200 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
Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS
We report on the detection of vinyl cyanide (CH2CHCN), cyanomethyl radical
(CH2CN), methylacetylene (CH3CCH) and thioformaldehyde (H2CS) in the C-rich
star IRC +10216. These species, which are all known to exist in dark clouds,
are detected for the first time in the circumstellar envelope around an AGB
star. The four molecules have been detected trough pure rotational transitions
in the course of a 3 mm line survey carried out with the IRAM 30-m telescope.
The molecular column densities are derived by constructing rotational
temperature diagrams. A detailed chemical model of the circumstellar envelope
is used to analyze the formation of these molecular species. We have found
column densities in the range 5 x 10^(12)- 2 x 10^(13) cm^(-2), which
translates to abundances relative to H2 of several 10^(-9). The chemical model
is reasonably successful in explaining the derived abundances through gas phase
synthesis in the cold outer envelope. We also find that some of these
molecules, CH2CHCN and CH2CN, are most probably excited trough infrared pumping
to excited vibrational states. The detection of these species stresses the
similarity between the molecular content of cold dark clouds and C-rich
circumstellar envelopes. However, some differences in the chemistry are
indicated by the fact that in IRC +10216 partially saturated carbon chains are
present at a lower level than those which are highly unsaturated, while in
TMC-1 both types of species have comparable abundances.Comment: 9 pages, 5 figures; accepted for publication in A&
Confirmation of circumstellar phosphine
Phosphine (PH3) was tentatively identified a few years ago in the carbon star
envelopes IRC+10216 and CRL2688 from observations of an emission line at 266.9
GHz attributable to the J=1-0 rotational transition. We report the detection of
the J=2-1 rotational transition of PH3 in IRC+10216 using the HIFI instrument
on board Herschel, which definitively confirms the identification of PH3.
Radiative transfer calculations indicate that infrared pumping to excited
vibrational states plays an important role in the excitation of PH3 in the
envelope of IRC+10216, and that the observed lines are consistent with
phosphine being formed anywhere between the star and 100 R* from the star, with
an abundance of 1e-8 relative to H2. The detection of PH3 challenges chemical
models, none of which offers a satisfactory formation scenario. Although PH3
locks just 2 % of the total available phosphorus in IRC+10216, it is together
with HCP, one of the major gas phase carriers of phosphorus in the inner
circumstellar layers, suggesting that it could be also an important phosphorus
species in other astronomical environments. This is the first unambiguous
detection of PH3 outside the solar system, and a further step towards a better
understanding of the chemistry of phosphorus in space.Comment: Accepted for publication in ApJ Letter
Reactivity of OH and CH3OH between 22 and 64 K: Modelling the gas phase production of CH3O in Barnard 1b
In the last years, ultra-low temperature chemical kinetic experiments have
demonstrated that some gas-phase reactions are much faster than previously
thought. One example is the reaction between OH and CH3OH, which has been
recently found to be accelerated at low temperatures yielding CH3O as main
product. This finding opened the question of whether the CH3O observed in the
dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and
OH. Several chemical models including this reaction and grain-surface processes
have been developed to explain the observed abundance of CHO with little
success. Here we report for the first time rate coefficients for the gas-phase
reaction of OH and CH3OH down to a temperature of 22 K, very close to those in
cold interstellar clouds. Two independent experimental set-ups based on the
supersonic gas expansion technique coupled to the pulsed laser photolysis-laser
induced fluorescence technique were used to determine rate coefficients in the
temperature range 22-64 K. The temperature dependence obtained in this work can
be expressed as k(22-64 K) = (3.6+/-0.1)e-12 (T/ 300)^(-1.0+/-0.2) cm3
molecule-1 s-1. Implementing this expression in a chemical model of a cold
dense cloud results in CH3O/CH3OH abundance ratios similar or slightly lower
than the value of 3e-3 observed in Barnard 1b. This finding confirms that the
gas-phase reaction between OH and CH3OH is an important contributor to the
formation of interstellar CH3O. The role of grain-surface processes in the
formation of CH3O, although it cannot be fully neglected, remains
controversial.Comment: Accepted for publication in The Astrophysical Journa
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
Laboratory And Astronomical Detection Of The Negative Molecular Ion C3N-
The negative molecular ion C3N- has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s(-1) or better in equivalent radial velocity. Four transitions of C3N- were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C3N- is 0.5% that of C3N, or approximately 20 times greater than that of C4H- relative to C4H. The C3N- abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C3N- relative to C3N (< 0.8%) and a limit for C4H- relative to C4H (< 0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration ofNRFKorean government MEST 2012R1A1A1014646, 2012M4A2026720Southeast Physics Network (SEP-Net)Science and Technology Facilities Council ST/F002858/1, ST/I000976/1Swedish Research Council 2009-4088U.S. NSF AST-0708176, AST-1009799NASA NNX07AH09G, NNG04G177G, NNX11AE09GChandra grant SAO TM8-9009XBiochemistr
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&
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