The IR-colour -- mass-loss relation of carbon-rich, dust-driven superwinds and a synthetic ( J-K, M_Bol) diagram

K.-P. Schröder, A. Wachter, and J.M. Winters

Astronomy & Astrophysics, 398, 229-237 (2003)

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Abstract:

We derive relations between mass-loss rates and IR-colours (J-K, H-K, K-L, L-M and K-12) for the carbon-rich, dust-driven stellar winds of extreme tip-AGB objects by applying a maximum-likelihood procedure to a representative set of 50 self-consistent, pulsating wind models. The J-K index shows the largest mass-loss related IR excess, which is consistent with observations. All synthetic IR-colours depend, in addition, on the luminosity of the individual model star. Consequently, the superwind mass-loss rates may be determined from observation by log(dot{M}) = -8.20 + 0.156 (J - K) - 0.463 MBol, for J-K > 4. As a case study for the interpretation of IR photometric data, we quantify the collective and individual tip-AGB mass-loss of the solar neighbourhood stellar population by means of a matching synthetic stellar sample, its IR properties and its present-day mass-loss distribution. The synthetic stars are generated on a grid of evolution tracks with a
consistent mass-loss description (see Schröder et al. 1999, and Wachter et al. 2002) and an IMF and SFR found in the local stellar population (Schröder & Sedlmayr, 2001). The display of the tip-AGB stars in a (J-K, MBol) diagram could be compared directly with observations once appropriate data become available. On a basis of 1.4 million stars brighter than MV = 4.0, our synthetic present-day sample includes 5067 giant stars with B-V > 1.4, and the collective mass-loss rate is 5.0*10-4 Msun/yr. There are 20 carbon-rich supergiants with an IR excess of J-K > 4.0 and a mass-loss rate well in excess of 10-6 Msun/yr, including 10 dust-enshrouded, extreme tip-AGB stars with J-K > 6.0, seen in their short-lived (approx. 30,000 yrs) superwind phase with dot{M} > 10-5
Msun/yr. They produce about 50% of the collective mass-loss of the whole sample.