319 research outputs found

    Deuterated molecules in DM Tau: DCO+, but no HDO

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    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

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    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

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    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

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    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 HC5_5H and cumulene carbene H2_2C5_5

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    The 13Σu−←X3Σg−1 ^3\Sigma_u^- \leftarrow X^3\Sigma_g^- transition of linear HC5_5H (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 HC2n+1_{2n+1}H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the 11A1←X1A11 ^1 A_1 \leftarrow X ^1 A_1 transition of the cumulene carbene pentatetraenylidene H2_2C5_5 (B).Comment: 7 pages, 4 figures, 5 table

    Tentative detection of phosphine in IRC+10216

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    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

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    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 C6_6H, 13^{13}C6_6H, and C6_6D

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    Electronic spectra of C6_6H are measured in the 18 950−21 10018\,950-21\,100 cm−1^{-1} domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C6_6H are presented, all probing the vibrational manifold of the B2Π^2\Pi electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for 13^{13}C6_6H and C6_6D), 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 0000_0^0 origin band, three non-degenerate stretching vibrations along the linear backbone of the C6_6H molecule are assigned: the Îœ6\nu_6 mode associated with the C-C bond vibration and the Îœ4\nu_4 and Îœ3\nu_3 modes associated with C≡\equivC triple bonds. For the two lowest Îœ11\nu_{11} and Îœ10\nu_{10} bending modes, a Renner-Teller analysis is performed identifying the ÎŒ2ÎŁ\mu^2\Sigma(Îœ11\nu_{11}) and both ÎŒ2ÎŁ\mu^2\Sigma(Îœ10\nu_{10}) and Îș2ÎŁ\kappa^2\Sigma(Îœ10\nu_{10}) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as ÎŒ2ÎŁ\mu^2\Sigma(Îœ9\nu_9) and ÎŒ2ÎŁ\mu^2\Sigma(Îœ8\nu_8) levels. In the excitation region below the first non-degenerate vibration (Îœ6\nu_6), some 2Π−2Π^2\Pi-^{2}\Pi transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a 2Π−2Π^2\Pi-^{2}\Pi transition found above the Îœ6\nu_6 level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B2Π^2\Pi state of C6_6H 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

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    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

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    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 17^{17}O/18^{18}O and 14^{14}N/15^{15}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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