12 research outputs found
Interstellar HOCN in the Galactic center region
Aims. Our aim is to confirm the interstellar detection of cyanic acid, HOCN,
in the Galactic center clouds. It has previously been tentatively detected only
in Sgr B2(OH).
Methods. We used a complete line survey of the hot cores Sgr B2(N) and (M) in
the 3 mm range, complemented by additional observations carried out with the
IRAM 30 m telescope at selected frequencies in the 2 mm band and towards four
additional positions in the Sgr B2 cloud complex in the 2 and 3 mm bands. The
spectral survey was analysed in the local thermodynamical equilibrium
approximation (LTE) by modeling the emission of all identified molecules
simultaneously. This allowed us to distinguish weak features of HOCN from the
rich line spectrum observed in Sgr B2(N) and (M). Lines of the more stable (by
1.1 eV) isomer isocyanic acid, HNCO, in these sources, as well as those of HOCN
and HNCO towards the other positions, were analysed in the LTE approximation as
well.
Results. Four transitions of HOCN were detected in a quiescent molecular
cloud in the Galactic center at a position offset in (R.A., decl.) by
(20'',100'') from the hot core source Sgr B2(M), confirming its previous
tentative interstellar detection. Up to four transitions were detected toward
five other positions in the Sgr B2 complex, including the hot cores Sgr B2(M),
(S), and (N). A fairly constant abundance ratio of ~ 0.3 - 0.8 % for HOCN
relative to HNCO was derived for the extended gas components, suggesting a
common formation process of these isomers
Submillimeter vibrationally excited water emission from the peculiar red supergiant VY CMa
Vibrationally excited emission from the SiO and H2O molecules probes the
innermost circumstellar envelopes of oxygen-rich red giant and supergiant
stars. VY CMa is the most prolific known stellar emission source in these
molecules. Observations were made to search for rotational lines in the lowest
vibrationally excited state of H2O. The APEX telescope was used for
observations of H2O lines at frequencies around 300 GHz. Two vibrationally
excited H2O lines were detected, a third one could not be found. In one of the
lines we find evidence for weak maser action, similar to known (sub)millimeter
H2O lines. We find that the other line's intensity is consistent with thermal
excitation by the circumstellar infrared radiation field. Several SiO lines
were detected together with the H2O lines.Comment: APEX A&A special issue, accepte
Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands
Hunting for the elusive methylene radical
CH2 transitions between 68 and 71 GHz were first detected toward the Orion-KL
and W51 Main SFRs. Given their upper level energies of 225 K, they were thought
to arise in dense, hot molecular cores near newly formed stars. However, this
has not been confirmed by further observations of these lines and their origin
has remained unclear. Generally, there is a scarcity of observational data for
CH2 and, while it is an important compound in the astrochemical context, its
actual occurrence in astronomical sources is poorly constrained. The present
study, along with other recent observations of the Orion region we report, rule
out the possibility of an association with gas that is both hot and dense. We
find that the distribution of the CH2 emission closely follows that of the
[CII] 158 um emission, while CH2 is undetected toward the hot core itself. The
observations suggest, rather, that its extended emission arises from hot but
dilute layers of PDRs and not from the denser parts of such regions as in the
case of the Orion Bar. This hypothesis was corroborated by comparisons of the
observed CH2 line profiles with those of CRRLs, well-known PDR tracers. In
addition, we report the detection of the 70 GHz fine- and hfs lines of o-CH2
toward the W51E, W51M, W51N, W49N, W43, W75N, DR21, and S140 SFRs, and three of
the fine- and hfs lines between 68-71 GHz toward W3 IRS5. Furthermore, using a
non-LTE radiative transfer analysis, we can constrain the gas temperatures and
H2 density to 163 K and 3.4e3 cm^-3, respectively. This analysis confirms our
hypothesis that CH2 originates in warm and dilute PDR layers. Our analysis
suggests that for the excitation conditions under the physical conditions that
prevail in such regions, these lines are masering, with weak level inversion.
