Pulsation-triggered dust production by asymptotic giant branch stars

Journal article, 2018

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] greater than or similar to 0.55 mag; AK Hya, V744 Cen, RU Crt, alpha Her). Stars below this limit were not detected (BQ Gem, epsilon 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 similar to 60 d, similar to the known 'superwind' trigger at similar to 300 d. Between similar to 60 and similar to 300 d, an approximately constant mass-loss rate and wind velocity of similar to 3.7 x 10(-7) M-circle dot yr(-1) and similar to 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 less than or similar to 6300 L-circle dot) 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.