201 research outputs found

    Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks

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    We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of N2_2H+^+ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N2_2H+^+ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N2_2. Using the full gas-grain network model, we reproduced the N2_2H+^+ ring in a disk model with millimeter grains. The chemical conversion of CO and N2_2 to less volatile species (sink effect hereinafter) is found to affect the N2_2H+^+ distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N2_2 abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulas, which are derived by analyzing the full-network model. The N2_2H+^+ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N2_2H+^+, however, is different among the full-network model and no-sink model. The column density of N2_2H+^+ in the no-sink model depends sensitively on the desorption rate of CO and N2_2, and the flux of cosmic ray. We also found that N2_2H+^+ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N2_2 are the same.Comment: accepted to Ap

    From Prestellar to Protostellar Cores II. Time Dependence and Deuterium Fractionation

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    We investigate the molecular evolution and D/H abundance ratios that develop as star formation proceeds from a dense-cloud core to a protostellar core, by solving a gas-grain reaction network applied to a 1-D radiative hydrodynamic model with infalling fluid parcels. Spatial distributions of gas and ice-mantle species are calculated at the first-core stage, and at times after the birth of a protostar. Gas-phase methanol and methane are more abundant than CO at radii r≲100r\lesssim 100 AU in the first-core stage, but gradually decrease with time, while abundances of larger organic species increase. The warm-up phase, when complex organic molecules are efficiently formed, is longer-lived for those fluid parcels in-falling at later stages. The formation of unsaturated carbon chains (warm carbon-chain chemistry) is also more effective in later stages; C+^+, which reacts with CH4_4 to form carbon chains, increases in abundance as the envelope density decreases. The large organic molecules and carbon chains are strongly deuterated, mainly due to high D/H ratios in the parent molecules, determined in the cold phase. We also extend our model to simulate simply the chemistry in circumstellar disks, by suspending the 1-D infall of a fluid parcel at constant disk radii. The species CH3_3OCH3_3 and HCOOCH3_3 increase in abundance in 104−10510^4-10^5 yr at the fixed warm temperature; both also have high D/H ratios.Comment: accepted to ApJ. 55 pages, 7 figures, 3 table

    Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band

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    We have conducted a mapping spectral line survey toward the Galactic giant molecular cloud W51 in the 3 mm band with the Mopra 22 m telescope in order to study an averaged chemical composition of the gas extended over a molecular cloud scale in our Galaxy. We have observed the area of 25′×30′25' \times 30', which corresponds to 39 pc ×\times 47 pc. The frequency ranges of the observation are 85.1 - 101.1 GHz and 107.0 - 114.9 GHz. In the spectrum spatially averaged over the observed area, spectral lines of 12 molecular species and 4 additional isotopologues are identified. An intensity pattern of the spatially-averaged spectrum is found to be similar to that of the spiral arm in the external galaxy M51, indicating that these two sources have similar chemical compositions. The observed area has been classified into 5 sub-regions according to the integrated intensity of 13^{13}CO(J=1−0J=1-0) (I13COI_{\rm ^{13}CO}), and contributions of the fluxes of 11 molecular lines from each sub-region to the averaged spectrum have been evaluated. For most of molecular species, 50 % or more of the flux come from the sub-regions with I13COI_{\rm ^{13}CO} from 25 K km s−1^{-1} to 100 K km s−1^{-1}, which does not involve active star forming regions. Therefore, the molecular-cloud-scale spectrum observed in the 3 mm band hardly represents the chemical composition of star forming cores, but mainly represents the chemical composition of an extended quiescent molecular gas. The present result constitutes a sound base for interpreting the spectra of external galaxies at a resolution of a molecular cloud scale (∼10\sim10 pc) or larger.Comment: Accepted for publication in Ap
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