2018MNRAS.481.4984M

Eleven nearby (<300 pc), short-period (50-130 d) asymptotic giant branch (AGB) stars were observed in the CO J = 2-1 line. Detections were made towards objects that have evidence for dust production (K_s - [22] >= 0.55 mag; AK Hya, V744 Cen, RU Crt, α Her). Stars below this limit were not detected (BQ Gem, ε Oct, NU Pav, II Hya, CL Hyi, ET Vir, SX Pav). K_s - [22] colour is found to trace mass-loss rate to well within an order of magnitude. This confirms existing results, indicating a factor of 100 increase in AGB-star mass-loss rates at a pulsation period of ∼60 d, similar to the known 'superwind' trigger at ∼300 d. Between ∼60 and ∼300 d, an approximately constant mass-loss rate and wind velocity of ∼3.7 x 10^–7 M_☉ yr^–1 and ∼8 km s^–1 is found. While this has not been corrected for observational biases, this rapid increase in mass-loss rate suggests a need to recalibrate the treatment of AGB mass-loss in stellar evolution models. The comparative lack of correlation between mass-loss rate and luminosity (for L <= 6300 L_☉) suggests that the mass-loss rates of low-luminosity AGB-star winds are set predominantly by pulsations, not radiation pressure on dust, which sets only the outflow velocity. We predict that mass-loss rates from low-luminosity AGB stars, which exhibit optically thin winds, should be largely independent of metallicity, but may be strongly dependent on stellar mass.