ALMA observations of CO isotopologues towards six obscured post-asymptotic giant branch stars

Le résumé fourni par la source

Context. Low- and intermediate-mass stars evolve through the asymptotic giant branch (AGB) when an efficient mass-loss process removes a significant fraction of their initial mass. For most sources, this mass-loss process relies on the interplay between convection, stellar pulsations, and dust formation. However, predicting the mass-loss history of a given star from first principles is complex and not yet feasible at present. At the end of the AGB, at least some stars experience a substantial increase in their mass-loss rate for unknown reasons, leading to the creation of post-AGB objects that are completely enshrouded in thick dusty envelopes. Recent studies have suggested that some of these sources may be the product of interactions between an evolved star with a close companion. Aims. We observed six obscured post-AGB stars (four C-rich and two O-rich sources) to constrain the properties of their circumstellar envelopes, recent mass-loss histories, and initial masses of the central stars. Methods. We used observations of the J = 2 − 1 line of 13CO, C17O, and C18O with the Atacama Large Millimeter/submillimeter Array (ALMA) to determine the circumstellar gas masses and the 17O/18O isotopic ratios, the latter of which can be used to infer initial stellar masses. These results were interpreted in the context of comparisons with stellar evolution models in the literature and existing observations of other post-AGB stars. Results. Based on the inferred 17O/18O isotopic ratios, we find all stars to have relatively low initial masses (< 2 M⊙), contrary to literature indications of higher masses for some sources. One of the C-rich sources, HD 187885, has a low 17O/18O ratio; coupled with a low metallicity, this would imply a relatively low mass (∼1.15 M⊙) for a carbon star. For all but one source (GLMP 950), we observe kinematic components with velocities of ≳30 km s−1, which are higher than typical AGB wind expansion velocities. For most sources, these higher velocity outflows display point-symmetric morphologies. The case of Hen 3-1475 is especially spectacular, with the high-velocity molecular outflow appearing to be interleaved with the high-velocity outflow of ionised gas observed at optical wavelengths. Based on the size of the emission regions of the slow components of the outflows, we derived typical kinematic ages associated with the C18O J = 2 − 1 emission of ≲1500 years and obtained relatively high associated mass-loss rates (≳10−4 M⊙ yr−1). The sources with known spectral types are found to have evolved faster than expected, compared to stellar evolutionary models.

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