The wind speeds, dust content, and mass-loss rates of evolved AGB and RSG stars at varying metallicity

Abstract

We present the results of our survey of 1612 MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud. We have discovered four new circumstellar maser sources in the LMC, and increased the number of reliable wind speeds from IR stars in the LMC from 5 to 13. Using our new wind speeds, as well as those from Galactic sources, we have derived an updated relation for dust driven winds: v exp ∝ ZL 0.4 . We compare the sub-solar metallicity LMC OH/IR stars with carefully selected samples of more metal-rich OH/IR stars, also at known distances, in the Galactic Centre and Galactic Bulge. For 8 of the Bulge stars we derive pulsation periods for the first time, using near-IR photometry from the VVV survey. We have modeled our LMC OH/IR stars and developed an empirical method of deriving gas-to-dust ratios and mass loss rates by scaling the models to the results from maser profiles. We have done this also for samples in the Galactic Centre and Bulge and derived a new mass loss prescription that includes luminosity, pulsation period, and gas-to-dust ra-tioṀ = 1.06 +3.5 −0.8 ·10 −5 (L/10 4 L ) 0.9±0.1 (P/500 d) 0.75±0.3 (r gd /200) −0.03±0.07 M yr −1 . The tightest correlation is found between mass loss rate and luminosity. We find that the gas-to-dust ratio has little effect on the mass loss of oxygen-rich AGB stars and RSGs within the Galaxy and the LMC. This suggests that mass loss of oxygen-rich AGB stars and RSGs is (nearly) independent of metallicity between a half and twice solar.

The evolution of red supergiant mass-loss rates

“…We also include results from NGC 2100 (BD16). Overplotted are also some commonly used M prescriptions (assuming a T eff of 3900K), including; De Jager et al (1988), Reimers (1975), Van Loon et al (2005) and Goldman et al (2017).…”

“…Reimers 1975;De Jager et al 1988;Van Loon et al 2005;De Beck et al 2010;Bonanos et al 2010;Goldman et al 2017) all showing significant scatter. For example the calibration of Mauron & Josselin (2011) has a large peak-topeak dispersion of a factor of ∼ 10.…”

“…the stars are of different masses and ages at the same luminosity). Goldman et al (2017) studied RSG winds across a range of metallicities, stating that lower metallicity environments yield slower wind speeds for stars. As M is directly proportional to the expansion velocity of the wind (see Equation 1), a lower wind speed will result in a lower derived M. Goldman et al (2017) measure a relation between expansion velocity of the wind and metallicity (v exp ∝Z L 0.4 ), with derived expansion velocities then being compared to mass-loss rates.…”

“…Goldman et al (2017) studied RSG winds across a range of metallicities, stating that lower metallicity environments yield slower wind speeds for stars. As M is directly proportional to the expansion velocity of the wind (see Equation 1), a lower wind speed will result in a lower derived M. Goldman et al (2017) measure a relation between expansion velocity of the wind and metallicity (v exp ∝Z L 0.4 ), with derived expansion velocities then being compared to mass-loss rates. From this we can estimate how large the effect of varying metallicity is on M. It is therefore possible that the v exp we have assumed for the RSGs in NGC 2100 (an LMC metallicity cluster) is systematically high.…”

Astrometry of OH/IR Stars Using 1612 MHz Hydroxyl Masers. I. Annual Parallaxes of WX Psc and OH138.0+7.2

“…Their respective pulsation periods are 650 and 1410 days (Engels et al 2015b), and both exhibit high mass loss rates of ∼10 −5 M ⊙ yr −1 (calculated based on the method in Goldman et al 2017). We describe the VLBI observations in Sect.…”