Dust-enshrouded asymptotic giant branch stars in the solar neighbourhood

A study is made of a sample of 58 dust-enshrouded asymptotic giant branch (AGB) stars (including two possible post-AGB stars), of which 27 are carbon-rich and 31 are oxygen-rich. These objects were originally identified by Jura & Kleinmann as nearby (within about 1kpc of the Sun) AGB stars with high mass-loss rates (M_⊙>10^-6M_solaryr^-1). Ground-based near-infrared photometry, data obtained by the IRAS satellite and kinematic data (radial and outflow velocities) from the literature are combined to investigate the properties of these stars. The light amplitude in the near-infrared is found to be correlated with period, and this amplitude decreases with increasing wavelength. Statistical tests show that there is no reason to suspect any difference in the period distributions of the carbon- and oxygen-rich stars for periods less than 1000d. There are no carbon-rich stars with periods longer than 1000d in the sample. The colours are consistent with those of cool stars with evolved circumstellar dust shells. Luminosities and distances are estimated using a period-luminosity relation. Mass-loss rates, estimated from the 60-μm fluxes, show a correlation with various infrared colours and pulsation period. The mass-loss rate is tightly correlated with the K-[12] colour. The kinematics and scaleheight of the sample show that the sources with periods less than 1000d must have low-mass main-sequence progenitors. It is argued that the three oxygen-rich stars with periods over 1000d probably had intermediate-mass main-sequence progenitors. For the other stars an average progenitor mass of about 1.3M_solar is estimated with a final white dwarf mass of 0.6M_solar. The average lifetime of stars in this high mass-loss AGB phase is estimated to be about 4×10⁴yr, which suggests that these stars will undergo at most one more thermal pulse before leaving the AGB, if current theoretical relations between thermal interpulse period and core mass are correct.