159 research outputs found
Gas-grain models for interstellar anion chemistry
Long-chain hydrocarbon anions CnH- (n=4, 6, 8) have recently been found to be
abundant in a variety of interstellar clouds. In order to explain their large
abundances in the denser (prestellar/protostellar) environments, new chemical
models are constructed that include gas-grain interactions. Models including
accretion of gas-phase species onto dust grains and cosmic-ray-induced
desorption of atoms are able to reproduce the observed anion-to-neutral ratios,
as well as the absolute abundances of anionic and neutral carbon chains, with a
reasonable degree of accuracy. Due to their destructive effects, the depletion
of oxygen atoms onto dust results in substantially greater polyyne and anion
abundances in high-density gas (with n_{H_2} >~ 10^5 cm^{-3}). The large
abundances of carbon-chain-bearing species observed in the envelopes of
protostars such as L1527 can thus be explained without the need for warm
carbon-chain chemistry. The C6H- anion-to-neutral ratio is found to be most
sensitive to the atomic O and H abundances and the electron density. Therefore,
as a core evolves, falling atomic abundances and rising electron densities are
found to result in increasing anion-to-neutral ratios. Inclusion of cosmic-ray
desorption of atoms in high-density models delays freeze-out, which results in
a more temporally-stable anion-to-neutral ratio, in better agreement with
observations. Our models include reactions between oxygen atoms and
carbon-chain anions to produce carbon-chain-oxide species C6O, C7O, HC6O and
HC7O, the abundances of which depend on the assumed branching ratios for
associative electron detachment
Millimeter-wave spectroscopy of titanium dioxide, TiO 2
a b s t r a c t The millimeter-wave rotational spectrum of TiO 2 in its ground vibrational state has been recorded in the frequency range from 248 to 345 GHz using the Cologne Supersonic Jet Spectrometer for Terahertz Applications (SuJeSTA). Forty-two b-type rotational transitions of the main isotopologue 48 TiO 2 and five transitions of 46 TiO 2 in natural abundance have been measured up to J = 22 and K a = 8 which corresponds to excitation temperatures of 170 K. TiO 2 was formed by laser ablation and adiabatically cooled in a supersonic jet of helium to rotational temperatures of 20 K. The new transitions have been analyzed together with previously reported data obtained from Fourier-transform microwave spectroscopy in the frequency range from 7 to 42 GHz. The improved and extended set of spectroscopic parameters provides accurate transition frequencies for future astronomical searches in the millimeter-wave region
Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae
We report on the discovery of an infrared emission band in the Spitzer
spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in
the circumstellar environment. We modelled the observed emission to derive the
temperature of the TiO molecules (\approx 600 K), an upper limit on the column
density (\approx 10^17.25 cm^{-2}) and a lower limit on the spatial extent of
the layer that contains these molecules. (\approx 4.6 stellar radii). This is
the first time that this TiO emission band is observed. A search for similar
emission features in the sample of S-type stars yielded two additional
candidates. However, owing to the additional dust emission, the identification
is less stringent. By comparing the stellar characteristics of NP Aur to those
of the other stars in our sample, we find that all stars with TiO emission show
large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These
characteristics might be necessary requirements for a star to show TiO in
emission, but they are not sufficient.Comment: 4 pages, 4 figures, letter to the edito
Search for corannulene (C20H10) in the Red Rectangle
Polycyclic Aromatic Hydrocarbons (PAHs) are widely accepted as the carriers of the Aromatic Infrared Bands (AIBs), but an unambiguous identification of any specific interstellar PAH is still missing. For polar PAHs, pure rotational transitions can be used as spectral fingerprints for identification. Combining dedicated experiments, detailed simulations and observations, we explore d the mm wavelength domain to search for specific rotational transitions of corannulene (C20H10). We performed high-resolution spectroscopic measurements and a simulation of the emission spectrum of ultraviolet-excited C20H10 in the environment of the Red Rectangle (RR), calculating its synthetic rotational spectrum. Based on these results, we conducted a first observational campaign at the IRAM 30-m telescope towards this source to search for several high-J rotational transitions of C20H10. The laboratory detection of the J = 112 ← 111 transition of corannulene showed that no centrifugal splitting is present up to this line. Observations with the IRAM 30-m telescope towards the RR do not show any corannulene emission at any of the observed frequencies, down to a rms noise level of Tmb= 8 mK for the J =135 → 134 transition at 137.615 GHz. Comparing the noise level with the synthetic spectrum, we are able to estimate an upper limit to the fraction of carbon locked in corannulene of about 1.0 × 10−5 relative to the total abundance of carbon in PAHs. The sensitivity achieved in this work shows that radio spectroscopy can be a powerful tool to search for polar PAHs. We compare this upper limit with models for the PAH size distribution, emphasizing that small PAHs are much less abundant than predicted. We show that this cannot be explained by destruction but is more likely related to the chemistry of their formation in the environment of the R
