Monitoring of dust-enshrouded AGB stars in the LMC

Twelve IRAS sources in the LMC with colours similar to those expected for dust-enshrounded AGB stars have been monitored in the infrared for 1880 days. The objects come from the list of Reid (1991). Nine of these objects are large amplitude variables with periods in the range 530 to 1295 days. The variability confirms their status as dust-enshrouded AGB stars. Combined with the results of Wood et al. (1992), a total of 15 large amplitude, dust-enshrouded AGB stars are now known in the LMC but it is estimated that many hundreds more await discovery. The objects studied in this paper are ∼2 magnitudes fainter in Mbol than the previously known variables. The low luminosities of six of the new objects suggest that they are evolving from the tip of the AGB of the dominant intermediate-age LMC population which has an initial mass∼1.5M and age∼23×109 years. It is suggested that these stars should be carbonrich although several of them have been tentatively classified as oxygen-rich. The period range of the objects is consistent with that expected from theoretical models of AGB evolution with mass loss. The observations presented here clearly demonstrate that once significant mass loss and dust formation occurs, large amplitude LPVs no longer fall on the tight (Mbol, logP ) or (K, logP ) relation found for the Mira variables with P <∼ 450 days.