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

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

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...183.5113s&link_type=abstract

American Astronomical Society, 183rd AAS Meeting, #51.13; Bulletin of the American Astronomical Society, Vol. 25, p.1371

Astronomy and Astrophysics

Astronomy

Scientific paper

We have searched for CO J=1-0 emission (on the NRAO 12-m) in an IRAS-selected sample of ``60 microns 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 ~ fewx10(17) cm, implying that mass-loss ceased in these stars fewx1000 yr ago. All of these objects are ``60 microns excess'' sources - the 60 to 25 microns 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 microns flux ratio >0.3 was required (galaxies and planetary nebulae were excluded by constraints on the 100 to 60 microns, and the 25 to 12 microns flux ratios, respectively). The survey cutoff was 3 Jy at 60 microns. 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 microns 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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