CH3OH abundance in low mass protostars

Astronomy and Astrophysics (Impact Factor: 5.08). 01/2005; 442(2):527-538. DOI: 10.1051/0004-6361:20052899
Source: arXiv

ABSTRACT We present observations of methanol lines in a sample of Class 0 low mass protostars. Using a 1-D radiative transfer model, we derive the abundances in the envelopes. In two sources of the sample, the observations can only be reproduced by the model if the methanol abundance is enhanced by about two order of magnitude in the inner hot region of the envelope. Two other sources show similar jumps, although at a lower confidence level. The observations for the other three sources are well reproduced with a constant abundance, but the presence of a jump cannot be ruled out. The observed methanol abundances in the warm gas around low mass protostars are orders of magnitude higher than gas phase chemistry models predict. Hence, in agreement with other evidence, this suggests that the high methanol abundance reflects recent evaporation of ices due to the heating by the newly formed star. The observed abundance ratios of CH3OH, H2CO and CO are in good agreement with grain surface chemistry models. However, the absolute abundances are more difficult to reproduce and may indicate the presence of multiple ice components in these regions.

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    ABSTRACT: Aims:We use observations and models of molecular D/H ratios to probe the physical conditions and chemical history of the gas and to differentiate between gas-phase and grain-surface chemical processing in star forming regions. Methods: As a follow up to previous observations of HDCO/H2CO and DCN/HCN ratios in a selection of low-mass protostellar cores, we have measured D2CO/H2CO and N2D^+/N2H+ ratios in these same sources. For comparison, we have also measured N2D^+/N2H+ ratios towards several starless cores and have searched for N2D+ and deuterated formaldehyde towards hot molecular cores (HMCs) associated with high mass star formation. We compare our results with predictions from detailed chemical models, and to other observations made in these sources. Results: Towards the starless cores and low-mass protostellar sources we have found very high N2D+ fractionation, which suggests that the bulk of the gas in these regions is cold and heavily depleted. The non-detections of N2D+ in the HMCs indicate higher temperatures. We did detect HDCO towards two of the HMCs, with abundances 1-3% of H2CO. These are the first detections of deuterated formaldehyde in high mass sources since Turner (1990) measured HDCO/H2CO and D2CO/H2CO towards the Orion Compact Ridge. Figures 1-5 are only available in electronic form at
    Astronomy and Astrophysics 01/2007; 471(3):849-863. · 5.08 Impact Factor
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    ABSTRACT: Context: High levels of deuterium fractionation in gas-phase molecules are usually associated with cold regions, such as prestellar cores. Significant fractionation ratios are also observed in hot environments such as hot cores or hot corinos, where they are believed to be produced by the evaporation of the icy mantles surrounding dust grains, and are thus remnants of a previous cold (either gas-phase or grain surface) chemistry. The recent detection of DCN towards the Orion Bar, in a clump at a characteristic temperature of 70 K, has shown that high deuterium fractionation can also be detected in PDRs. The Orion Bar clumps thus appear to be a good environment for the observational study of deuterium fractionation in luke warm gas, allowing us to validate chemistry models for a different temperature range, where dominating fractionation processes are predicted to differ from those in cold gas (
    Astronomy and Astrophysics 01/2009; 508(2):737-749. · 5.08 Impact Factor
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    ABSTRACT: We investigate the molecular abundances in protostellar cores by solving the gas-grain chemical reaction network. As a physical model of the core, we adopt a result of one-dimensional radiation-hydrodynamics calculation, which follows the contraction of an initially hydrostatic prestellar core to form a protostellar core. Temporal variation of molecular abundances is solved in multiple infalling shells, which enable us to investigate the spatial distribution of molecules in the evolving core. The shells pass through the warm region of T ~ 20–100 K in several 104 yr and falls onto the central star in ~100 yr after they enter the region of T > 100 K. We found that the complex organic species such as HCOOCH3 are formed mainly via grain-surface reactions at T ~ 20–40 K, and then sublimated to the gas phase when the shell temperature reaches their sublimation temperatures (T ≥ 100 K). Carbon-chain species can be re-generated from sublimated CH4 via gas-phase and grain-surface reactions. HCO2+, which is recently detected towards L1527, are abundant at r = 100–2,000 AU, and its column density reaches ~1011 cm−2 in our model. If a core is isolated and irradiated directly by interstellar UV radiation, photo-dissociation of water ice produces OH, which reacts with CO to form CO2 efficiently. Complex species then become less abundant compared with the case of embedded core in ambient clouds. Although a circumstellar (protoplanetary) disk is not included in our core model, we can expect similar chemical reactions (i.e., production of large organic species, carbon-chains and HCO2+) to proceed in disk regions with T ~ 20–100 K.
    Proceedings of the International Astronomical Union 01/2008; 4:129 - 136.

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