Evolution of the Circumstellar Atomic and Molecular Envelopes Around Evolved Stars
The asymptotic giant branch (AGB) mass loss history of stars is recorded in the structure of their circumstellar envelope (CSE). For the more evolved protoplanetary nebula (PPN) and planetary nebula (PN) phases, the CSE also chronicles its interaction with fast winds and photodissociating/photoionizing radiation. Most AGB stars lose mass in spherically symmetric outflows, whereas PNe are largely axisymmetric. To investigate the physical properties of these winds and the processes that shape the CSE prior to the PN stage, the neutral atomic and molecular gas are probed. Here we present observations of the atomic and molecular components of the CSE for objects at different epochs of evolution. Far-infrared (FIR) spectra were taken for 24 evolved stars, including the line emission of [O I], [C II], [Si I], [Si II], [S I], [Fe I], [Fe II], and [Ne II]. Atomic emission was found only in those sources where Teff ≥ 10000 K. With higher T eff the number of detectable lines increase as well as the intensity of the emission, which imply that these atomic lines originate from photodissociation regions (PDRs). The line measurements agree reasonably well with intensities predicted by PDR models. Shock models, however, do not compare well with the observed line intensities. The kinematics of the atomic gas are comparable to the molecular expansion velocities, expected for cooling lines associated with circumstellar PDRs. PDR mass estimates ranging from ∼0.01--1.3 M⊙ were derived from the [C II] 158 mum line emission. Half of these objects were selected for a complementary, study of the molecular gas component. In the 12CO J = 1 → 0 imaging survey, high resolution maps of their full molecular envelopes are presented. A variety of different nebular morphologies and kinematic structures were encountered. For the PPNe and PN, the neutral molecular envelopes are compared with images taken at optical, near-IR, and mid-IR wavelengths. These trace the different components of the envelope and show similar and/or complementary features with the CO maps. The presence of outflows clearly shape the surrounding envelopes; however, outflows are not present in some objects with unusual protruding morphologies. The origin of collimated molecular outflow does not appear to emerge at a particular evolutionary epoch, but can appear as early as the AGB phase. No matter what stage of evolution, a common feature among these evolved stars is an extended, expanding envelope. Full radiative transfer models were constructed for the CO line emission to constrain the physical conditions of the CSEs and to determine their history of mass loss.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign