and the role of the C + NH3 reaction
We carried out an observational search for the recently discovered molecule
H2NC, and its more stable isomer H2CN, toward eight cold dense clouds (L1544,
L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH3, L1498, and L1641N) and two diffuse
clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in
different types of interstellar regions and shed light on its formation
mechanism. We detected H2NC in most of the cold dense clouds targeted, 7 out of
8, while H2CN was only detected in 5 out of 8 clouds. The column densities
derived for both H2NC and H2CN are in the range 1e11-1e12 cm-2 and the
abundance ratio H2NC/H2CN varies between 0.51 and >2.7. The metastable isomer
H2NC is therefore widespread in cold dense clouds where it is present with an
abundance similar to that of H2CN. We did not detect either H2NC or H2CN in any
of the two diffuse clouds targeted, which does not allow to shed light on how
the chemistry of H2NC and H2CN varies between dense and diffuse clouds. We
found that the column density of H2NC is correlated with that of NH3, which
strongly suggests that these two molecules are chemically linked, most likely
ammonia being a precursor of H2NC through the C + NH3 reaction. We performed
electronic structure and statistical calculations which show that both H2CN and
H2NC can be formed in the C + NH3 reaction through two different channels
involving two different transition states which lie very close in energy. The
predicted product branching ratio H2NC/H2CN is very method dependent but values
between 0.5 and 0.8 are the most likely ones. Therefore, both the astronomical
observations and the theoretical calculations support that the reaction C + NH3
is the main source of H2NC in interstellar clouds.Comment: Accepted for publication in A&
A new protonated molecule discovered in TMC-1: HCCNCH+
In recent years we have seen an important increase in the number of
protonated molecules detected in cold dense clouds. Here we report the
detection in TMC-1 of HCCNCH+, the protonated form of HCCNC, which is a
metastable isomer of HC3N. This is the first protonated form of a metastable
isomer detected in a cold dense cloud. The detection was based on observations
carried out with the Yebes 40m and IRAM 30m telescopes, which revealed four
harmonically related lines. We derive a rotational constant B = 4664.431891 +/-
0.000692 MHz and a centrifugal distortion constant D = 519.14 +/- 4.14 Hz. From
a high-level ab initio screening of potential carriers we confidently assign
the series of lines to the ion HCCNCH+. We derive a column density of (3.0 +/-
0.5)e10 cm-2 for HCCNCH+, which results in a HCCNCH+/HCCNC abundance ratio of
0.010 +/- 0.002. This value is well reproduced by a state-of-the-art chemical
model, which however is subject to important uncertainties regarding the
chemistry of HCCNCH+. The observational and theoretical status of protonated
molecules in cold dense clouds indicate that there exists a global trend in
which protonated-to-neutral abundance ratios MH+/M increase with increasing
proton affinity of the neutral M, although if one restricts to species M with
high proton affinities (>700 kJ/mol), MH+/M ratios fall in the range 0.001-0.1,
with no apparent correlation with proton affinity. We suggest various
protonated molecules that are good candidates for detection in cold dense
clouds in the near future.Comment: Accepted for publication in A&A Letter