TIMASSS: The IRAS16293-2422 Millimeter And Submillimeter Spectral Survey. I. Observations, calibration and analysis of the line kinematics
While unbiased surveys observable from ground-based telescopes have
previously been obtained towards several high mass protostars, very little
exists on low mass protostars. To fill up this gap, we carried out a complete
spectral survey of the bands at 3, 2, 1 and 0.8 mm towards the solar type
protostar IRAS16293-2422. The observations covered about 200\,GHz and were
obtained with the IRAM-30m and JCMT-15m telescopes. Particular attention was
devoted to the inter-calibration of the obtained spectra with previous
observations. All the lines detected with more than 3 sigma and free from
obvious blending effects were fitted with Gaussians to estimate their basic
kinematic properties. More than 4000 lines were detected (with sigma \geq 3)
and identified, yielding a line density of approximatively 20 lines per GHz,
comparable to previous surveys in massive hot cores. The vast majority (~2/3)
of the lines are weak and due to complex organic molecules. The analysis of the
profiles of more than 1000 lines belonging 70 species firmly establishes the
presence of two distinct velocity components, associated with the two objects,
A and B, forming the IRAS16293-2422 binary system. In the source A, the line
widths of several species increase with the upper level energy of the
transition, a behavior compatible with gas infalling towards a ~1 Mo object.
The source B, which does not show this effect, might have a much lower central
mass of ~0.1 Mo. The difference in the rest velocities of both objects is
consistent with the hypothesis that the source B rotates around the source A.
This spectral survey, although obtained with single-dish telescope with a low
spatial resolution, allows to separate the emission from 2 different
components, thanks to the large number of lines detected. The data of the
survey are public and can be retrieved on the web site
http://www-laog.obs.ujf-grenoble.fr/heberges/timasss.Comment: 41 pages (26 pages of online Tables), 7 Tables and 6 Figure
Accurate laboratory rest frequencies of vibrationally excited CO up to and up to 2 THz
Astronomical observations of (sub)millimeter wavelength pure rotational
emission lines of the second most abundant molecule in the Universe, CO, hold
the promise of probing regions of high temperature and density in the innermost
parts of circumstellar envelopes. The rotational spectrum of vibrationally
excited CO up to \varv = 3 has been measured in the laboratory between 220
and 1940 GHz with relative accuracies up to , corresponding
to kHz near 1 THz. The rotational constant and the quartic
distortion parameter have been determined with high accuracy and even the
sextic distortion term was determined quite well for \varv = 1 while
reasonable estimates of were obtained for \varv = 2 and 3. The present
data set allows for the prediction of accurate rest frequencies of
vibrationally excited CO well beyond 2 THz.Comment: Astron. Astrophys, accepted; 5 pages, 2 Figures, 2 Table
Circumstellar molecular composition of the oxygen-rich AGB star IK~Tau: II. In-depth non-LTE chemical abundance analysis
Aims: Little information exists on the circumstellar molecular abundance
stratifications of many molecules. The aim is to study the circumstellar
chemical abundance pattern of 11 molecules and isotopologs (CO,
CO, SiS, SiO, SiO, SiO, HCN, CN, CS, SO, SO) in
the oxygen-rich evolved star IK~Tau. Methods: We have performed an in-depth
analysis of a large number of molecular emission lines excited in the
circumstellar envelope around IK~Tau. The analysis is done based on a non-local
thermodynamic equilibrium (non-LTE) radiative transfer analysis, which
calculates the temperature and velocity structure in a self-consistent way. The
chemical abundance pattern is coupled to theoretical outer wind model
predictions including photodestruction and cosmic ray ionization. Not only the
integrated line intensities, but also the line shapes, are used as diagnostic
tool to study the envelope structure. Results: The deduced wind acceleration is
much slower than predicted from classical theories. SiO and SiS are depleted in
the envelope, possibly due to the adsorption onto dust grains. For HCN and CS a
clear difference with respect to inner wind non-equilibrium predictions is
found, either indicating uncertainties in the inner wind theoretical modeling
or the possibility that HCN and CS (or the radical CN) participate in the dust
formation. The low signal-to-noise profiles of SO and CN prohibit an accurate
abundance determination; the modeling of high-excitation SO lines is
cumbersome, possibly related to line misidentifications or problems with the
collisional rates. The SiO isotopic ratios (SiO/SiO and
SiO/SiO) point toward an enhancement in SiO compared to
results of classical stellar evolution codes. Predictions for HO lines in
the spectral range of the Herschel/HIFI mission are performed. [abbreviated]Comment: 24 pagees, accepted for publication in Astronomy & Astrophysic
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