The resulting amplification of the lines' spontaneous emission greatly aids in
their detection.Comment: Accepted in A&A, 25 pages, 18 figures, 5 table
LABORATORY DETECTION OF HOCN AND TENTATIVE IDENTIFICATION IN Sgr B2
The rotational spectrum of cyanic acid, HOCN, has been detected in the centimeter-wave band in a molecular beam by Fourier transform microwave spectroscopy and in the millimeter-wave band by conventional spectroscopy in a low-pressure laboratory discharge. Spectroscopic constants, including the nitrogen hyperfine coupling constant, derived from 31 a-type transitions between 21 and 360 GHz with J up to 17 and K-a <= 2 allow the spectrum in the only rotational ladders populated in the interstellar gas to be calculated well into the submillimeter-wave band to 1 km s(-1) or better in equivalent radial velocity. Four consecutive transitions of HOCN between 83.9 and 146.8 GHz were tentatively identified in published spectral line surveys of Sgr B2(OH). An approximate column density in Sgr B2(OH) of 6 x 10(12) cm(-2) implies that the HOCN/HNCO ratio is about 0.5%. Because HNCO is widely distributed in the interstellar gas, HOCN may be as well
Accurate High-N Rest Frequencies for CO+, an Ideal Tracer of Photon-Dominated Regions
The submillimeter-wave rotational spectra of CO+, (CO+)-C-13 and (CO+)-O-18 in the nu = 0 and 1 vibrational states were measured through a hollow cathode dc discharge in a cryogenic cell cooled to liquid nitrogen temperature. In addition, a few transitions Of the main isotopic species have been measured between 1.1 and 1.3 THz. An updated isotopically invariant fit, including Born-Oppenheimer breakdown corrections,. is presented: the derived set of independent molecular parameters, valid for all the isotopologues of the molecule included in the fit, allows to predict the rotational spectrum with calculated 1 sigma uncertainty of 280 kHz at 2 THz
Pure rotational spectra of TiO and TiO2 in VY Canis Majoris
We report the first detection of pure rotational transitions of TiO and TiO2 at (sub-)millimeter wavelengths towards the red supergiant VY CMa. A rotational temperature, Trot, of about 250 K was derived for TiO2. Although Trot was not well constrained for TiO, it is likely somewhat higher than that of TiO2. The detection of the Ti oxides confirms that they are formed in the circumstellar envelopes of cool oxygen-rich stars and may be the "seeds" of inorganic-dust formation, but alternative explanations for our observation of TiO and TiO2 in the cooler regions of the envelope cannot be ruled out at this time. The observations suggest that a significant fraction of the oxides is not converted to dust, but instead remains in the gas phase throughout the outflow.
Based on observations carried out with the Submillimeter Array and IRAM Plateau de Bure Interferometer.Plateau de Bure data (FITS file) is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/551/A113status: publishe
THE ROTATIONAL SPECTRA OF (SiC2)-Si-29 AND (SiC2)-Si-30
The rotational spectra of (SiC2)-Si-29 and (SiC2)-Si-30, two silicon isotopic species of the abundant astronomical ring (SiC2)-Si-28, have been characterized in the millimeter band between 140 and 360 GHz in a low pressure discharge through SiH4, C2H2, and Ar. Precise rotational and centrifugal distortion constants have been derived for both species by fitting a standard asymmetric top Hamiltonian to 38 a-type transitions of (SiC2)-Si-29 and 35 of (SiC2)-Si-30; the data sets include transitions up to K-a = 8 and at least J = 16. With these new measurements in hand, the most intense radio transitions of both species either have been measured or can now be predicted to better than 1 km s(-1) in equivalent radial velocity up to 500 GHz, more than adequate accuracy for spectral line identifications in circumstellar shells of evolved carbon stars such as IRC+10216 where SiC2 is conspicuous. More than 10 new lines of (SiC2)-Si-29 and (SiC2)-Si-30 have been identified between 295 and 354 GHz in the interferometric spectral line survey of IRC+10216 with the Submillimeter Array
Exploring a dynamical path for C2H− and NCO− formation in dark molecular clouds
This paper deals with the possible formation of two molecular anions often considered
likely components in the physical environments of the interstellar medium (ISM):
C2H− and NCO−. They are both discussed here by computationally
following the radiative association (RA) mechanism starting from C2−, H, N− and O as partners. The corresponding RA total cross
sections produced by the calculations are in turn employed to generate the overall
association rates over the relevant range of temperatures. The latter are found to be in
line with other molecular ions formed by RA but not large enough to uniquivocally suggest
this path as the main route to the anions formation in the ISM. Other possible paths of
formation are also analysed and discussed. The presence of resonant structures during the
association dynamics for both systems is found by the calculations and their consequences
are discussed in some detail in the present study