320 research outputs found
Deuterated molecules in DM Tau: DCO+, but no HDO
We report the detection of the J=2-1 line of DCO+ in the proto-planetary disk
of DM Tau and re-analyze the spectrum covering the 465 GHz transition of HDO in
this source, recently published by Ceccarelli et al. (2005). A modelling of the
DCO+ line profile with the source parameters derived from high resolution HCO+
observations yields a DCO+/HCO+ abundance ratio of about 0.004, an order of
magnitude smaller than that derived in the low mass cores. The re-analysis of
the 465 GHz spectrum, using the proper continuum flux (0.5 Jy) and source
systemic velocity (6.05 km/s), makes it clear that the absorption features
attributed to HDO and C6H are almost certainly unrelated to these species. We
show that the line-to-continuum ratio of an absorption line in front of a
Keplerian disk can hardly exceed the ratio of the turbulent velocity to the
projected rotation velocity at the disk edge, unless the line is optically very
thick (tau > 10 000). This ratio is typically 0.1-0.3 in proto-planetary disks
and is about 0.15 in DM Tau, much smaller than that for the alleged absorption
features. We also show that the detection of H2D+ in DM Tau, previously
reported by these authors, is only a 2-sigma detection when the proper velocity
is adopted. So far, DCO+ is thus the only deuterated molecule clearly detected
in proto-planetary disks
Photochemistry in the inner layers of clumpy circumstellar envelopes: formation of water in C-rich objects and of C-bearing molecules in O-rich objects
A mechanism based on the penetration of interstellar ultraviolet photons into
the inner layers of clumpy circumstellar envelopes around AGB stars is proposed
to explain the non-equilibrium chemistry observed in such objects. We show
through a simple modelling approach that in circumstellar envelopes with a
certain degree of clumpiness or with moderately low mass loss rates (a few
10^(-7) solar masses per year) a photochemistry can take place in the warm and
dense inner layers inducing important changes in the chemical composition. In
carbon-rich objects water vapor and ammonia would be formed with abundances of
10^(-8) - 10(^-6) relative to H2, while in oxygen-rich envelopes ammonia and
carbon-bearing molecules such as HCN and CS would form with abundances of
10^(-9) - 10^(-7) relative to H2. The proposed mechanism would explain the
recent observation of warm water vapor in the carbon-rich envelope IRC +10216
with the Herschel Space Observatory, and predict that H2O should be detectable
in other carbon-rich objects.Comment: 5 pages, 3 figures; accepted for publication in ApJ Letter
Detection of C5N- and vibrationally excited C6H in IRC +10216
We report the detection in the envelope of the C-rich star IRC +10216 of four
series of lines with harmonically related frequencies: B1389, B1390, B1394 and
B1401. The four series must arise from linear molecules with mass and size
close to those of C6H and C5N. Three of the series have half-integer rotational
quantum numbers; we assign them to the 2Delta and 2Sigma vibronic states of C6H
in its lowest (v_11) bending mode. The fourth series, B1389, has integer J with
no evidence of fine or hyperfine structure; it has a rotational constant of
1388.860(2) MHz and a centrifugal distortion constant of 33(1) Hz; it is almost
certainly the C5N- anion.Comment: Accepted for publication in ApJ Letter
Mapping the cold dust in edge-on galaxies at 1.2 mm wavelength
Using the IRAM 30-m telescope, we have mapped the 1.2mm continuum emission in
NGC 891, NGC 5907 and NGC 4565. In particular this latter galaxy shows a weak,
extended plateau that is correlated with HI in addition to the central peak and
ring structure common to all these spirals. Moreover, the outer part of this
dust emission of NGC 4565 is clearly warped. The average dust temperature in
the outer parts is 15 K and the derived dust absorption cross section is very
close to that predicted for the local diffuse clouds.Comment: 4 pages LaTeX plus gzipped tar-file, including style and 2
ps-figures; to be published in the proceeedings of the "Dust-Morphology"
Conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
Electronic spectra of linear HCH and cumulene carbene HC
The transition of linear HCH
(A) has been observed in a neon matrix and gas phase. The assignment is based
on mass-selective experiments, extrapolation of previous results of the longer
HCH homologues, and density functional and multi-state CASPT2
theoretical methods. Another band system starting at 303 nm in neon is assigned
as the transition of the cumulene carbene
pentatetraenylidene HC (B).Comment: 7 pages, 4 figures, 5 table
Tentative detection of phosphine in IRC+10216
The J,K = 1,0-0,0 rotational transition of phosphine (PH3) at 267 GHz has
been tentatively identified with a T_MB = 40 mK spectral line observed with the
IRAM 30-m telescope in the C-star envelope IRC+10216. A radiative transfer
model has been used to fit the observed line profile. The derived PH3 abundance
relative to H2 is 6 x 10^(-9), although it may have a large uncertainty due to
the lack of knowledge about the spatial distribution of this species. If our
identification is correct, it implies that PH3 has a similar abundance to that
reported for HCP in this source, and that these two molecules (HCP and PH3)
together take up about 5 % of phosphorus in IRC+10216. The abundance of PH3, as
that of other hydrides in this source, is not well explained by conventional
gas phase LTE and non-LTE chemical models, and may imply formation on grain
surfaces.Comment: 4 pages, 2 figures; accepted for publication in A&A Letter
Detection of the linear radical HC4N in IRC+10216
We report the detection of the linear radical HC4N in the C-rich envelope of
IRC+10216. After HCCN, HC4N is the second member of the allenic chain family
HC_(2n)N observed in space. The column density of HC4N is found to be 1.5
10**12 cm**(-2). The abundance ratio HC2N/HC4N is 9, a factor of two larger
than the decrement observed for the cyanopolyynes HC$_(2n+1)N/HC_(2n+3)N.
