A CO search for interrupted mass-loss envelopes in AGB stars

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Session 51 -- Planetary Nebulae
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[51.13] A CO search for interrupted mass-loss envelopes in AGB stars

R. Sahai (JPL/ Caltech, Pasadena), A. Wootten (NRAO, Charlottesville, VA), J. DuPuy (Georgetown University, Washington, DC), K. Hinkle (NOAO, Tucson, AZ)

We have searched for CO J=1-0 emission (on the NRAO 12-m) in an IRAS-selected sample of ``60 $\mu$m excess'' AGB stars, in order to look for evidence of interrupted mass-loss. A small number of such stars have been found recently (Olofsson et al. 1990, Astr.Ap. 230, L13) - the mm-wave CO line shapes (doubly-peaked) and maps show the presence of a cold extended hollow shell with radius $\sim$ fewx10$^{17}$ cm, implying that mass-loss ceased in these stars fewx1000 yr ago. All of these objects are ``60 $\mu$m excess'' sources - the 60 to 25 $\mu$m flux ratio is significantly larger than the average ratio for AGB stars, suggesting that relatively little hot dust exists close to the star and cooler dust lies further out, in a shell. van der Veen and Habing (1988, Astr.Ap. 194,125) suggest that the mass-loss may get interrupted by a He-shell flash, thought to occur periodically in these stars - this explanation is consistent with the carbon-rich nature of these interrupted mass-loss stars. Detection of further examples of such objects envelopes has important implications for our understanding of the thermal-pulse phenomenon in AGB stars as a possible cause for modification of mass loss rates. To select for cold shells in our survey sample, a 60 to 25 $\mu$m flux ratio >0.3 was required (galaxies and planetary nebulae were excluded by constraints on the 100 to 60 $\mu$m, and the 25 to 12 $\mu$m flux ratios, respectively). The survey cutoff was 3 Jy at 60 $\mu$m. The survey list contains 97 sources (only a few of which do not appear to be stars), with both oxygen-rich and carbon-rich classifications, based on the IRAS LRS spectra. 23 new objects were detected, out of a total of 66 objects observed (in one or both of the 1-0 and 2-1 lines). A few objects have profiles indicating interrupted mass-loss. Mass-loss rates and dust-to-gas ratios have been estimated. High-resolution spectra of the 4.6 $\mu$m CO V=0-1 lines have been obtained (using the KPNO 4m/ FTS) for stars in our sample with M mag. <~0, as an independent probe of episodic mass-loss events.

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