Transitory O-rich chemistry in heavily obscured C-rich post-AGB stars

[摘要] Spitzer/IRS spectra of eleven heavily obscured C-rich sources rapidly evolving from asymptotic giant branch (AGB) stars to Planetary Nebulae are presented. IRAM 30m observations for three of these post-AGBs are also reported. A few (3) of these sources are known to exhibit strongly variable maser emission of O-bearing molecules such as OH and H2O, suggesting a transitory O-rich chemistry because of the quickly changing physical and chemical conditions in this short evolutionary phase. Interestingly, the Spitzer/IRS spectra show a rich circumstellar carbon chemistry, as revealed by the detection of small hydrocarbon molecules such as C2H2, C4H2, C6H2, C6H6, and HCN. Benzene is detected towards two sources, bringing up to three the total number of Galactic post-AGBs where this molecule has been detected. In addition, we report evidence for the possible detection of other hydrocarbon molecules like HC3N, CH3C2H, and CH3in several of these sources. The available IRAM 30m data confirm that the central stars are C-rich - in despite of the presence of O-rich masers - and the presence of high velocity molecular outflows together with extreme AGB mass-loss rates (∼10-4Mo/yr). Our observations confirm the polymerization model of Cernicharo [1] that predicts a rich photochemistry in the neutral regions of these objects on timescales shorter than the dynamical evolution of the central HII region, leading to the formation of small C-rich molecules and a transitory O-rich chemistry.

[发布机构] Instituto de Astrofísica de Canarias, C/Via Láctea s/n, La Laguna; E-38205, Spain^1;Departamento de Astrofísica, Universidad de la Laguna (ULL), La Laguna; E-38206, Spain^2;Herschel Science Centre. European Space Astronomy Centre, Research and Scientific Support Department of ESA, Villafranca del Castillo, Madrid; E-28080, Spain^3;Group of Molecular Astrophysics, ICMM, CSIC, C/Sor Juana Inés de La Cruz N3, Madrid; E-28049, Spain^4;Hamburger Sternwarte, Gojenbergsweg 112, Hamburg; D-21029, Germany^5;European Space Astronomy Centre, INSA S.A., P.O. Box 78, Madrid; E-28080, Spain^6

[效力级别] 天文学 [学科分类] 天文学(综合)

[关键词] Asymptotic giant branch stars;Chemical conditions;Dynamical evolution;Evolutionary phase;Hydrocarbon molecules;Molecular outflows;Polymerization model;Small hydrocarbons