Linear HC_4N has a 3-Sigma electronic ground state and is one of the 3
low-energy isomeric forms of this molecule. We have searched for the bent and
ringed HC4N isomers, but could only derive an upper limit to their column
densities of about 3 10**(12) cm**(-2).Comment: Preprint of 10 page
A Spectroscopic Survey of Electronic Transitions of CH, CH, and CD
Electronic spectra of CH are measured in the cm
domain using cavity ring-down spectroscopy of a supersonically expanding
hydrocarbon plasma. In total, 19 (sub)bands of CH are presented, all
probing the vibrational manifold of the B electronically excited state.
The assignments are guided by electronic spectra available from matrix
isolation work, isotopic substitution experiments (yielding also spectra for
CH and CD), predictions from ab initio calculations as well as
rotational fitting and vibrational contour simulations using the available
ground state parameters as obtained from microwave experiments. Besides the
origin band, three non-degenerate stretching vibrations along the
linear backbone of the CH molecule are assigned: the mode
associated with the C-C bond vibration and the and modes
associated with CC triple bonds. For the two lowest and
bending modes, a Renner-Teller analysis is performed identifying the
() and both () and
() components. In addition, two higher lying bending
modes are observed, which are tentatively assigned as ()
and () levels. In the excitation region below the first
non-degenerate vibration (), some transitions are
observed that are assigned as even combination modes of low-lying bending
vibrations. The same holds for a transition found above the
level. From these spectroscopic data and the vibronic analysis a
comprehensive energy level diagram for the B state of CH is derived
and presented.Comment: Accepted for publication in The Journal of Physical Chemistry A (26
July 2016
Distribution of the molecular absorption in front of the quasar B0218+357
The line of sight to the quasar B0218+357, one of the most studied lensed
systems, intercepts a z=0.68 spiral galaxy, which splits its image into two
main components A and B, separated by ca. 0.3'', and gives rise to molecular
absorption. Although the main absorption component has been shown to arise in
front of image A, it is not established whether some absorption from other
velocity components is also occuring in front of image B. To tackle this
question, we have observed the HCO+(2-1) absorption line during the
commissioning phase of the new very extended configuration of the Plateau de
Bure Interferometer, in order to trace the position of the absorption as a
function of frequency. Visibility fitting of the self-calibrated data allowed
us to achieve position accuracy between ~12 and 80 mas per velocity component.
Our results clearly demonstrate that all the different velocity components of
the HCO+(2-1) absorption arise in front of the south-west image A of the
quasar. We estimate a flux ratio fA/fB = 4.2 (-1.0;+1.8 at 106 GHz.Comment: accepted for publication in A&A Letter special issue for the new
extended configuration of the Plateau de Bure Interferomete
Probing isotopic ratios at z = 0.89: molecular line absorption in front of the quasar PKS 1830-211
With the Plateau de Bure interferometer, we have measured the C, N, O and S
isotopic abundance ratios in the arm of a spiral galaxy with a redshift of
0.89. The galaxy is seen face-on according to HST images. Its bulge intercepts
the line of sight to the radio-loud quasar PKS 1830-211, giving rise at mm
wavelengths to two Einstein images located each behind a spiral arm. The arms
appear in absorption in the lines of several molecules, giving the opportunity
to study the chemical composition of a galaxy only a few Gyr old. The isotopic
ratios in this spiral galaxy differ markedly from those observed in the Milky
Way. The O/O and N/N ratios are low, as one would
expect from an object too young to let low mass stars play a major role in the
regeneration of the gas.Comment: accepted in A&
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