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Register a new userMagnesium radicals MgC$_5$N and MgC$_6$H in IRC+10216
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After the previous discovery of MgC$_3$N and MgC$_4$H in IRC+10216, a deeper Q-band (31.0-50.3 GHz) integration on this source had revealed two additional series of harmonically related doublets that we assigned on the basis of quantum mechanical calculations to the larger radicals MgC$_5$N and MgC$_6$H. The results presented here extend and confirm previous results on magnesium-bearing molecules in IRC,+10216. We derived column densities of (4.7$pm$1.3)$times$10$^{12}$ for MgC$_5$N and (2.0$pm$0.9)$times$10$^{13}$ for MgC$_6$H, which imply that MgC$_5$N/MgC$_3$N=0.5 and MgC$_6$H/MgC$_4$H = 0.9. Therefore, MgC$_5$N and MgC$_6$H are present with column densities not so different from those of the immediately shorter analogs. The synthesis of these large magnesium cyanides and acetylides in IRC+10216 can be explained for their shorter counterparts by a two-step process initiated by the radiative association of Mg$^+$ with large cyanopolyynes and polyynes, which are still quite abundant in this source, followed by the dissociative recombination of the ionic complexes.
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The evidence for benzonitrile (C$_6$H$_5$CN}) in the starless cloud core TMC-1 makes high-resolution studies of other aromatic nitriles and their ring-chain derivatives especially timely. One such species is phenylpropiolonitrile (3-phenyl-2-propynenitrile, C$_6$H$_5$C$_3$N), whose spectroscopic characterization is reported here for the first time. The low resolution (0.5 cm$^{-1}$) vibrational spectrum of C$_6$H$_5$C$_3$N} has been recorded at far- and mid-infrared wavelengths (50 - 3500 cm$^{-1}$) using a Fourier Transform interferometer, allowing for the assignment of band centers of 14 fundamental vibrational bands. The pure rotational spectrum of the species has been investigated using a chirped-pulse Fourier transform microwave (FTMW) spectrometer (6 - 18 GHz), a cavity enhanced FTMW instrument (6 - 20 GHz), and a millimeter-wave one (75 - 100 GHz, 140 - 214 GHz). Through the assignment of more than 6200 lines, accurate ground state spectroscopic constants (rotational, centrifugal distortion up to octics, and nuclear quadrupole hyperfine constants) have been derived from our measurements, with a plausible prediction of the weaker bands through calculations. Interstellar searches for this highly polar species can now be undertaken with confidence since the astronomically most interesting radio lines have either been measured or can be calculated to very high accuracy below 300 GHz.
We report the detection in IRC+10216 of lines of HNC $J$=3-2 pertaining to 9 excited vibrational states with energies up to $sim$5300 K. The spectrum, observed with ALMA, also shows a surprising large number of narrow, unidentified lines that arise in the vicinity of the star. The HNC data are interpreted through a 1D--spherical non--local radiative transfer model, coupled to a chemical model that includes chemistry at thermochemical equilibrium for the innermost regions and reaction kinetics for the external envelope. Although unresolved by the present early ALMA data, the radius inferred for the emitting region is $sim$0.06 (i.e., $simeq$ 3 stellar radii), similar to the size of the dusty clumps reported by IR studies of the innermost region ($r <$ 0.3). The derived abundance of HNC relative to H$_2$ is $10^{-8} <$ $chi$(HNC) $< 10^{-6}$, and drops quickly where the gas density decreases and the gas chemistry is dominated by reaction kinetics. Merging HNC data with that of molecular species present throughout the inner envelope, such as vibrationally excited HCN, SiS, CS, or SiO, should allow us to characterize the physical and chemical conditions in the dust formation zone.
A single dish monitoring of millimeter maser lines SiS J=14-13 and HCN nu_2 = 1^f J=3-2 and several other rotational lines is reported for the archetypal carbon star IRC+10216. Relative line strength variations of 5%~30% are found for eight molecular line features with respect to selected reference lines. Definite line-shape variation is found in limited velocity intervals of the SiS and HCN line profiles. The asymmetrical line profiles of the two lines are mainly due to the varying components. Their dominant varying components of the line profiles have similar periods and phases as the IR light variation, although both quantities show some degree of velocity dependence; there is also variability asymmetry between the blue and red line wings of both lines. Combining the velocities and amplitudes with a wind velocity model, we suggest that the line profile variations are due to SiS and HCN masing lines emanating from the wind acceleration zone. The possible link of the variabilities to thermal, dynamical and/or chemical processes within or under this region is also discussed.
A new chemical model is presented for the carbon-rich circumstellar envelope of the AGB star IRC+10216. The model includes shells of matter with densities that are enhanced relative to the surrounding circumstellar medium. The chemical model uses an updated reaction network including reactions from the RATE06 database and a more detailed anion chemistry. In particular, new mechanisms are considered for the formation of CN-, C3N- and C2H-, and for the reactions of hydrocarbon anions with atomic nitrogen and with the most abundant cations in the circumstellar envelope. New reactions involving H- are included which result in the production of significant amounts of C2H- and CN- in the inner envelope. The calculated radial molecular abundance profiles for the hydrocarbons C2H, C4H and C6H and the cyanopolyynes HC3N and HC5N show narrow peaks which are in better agreement with observations than previous models. Thus, the narrow rings observed in molecular microwave emission surrounding IRC+10216 are interpreted as arising in regions of the envelope where the gas and dust densities are greater than the surrounding circumstellar medium. Our models show that CN- and C2H- may be detectable in IRC+10216 despite the very low theorised radiative electron attachment rates of their parent neutral species. We also show that magnesium isocyanide (MgNC) can be formed in the outer envelope through radiative association involving Mg+ and the cyanopolyyne species.
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.
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